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.
at the time of starting slip is maximum that is <1> and rotor speed is zero emf depends upon synchronous speed and actual speed generally this starting is 5 to 8 times of full load current. this current damages the circuit thats why the starter is required
T=FLUX*I*POWER FACTOR, as time of starting s=1
so reactance is very large in comperision to
resistance(normaly is very low) for constant load(and
inertia) the power factor(theta=reactance/resistance is
large) is very less(flux is constant) so starting current
is very large so Starter is used to reduce starting current.