Induction Motor Interview Question

Induction Motor Interview Question:


What is an Induction Motor?
An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.

What is an Electric Motor?
An Electric Motor converts electrical power to mechanical power in its rotor.

How to supply power to rotor?
In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device.

Why an Induction Motor sometimes called Rotating transformer?
An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.

Who invented Induction Motor?
Nikola Tesla.

What is the basic difference between Synchronous motor and an Induction Motor?
The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

Why stator windings are arranged around the rotor?
the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern can induce currents in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and the rotor will turn.

Why the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different?
the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced.

What is the SLIP?
This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is calledslip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor to the speed of the rotating field. Due to this an induction motor is sometimes referred to as an asynchronous machine.

How many types of Induction Motor ?

  1. Based on type of phase supply
    1. Three phase induction motor (self starting in nature)
    2. Single phase induction motor (not self starting)
  2. Other
    1. Squirrel-cage induction motor
    2. Slip ring induction motor

What is the relationship between the supply frequency number of poles and sycnchronous speed?
f = \frac{Pn_s}{2}.

What is a Rotor Speed?

\mbox{Rotor speed, }n_r = n_s(1-s)\,\!.

How to calculate the Slip?

Slip is calculated using:

s = \frac{n_s-n_r}{n_s}.

What is a Stator?
The stator consists of wound ‘poles’ that carry the supply current that induces a magnetic field in the conductor. The number of ‘poles’ can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).

How many types of Rotor are there?
There are two types of rotors

Squirrel Cage rotor.
Slip Ring rotor.

What techniques is used to produce a desired Speed?
The most commonly used technique is Pulse Width Modulation in which a DC signal is switched on and off very rapidly, producing a sequence of electrical pulses to the inductor windings.

What is the difference between DC motors and the Induction motors?
The induction motor has no brushes and is easy to control, many older DC motors are being replaced with induction motors and accompanying inverters in industrial applications.

How an Induction Motor is started? Why the Starter is Used?
When the motor is started, the slip is equal to 1 as the rotor speed is zero, so the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. Similarly, when an induction motor starts, a very high current is drawn by the stator, on the order of 5 to 9 times the full load current. This high current can damage the motor windings and because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, the starting current should be limited. A soft start starter is a device which limits the starting current by providing reduced voltage to the motor. Once the rotor speed increases, the full rated voltage is given to it.

What is a Rotor?
The rotor is the non-stationary part of a rotary electric motor or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor’s axis. In some designs, the rotor can act to serve as the motor’s armature, across which the input voltage is supplied. The stationary part of an electric motor is the stator. A common problem is called cogging torque.

What is a Stator?

The stator is the stationary part of an electric generator or electric motor. The non-stationary part on an electric motor is the rotor.

Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving field coils on the rotor.


What is Commutator?
commutator is an electrical switch that periodically reverses the current direction in an electric motor or electrical generator. A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor’s armature, a steady rotating force (torque) is produced. Similarly, in a generator, reversing of the coil’s connection to the external circuit produces unidirectional current in the circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832,


What is an Armature?
An armature is one of the two principal electrical components of an electromechanical machine–a motor or generator. The other is the field winding, field magnet. The role of the “field” component is simply to create a magnetic field (magnetic flux) for the armature to interact with, so this component can comprise either permanent magnets, or electromagnets formed by a conducting coil. The armature, in contrast, must carry current so it is always a conductor or a conductive coil, oriented normal to both the field and to the direction of motion, torque(rotating machine), or force (linear machine). The armature’s role is two-fold: (a) to carry current crossing the field, thus creating shaft torque (in a rotating machine) or force (in a linear machine), and (b) to generate an electromotive force (“EMF”).


What is a Cogging Torque?
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. Also termed as detent or ‘no-current’ torque, it is an undesirable component for the operation of such a motor. It is especially prominent at lower speeds, with the symptom of jerkiness.


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