In this topic, you study Starting of Three Phase Induction Motor and types of Starting Methods.
In the case of a three phase induction motor, the magnitude Of the induced emf in the rotor winding depends on the relative speed between the rotating magnetic field produced by the stator and the rotor conductors. Initially when the rotor is stationary, this relative speed is maximum. Therefore, a large emf is induced in the rotor conductors. Since all the rotor conductors together form a closed circuit, very large current is circulated through the rotor winding. Consequently, the motor draws a very large current of the order of 5 to 7 times its full-load current for a short period from the supply. In this condition, the motor can be regarded as a transformer with a short-circuited secondary. Once the rotor starts running, the relative speed between the rotor conductors and stator field reduces thereby reducing the emf developed in the rotor. Thus, the rotor current and hence the current drawn by the motor is automatically reduced.
The large starting current drawn by the motor may not cause harm to the motor due to its rugged construction but causes a large voltage drop in the lines. This affects the working of the other equipments connected to the same line. Many supply authorities, therefore, limit the size of the induction motor which can be started directly on line to three or sometimes five kilowatts. Hence, use Of starters becomes essential for the motors above this rating for reducing their starting current.
Thus, induction motor starter as in the case of a dc motor, mainly limits the starting current to the safe value without, at the same time, reducing the starting torque to a value less than that required to start and accelerate the motor and the load. In addition to this, it also provides protection to the motor against over-load and low voltage. For this, it is always fitted with no-volt and over-load releases. It may also be fitted with single phasing preventer for providing protection to the motor against single phasing.
The starting current of the induction motor can be reduced either by applying a reduced voltage to the stator during the starting period or by increasing the resistance of the rotor circuit. Various types Of starters have been developed for this purpose to suit squirrel cage and slip-ring induction motors.
Starting of Squirrel cage Induction Motors
In general, the starting methods for squirrel cage induction motors may be broadly classified as :
- Direct on line starting,
- Reduced-voltage starting.
Starting of Slip-Ring Induction Motors
Slip-ring induction motors are invariably started with full line voltage applied across the Stator terminals. The starting current is limited by introducing a variable resistance in the rotor circuit. The increased rotor resistance not only limits the starting current but also gives high starting torque.
