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Electric Interview Questions:

In this article we are going to see the electrical interview questions from various subject such as transformer, measurements, electronics, circuit, generator, motor etc.

Electric Interview Questions In Transformer

What is the principle of power transformer?

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Generally there are two principles:

  • An electric current can produce a magnetic field in other coil with the principle of mutual induction typically Faraday’s Law of Electromagnetic induction
  • A changing magnetic field within a coil of wire induces a voltage across the ends of the coil. It is also called as electromagnetic induction.

 

What is the minimum rating of power transformer?

 

  • Generally, most of the power transformers are usually rated at 500KVA and above.
  • Without Direct connectivity of convention loads such transformer is called power transformer

 

When CT saturation happens?

 

External fault currents can lead to CT saturation. This can cause relay operating current to flow due to distortion of the saturated CT current

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Why we need over current protection in power X,MER?

 

Over current protection is used for the purpose of providing back up protection for large transformer or above 5MVA. Two phase fault and one ground fault relay is sufficient to provide overcurrent protection to star delta X,MER.

 

Do you about incipient faults?

 

Faults which are not serious at the beginning but which slowly develops into serious faults are known as incipient faults

 

Do you know the operating time of Buchholz Relay?

 

Buchholz relay is slow acting device. The minimum operating time of this relay is 0.1 sec and average time is 0.2 sec. It is mechanical operated relay.

Electric Interview Questions In Protection

 

What is the use of pressure relief valve is power X,MER?

An oil pressure relief valve is fitted at the top of the transformer tank. It is a spring controlled valve located at the end of an oil relief pipe protruding from the top of the tank. Whenever a surge in the oil is developed, it bursts, thereby allowing the oil to discharge rapidly. It operates when the pressure exceeds 10 PSI but closes automatically when the pressure falls below the critical level. This avoids the explosive rupture of the tank and the risk of fire.

 

Which type of fault occurs in transformer?

 

  • 1. Ground faults
  • 2. Phase to phase fault
  • 3. Inter turn faults
  • 4. Core faults
  • 5. Overload faults
  • 6. External short circuit faults
  • 7. Magnetizing inrush faults
  • 8. Ferro resonance faults
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Which types of characteristic can detect transformer internal faults?

 

There are three types of characteristic generally provide detection of transformer faults:

  1. An increase in phase current
  2. An increase in differential current
  3. Gas formation caused by fault arc

 

Explain unit protection and non-unit protection?

 

Unit protection are operated only for fault in the protected transformer\n Non unit protection are operated for a fault outside the transformer. It is also called as a backup protection of transformer.

 

Why we need biased or percentage differential protection?

 

A normal circulating current differential protection cannot be applied to a transformer due to the factors like ratio, tap position and magnetizing inrush etc.\n Hence it is necessary to incorporate the percentage bias in the differential circuit.

 

Explain restricted earth fault protection?

 

This protection is applied to single winding of the transformer and quite sensitive and straight forward. The relay is high impedance differential relay and remain stable for all out of zone faults. This protection is insensitive to the inter turn faults.

 

What is back up earth fault protection?

 

The backup earth fault protection take current signal from the neutral CT of the transformer. This the last back up protection for uncleared ground faults.

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Which the main reasons are for inter turn faults in transformer?

 

There are generally three types of main reasons for inter turn faulted in transformer:\n

  • Mechanical force on the winding due to external short circuits.
  • Excessive moisture in the paper insulation of the windings.
  • Structural failure of paper insulation due to aging.

Electric Interview Questions In Generator

What is generators? What its types?

 

A generator is a machine that transforms mechanical energy into electric power. Prime movers such as engines and turbines convert thermal or hydraulic energy into mechanical power.\n Thermal energy is derived from the fission of nuclear fuel or the burning of common fuels such as oil, gas or coal. The alternating current generating units of electric power utilities generally consist of steam turbine generators, gas combustion turbine generators, hydro generators, and internal combustion engine generators.

 

What is prime movers in electrical generators?

 

The prime movers used for utility power generation are predominantly steam turbines and internal combustion machines. High pressure / high temperature and high speed (1800 to 3600 rpm) steam turbines are used primarily in large industrial and utility power generating stations. Internal combustion machines are normally of the reciprocating engine type. The diesel engine is the most commonly used internal combustion machine, although some gasoline engines are also used.

 

 

Do you know about generator capacity?

 

Turbine units can be built for almost any desired capacity. The capacity of steam turbine driven generators in utility plants range from 5MW to 1000MW. Most of the installed steam turbine generators are rated less than 500MW.\n Gas turbine generators for electric power generation generally have capacities ranging from 100kW to 20 MW (but are used in multiple installation). The application of gas turbine generators include both continuous and peak load service.\n Diesel engine generator sets have capacities ranging from 500kW to 6500kW. These units are widly used in auxiliary or standby service in portable or stationary installations, but they may be used as the primary power source in some locations.\n Smaller units (steam turbine, gasoline or diesel engine) are also available for special applications or industrial plants.

 

What are the ratings of generator voltage?

 

Large generators used for commercial utilities are usually designed with output voltages rated between 11 and 18kV. Industrial plant generators are normally rated 2.4kV to 13.8kV, coinciding with standard distribution voltages. The generated voltage is stepped up to higher levels for long distance power transmission.

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Do you know the standard value of generator frequency?

 

Power generation in United States is standardized at 60Hz. The standard frequency is 50Hz in most foreign countries. Generators operating at higher frequencies are available for special frequencies.

 

How can we control generator voltage?

 

The terminal voltage of a generator operating in isolation is a function of the excitation on the rotor field winding. The generator output terminal voltage is normally maintained at the correct level by an automatic voltage regulator that adjusts the field current.

 

How to control frequency of electrical generators?

 

Electrical frequency is directly proportional to the rpm of the rotor which is driven by the prime mover. Because of this relationship, prime movers are controlled by governors that respond to variation in speed or frequency. The governor is connected to the throttle control mechanism to regulate speed, accomplishing frequency control automatically.

 

What do you know about parallel operation of generators?

 

Large power plants normally have more than one generator in operation at the same time. When generators are to be paralleled, it is necessary to synchronize the unit,s before closing the paralleling circuit breaker. This mean that the generators must be bought to approximately the same speed, the same phase rotation and position, and the same voltage. Proper synchronization is accomplished with the aid of a syncroscope, an instrument which indicates the difference in phase position and in frequency of two sources. Paralleling of generators is accomplished either manually or automatically with one incoming unit at a time.

 

How to generate DC power?

 

The requirement for direct current power is limited largely to special loads, for example, electrochemical processes, railway electrification, cranes, automotive equipments and elevators. Direct current power may be generated directly as such, but is more commonly obtained by conversion or rectification of AC power near the load.

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What is the general system requirement of alternators?

 

For the generation of EMF, there should be two basic systems\n

  •  Magnetic field system to produce the magnetic field.
  •  Armature system which houses the conductors on which the EMF, is to be induced.

 

Will the alternators have rotating armature system or stationary armature system?

 

Generally in alternators, the armature is stationary and the field rotates. Small low- voltage alternators often have a rotating armature and a stationary field winding. But it large alternators rotating armature field type is used.

