Page 699 - Basic Electrical Engineering

As speed increases, i.e., slip decreases, I will go on decreasing; accordingly
2
the current drawn from the supply will also be gradually reducing as has been

shown in Fig. 8.14 (c). An induction motor at starting, therefore, will draw
very high current if started with full voltage applied across its stator

terminals. The starting current may be as high as six-times its full-load
current. Thus, it becomes necessary to limit the starting current of an

induction motor.
Starting of three-phase induction motors by applying full voltage directly

to the stator windings is restricted to small motors upto 5 kW rating. If large
motors are started this way, heavy current will be drawn (usually six to eight
times the rated current) by the motors. This will not only be harmful to the

motors in the long run but will also create heavy voltage drop in the electrical
distribution lines, which will disturb the working of other electrical gadgets

and machines connected to the line. Higher the rating of the motor, higher
will be the disturbance of the line voltage. Starting of motors upto 5 kW

rating may be done by applying full voltage. This is called direct-on-line or
DOL starting of motors. Reduced voltage starting should be done for larger

motors by using a three-phase auto-transformer or by connecting the stator
windings of the motor first in star formation and giving the supply and as the
motor picks up sufficient speed, connecting the windings in delta formation.

Thus, there are three types of starters used in starting of three-phase
induction motors. They are

i. direct-on-line starters;
ii. star–delta starters;
iii. auto-transformer starters.

8.14.1 Direct-on-Line Starting

Small induction motors upto the rating of 5 kW are allowed to be started
direct-on-line by the electricity boards. For large motors, starters have to be