Page 700 - Basic Electrical Engineering

used.
The simplest method of starting a three-phase induction motor is to
connect it to a three-phase supply through a switch as shown in Fig. 8.15 (a).

However, in case of short-circuit or overload condition due to any wrong
connection or excessive loading, the motor will not be protected. So, in a

DOL starter provision for overload protection and short-circuit protection
must be made. For short-circuit protection, cartridge-type fuses are used.

These are connected in series with the lines supplying power to the motor.
For overload protection, a thermal overload relay is used. The contact of the

relay will open when, due to overload current, the relay will get heated up.
The switching of supply to the motor is done through a contactor and not
by a manual switch. Push-button switches are provided for starting and

stopping the motor. When the START push button is pressed, the contactor
coil gets energized, its main contacts close, and the motor starts running. For

stopping the motor the OFF push button is to be pressed due to which the
contactor coil will not receive supply and will be de-energized. The contacts

of the contractor will open, thereby stopping the motor. In case of over load
also the contactor will get de energized due to opening of the contact of the

overload relay. The working of the DOL starter is further explained using
Fig. 8.15 (b) as follows.