 

What are the advantages of stationary armature and rotating field system?

 

1. The stationary armature coils can be insulated easily. 2. Higher peripheral speed can be achieved in the rotator. 3. Cooling of the winding is more efficient. 4. Only two slip rings are the required to give DC supply to the field system. 5. Output current can be easily supplied to the load circuit slip-rings and brushes are not necessary.

 

What is the meant by stator? What is the meant by rotator?

 

In any electrical machine (AC/DC motor or generator) the stationary member is called as stator similarly in all machines the rotating member is known as rotator.

 

What are the advantages of three phase motor over singal phase motor?

 

Three phase motor are having 1. Higher starting torques. 2. Improved speed regulation. 3. Less vibration. 4. Quieter operation compared to the single phase motors.

 

What is the basic principle of operation of alternators / DC generator?

 

They are working on the fundamental principle based on Faraday,s Laws of electromagnetic induction. This law states that, when a current carrying conductor moves in magnetic field, it induced an EMF.

 

How to minimize the eddy current losses?

 

When the core is laminated and insulated from each other with paper or varnish the eddy current loss is minimized.

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What meant by turbo alternator?

 

Turbo alternators are high speed alternator. Because of high speed of rotation, the rotor diameter is reduced and the axial length is increased. Two or four poles are generally used and steam turbines are used as prime movers.

 

What are the various types of rotors used in the alternators?

 

1. Salient pole rotor. 2. Non- salient pole rotor or cylindrical rotor.

 

Define pole pitch?

 

The distance of between the centers of two adjacent pole is called pole pitch. One pole pitch is equals to 180 electrical degrees. It is also defined as the number of slots per pole.

 

Electric Interview Questions In Switchgear

 

What is meant by reach point of the relay?

 

The farthest point from the relay location, which is still inside the zone of protection, is called the reach point.

 

What is difference between fuse and breaker?

 

Fuses are burnt when over current flows in the circuit but circuit breaker will just open during over current. Thus fuses are used only once but breaker can used by multiple number of times.

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What is the difference between isolator and electrical CB?

 

Isolator are mainly for switching purpose under normal conditions but they cannot operate in fault conditions. Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected.

 

Explain the working of an OC relay and mention the type of it?

 

An over-current (OC) relay has single input in the form of AC current. The output of the relay is normally- open contact which changes over to closed state when the relay trips. The relay has two settings. These are the time setting and the plug setting. The time setting decides the operating time of the relay while the plug setting decides the current required for the relay to pick up. There are three types of over current relay. 1. Instantaneous over-current relay 2. Definite time over-current relay 3. Inverse time over-current

 

Explain the working of an IDMT relay?

 

In an IDMT (Inverse definite minimum time) relay the operating principle is based on the idea that with the more severe a fault is the faster it should be cleared to be avoid damage to the apparatus. It is inverse in the sense the tripping time will decrease as the magnitude of the fault current increase.

 

How to select the pickup value of a relay?

 

We can set the pickup value of the relay keeping in mind that the relay should allow normal load as well as a certain degree of overload to be supplied. Thus the pickup value of the relay should be more than the allowable maximum load. At the same time the relay should be sensitive enough to respond to the smallest fault. Thus the pickup value should be less than the smallest fault current.

 

How to set the operating time of relay?

 

A relay must get an adequate chance to protect the zone under is primary protection. Only if the primary protection does not clear the fault the back-up protection should initiate tripping. Thus as soon as the fault takes place it sensed by both the primary and the back-up protection. Naturally the primary protection is the first to operate its operating time being less than that the back-up relay. We have to allow for the overshoot of the primary relay so that there is proper coordination between the primary and the back-up.

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When do we use IDMT relays and DTOS relays?

 

IDMT relays have better fault clearing times over DTOS relay. But DTOS relays are used for the lines which are short in length as Zs greater then Zi and if remains almost same.

 

Explain different types of fault in circuit breakers?

 

There are basically two types of fault. 1. Shunt faults: Basically shunts faults are short circuits. When the path of the load current is cut short because of breakdown of insulation we say that ,short circuit, has occurred which is nothing but a shunt fault. 2. Series fault: series faults are nothing but a break in the path of current. It is observed in practice that most of the open conductor faults sooner or later develop into some or the other short-circuit fault.

 

What are the various states of operation of a power system?

 

Normal, Alert, Emergency, In extremis and Restorative are various states of operation of power system.

 

Explain the working of a current transformer?

 

The current transformer has two jobs to do. Firstly, it steps down the current to such a level that it can be easily handled by the relay current coil. Secondly it insolates the relay circuitry from the high voltage of the EHV system. A CT whose primary is in series with the line in which current is to be measured.

 

Why the secondary of CT should not be open?

 

If the secondary of a CT is open current through it is zero and hence the ampere turns produced by secondary which generally oppose the primary ampere turns become zero. As there is no counter m.m.f, primary m.m.f produces high flux in the core which leads to excessive core losses and overheating.

 

Explain the working of a potential transformer?

 

The voltage transformer or potential transformer steps down the high voltage of the line to a level safe enough for the relaying system (pressure coil of relay) and personnel to handle. A PT primary is connected in parallel at the point where measurement is desired.

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Explain the working principal of the circuit breaker?

 

The circuit breaker is an electrically operated switch, which is capable of safely making as well as breaking, short-circuit currents.it has two contacts namely fixed contact and moving contact. Under normal conditions the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal and faulty conditions (when current exceed the rated value) an arc is produced between the fixed and moving contacts and thereby it forms the open circuit arc is extinguished by the arc Quenching media like air, oil, vacuum etc.

 

What are the different types of circuit breaker?

 

1. Air Break circuit breaker. 2. Oil circuit breaker. 3. Minimum oil circuit breaker. 4. Air blast circuit breaker. 5. Vacuum circuit breaker. 6. SF6 circuit breaker.

Electric Interview Questions In Relay

 

What are the functions of protective relays?

 

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.

What is back up protection?

 

Back up protection is the second line of defense, which operates if the primary protection fails to activate within a definite time delay.

 

How does the over voltage surge affect the power system?

 

The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generator and the other equipment connected to the line.

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What are symmetrical components?

 

It is a mathematical tool to resolve unbalanced components into balanced components.

 

Define negative sequence components?

 

It has three vectors equal in magnitude and displaced from each other by an angle 120 degree and has the phase sequence in opposite to its original phasors.

 

Give the consequences of short circuit?

 

Whenever short circuit occurs the current flowing through the coil increases to an enormous value. If protective relays are present, a heavy current also flows through the relay coil, causing it to operate by closing its contacts. The trip circuit is then closed, the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage system connected to the supply.

 

What is the need of relay coordination?

 

The operation of a relay should be fast and selective, i.e. It should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate there should be sufficiently quick back up protection so that the rest of the system is protected. By coordinating relays, fault can always be isolated quickly without serious disturbance to the rest of the system.

 

State the various types of earthing?

 

Solid earthing, resistance earthing, reactance earthing, voltage transformer earthing and zig- zag transformer earthing are the various types of earthing.

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State the three sequence components?

 

Positive sequence components, negative sequence components and zero sequence components.

 

State the different types of fault?

