Starters Reduced-voltage starters are generally used for motors rated above 50 horsepower. Reduced-voltage starters are designed to reduce the current draw of the motor during the starting period only. They use either an autotransformer or resistor, both using the same basic principles.
Figure 7-53 is a schematic drawing of an autotransformer reduced-voltage starter. The autotransformer starter provides greater starting torque per ampere of starting current drawn from the line than any other reduced-voltage motor starter. But this type of starter is not always desirable, because, with the changing of the S and R relays, the motor is without power for a short time. Therefore, a resistance-reduced- voltage starter may be used. Resistance starters are sometimes applied where the circuit should not be opened during the transition from reduced to full voltage. They are particularly desirable when sudden mechanical shock to the driven load must be avoided.
Figure 7-54 shows a typical resistance-reduced- voltage starter. Pressing the start button energizes the S relay. The S contacts close, connecting power through the resistors to the motor. Voltage is dropped across the resistors, lowering the voltage to the motor. After a period of time, the T contact closes, energizing the R relay. The R relay contacts close, shunting around the resistors, to apply full voltage to the motor. The contact device may be a time delay relay or even a centrifugal switch, operated from the speed of the motor. Protective devices used in reduced-voltage starters are determined in the same way as we previously described.
Figure 7-53. - Autotransfromer reduced-voltage starter.Continue Reading