DOL Starter | Direct Online Starter Diagram, Construction, Advantages

DOL – Direct Online Starter:

A Direct On Line (DOL) or across the line starter is an electrical device used to apply a full three-phase line voltage to the electrical motor terminals. In order to avoid direct switching, DOL starters are implemented. During normal condition, it acts as a switch.

Direct Online starter comprises MPCB or MCCB or MCB, thermal overload relays, contactors, start and stop pushbutton, etc. For starting medium and high voltage motor, the circuit breaker is used as switching devices instead of electrical contactors.

DOL starter is the simplest form of motor starter which is used for smaller size motor typically below 10HP. For larger motor, we need some specialized starting equipment like star-delta starter, auto transformer starter, soft starter, solid-state devices VFDs, etc.

Do you know! What is meant by an electrical starter?

The electrical motor starter is nothing but a combination of electrical equipment uses to accelerate the electrical motor at the rated RPM (revolution per minute). It works based on Faraday’s electromagnetic principle.

A contractor that is associated with the electrical starter ensures the contact between the motor terminal and the power source.

There are 5 types of starters are used to start electrical such as

  • DOL – Direct online starter – Motor range below 10 HP
    • Stator resistance starter-
    • Rotor resistance starter – High starting torque motor
    • Forward & reverse starter
  • Star- Delta starter – above 10 HP Motor
  • Autotransformer starter – above 1 HP
  • Soft starter – motor range above 150 HP
  • VFD – Any range of motor

In this article we are going to see a starter, why the starter is required, Inrush current, What is the direct online starter, DOL starter wiring diagram, DOL working principle, DOL protection circuit, single-phase DOL, three-phase DOL starter, Application, merits, and demerits.

Why DOL- Direct Online starter is not preferred more than 10 Hp motor.

Before asking this question, you must know what is inrush current or starting current.

Inrush current is nothing but a flow of instantons peaks current at the time of electrical motor starting. Typically, starting current will be 8 to 10 times of the full load current. Due to this high current, the voltage drop happens across the feeder. This voltage drop creates a disturbance in the nearest feeder.

You know the value of the inrush current Istart is

inrush current in DOL Starter

At starting the back EMF E is zero, here the stator resistance is very low, less than 0.2 ohms. Hence the motor draws a large amount of current from the feeder.

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Lets have simple example,

Ra= 0.123 Ohms, Vin= 415Volts, full load current = 500A  and calculate Istart =?

Let we apply them in the Istart = 3373 Amps. It is about 6 times higher than the starting current.

A starting high current damages the winding insulations, conductors and as well as the machine.

Here, smaller size motors are generally manufactured with “L – Larger core” Frame, it means the rotor is designed with high axial length and the motor reaches the rated speed at a shorter time.

Also, most of the larger rating motor is designed with “S-Frame – Shorter core”, hence it has smaller axial lengths. If we apply three-phase to bigger size motor (above 10 HP), it would not run smoothly and will be damaged. However, for large-rated motors, we can use starter resistance DOL starter or star-delta starter.

That’s why smaller motor can be operated with the DOl Starter.

The above fundamental data is used to design the DOL starter for larger size high voltage motor

Direct Online Starter Circuit Diagram:

Generally, Two kinds of Direct online starter wiring diagrams are implemented; one is a conventional type and another one is the industrial type.

Conventional DOL starter has own start/stop circuit. The Power and control circuit will be wired in the same circuit. Here, remote operation is not possible.

DOL Starter Circuit Explanation
DOL Starter Circuit – conventional model

Industrial type dol starter has separate power and control circuit.  It means the power and control circuit has different power sources. We can operate the circuit locally as well as remotely. Refer to the below diagram, The start and stop pushbuttons are placed outside.

Dol circuit wiring diagram
Dol starter circuit wiring diagram – Industrial model

DOL starter Wiring Diagram:

DOL – Direct online starter has four-section such as input, contact, protection and closure section.

DOL starter basic Diagram

Input section: it comes with one MCCB (Molded case circuit breaker) or MPCB (Motor protection circuit breaker) or MCB (Miniature circuit breaker) or SFU (Switch Fuse unit.).

The rating of the MCCB/MPCB should be higher than the motor rating. It is used to protect the motor under short circuit conditions.

Contact section:

It consists of an electromagnetic contactor and push button. A contactor is nothing but an electrically controlled electromagnetic device. It creates contact between the motor terminal and a power source. For larger motor circuit breakers are used. It will be three-pole or 4 poles.

3- pole contactor => Only Power to the motor

4- pole contactor => 3 Pole for power + one pole for auxiliary contact. The auxiliary contact electrically coupled with power contact.

According to the breaking capacity of the Contactor, the rating will be varied. AC3 contactor is the best choice for the DOL starter application.

The contactor consists of one coil (terminal is A1 or A2) and one auxiliary NO contact (Normally Open). This NO is used to hold the contact terminal continuously when the motor start button is released. It ensures the continuous power supply to the coil.