 

Symmetrical faults and unsymmetrical faults and open conductor faults are the main types of faults.

 

Define protected zone?

 

Protected zone are those which are directly protected by a protective system such as relays, fuses or switchgears. If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

 

What are the various faults that would affect an alternator?

 

(a) Stator faults: 1. Phase to phase fault 2. Phase to earth fault 3. Inter turn faults (b) 1. Earth faults 2. Fault between turns 3. Loss of excitation due to fuel failure (c) 1. Over speed 2. Loss of drive 3. Vacuum failure resulting in condenser pressure rise, resulting in shuttering of the turbine low pressure casing (d) 1. Fault on lines 2. Fault on bus bars

 

What are arcing grounds?

 

The presence of inductive capacitive currents in the isolated neutral system leads to formation of arcs called as arcing grounds.

 

Define positive sequence component and zero sequence components?

 

Positive sequence components are 3 vector equal in magnitude and displaced from each other by an angle 120 degree and having the phase sequence as original vector. Zero sequence components have 3 vectors having equal magnitudes and displaced from each other by an angle zero degrees.

 

State the various types of unsymmetrical faults?

 

Line to ground, line to line, and double line to ground faults are the types of unsymmetrical faults.

 

What are unit system and non-unit system?

 

A unit protective system is one in which only faults occurring within its protected zone are isolated. Faults occurring elsewhere in the system have no influence on the operation of a unit system. A non-unit system is a protective system which is activated even when the faults are external to its protected zone.

 

Electric Interview Questions In Power System

Define single line diagram?

 

Representation of various power system components in a single is defined as single line diagram.

 

What is arc suppression coil?

 

A method of reactance grounding used to suppress the arc due to arcing grounds.

 

State the significance of double line fault?

 

It has no zero sequence components and the positive and negative sequence networks are connected in parallel.

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Mention the withstanding current in our human body?

 

9mA is the withstanding current in our human body.

 

What is primary protection?

 

Primary protection is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker. It serves as the first line of defense.

 

What are the different types of earthing?

 

I) resistive earthing II) reactance earthing III) resonant earthing are different types of earthing.

 

State the significance of single line to ground fault?

 

In single line to ground fault all the sequence networks are connected in series. All the sequence currents are equal and the fault current magnitude is three times its sequence currents.

 

Difference between a fuse and circuit breaker?

 

Fuse is a low current interrupting device. It is a copper or an aluminum wire. Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.

 

Define per unit value?

 

It is defined as the ratio of actual value to its base value.

 

What is surge absorber? How do they differ from surge diverter?

 

Surge absorber is a device designed to protect electrical equipment from transient high voltage to limit the duration and amplitude of the following current. Surge diverter discharge impulse surge to the earth and dissipates energy in the form of heat.

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Define the terms “insulation coordination”?

 

Grading of withstand level of apparatus/ equipment with the protective levels of surge arresters and coordination at entire voltage level and various other voltage levels.

 

Name the different kinds of over current relay?

 

Induction type non- directional over current relay induction type directional over current relay and current differential relay.

 

Define energizing quantity?

 

It refer to the current or voltage which is used to activate the relay into operation.

 

Define operating time of relay?

 

It is defined as the time period extending from the occurrence of the fault through the relay detecting the fault to the operation of the relay

 

Define resetting time of relay?

 

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

Also Know About:   Why Capacitor Has Leading Power Factor

 

What are over and under current relays?

 

Over current relays are those that operate when the current in a line exceeds a predetermined value. (e.g. induction type non – directional / directional over current relay, differential over current relay) Whereas undercurrent relays are those which operate whenever the current in a circuit/ line drops below a predetermined value.(eg.: differential over voltage relay)

 

Mention any two applications of differential relay?

 

Protection of generator and generator transformer unit; protection of large motors and bus bar.

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What is biased differential beam relay?

 

The beam baised relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over current balanced beam relay type with an additional restraining coil. The restraining coil produces a bais force in the opposite direction to the operating force.

 

What is the need of relay coordination?

 

The operation of a relay should be fast and selective, I e it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also if a relay fails to operate there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays fault can always be isolated quickly without serious disturbance to the rest of system.

 

Give the limitation of Merz price protection?

 

Since neutral earthing resistances are often used to protect circuit from earth fault currents it becomes impossible to protect the whole of a star –connected alternator. If an earth –fault occurs near the neutral point the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CTs. In addition to this there always an inherent phase difference between the primary and the secondary quantities and a possibility of current.

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What is an under frequency relay?

 

An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.

 

Define the term pilot with reference to power line protection?

 

Pilot wires refer to the wires that connect the CT s placed at the end of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms

 

Mention any two disadvantages of carrier current scheme for transmission lineonly?

 

The program time (i.e. the time taken by the carrier to reach the other end- up to .1 percent miles) the response time of band pass filter capacitance phase – shift of the transmission line.

 

What are the features of directional relay?

 

High speed operation; high sensitivity; short- time thermal ratio; burden must not be excessive.

 

Explain mechanical losses in transformer?

 

The alternating magnetic field causing fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.

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Explain stray losses in transformer?

 

A portion of the leakage flux may induce eddy currents with in nearby conductive objects such as the transformer,s support structure be converted to heat.

 

Why we need power transformer?

 

Power transformers are used in transmission network of higher voltages for the step- up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33 kV) and are generally rated above 200 MVA.

 

Why we need distribution transformer?

 

Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11 kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200MVA.

 

Application of power transformer?

 

1. Power transformer used in in transmission network so they do not directly connect to the consumers, so load fluctuations are very less. 2. These are loaded fully during 24 hours a day so Cu losses and Fe losses takes place throughout day.

 

Application of distribution transformers?

 

1. Distribution transformer directly connected to the consumer so load fluctuation are very high. 2. Distribution transformer not loaded fully at all time so iron losses takes place 24 hour a day and Cu losses takes place based on load cycle.

 

Maximum efficiency of distribution transformer?

 

Distribution transformer is designed for maximum efficiency at 60 percent to 70 percent load as normally does not operate at full load all the time. Its load depends on distribution demand.

 

Maximum efficiency of power transformer?

 

Power transformer is designed for maximum efficiency at 100 percent load as it always runs at 100 percent load being near to generating station.

 

Importance of transformer winding?

 

1. Thickness of winding also plays a major part in deigning process. 2. The voltage that each winding sees determine the wires insulation thickness. 3. Once this voltage is known, the diameter of the selected insulated wire can be used 4. By knowing the wire diameter the number of turns per layer is calculated 5. Number of layers are calculated using the window height and winding margins.

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What is percent impedance voltage?

 

The percent impedance is the percent voltage required to circulate rated current flow through one transformer winding when another winding is short circuited at the rated voltage tap at rated frequency. This drop in voltage is due to the winding resistance and leakage reactance.

 

What do you know about corona

 

A luminous discharge due to the ionization of gas surrounding a conductor caused by a voltage gradient exceeding a certain critical value. It does not greatly heat the conducter, and it is limited to the region surrounding the conductor. While corona is a low energy process, over periods of time, it can degrade insulators, causing a system to fail due to dielectric breakdown.

 

What is creepage?

 

Current flowing between two conductors along a surface that is in contact with both conductors. Generally this can be neglected up until the voltage where corona or flashover occurs.

