Induction Motor Startup

By 19 July 2017

The main objectives while starting an induction motor are:

• To handle high-starting current
• To achieve high-starting torque.

As discussed earlier, rotor resistance determines starting torque. Usually, this rotor resistance is small, giving small starting torque, but good running conditions. So, the squirrel-cage motor can run only with low-starting loads.

If the rotor resistance is increased by some means, then the slip and speed at which maximum torque occurs can be shifted. For that purpose, external resistance can be introduced in the rotor circuit, which is done in the case of slip ring or wound rotor type motors.

When power is applied to a stationary rotor, excessive current will start flowing. This happens due to the fact that there is a transformer action between the stator winding and the rotor winding, and the rotor conductors are short-circuited. This causes heavy current flow through the rotor.

If, for reducing this heavy starting current, starting voltage applied is reduced then it affects the starting torque as well.

To get everything out, the following method of starting is generally used:

• DOL starting
• Auto transformer starting
• Star–delta starting.