DOL starter- Direct On Line Starter Wiring Diagram for 3 Phase Induction Motor

What is DOL Starter?

DOL starter is a simple starter, used to start 3 Phase induction motor. In this starter, 3 phase voltage directly applied to motor terminals (Without reducing the voltage by any method).

DOL Starter Capacity

We can use DOL Starter, where the starting current of motor will not affect the main power supply voltage. In industries, DOL starter is used to run the motors upto 15kW (Until and unless motor supplier is not recommending Star-Delta).

Please find the following diagrams of DOL Starter to know more.

Single Line Diagram DOL Starter (Direct On Line Starter) and Power Wiring Diagram of a DOL Starter (Direct On Line Starter)

Single Line Diagram DOL Starter (Direct On Line Starter) and
Power Wiring Diagram of a DOL Starter (Direct On Line Starter)

If you unable to understand above drawing, read below step-by-step to understand the power diagram of DOL Starter.

  • As you can see in the above picture, the 3phase voltage is apply to MCCB (Q1), it is a short-circuit protection device. After switching “On” the MCCB (Q1) supply goes to contactor.
  • Contactor (K1), it is a switching device (ON-OFF). A contactor is having a coil in it to operate. When we apply Control Voltage to the coil of the contactor. The power contacts of the contactor get ‘closed’, and gives 3phase power supply to Overload Relay.
  • Overload relay (Z1) provides over-current protection and single phasing protection. When it sense the fault current, it gives the command to control circuit to switch off the control power supply of contactor (K1). There is no switching of 3 phase power supply in overload relay, so as soon as power reach at the input of the over load relay, it directly reach to the motor terminals through cable, and motor starts.

Note: At the time of starting; the rotor is in rest position, so to rotate the rotor, motor takes 6-8 times more current than rated current; in starting for a few milliseconds. That is the reason it drops the main supply voltage for a few milliseconds.

Control Wiring Diagram of a DOL Starter (Direct On Line Starter)

Control Wiring Diagram of a DOL Starter (Direct On Line Starter)

If you unable to understand above drawing, read below to understand the control diagram of DOL Starter.

  • As you can see in the above picture, we have taken the control voltage as 230VAC (kindly note that according to electrical panel’s control voltage, it can be 440VAC, 110VAC, 110VDC, 230VDC etc)
  • Phase (P) is connected to control MCB (This MCB is act as an isolation device and short circuit device for control wiring.) then Phase goes to overload relay’s (Z1) NC contact. (Wire no. P to 1)
  • Through (NC contact) of overload relay (Z1) phase goes to STOP PUSH BUTTON’s (NC contact). (Between Wire no. 1 to 2)
  • Through (NC contact) of STOP PUSH BUTTON Phase goes to Start PUSH BUTTON (NO contact) and contactor (K1) (NO contact.) (Wire no. 2 to 3)
  • From the output of the Start PUSH BUTTON (NO contact) phase reach to the A1 point of the contactor coil (K1). (Wire no. 3 to 4)

DOL Starter Working Principle

If you have read the above details, you definitely know the purpose of each device of the DOL starter. Let’s start the motor…

  • Press the START Push button to start the motor.
  • When we press START Push button. It gives the control voltage to the contactor coil (K1) and contactor (K1) will ‘Close’ (Energised/engaged/ON). Same time the K1’s NO contact, which is between wires number 3 and 4(i.e. also connected in parallel to the START Push button) will also get ‘Close’.    
  • When we will release the START Push button, the control voltage will be still receive by the contactor’s coil (K1), through K1’s NO contact, which is between wires number 3 and 4. It is called LATCH.
  • Now, motor will STOP only if the STOP Push button is pressed OR motor trips OR in case of control voltage failure.
  • If we want to STOP the motor, just press the STOP Push button, it will break the control voltage of contactor’s coil and contactor (K1) will be ‘Open’ (de-energized/ disengaged/OFF). 

Advantages of DOL Starter

3 major advantage of DOL Starter

  1.  It is the simplest starter.
  2. Cost effective.
  3. Trouble shooting is very easy

Disadvantages of DOL Starter

3 major disadvantages of DOL starter:

  1. High Starting current (6-8 times of the rated current)
  2. Voltage can drop in the main power supply during starting.
  3. Produce unnecessary high torque in starting due to high starting current.

DOL Starter Applications (Where DOL Starter is used?)

Despite the disadvantages we use DOL Starter for many applications. We use motors for small pumps, screw feeders, small fans, conveyor belts, motorised valves etc.

Wanna know more just give a comment!

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