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The contactor coil voltage will be 230 Volts to 440 Volts.

Asian – 440 Volts

European – 230 Volts.

Two Pushbuttons such as green and red are used. Here, green uses to start the motor and is red for stopping the motor.

Push buttons are nothing but a NO and NC contact for start or stopping the motor.

Green => NO (Normally Open)

Red => NC (Normally Closed)

But conventional DOL starter does not manufacture with separate NO or NC for start and stop operation.

Here for starting, Contactor KNO is used for starting the motor. we can achieve it by pressing the contact manually.

In such case, the Start button is coupled with the contactor, Example Siemens 3 TW42 model DOL starter.

Most of the industry uses individual NO and NC type, but for a convention, the DOL starter does not come with the NO and NC type push button. They are manually operated and mechanically coupled with the contactor.

Absence of power supply due to any fault in the motor or feeder, the contactor coil will be de-energized. Hence the motor will be disconnected from the source.

Overcurrent Protection – DOL starter:

Electrical motor’s insulation and winding may get the damage under fault conditions such as over current, short circuit, earth fault, phase losses, and instantons fault. For any reason, if the motor current exceeds the maximum allowable current means we need to isolate the motor. In order to protect the electrical motor, the following devices are implemented.

Protection against overload or over current => Thermal overload relay or CT operated thermal overload relay

Protection against earth fault, short circuit, and instantons over current => MCCB are used

Note: If you want to trip the MCCB under earth fault condition, then you should design or purchase the MCCB for tripping LISG fault.

Phase Losses Protection: MPCB is used.

DOL starter is inbuilt with thermal overload relay. Please refer to the below diagram. Thermal overload relay comes with the current setting adjustment.

Using the adjustment, we can control the current flow to the motor as per our requirement. It works based on i2r losses. When the current exceeds the setting value, the relay trips the circuit.

Some OLR comes along with the short circuit protection with the help of magnetic tripping. Since under heavy fault conditions, the thermal overload relay cannot trip the circuit.

To protect the motor is such condition, magnetic coil overload relays are used. Also, magnetic tripping mechanisms are not affected by temperature.

Example: L&T MPR relays – It has overload protection and short circuit protection

Enclosure section:

As per IEC, all the electrical transition should be done in the closed area.

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The enclosure is nothing but a metal cover used to cover the circuit. The enclosure should be made of steel or non-flexible material with adequate thickness.

As per IEC, if it is steel then the enclosure should be a minimum of 1.6mm thickness. It has to be connected with the proper earthing system.

DOL Starter Working:

DOL starter working as two circuits such as a power circuit and a control circuit. The power circuit helps us to flow the power to the motor terminal.

The power circuit is responsible for connecting power to the motor and control circuit is responsible for controlling the starter circuit

Power flows = MCCB => Contactor => Thermal protection (OLR) => Motor

Here refer below the DOL starter power diagram, the three-phase line is directly connected to the circuit breaker, then the output of the breaker is connected to overload protection and Motor.

DOL Starter Circuit Control Diagram
DOl starter – Power and Control Diagram

Consider the dol starter’s start push button is pressed, Then the control current pass through terminal A => B => C => A1 => A2, it means the control current goes through start NO => Stop NC => OLR NC => Contactor coil. The contactor coil is energized, the power contacts are closed and then the motor starts rotating.

DOL starter working sequence:

While Pressing the Starter Push button:

  • Start Pushbutton NO become NC
  • Stop push-button NC become NC – No changes
  • Kno Become Knc
  • OLR NC becomes NC – No changes.

If the contactor is energized means, the Kno becomes Knc, Then the current reaches the contactor continuously.

After releasing Start Push Button:

  • Start Pushbutton NO become NO
  • Stop push-button NC become NC
  • Kno become Knc
  • OLR NC become NC

When Stopping the motor:

  • Start Pushbutton NO become NO
  • Stop push-button NC become NO
  • Kno become Kno
  • OLR NC becomes NC – No changes.

While Tripping the Motor:

  • Start Pushbutton NO become NO
  • Stop push-button NC become NC
  • Kno become Kno
  • OLR NC become NO

Advantage & Disadvantages

Advantages of DOL Starter:

  • Direct Online starters are lower cost than other starters such as VFD, Star-delta starter and soft starter.
  • We reach full speed in few milliseconds
  • Very easy operation and does not required any technical skill. Layman can operate this DOL starter
  • Troubleshooting is simple.

Disadvantages of DOL starter:

  • Cannot be used more than 10 HP motor
  • For Larger motor, we need to design a system to withstand on under-voltage condition.
  • In Larger size motor, we need to connect the motor at star connection. So we cannot achieve 100% Torque. For achieving 100% torque, we need to go for DOL-Rotor resistance starter.

Applications of DOL Starter

It is mainly focusing on low power applications such as motors, heater, air compressors etc, where starting currents do not cause any damage.

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