 

Two bulbs of 100W and 40W connected in series across a 230V supply. Whichbulb will glow bright and why?

 

Since two bulbs are in series, they will get equal amount of electrical current but as the supply voltage is constant across the bulb, so the resistance of 40W bulb is greater than t and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

 

If one lamp connect between two phases, it will glow or not?

 

If the voltage between the two phases is equal to the lamp voltage then the lamp will glow. When the voltage difference is very greater than it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

 

Why series motor cannot be started on no-load?

 

Series motor cannot be started without load because of high starting torque. Series motor are used in trains, crane etc.

 

What is automatic voltage regulator (AVR)?

 

AVR is an abbreviation for automatic voltage regulator. It is important part in synchronous generator. It controls the output voltage of the generator by controlling its excitation current. Thus it can control the output reactive power of the generator.

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Why we can’t store AC in Batteries instead of DC or can we store AC in batteries instead of DC?

 

We cannot store AC in batteries because AC changes their polarity up to 50 (When frequency = 50 Hz) or 60 (When frequency = 60 Hz) times in a second. Therefore the battery terminals keep changing Positive (+Ve) becomes Negative (-Ve) and vice versa, but the battery cannot change their terminals with the same speed so that,s why we can,t store AC in Batteries. In addition, when we connect a battery with AC Supply, then It will charge during positive half cycle and discharge during negative half cycle, because the Positive (+Ve) half cycle cancel the negative (-Ve) half cycle, so the average voltage or current in a complete cycle is Zero. So there is no chance to store AC in the Batteries.

 

Why AC systems are preferred over DC systems?

 

Due to following reasons, AC systems are preferred over DC systems: a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution. b. Plant cost for AC transmission (circuit breakers, transformers etc.) is much lower than the equivalent DC transmission c. From power stations, AC is produced so it is better to use AC then DC instead of converting it. d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

 

What are the various kind of cables used for transmission?

 

Cables, which are used for transmitting power, can be categorized in three forms: • Low-tension cables, which can transmit voltage up to 1000 volts. • High-tension cables can transmit voltage up to 23000 volts. • Super tension cables can transmit voltage 66 kV to 132 kV.

 

State Thevenin’s Theorem:

 

According to thevenin,s theorem, the current flowing through a load resistance Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).

 

State Norton’s Theorem.

 

The Norton,s theorem explains the fact that there are two terminals and they are as follows: • One is terminal active network containing voltage sources • Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance. The Norton,s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.

 

Why Power Plant Capacity Rated in MW and not in MV

 

For the following reasons, a Power plant capacity rating may be expressed in MW instead of MVA. In a Generating station, the prime mover (Turbine) generates only and only Active Power. That,s why we rated a power plant capacity in MW instead of MVA. Its mean no matter how large your generator is, but it depends on the capacity of the engine (Prime mover/Turbine) I.e. a 50MW turbine connected to a 90MVA alternator in a power plant will generate only 50MW at full load. In short, a power plant rating is specified in terms of prime mover /Turbine and not by the alternator set coupled to it.

Another thing is that, electric power company charges their consumer for kVA while they generate kW (or MW) at the power station. They penalize their consumer for low Power factor because they are not responsible for low power factor and kVA but you. Moreover, in power plant, power factor is 1 therefore MW is equal to MVA …… (MW = MVA x P.F).

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Why Motor rated in kW/Horsepower instead of kVA?

 

We know that Transformer rating may be expressed in kVA as well as Generator and Alternator rated in kVA Designer doesn,t know the actual consumer power factor while manufacturing transformers and generators i.e. the P.F of Transformer and Generator depends on the nature of connected load such as resistive load, capacitive load, and inductive load as Motors, etc. But Motor has fixed Power factor, i.e. motor has defined power factor and the rating has been mentioned in KW on Motor nameplate data table. That,s why we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA.

In addition, Motor is a device which converts Electrical power into Mechanical power. In this case, the load is not electrical, but mechanical (Motor,s Output) and we take into the account only active power which has to be converted into mechanical load. Moreover, the motor power factor does not depend on the load and it works on any P.F because of its design.

 

Why AC rated in Tons, Not in kW?

 

AC (Air-conditions and Refrigeration are always rated in Tons. Air conditioners are always rated in Tons capacity instead of kW because Air conditioners are designed on the basis of quantity of heat removal from room, hall or specific area. Quantity of heat is termed in Tons means if an air conditioner is able to remove 1000 kilocalories of heat or 4120 kilojoules or 12000 BTU of heat in an hour that AC rated as 1 Ton of AC because 1000 Kilocalories or 4120 kilojoules or 12000 BTU equal to one Ton of heat. Also, this is the same case for freezer and refrigerator i.e. refrigeration system

 

What is The Main Difference between Active and Passive Components?

 

Active Components: Those devices or components which required external source to their operation is called Active Components. For Example: Diode, Transistors, SCR etc… Explanation and Example: As we know that Diode is an Active Components. So it is required an External Source to its operation. Because, If we connect a Diode in a Circuit and then connect this circuit to the Supply voltage., then Diode will not conduct the current Until the supply voltage reach to 0.3(In case of Germanium) or 0.7V(In case of Silicon). Passive Components: Those devices or components which do not required external source to their operation is called Passive Components.

For Example: Resistor, Capacitor, Inductor etc… Explanation and Example: Passive Components do not require external source to their operation. Like a Diode, Resistor does not require 0.3 0r 0.7 V. I.e., when we connect a resistor to the supply voltage, it starts work automatically without using a specific voltage. If you understood the above statement about active Components, then you will easily get this example.

In other words: Active Components: Those devices or components which produce energy in the form of Voltage or Current are called as Active Components Passive Components: Those devices or components which store or maintain Energy in the form of Voltage or Current are known as Passive Components In very Simple words; Active Components: Energy Donor Passive Components: Energy Acceptor Also Passive Components are in linear and Active Components are in nonlinear category.

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How can eddy current loss be minimized?

 

By laminating the core, eddy current loss can be minimized.

 

Does the transformer draw any current when its secondary is open?

 

Yes, no-load primary current.

 

Why, when birds sit on transmission lines or current wires does not get shock?

 

It,s true that if birds touch the single one line (phase or neutral) they do not get shock. If birds touch 2 lines than the circuit is closed and they get shock.

 

What is meant by armature reaction?

 

The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

 

What happen if we give 220 volts dc supply to bulb or tube light?

 

Bulbs for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it my damage the bulb element.

 

Which motor has high starting torque and starting current DC motor, induction motor or synchronous motor?

 

DC series motor has high starting torque. We cannot start the induction motor and synchronous motors on load, but cannot start the DC series motor without load.

 

What is ACSR cable and where we use it?

 

ACSR means Aluminum conductor steel reinforced, this conductor is used in transmission and distribution.

 

What is vacuum current breaker? Define with cause and where we use it device?

 

A breaker is normally used to break the circuit, while breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionized and result in the arc. Various medium are used to quench this arc in represented CB,s. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure the arc formation is interrupted. VCB,s can be used up to kV.

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What will happen when power factor is leading in distribution of power?

