Starters for AC motors


Starters for AC motors

  • The Device which is used for starting of 3 phase induction motor is called starters.
  • the function of starters is limit a starting high current to a safe value.
  • The starter is a safety and a protecting device by which we can control or minimize the starting current in order to prevent the motor winding of the respective motor.


Rotor current at stand still condition,


In a three phase induction motor, the magnitude of an induced e.m.f. in the rotor circuit depends on the slip of the induction motor. This induced e.m.f. effectively decides the magnitude of the rotor current. The rotor current in the running condition is given by,

rotor current under running condition

In induction motor large current at starting is due to the absence of back emf. If an induction motor is directly switched on from the supply, it takes 5 to 7 times its full load current . But it develops a torque is only 1.5 to 2.5 times to full load torque. This large starting current produces a large voltage drop in the line, which may affect the operation of other devices connected to the same line. Hence, it is not advisable to start induction motors of higher ratings (above 25KW) directly from the mains supply.

The initial rush of current is controlled by applying a reduced voltage to the stator (motor) winding.  During the starting period and then the full normal voltage is applied when the motor has run up to speed. For the small capacity motors say up to 3HP, normal voltage can be applied at the start. How ever to start and stop the motor and to protect the winding from the over load current and low voltages, a starter is required in the motor circuit. In addition to this the starter may also reduce the applied voltage to the motor at the time of starting.

Various starts of Squirrel cage induction motors

  1. Direct on line Starters (DOL Starters)
  2. Rotor resistance starters
  3. Star-Delta Starters
  4. Auto-transformer starters

Direct on line Starters (DOL Starters)


direct online starter

The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Starter (DOL) consist a Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.

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. The contactor is electrically latched closed while the motor is operating.

In this starter , mo current limiting device to limit starting high current , only protection against under voltage and over current is provided.


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