Direct Online Starter (DOL Motor Starter) : Circuit Diagram and Working Principle

Direct online starter is method of starting of an induction motor. The stator of the motor receives the full supply voltage in DOL starter. We prefer a Direct Online Starter for starting of small rating three phase induction motors. We will also discuss the DOL power circuit diagram. DOL starter control circuit diagram is also very important for understanding the working of DOL starter.

Types of Starters for an Induction Motor

There are three types of starters mainly used for starting a squirrel cage induction motor.

  • DOL( Direct Online Starter)
  • Star Delta Starter
  • Soft Starter
  • V/f Drive ( Used for soft starting and speed control)

Direct online or DOL Motor Starter

The Direct On Line starter is the simplest form of motor starter. The direct online starter is a basic and simple starter used for starting the induction motor. In DOL Starter, we connect the stator winding of the induction motor directly  to  three phase supply voltage. Thus, the stator winding receives full line voltage. 

The DOL motor starter is suitable for starting small rating motors because the motor draws about 6 to 8 times of its full rated current at starting time when stator winding gets the full rated voltage. 

Why induction motor draws 6 times starting current with DOL starter?

The reasons of drawing high starting current are as follows;

  • The rotor is highly inductive because the rotor current frequency is slip times the stator frequency. The slip of the rotor current is unity at start. Thus, the rotor reactance is very high(XL =2πsfs). Therfore, the rotor of the induction motor draws very high current at start. 
  • At start, the induced voltage across the stator winding is zero and there is no opposing EMF against the applied voltage, and thus the current limiting is through the stator winding resistance only.
EMF formula of induction motor

In the absence of EMF at the time of starting, the motor draws about 6 to 8 times current of its full load current.

Why induction motor power factor poor with DOL starter?

Moreover, the power factor of the motor is very low and it draws large magnetizing current. The motor takes about 6-8 times when it is started and the magnitude of the current decreases as the motor gets accelerated towards its base speed(full rated speed). Even though motor draws large current at start, it delivers very low starting torque. The DOL starting is suitable for the small rating induction motors.

If large rating induction motors are started with DOL starting method, a heavy starting current can cause voltage dip in the supply system and may damage the motor. 

The large rating induction motor can  be started on DOL if the capacity of power network permits. If the large rating transformer is used for distribution of three phases 440 volts, the large rating motor up to 90 KW can be started on DOL. However, in domestic connections the DOL starting of 5.5 KW motor is feasible.

The Direct on Line Starter usually mainly consists of two parts:

  • Control Part      –  Contactors and Push Buttons
  • Protection Part   – Overload Relay

Working Principle of DOL Starter 

The full line voltage is applied to stator for starting of three phase induction motor. The DOL starter is suitable for small rating three phase squirrel cage induction motor.

Direct Online Starter Circuit Diagram

The Direct online starter circuit diagram can be divided into two parts.

  1. Power Circuit Diagram
  2. Control Circuit Diagram

DOL Starter Power Circuit Diagram

The Power Circuit Diagram of the DOL starter consists following parts.

  1. MCCB or Switch Fuse Unit
  2. Motor Protection Circuit Breaker(MPCB) Or O/L Relay
  3. Power Contactor of AC3 rating
dol starter power circuit diagram

The motor protection circuit breaker protects the motor under short circuit and the overloading condition.

DOL Starter Control Circuit Diagram

The control circuit of the direct online Starter is as given below.The control circuit has following components.

1. Auxiliary contactor
2. Start Push button
3. Stop Push Button
4. Indication Lamps- ON/OFF/Over Load

control circuit of dol starter

Working of DOL Starter 

When the start push button is pressed the contactor C2(main Contactor of AC3 rating ) operates if overload relay is healthy and stop push button is in released state, then the full voltage is applied to the motor. The main contactor C2 remains in its on state through holding contact of the main contactor C2, after releasing the start push button. Thus the supply to stator is available after releasing the start button. We can stop the motor by pressing the stop switch. If load on the motor exceeds above its rated capacity, the overload relay signals and trip the contactor C2 and the motor trips.

The wiring diagram of  Direct online motor starter is as given below.

wiring diagram of dol motor starter

The motor draws large current and it starts reducing when the motor attains its base speed. The motor  current and torque is as given below.

DOL starter current and torque

When stop push button is pressed the main contactor C2 drops and motor stops.

In the case of jamming from the load side or problem in the motor like single phasing or unbalance voltage, the motor draws a large current above its full load current(FLC) and, the overload relay will get tripped with overloading fault. The contact of the overload relay QM1 drops the main contractor C2 and thus the motor stops due to overloading.The overload relay can be thermal over load type or electronic relay.

In electronic overload relay apart from the overload protection, the negative sequence current fault(I2) and earth fault protection(Io) and stall protection are also available and motor can be protected if fault occurs.

Advantages of DOL Starter

  • Simple, most economical, easy to understand starter.
  • More comfortable to design, operate and control.
  • Provides nearly full starting torque at starting.
  • Easy to troubleshoot.
  • DOL starter directly connects the supply to the delta winding of the motor so there is no complexity of the  circuit diagram

Disadvantages of DOL Starter

  • High starting current (5-8 times of full load current). The higher rating of switch gears are required to be installed.
  • Direct online Starter causes a significant dip in voltage hence suitable only for small motors. However, it can be used for starting a large rating motor up to 90 KW if the power transformer capacity permits. 
  • DOL Starter reduces the lifespan of the machine because of higher thermal and mechanical stresses on the system.

Applications of  direct online Starter

DOL starters are used to start the small rating motor like water pumps, compressors,conveyors etc. The compressor motor starts at no load therefore it is suitable for compressor motor. The Squirrel cage induction motor coupled through fluid coupling can also be started with DOL starter.  

Related Posts

  1. Star Delta Starter | Working Principle, Power and Control Circuit Diagram
  2. Why is Star Delta Starter Preferred with an Induction Motor?
  3. What is the Difference between Soft Starter and VFD?
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