1- Star Delta Starter The method achieved low starting current by first connecting the stator winding in star configuration, and then after the motor reaches a certain speed, throw switch changes the winding arrangements from star to delta configuration. By connecting the stator windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to one-third as compared to starting current with the windings connected in delta. At the time of starting when the stator windings are start connected, each stator phase gets voltage V L / 3, where V L is the line voltage. Since the torque developed by an induction motor is proportional to the square of the applied voltage, star- delta starting reduced the starting torque to one-third that obtainable by direct delta starting. 1
2- Auto transformer starter One way to reduce the motor's terminal voltage during starting by using autotransformers to step its terminal voltage down. During starting, contacts 1 and 3 are shut, supplying a lower voltage to the motor. Once the motor is nearly up to speed, those contacts are opened and contacts 2 are shut. These contacts put full line voltage across the motor. 2
3- Rotor resistance starter This method of starting is only used for slip ring induction motors because in case of squirrel cage induction motors, rotor has copper bars, which are short circuited at both ends. Hence it is not possible to add any external resistance in the rotor circuit. In case of slip ring induction motors, rotor has 3-phase star connected winding whose terminals are connected to three respective slip rings. In rotor resistance starter the three terminals of the rotor winding are connected to a variable external resistances through slip rings. Full supply voltage is applied across the stator. Resistances are fully in the circuit at starting, so that the starting current is reduced. The External variable resistance connected in each phase of the rotor circuit not only reduces the current at starting but increases the starting torque also due to improvement in power factor. The rotor circuit resistance is gradually cut out, as the motor speeds up and during normal running condition, the rotor circuit resistance is completely cut out and the slip rings are short circuited. 3
4- Full-voltage or across-the-line magnetic induction motor starter. When the start button is pressed, the relay (or contactor) coil M is energized, causing the normally open contacts M I, Mb and M3 to shut. When these contacts shut, power is applied to the induction motor, and the motor starts. Contact M4 also shuts, which shorts out the starting switch, allowing the operator to release it without removing power from the M relay. When the stop button is pressed, the M relay is de-energized, and the M contacts open, stopping the motor. A magnetic motor starter circuit of this sort has several built-in protective features: 1. Short-circuit protection 2. Overload protection 3. Under-voltage protection 4
Speed Control of Induction Motors 1- Induction Motor Speed Control by Pole Changing. 2- Induction Motor Speed Control by Changing the line frequency. 3- Induction Motor Speed Control by Changing the line voltage. 4- Induction Motor Speed Control by Changing the Rotor Resistance. 5
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