 

If there is high power factor, i.e. if the power factor is close to one: 1. Losses in form of heat will be reduced. 2. Cable becomes less bulky and easy to carry, and very cheap to afford. 3. It also reduces over heating of transformers.

 

What the one main difference between UPS and inverter?

 

Uninterrupted power supply (UPS) is mainly use for short time. Means according to UPS VA it gives backup. UPS is also two types: on line and offline. Online UPS having high volt and amp for long time backup with high DC voltage but UPS start with 2V DC with 7 amp. But inverter is start with 2V, 24amp DC to 36V DC and 20amp to 80amp battery with long time backup.

 

What is 2 phase motor?

 

A two phase motor is a motor with the starting winding and the running winding having a phase split e.g. servo motor where the auxiliary winding and the control winding have a phase split of 90 degree.

 

What is the significance of vector grouping in power transformers?

 

Every transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage e.g. DYN means delta primary, wye secondary and the current is at o clock referred to the voltage.

 

Which type of A.C motor is used in the fan which are find in the house?

 

Its single phase induction motor which mostly squirrel cage rotor.

 

Give two basic speed control scheme of DC shunt motor?

 

By using flux control method: In this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since the speed is inversely proportional to flux speed can be controlled.NArmature control method: In this method a rheostat is connected across armature winding by varying the resistance, the value of resistive drop can be varied and since speed is directly proportional to resistive drop so speed can be controlled.

 

What is meant by armature reaction?

 

The effect of armature flux to main flux is called armature reaction. The armature flux may support main flux or opposes main flux

 

What is the difference between synchronous generator and asynchronous generator?

 

In simple, synchronous generator supply,s both active and reactive power but asynchronous generator (induction generator) supply,s only active power and observe reactive power for magnetizing. This type of generators are used in windmills.

 

What is the polarization index value?

 

Its ratio between installation resistance (IR) i.e. meggar value for omin to insulation resistance for min. it ranges from 5-7 for new motor and normally for motor to be in good condition it should be greater than 0.5.

 

Why synchronous generators are used for the production of electricity?

 

Synchronous machine have capability to work on different power factor or say different imaginary power varying field emf. Hence synchronous generator used for the production of electricity.

 

What are the types of DC generator?

 

DC generator are classified into two types: (i) Separately excited DC generator (ii) Self-excited DC generator

 

What is an exciter and how it work?

 

There are two types of exciter. Static exciter and rotatory exciter. Purpose of exciter is to supply the excitation DC voltage to the fixed poles of generator.

 

Why use the VCB at high transmission system? Why cannot use ACB?

 

Actually the thing is vacuum has high arc quenching property compare to air because in VCB the die electric strengths equal to 8 times of air. That,s why always vacuum used as in HT breaker and air used as in LT.

 

What is the difference between surge arrestor and lightning arrestor?

 

Lightning arrestor is installed outside and effect of lightning is grounded, where s surge arrestor installed inside panels comprising of resistors consumes the energy and nullify the effect of surge.

 

What happen if I connect a capacitor to a generator load?

 

Connect a capacitor across a generator always improves power factor, but it will help depends upon the engine capacity of the alternator, otherwise the alternator will be over loaded due to extra watts consumed due to the improvement on PF. Secondly, do not connect a capacitor across an alternator while it is picking up or without any other load.

 

Why the capacitors works on AC only?

 

Generally capacitor gives infinite resistance to DC component (i.e. block the DC components). It allows the AC components to pass through.

 

 

Explain the working principal of the circuit breaker?

 

Circuit breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact and moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal and faulty conditions an arc is produced between the fixed and moving contacts and thereby it forms the open circuit arc is extinguished by the arc quenching media like air, oil, vacuum etc.

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Write the types of cooling system in transformer?

 

ONAP (oil natural, air natural) ONAF (oil natural, air forced) OFAF (oil forced, air forced) ODWF (oil direct, water forced) OFAN (oil forced, air natural)

 

What is the function of anti-pumping in circuit breaker?

 

When breaker is close at one time by close push button, the anti-pumping contactor prevent reclose the breaker by close push button after if it already close.

 

What is stepper motor? What is its uses?

 

Stepper motor is the electrical machine which act upon input pulse applied to it. It is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle. So, in automation parts it is used

 

 

Tell me in detail about CT and PT?

 

The term CT means current transformer and the term PT means potential transformer. In circuit where measurements of high voltage and high current is involved, they are used there. Particularly when a measuring device like volt meter or ammeter is not able to measure such high value of quantity because of large value of torque. Due to such high value it can damage the measuring device, so CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagnetic flux produced by primary with secondary.

 

 

If we have a transformer and an induction machine. These two have same supply. For which device the load current will be maximum and why?

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The motor has maximum load current compare to that of transformer because the motor consumes real power and transformer is only producing the working flux and it,s not consuming. Hence the current in the transformer is because of core loss so it is minimum.

 

 

What is power factor? Whether it should be high or low? Why?

 

It is the ratio of true power to apparent power it has to be ideally 1. If it is too low then cables overheating and equipment over loading will occur. If it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping. Power factor should be high in order to get smooth operation of the system. Low power factor means losses will be more.

 

 

What is the difference between isolator and circuit breaker?

 

Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of ant maintenance on downstream circuit. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load first the load on it must be made zero and then it can safely operate. Its specification only rated current is given. But circuit breaker in on load automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc. it has three specification. 1st is rated current. 2nd is short circuit breaking capacity nad the 3rd is instantaneous tripping current.

 

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What is frantic effect?

 

Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

 

 

What is meant by insulation voltage in cables?

 

It is property of a cable by virtue of it can withstand the applied voltage without rupturing. It is known as insulation level of the cable.

 

 

What is SF6 circuit breaker?

 

  • SF6 is sulpher hexa fluoride gas. If this gas is used as arc quenching medium in a circuit breaker then it is called SF6 circuit breaker.

 

 

Why we do two types of earthing on transformer i.e. body earthing and neutral earthing?

 

The two types of earthing are familiar s equipment earthing and system earthing. In equipment earthing: body (non conducting part) of the equipment should be earthed to safe guard the human beings. In system earthing: neutral of the supply source (transformer or generator) should be grounded.

 

 

Where the lightning arrestor should be placed in distribution lines?

 

Near the distribution transformers and outgoing feeders of 11 kV and incoming feeders of 33kV and near power transformer in sub stations

 

 

What is IDMT relay?

 

It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense, the tripping time will decrease as the magnitude of fault current increase.

 

 

What are the transformer losses?

 

Transformer losses have two source, copper loss and magnetic loss. Copper losses are caused by the resistance of wire. Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measureable loss. Hysteresis loss is constant for a particular voltage and current. Eddy current loss, however is different for each frequency passed through the transformer.

 

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What is boucholz relay and what is its significance in transformer?

 

Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay, whenever any internal fault occurs in a transformer, it at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

 

 

What is meant by regenerative breaking?

 

When the supply is cutoff for running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature windings and the motor should have maintained continuous field supply, so that back emf voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called regenerative breaking.

 

 

Why the starting current is high in a DC motor?

