To reduce the voltage across the motor terminals during the starting period, an autotransformer-type starter generally has two autotransformers connected in open delta. In this method of connection, only two windings are used and connected as shown in Figure.
This arrangement is generally used, because it is cheaper, although the currents are unbalanced during starting period. This is, however, not much objectionable, because the current imbalance is about 15 percent and balance is restored as soon as running conditions are attained.
The autotransformer starters can be used for both star and delta connected motors. Most of the auto-starters are provided with 3 sets of taps, so as to reduce the voltage to 80, 65 or 50 percent of the line voltage, to suit the local conditions of supply.
- Most squirrel cage induction motors can be successfully started at 65 percent of line voltage. Where this fraction of line voltage does not provide sufficient starting torque, the 80 percent tap can be used.
- Where 50 percent tap creates excessive voltage dip, the 65 percent taps can be used.
At the time of starting, a reduced voltage is applied across the motor terminals. With the lower starting voltage, the motor draws less current and develops less starting torque than if it were connected to the full line voltage.
When the motor attains 80% of its normal speed, auto-transformers are cut out and the full supply voltage is applied to the motor. The switch making these changes from ‘start’ to ‘run’ may be airbreak (for small motors) or may be oil-immersed (for large motors) to reduce sparking.
Autotransformer Starter Working
Power circuit and control circuit for a two coil autotransformer starter is shown in Figure. The working of the starter is as under:
- When the start-button is pressed a circuit is completed to the coil of control relay CR, causing all CR contacts to close.
- When the coil of S contactor energizes, all S contacts change position. The normally closed S contact connected in series with R coil opens to prevent both S and R contactors from being energized at the same time.
- When the S load contacts close, the motor is connected to the power line through the autotransformers and 65% of the supply voltage is applied to the motor.
- When the time sequence for TR timer is completed, both TR contacts change position.
- The normally closed TR contact opens and disconnects contactor S from the line causing all S contacts to return to their normal position.
- The normally open TR contact closes and supplies power through the now closed S contact to coil R.
- When contactor R energizes, all R contacts change position.
- The normally closed R contact connected in series with S coil opens to provide interlocking for the circuit.
- The R load contacts closed and connect the motor to full voltage.
- When the stop-button is pressed, control relay CR de-energizes and opens all CR contacts. This disconnects all other control components from the power line and the circuit returns to its normal position.
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