Page 704 - Basic Electrical Engineering

or,

Thus, current drawn during starting is reduced to one-third if the windings are
first star connected and then delta connected. Note that in the manual star–

delta starter shown in Fig. 8.16, over load protection device has not been
shown.
Star–delta starters are available in automatic form. The windings are first

connected in star before full voltage is applied. After the rotor picks up
sufficient speed, a time-delay relay (TDR) operates and then the windings get

connected in delta. Such starters are called push-button-operated star–delta
starters. The operating time of the TDR can be adjusted according to the time

taken by the rotor to pick up sufficient speed.
In auto-transformer starters, reduced voltage is applied to the stator

windings at starting with the help of a three-phase auto-transformer.

8.15 SPEED CONTROL OF INDUCTION MOTORS

Different types of electric motors are manufactured for use as drives in

various kinds of industrial applications. The selection criterion for use are
their ruggedness, cost, ease of use, supply requirement, and control of speed.

Induction motors have become universally accepted as a first choice because
of their satisfying all the above-mentioned requirements. Because of

availability of power electronic control devices, speed control of induction
motors has become easy now. The basic methods of speed control of
induction motors will be discussed. The electronic method of speed control is

beyond the scope of this book. The details of use of electronic control are
dealt with in a separate subject of power electronics.

We have known that the slip of an induction motor is expressed as