Construction And Working:
In the auto-transformer starting method, the reduced voltage is obtained by taking tappings at suitable points from a three-phase auto-transformer, as shown in fig.
The autotransformers are generally tapped at the 50, 60, and 80 percentage points, so that adjustment at these voltages may be made for proper starting torque requirements. Since the contacts frequently break large values of current, arcing is sometimes quenched effectively by having them assembled to operate in an oil bath.
Auto-transformer starters may be either manually or magnetically operated. The wiring diagram of an auto-transformer starter is shown in fig.
The operating handle is spring loaded lever mounted on the outside of a steel cabinet to stand vertically in the "off" position.
When the operating handle is moved to the start position, the movable contacts are moved against the starting contacts.
This energizes the star-connected auto-transformer and impresses reduced voltage (50 or 60 or 80 percent according to the setting on auto-transformer) across the motor.
After the motor has accelerated to about full speed, the operating handle is moved to the run position.
This instantly opens the starting contacts (or disconnects the auto-transformer from the line) and connects the motor directly to the line through the running contacts.
A latch, fixed to the mechanism, is made to drop into a notch so that the operator is prevented from throwing the handle accidently to the run position first, however, when the handle is quickly pushed from the start to the run' position, the latch is kicked up to make the lever free for its forward motion.
The operating handle is held in the run position by an electromagnet or under voltage (latch) coil until the stop button is pressed.
If the supply fails or supply voltage drops to a low value, the electro-magnet will release and trip the holding mechanism. Overload protection is provided by thermal overload relays.
Let the motor be started by an auto-transformer having a transformation ratio, k. If Isc, is the starting current when normal voltage is applied, and applied voltage to stator winding at starting = kV
then motor input current, Is, = k Isc
Supply current = Primary current of auto-transformer
= k x secondary current of auto-transformer = K2Isc
= K2 X Torque is obtained by switching the motor directly.
Hence the line current and the starting torque are reduced in the square ratio.
Advantages:
1. The advantages of this method of starting of cage induction motors lie in the fact that the voltage is reduced by transformation and not by dropping the voltage in the resistors (or reactors) and, therefore, the current and power drawn from the supply mains are also reduced in comparison to the primary resistor (or reactor) starting.
2. The internal losses of the starter itself are small during long starting periods. availability of highest torque per ampere of supply current
3. adjustment of starting voltage by a selection of proper tap on the auto-transformer
4 suitability for long starting periods
5. closed transition starting and
6. motor current larger than supply current.
Disadvantages:
The drawbacks of such starters are low power factor and higher cost in case of lower output rating motors.
Applications:
This method can be employed for starting of star-connected as well as delta-connected motors. For starting of large cage motors (of output rating exceeding 20 kW) this method of starting is often used.