The autotransformer reduced-voltage starter places the motor on the secondary of the autotransformer while starting. The taps on the autotransformer limit the voltage applied to the motor to 50%, 65% or 80% of the nominal voltage. With autotransformer starting, the line current is always less than the motor current during starting because the motor is on the secondary of a transformer during acceleration. If a motor is connected to the 50% tap of the autotransformer, the motor current would be reduced to 50% of the normal starting value, but the line current would be only 25% of the normal starting current. The difference between line and motor current is due to the transformer in the circuit.
The lower line current is the reason the autotransformer starter is a very popular type of reduced-voltage starter. Since the motor starting current is greater than the line current with an autotransformer starter, the starter produces more torque-per-ampere of line current than any other type of reduced-voltage starter. Most motors can be started at 65% of line voltage. If the torque that the motor supplies to the driven equipment is not sufficient on the 65% voltage tap, a higher torque on the 80% tap is available.
Similarly, if too much torque is applied to the load with 65% voltage, or if the voltage dip associated with 65% starting is too high, the 50% tap is available. This versatility makes the autotransformer starter very popular. Figure 6 shows the typical schematic diagram for a two coil autotransformer starter. The circuit for a three coil autotransformer starter is similar except that an additional transformer winding would be inserted in the L2 leg. It should be pointed out that there are no significant disadvantages to using a two coil autotransformer design since the starting currents will be approximately balanced in each phase.
Up to 1500 KW rating
An auto-Transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting.
The motor starts at the voltage reduced by the transformer, with a correspondingly smaller current.
Which means the feeding current in comparison to direct starting would be reduced by the square of the transformer voltage ratio; nevertheless, it is in most cases noticeably higher, as it also covers the relatively high transformer losses.