The most common causes of electrical failures in motors include:
• Grounded, open, or shorted windings
• Open internal overloads
Grounded, Open, or Shorted Windings — Motors are rugged and reliable, but can fail in several ways. The motor may fail when the wires in one or more of the motor windings break, thereby opening the motor circuit. It may fail grounded, where the wires touch the motor stator or housing. It can also fail due to an internal short circuit where one or more windings short together. Any one of these problems justifies the replacement of a motor. The electrical tests used to determine if a motor is open, shorted, or grounded are given in the detailed procedure at the end of this section.
Open Internal Overloads — Most motors have some type of internal overload. Single-phase motors usually have internal overloads (Figure SP-10-2) that sense both motor current and winding temperature. If the overload opens, it breaks the current path and shuts down the motor.
Should an overload fail in the open position, motor replacement is required. Before replacing a motor because of an open winding or internal overload, always make sure that the motor has cooled down enough to allow any internal overload device to reset. It may require more than an hour for the overload to reset. Generally, the motor must be below 115° F before checking the continuity o f the motor windings through an internal overload device.
One way to ensure that sufficient cool-down time has elapsed is to advise the customer to turn the thermostat switch to OFF immediately. The time between the customer’s call and the service visit is usually long enough for the motor to cool down.