Electrical Engineering ⇒ Topic : Starting of Induction Motors

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 distribution 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 motor 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

                                                                                       .............  (1)

When the motor is direct-switched On to normal votlage, then starting current is the short circuit current

                                                                                            ..........(2)

Some of the starting devices for starting induction motors are discussed below :

• Squirrel-Cage Motors 
• Stator rheostate starter
• Auto-transformers (auto-starters)
• Star-delta starter

STARTING OF INDUCTION MOTORS

Small induction motors (up to 2 kW) capacity may be connected directly to the supply mains.Higher capacity machine should not be connected directly to the supply mains because the starting current of the induction motor is 5 to 7 times the full-load current. This excessive current has two major upsetting effects, namely, it causes a large voltage drop in the distribution network and stops the other machines, which are already running on the supply mains. So the large capacity induction motor should not be directly connected to the supply mains.