As we know at the time of starting an induction motor the rotor is stand still hence the slip is 1.if slip is 1 the rotor induced voltage is maximum hence drives maximum current in the rotor circuit.This rotor current current has impact on stator current(As per LENZ's law).This is the reason why an induction motor takes high current at starting(About 5 to 7 times the full load current).
This high current causes voltage fluctuation,unwanted power loss & unwanted stress to the system or equipment.So it has to be reduced by using different starting methods.
There are two possible ways to limit the starting current
1.Reduce the applied voltage.
2.Increase the rotor resistance.
small induction motors up to 3H.P.,may directly be switched on to supply mains ,but those of larger capacity must use some type of starting device or starter.the function of starter is to limit the current at the time of start, which is about 5 times the full load current this excessive current has two major upsetting effects 1. it produces large voltage drop in the distribution network and causes voltage regulation of the supply system 2.it may cause disturbance in stopping of the motor which are already running on the same supply line Four types of starting methods are employed:
2.Auto transformer method
3.Primary resistor starting.
Starters are used in Squirrel Cage Induction Motors for the
1. To provide several switching on and off operations by
means of the Contactor whose special contacts can withstand
high starting currents and switching off surges.
2. To provide lower starting currents at starting for
larger HP Motors, thereby preventing severe voltage drops in
the line which may affect other running equipments.
3. To provide protection against overload and single
phasing by means of the thermal magnetic Overload relay
provided in the Starter.
at starting, The relative speed between the field and the stationery conductor (the rotor) is maximum and the resistance of winding is very small. If rated voltage is applied to the motor a current of 500%-800% of nominal current is drawn from the the supply. The starting current will be diminished when the rotor picks up speed. This high starting current will heat up the motor and may also damage insulation. The starting toque, which is proportional to the squared of the voltage will also be massive and might damage the load connected to the shaft of the motor. Another Important factor is the voltage dip in the network due to the sudden withdrawal of large current from the network. To overcome all these problems, a starter is used especially for big industrial motors. Factors, such as size of the motor, type of load, duty cycle and the environment are important when selecting a particular starter for a motor. The best starters are soft-starters and variable speed drives (V.S.D). With the use of VSDs, you can limit the starting current and also regulate the torque -speed characteristics of the motor as per requirements of the load.