HEATER TROUBLESHOOTING.- A heater must be manually reset at the motor starter. If the magnetic starter fails to energize, the trouble is within the control circuit. However, if the coil should energize but the motor fails to run, the trouble must be within the load circuit or motor. The load circuit can be checked at terminals TI, T2, and T3. If the proper voltage requirements is there, the trouble is most likely in the motor.

Push-Button Stations An example of a push-button station with overload protection is shown in figure 7-52. In this case, the controller is connected to a 208-volt single-phase motor. This controller is a single-phase, double-contact device which connects or disconnects both undergrounded conductors to the motor. It has a start and stop button that mechanically opens or closes the contacts. Pressing the start button closes both contacts, and pressing the stop button opens both contacts. The control has two overload devices connected in series with the contacts. If an overload condition occurs, either overload device will open both sets of contacts. A typical application of this type control would be to control small machine tools.

Full-Voltage Reversing Starters Reversing magnetic controllers use two magnetic across-the-line starters whose power leads are electrically interconnected to reverse two of the three phases. The two motor starters are generally contained in one box and are mechanically interlocked so that one cannot close without the other opening. They are sometimes also electrically interlocked to help prevent closing both starters at the same time.

Figure 7-52.- Schematic for a single-phase manual controller with overload protection.

Reduced-Voltage 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.

Part-Winding Starters

Figure 7-54.- Resistance reduced-voltage starter.

Part-winding starters use two magnetic starters and operate like a resistance start controller. Figure 7-55 is a schematic drawing of a wye-connected, three-phase motor. The control circuits for the S and R relays are the same as for a resistance reduced-voltage starter, and so they are not shown. The S relay is energized first, connecting voltage to only part of the winding. The motor starts to run but develops little torque. At a predetermined time, the R relay closes. This action parallels the windings in the motor, reducing their resistance and causing increased current flow and more torque.