 

In DC motors, voltage equation is: V = Eb – IaRaWhere V = terminal voltage, Eb = back emf in motor, Ia = armature current, Ra = armature resistance. At starting Eb is zero. Therefore, V = IaRa, Ia = V/Ra, where Ra is very less. i.e Ia will become enormously increased.

 

 

What are the advantages of star delta starter with induction motor?

 

The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of direct online starting. Hence the starting current is reduced, the voltage drops during the starting of motor in systems are reduced.

 

 

Why star delta transformer are used for lighting loads?

 

For lighting loads, neutral conductor is must and hence the secondary must be star windings and this lighting lod is always unbalanced in all three phases. To minimize the current unbalance in the primary, we use delta windings in the primary, so delta star transformer is used in lighting loads.

 

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Why in a three pin plug the earth pin is thicker and longer than the other pins?

 

It depends upon R = rho 1/a where area is inversely proportional to resistance so if area increases, R decreases and if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the first to make the connection and last to disconnect should be earth pin. This assures safety for the person who uses the electrical instrument.

 

 

How electrical power is generated by n AC generator?

 

For the generation of electrical power we need a prime mover which supplies mechanical power input to the alternator can be steam turbines or hydro turbines. When poles of the rotor moves under the armature conductor which are placed on the stator, field flux cut the armature conductor, therefore voltage is generated and is of sinusoidal in nature… due to polarity change of rotor poles i.e. N-S-N-S.

 

 

Why temperature rise is conducted in bus bars and isolators?

 

Bus bars and isolators are rated for continuous power flow that means they carry heavy current which rises their temperature. So it is necessary to test this devices for temperature rise.

 

 

When voltage increases then current also increases. Then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

 

No, we cannot sense the over voltage by just measuring the current only because the current increases not only for over voltage but also for under voltage. So, the over voltage protection and over current protection are completely different. Over voltage relay meant for sensing over voltages and protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure and risk of fire, so for a better protection of the system. It should have both over voltage and over current relay.

 

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How do you select a cable size (Cu and Al) for a particular load?

 

At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and lying of cable) after choose the cable size from cable catalog considering derating electrical current. After that measure the length of cable required from supply point of load to lod point. Calculate the voltage drop which will maximum 3 percent(resistance and reactance of cable found from cable catalog of selecting cable) if voltage drop greater than 3 percent then choose next higher size of cable

 

 

What are HRC fuses and where it is used?

 

HRC stand for (high rupturing capacity) and it is used in distribution system for electrical transformer.

 

 

Which power plant has high load factor?

 

All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load, we can reduce the cost of generation. Hydel power plants have a higher efficiency than thermal and nuclear plants.

 

 

Mention the methods for starting an induction motor?

 

Different methods are mention below: 1. DOL: direct online starter 2. Star delta starter 3. Auto transformer starter 4. Resistance starter 5. Series reactor starter

 

 

What is the difference between earth resistance and earth electrode resistance?

 

Only one of the terminal is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: earth resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away. The resistance of the electrode has the following components: 1. The resistance of the metal and that of connection to it. 2. The contact resistance of the surrounding earth to the electrode.

 

 

What is the use of lockout relay in HT voltage?

 

A lockout relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by same electrical source as the control power is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

 

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What is the power factor of an alternator at no load?

 

At no load synchronous impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

 

 

How to determine capacitor tolerance codes?

 

In electronic circuits, the capacitor tolerance can be determine by a code that appear on the casing. The code is a letter that often follows a three digit number (such as 130Z). The first two are the 1st and 2nd significant digits and the third is the multiplier code. Most of the time, the last digits tells you how many zero,s to write after the first two digits nd these are read s Pico-farads.

 

 

What is meant by knee point voltage?

 

Knee point voltage is calculated for electrical current transformers and is very important factor to choose a CT. it is the voltage at which a CT gets saturated.

 

 

What is reverse power relay?

 

Reverse power flow relay are used in generating stations protection. A generating stations is supposed to fed power to the grid and in case generating units are off, there I no generation in the plant then plant my take power from grid. To stop the flow of power from grid to generator, we use reverse power relay.

 

 

Why star delta starter is preferred with induction motor?

 

Star delta starter is preferred with induction motor due to following reasons: • Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses. • Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused. • In addition, starting torque is increased and it prevents the damage of motor winding.

 

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State the difference between generator and alternator.

 

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

 

 

How can you relate power engineering with electrical engineering?

 

Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipment,s also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.

 

 

Why back emf used for a dc motor? Highlight its significance.

 

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.

 

 

What is slip in an induction motor?

 

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol S.

 

 

Explain the application of storage batteries.

 

Storage batteries are used for various purposes, some of the applications are mentioned below:

• For the operation of protective devices and for emergency lighting at generating stations and substations.

• For starting, ignition and lighting of automobiles, aircrafts etc. • For lighting on steam and diesel railways trains.

• As a supply power source in telephone exchange, laboratories and broad casting stations.

• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

 

 

Explain advantages of storage batteries.

 

Few advantages of storage batteries are mentioned below:

  • • Most efficient form of storing energy portably.
  • • Stored energy is available immediately because there is no lag of time for delivering the stored energy.
  • • Reliable source for supply of energy.
  • • The energy can be drawn at a fairly constant rate.

 

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What are the different methods for the starting of a synchronous motor?

 

Starting methods: Synchronous motor can be started by the following two methods:

  • • By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.
  • • By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.

 

 

Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.

 

Following motors are used:

  • • Vacuum cleaners- Universal motor.
  • • Phonographic appliances – Hysteresis motor.
  • • Vending machines – Shaded pole motor.
  • • Refrigerators – Capacitor split phase motors.
  • • Rolling mills – Cumulative motors.
  • • Lathes – DC shunt motors.
  • • Power factor improvement – Synchronous motors.

 

State Maximum power transfer theorem.

 

The Maximum power transfer theorem explains about the load that a resistance will extract from the network. This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.

 

 

Explain different losses in a transformer.

 

There are two types of losses occurring in transformer:

• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:

  • 1. eddy current loss
  • 2. Hysteresis loss.

These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.

• Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.

 

 

Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.

 

Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of compoles or interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.

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Comment on the working principle of operation of a single-phase transformer.

 

Working principle of operation of a single-phase transformer can be explained as an AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday,s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self-inductance and known as self-induced emf and according to Lenz,s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.

 

 

State the methods of improving power factor?

 

Methods of improving power factor:

  • • By connecting static capacitors in parallel with the load operating at lagging power factor.
  • • A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
  • • By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor

 

 

How does Zener phenomenon differ from Avalanche breakdown?

 

The phenomenon when the depletion region expands and the potential barrier increases leading to a very high electric field across the junction, due to which suddenly the reverse current increases under a very high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur independently or both of these may occur simultaneously. Diode junctions that breakdown below 5v are caused by Zener Effect. Junctions that experience breakdown above 5v are caused by avalanche-effect. The Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

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Define the following: Average demand, Maximum demand, Demand factor, Load factor.

 

  • Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.
  • Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.
  • Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.
  • Load Factor: It is defined as the ratio of the average power to the maximum demand.

 

 

Compare JFE,’s and MOSFET,s.

 

Comparison of JFET,s and MOSFET,s: • JFET,s can only be operated in the depletion mode whereas MOSFET,s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial. • MOSFET,s have input impedance much higher than that of JFET,s. Thus is due to negligible small leakage current. • JFET,s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance. • When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.

