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NORA Oilheat Technicians Manual

PSC motors are also frequently used in air handlers, fans, and blowers and other cases where a variable speed is desired. By changing taps on the running winding, but keeping the load constant, the motor can be made to run at different speeds. Troubleshooting ‘dead spots’ ‘Dead spot’ is a common term for a certain orientation of the rotor at which the motor will not start. Two things can cause dead spots. First, there could be a fault in the manufacturing process. If so, the motor may not have enough torque to start the burner. This condition is rare and cannot be repaired. Second, if the start switch of a split phase motor is unevenly worn or has shifted out of position, the contacts may become slightly separated when the rotor is in a particular location. No current will flow through the starting winding and the motor will not start. If this is the problem, the motor should be replaced. Troubleshooting the start switch To check a faulty start switch, you need an ohmmeter. The motor starting windings have a much lower resistance than the run windings do. Use this difference to determine if the start switch is defective. When the start switch is functioning properly, both the start and run windings are connected, resulting in lower resistance. If the start switch is faulty, the start windings will not be connected or will intermittently be connected. This results in higher resistance or fluctuating resistance as the motor shaft is rotated. Set the scale on your ohmmeter at its lowest setting and zero out the meter by touching the test leads together and adjusting the meter to show zero ohms. Turn off the power, disconnect the motor input leads and remove the motor from the housing. Place the motor on its back, shaft up. Connect your meter across the black and white motor input leads. If the start switch is functioning properly, the resistance Chapter 10 Motors Figure 10-12: Multimeter with a capacitance range. should be about 2 to 4 ohms. If the resistance is much higher (in the range of 7 to 10 ohms) then the start switch is probably bad. To double-check the measurement, pull up on the shaft and note if the resistance drops to the proper 2-4 ohms. If it does, the switch is definitely bad and the motor should be replaced. The most important function of this test is to make sure the resistance across the motor leads does not change when the motor shaft is pushed down or rotated in a full circle. Rotate the shaft slowly and note the resistance. If it goes up to 7-10 ohms at any point in the rotation, you have found your dead spot. If it is the kind of motor where you can see the start switch, push down on the switch and the resistance should increase from 2-4 to 7-10 ohms. Troubleshooting PSC motors PSC motors have two major areas to troubleshoot—the capacitor and the windings. Both are relatively simple to check and require only a multimeter (Figure 10-12) with a capacitance range. On page 10-17, from RW Beckett, is a PSC motor trouble shooting checklist. Checking capacitors—A failed capacitor will cause a PSC motor to either stop or run more slowly than designed. The thermal protector will trip if a restart is attempted. To check a capacitor, perform the following steps: 1. Remove power from the burner and carefully disconnect the two leads from the capacitor terminals. 2. Discharge the capacitor. To safely discharge the capicitor follow the instructions on page 8-21 and Figure 8-25. Caution: Capacitor discharge can cause physical harm due to electrical shock. Chapter 10—Motors 10-11


NORA Oilheat Technicians Manual
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