Starters For AC Starters

STARTERS FOR AC STARTERS

1. NECESSITY FOR STARTER

·                  At starting when full voltage is connected across the stator terminals of an induction motor, large current is drawn by the windings.

·                  This is because, at starting (i.e before the rotor starts rotating) the induction motor behaves as a short circuited transformer.

·                  This induced emf of the rotor will circulate a very large current through its windings, due to short.

·                  The primary will draw very large current nearly 7 times of the rated current from the supply main to balance the rotor ampere turns.

·                  This current will however be gradually decreasing as the motor will pick up speed.

·                  Hence if induction motors are started direct-online heavy current is drawn by the motor, such as heavy starting current of short duration may not cause harm to the motor since the construction of induction motors are rugged.

·                  Moreover, it takes time for intolerable temperature rise to endanger the insulation of the motor windings. But this heavy in high of current will cause a large voltage drop in the lines leading to the motor.

·                  Other motors and equipment connected to the supply lines will receive reduced voltage.

·                  In industrial installation, however, it a number of large motors are started direct on-line, the voltage drop will be very high and may be really objectionable for the other types of loads connected to the system.

·                  The amount of voltage dro0p will not only dependent on the size of the motor but also on factors like the capacity of the power supply system, the size and length of the line leading to the motors, etc.

·                 Types of starters available for induction motors are:

2. PRIME PURPOSE OF A STARTER FOR MOTORS

·      When induction motor is switched on to the supply, it takes about 5 to 8 times full load current at starting.

·      This starting current may be of such a magnitude as to cause objectionable voltage drop in the lines.

·      So Starters are necessary

3. NEED FOR STARTER IN AN INDUCTION MOTOR

At starting when full voltage is connected across the stator terminals of an induction motor, large current is drawn by the windings.

This is because, at starting (i.e before the rotor starts rotating) the induction motor behaves as short circuited transformer.

At starting, when the rotor is at stand still emf is induced n the rotor circuit exactly similar to the emf induced in the secondary windings of a transformer.

This induced emf of the rotor will circulate a very large current through its windings, due to short.

The primary will draw very large current nearly 5 – 7 times of the rated current from the supply mains to balance rotor ampere turns.

This current will however be gradually decreasing as motor will pick up speed.

In order to reduced starting current starters are used.

Induction motor starter will supply reduced voltage to the stator of induction motors.