 

 

Explain thin film resistors and wire-wound resistors.

 

a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

 

 

What is a differential amplifier? Also, explain CMRR.

 

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal. CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.

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Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.

 

Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward-resistance. DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it. AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

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How is magnetic leakage reduced to a minimum in commercial transformers?

 

By interleaving the primary and secondary windings.

 

In practice, what determines the thickness of the laminae or stampings?

 

Frequency.

 

 

Is Cu loss affected by power factor?

 

Yes, Cu loss varies inversely with power factor.

 

 

Why Cu loss affected by power factor?

 

Cu loss depends on current in the primary and secondary windings. It is well-known that current required is higher when power factor is lower.

 

 

Mention the factors on which hysteresis loss depends?

 

(i) Quality and amount of iron in the core(ii) Flux density(iii) Frequency

 

 

What effects are produced by change in voltage?

 

  • Iron loss………varies approximately as V2.
  • Cu loss………it also varies as V2 but decreases with an increase in voltage if constant kVA output is assumed
  • Efficiency………for distribution transformers, efficiency at fractional loads decreases with increase in voltage while at full load or overload it increases with increase in voltage and vice versa.
  • Regulation………it varies as V2 but decreases with increase in voltage if constant kVA output is assumed.
  • Heating………for constant kVA output, iron temperatures increase whereas Cu temperatures decrease with increase in voltages and vice-versa

 

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How does change in frequency affect the operation of a given transformer?

 

  • 1. Iron loss ………increases with a decrease in frequency. A 60-Hz transformer will have nearly 11 percent higher losses when worked on 50Hz instead of 60 Hz. However, when a 25-Hz transformer is worked on 60 Hz, iron losses are reduced by 25 percent.
  • 2. Cu loss………in distribution transformers, it is independent of frequency.
  • 3. Efficiency………since Cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one.
  • 4. Regulation………regulation at unity power factor is not affected because IR drop is independent of frequency. Since reactive drop is affected, regulation at low power factors decreases with a decrease in frequency and vice-versa. For example, the regulation of a 25-Hz transformer when operated at 50-Hz and low power factor is much poorer.
  • 5. Heating………since total loss is greater at a lower frequency, the temperature is increased with decrease in frequency.

 

 

What is Main Difference b/w Electrical and Electronics Engineering?

 

According to Wikipedia; Electrical Engineering is the field of Engineering that generally deals with the study and application of electricity, electronics, and electromagnetism. And, Electronics Engineering is an Engineering discipline where non-linear and active electrical and electronics components and devices such as electron tubes, and semiconductor devices, especially transistors, diodes and integrated circuits, etc. are utilized to design electronic circuits, devices and systems.

But below is the main difference between Electrical and Electronics Engineering which prevents such a confusion between Electrical and Electronics Engineering Main difference between Electrical and Electronics Engineering. Electrical Engineering = Study and Utilization/Application of Flow of Electrons. Electronics Engineering = Study and utilization/Application of Flow of Charge (Electron and Holes). As we know that we study only the flow of Electrons in a Conductor and insulator, but in case of Semiconductor, we study both of flow of electrons (Negatively Charges) and holes (Positively Charge). Also Note that Electronics Engineering is one of the Field/branch of Electrical Engineering in other words, Electronics Engineering is Son of Electrical Engineering :).

 

 

Why Power in pure Inductive Circuit is Zero (0).

 

We know that in Pure inductive circuit, current is lagging by 90 degree from voltage (in other words, Voltage is leading 90 Degree from current)

i.e. the phase difference between current and voltage is 90 degree. So If Current and Voltage are 90 Degree Out of Phase, then the Power (P) will be zero.

The reason is that, we know that Power in AC Circuit

P= V I Cos φ

if angle between current and Voltage are 90 (φ = 90) Degree.

Then Power P = V I Cos (90) = 0 [Note that Cos (90) = 0]

so if you put Cos 90 = 0→Then Power will be Zero (In pure Inductive circuit)

 

 

What will happen if DC supply is given on the primary of a transformer?

 

Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out.

 

 

What is the difference between isolators and electrical circuit breakers?

 

Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions. Actually they used for isolating the CBs for maintenance. Whereas circuit breaker gets activated under fault conditions according to the fault detected.

 

 

What is bus bar?

 

Bus bar is nothing but a junction where the power is getting distributed for independent loads.

 

 

What are the advantage of freewheeling diode in a full wave rectifier?

 

It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in an inductive load.

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What is the function of interposing current transformer?

 

The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers. Interposing current transformer are equipped with a wide range of taps that can be selected by the user to achieve the balance required.

 

What is power quality meter?

 

Power quality meter are common in many industrial environment. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within the building or by a single machine or appliance. In some situations, equipment function and operating is monitored and controlled from a remote location where communication is via modem, or high speed communication lines. So we can understand the importance of power measurement through power quality meters.

 

 

What is the difference between digital phase convertor and ordinary phase convertor?

 

Ordinary phase convertor are a recent development in phase convertor technology that utilizes proprietary software in a power microprocessor to control solid state power switching components. This microprocessor, called digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.

 

 

What is the operation of variable frequency transformer?

 

A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variable frequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three phase wound rotor, connected by slip rings to one external AC power circuit. A direct current torque motor is mounted on the same shaft. Changing the direction of the torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. The variable frequency transformer behaves as a continuously adjustable phase networks.

 

 

What is the main use of rotary phase convertor?

 

Rotary phase convertor will be converting single phase power into true balanced 3 phase power, so it is often called as single phase to three phase convertor. Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

 

 

Which type of oil is used as a transformer oil?

 

Transformer oil or insulating oil is usually a high refined mineral oil that is stable at high temperature and has excellent electrical insulating properties. It is used in oil filled transformer, some types of high voltage capacitors, fluorescent lamp ballasts and some types of high voltage switches and circuit breakers.

Its functions are to insulate, suppress corona and arcing and to serve as a coolant. Well into the 170s, polychlorinated biphenyls (PCB) were often used as dielectric fluid since they are not flammable. They are toxic and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 170s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicon based or fluoridated hydrocarbons are used, where the added expense of a fire resistant vegetable oil based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.

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What is the full form of KVAR?

 

We know there are three types of power in electrical as active, apparent and reactive. So KVAR is stand for, Kilo Volt Amperes with Reactive component.

 

 

What is excitation?

 

Excitation is applying an external voltage to DC shunt coil in DC motors.

 

 

In three pin plug 6 amp, 220 V AC rating, why earth pin diameter is higher than other two pins? What its purpose?

 

Because current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high current bypassed in the earthing terminal. (R=PL/a, area of the conductor increases resistance value decreases)

 

 

What is the difference between meggar test equipment and contact resistance meter test instruments?

 

Meggar test equipment used to measure cable electric resistance, conductor continuity, and phase identification whereas contact resistance meter test instruments used to measure low resistance like relays, contactors.

 

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When we connect the large capacitor bank in series?

 

We connect large bank capacitor in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedence of the line. So in order to bring the voltage at the load terminals within its limits (i.e. (+ or – 6 percent) of the rated high terminal voltage) the large capacitor bank is used in series.

