Direct On Line Starter

There are different starting methods are employed for starting induction motor. It is more important because induction motor draws more starting current during starting. To prevent damage of windings due to this high starting current flow, we employ different types of starter.

From this all types of method, the simplest one is Direct On Line Starter. It consist a MCCB or circuit breaker, contactor, overload relay for protection. Electromagnetic conductor which can be opened by the thermal overload relay under fault conditions. Typically, the contactor will be controlled by separate start and stop buttons and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating. It has some advantage and disadvantage also-

Advantages of DOL Starter

• It is more economical and cheapest starter.
• Simple to establish, operate and maintain.
• Simple control circuitry.
• Easy to understand and trouble shoot.
• It can provide 100% torque at the starting time.
• Only one set or cable is required from starter to motor.

Disadvantage of DOL Starter

• It does not reduce the starting current of the motor.
• High starting current.
• Unnecessary high starting torque when not required by the load. Thereby increase the mechanical stress on the mechanical systems. Such as rotor, shaft, bearing, gearbox, coupling, chain drive, connected equipment etc.
• Due to thermal stress on the motor, thereby reduce its lifetime.
• There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same line and therefore not suitable for high size squirrel cage motor.

In case of its more advantage it is used in small water pump, conveyors belt, fan, compressor etc.

Edited By
Jeion Ahmed
B.Sc. in Electrical & Electronic Engineering (EEE)
Chittagong University of Engineering & Technology (CUET)