20. STARTING OF INDUCTION MOTORS
Small induction motors (up to 2 kW) capacity may directly be switched on to the supply mains, but those of higher capacity must use some type of starting device, or starters as they are commonly called. The function of these starters is to restrict the initial rush of current, which, in the case of induction motors, is about 5 times the full-load current. This excessive current has two major upsetting effects, namely, a large voltage drop in the distributing network and causing stoppage of machines which are already running on the supply mains. Hence the Electrical Undertaking Authorities forbid the users of large capacity induction motors to directly switch on their machines.
The principle of all starting devices is to impress lower voltage on stator phases at the time of starting, or if the motor is slip ring or wound rotor, then to include external resistance in each rotor phase to keep the initial rotor current to a low value, this consequently means less current in the stator phases and therefore in the supply mains.
Direct-on-line starting of induction motors. This method means switching the motor
directly on to the supply without using any device for reducing the starting current. The method is restricted to small motors up to about 2 kW. For these small motors, the starting torque is about twice the full-load torque. Hence the starting period lasts only a few seconds.
We know that,
Rotor input = 2ΠNsT = kT
Also, rotor copper loss = s × rotor input
3I22R2 = skT
Some of the starting devices for starting induction motors are discussed below:
(i) Stator rheostat starter.
(ii) Auto-transformers (auto-starters).
(iii) Star-delta starter.
Slip Ring Motors:
(1) Rotor rheostat.