 

 

What is electrical diversity factor in electrical installations?

 

Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of the system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor usually more than one.

 

 

Why field rheostat is kept in minimum position while armature rheostat at maximum position?

 

In motors at the time of starting the armature resistance is introduced to reduce the high starting current ant the field resistance is kept minimum to have high starting torque.

 

 

Why computer humming sound occurred in HT transmission line?

 

This computer humming sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect and it is considered as power loss

 

What is rated speed?

 

At the time of motor taking normal current (rated current) the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.

 

 

What is different between resistance grounding system and resistance earthing system?

 

Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect the equipment in occurrence of fault in the system.

 

 

Why should be the frequency 50 Hz and 60 Hz only? Why not others like 45, 95,56 or anything, why should we maintain the frequency constant if so why it is only 50 Hz and 60 Hz?

 

We can have the frequency at any frequency you like, but then you must also make your own motors, high voltage transformers or any other equipment you want to use. We maintain the frequency at 50 Hz or 60 Hz because the world maintains a standard at 50/60 Hz and the equipments are made to operate at these frequency.

 

 

How to determine alternating current frequency?

 

Zero crossing of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are, n, pulses per second. Each with a constant energy. The more pulses there are per second, the more the energy.

The pulses are integrated (filtered are averaged) to get a study DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.

 

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Why electricity in India is in multiple of 11 like 11Kv, 22Kv, 33Kv?

 

Transformer induced voltage equating contains 4.44 factor. E = 4.44*f*T*phi Where E induced emf per phase, T number of turns, f frequency and phi is maximum flux per pole. From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11. So always transmission voltage is multiple of 11.

 

 

Why we use AC system in Pakistan and India? Why not DC?

 

Firstly, the output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. Thirdly the cost of plant associated with AC transmission (circuit breaker, transformer etc.) is much lower than the equivalent of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs.in an AC system this becomes much easier to interrupt as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

 

 

Which type of motor is used in trains, what is the rating of supply used. Explain working principal.

 

DC series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500V DC.

 

 

What is inrush current?

 

Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltage.

 

 

In a Tap changing transformer where is the tap connected in the primary side or secondary side?

 

Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, spark will produce while tap changing operation due to high current.

 

 

Why transformer ratings are in KVA?

 

Since the power factor of transformer is dependent on load we only define VA rating and does not include Power factor. In case of motors, power factor depend on construction and hence rating of motors is in KW and include power factor.

 

 

What is difference between fuse and breaker?

 

Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.

 

Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?

 

As we know that electrical is having two type of load, active and reactive. Capacitor is a reactive load which is not considering as a load and its factor is lSin@. Meter is design based on current RMS value because of it meter is showing the current RMS value.

 

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What, s electrical traction?

 

Traction implies with the electric power for traction system. I.e. for railways, trams, trolleys etc. electric traction implies use of the electricity for all these. Now a day, magnetic traction is also utilized for bullet trains. Essentially DC motors are utilized for electric traction systems.

 

 

What is ,,pu,, in electrical engineering?

 

Pu stands for per unit in power system. (Pu = actual value/base value)

 

Define stepper motor. What is the use of stepper motor?

 

The motor which work or act on the applied input pulse in it, is called as stepper motor. This stepper motor Is under the category of synchronous motor, which often does not fully depend of complete cycle. It likes to works in either direction related to steps. For this purpose it mainly used in automation parts.

 

 

What is differential amplifier?

 

The amplifier which is used to amplify the voltage difference between two input lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise which is injected into the amplifier, because any noise appearing simultaneously on both the input terminals s the amplifying circuitry rejects it being a common mode signal.

 

 

What is CMRP?

 

It is the ratio of differential voltage gain to common made voltage gain. If a differential amplifier is perfect, CMRP will be infinite because in that case common mode voltage gain would be zero.

 

 

What is use of lockout relay in HT voltage?

 

A lock out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power which is operated by a key lock switch. This relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

 

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How can you start-up the 40W tube lite with 230 AC/DC without using any choke or coil?

 

Its possible with electronic choke, otherwise its not possible to ionize the particles in tube lite with normal voltage.

 

 

What types domain of Laplace transforms? What behavior can Laplace transform predict how the system work?

 

The armature flux has an important role for the running condition. This armature flux can oppose the main flux or it may support the main flux for better running condition. This effect of supporting and opposing of main flux to armature flux is called armature reaction.

 

 

What is the difference between thin film resistors and wire wound resistors?

 

Thin film resistors: it is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.

Wire wound resistors: length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel.

 

 

What is the difference between electronic regulator and ordinary rheostat regulator for fans?

 

The difference between the electronic and ordinary regulator is the fact that in electronic regulator, power losses tend to be less because as we minimize the speed the electronic regulator give the power necessary for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved.

In electronic regulator triac is employed for speed control. By varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.

 

 

What is two phase motor?

 

A two phase motor is often a motor with the starting winding and the running winding have a phase split. E.g. AC servo motor, where the auxiliary windings and the control winding have a phase split of 90 degree.

 

 

What are the types of power in electrical power?

 

There are normally three types of power in electrical power. (i) Apparent power (ii) Active power (iii) Reactive power

 

 

What are the advantages of VSCF wind electrical system?

 

Advantages of VSCF wind electrical system are:

  1. (i) No complex pitch changing mechanism is needed.
  2. (ii) Aero turbine always keeps going at maximum efficiency point.
  3. (iii) Extra energy in the high wind speed region of the speed- duration can be extracted
  4. (iv) Significant reduction in aerodynamic stresses, which are associated with constant-speed operation.

 

 

Why link is provided in neutral of an AC circuit and fuse in phase of AC circuit?

 

Link is provided at a neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of fuse in the phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.

 

 

Why the core of transformer is laminated?

 

To reduce eddy current loss. They are made up of thin laminated sheet and are insulated from each other. Finally the solid structure is pressed to remove presence of air gap.

 

 

Explain the effect of cooling in transformer.

 

To control the temperature of transformer we use some cooling mechanism like cooling fins. It is attached with tank to prevent the harmful effect of excessive temperature.

 

 

Explain auto and audio transformer.

 

It is smaller, lighter, and cheaper as compare to dual winding transformer. In this single winding act as primary and secondary windings. It does not provide electrical isolation.

 

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What is meant by crawling in the induction motor?

 

In induction motors, particularly squirrel cage type induction motors, sometimes exhibit a tendency to run stably at speeds as low as one seventh of their synchronous speed Ns. This phenomenon is known as crawling of an induction motor and the speed is called as crawling speed.

 

 

What are the various methods of measuring slip?

 

By actual measurement of rotor speed  By measurement of rotor frequency  stroboscopic method

 

 

What is meant by cogging in the induction motor? How to prevent the cogging?

 

When the number of teeth in stator and rotor are equal, the stator and rotor teeth have a tendency to align themselves exactly opposite to ech other, since this corresponds to minimum reluctance position. In such case therotor may refuse to accelerate. This phenomenon is called as magnetic locking or cogging. This problem can be prevented by proper choice of stator and rotor slots and also by skewing the rotor slots by one slot pitch.

 

 

 

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