Understanding the Direct-On-Line (DOL) Starter in Induction Motor Control
A DOL starter is the simplest and most commonly used method for starting either three-phase or single-phase induction motors in industries and homes. The single-phase motors referred to here include water pumps commonly used in households. They all operate using a DOL starter.
The DOL starter directly connects the full supply voltage to the motor, which is why it’s called a Direct-On-Line Starter.
Main Components of a DOL Starter
- Contactor
- Overload relay
- Single start push button
- Single stop push button
- Protective devices, such as a three-pole breaker and a control breaker, to safeguard the circuit
- Indicator lights (optional) for monitoring:
- Red: Stop / Not running
- Green: Start / Running
- Amber: Tripped overload or fault
Operation of a DOL Starter
- When the start push button is pressed, power is applied to the contactor coil.
- The contactor energizes, closing its contacts to directly apply the full AC supply voltage to the motor. The motor starts running.
- When the stop push button is pressed, power is cut to the contactor coil.
- The contactor deenergizes, opening its contacts, which cuts supply voltage to the motor and brings it to a halt.
Why the Overload Relay?
The overload relay protects the motor from excessive current caused by:
- Motor overload
- Seized bearings
- Misalignment on couplings
When excess heat is detected due to overcurrent, the relay deenergizes the contactor, stopping the motor to prevent damage.
Why the Circuit Breakers?
Circuit breakers are installed to cut the supply voltage to the motor in case of a short circuit, protecting both the motor and the circuit.
Advantages of a DOL Starter
- Simple design and easy to operate
- Less costly to design compared to other starters
- Provides full starting torque
Limitations / Disadvantages of a DOL Starter
- High inrush current
- High voltage drop, which can affect other loads on the same supply line
Applications
- Water pumps
- Fans and blowers
- Conveyors
- Other small to medium industrial machines
Learn How to Wire a DOL Starter
Do you want to know how to wire this DOL starter? Click the link below to watch the full tutorial on my YouTube channel:
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