Grounded, Open, or Shorted Motor
Hermetic motors are rugged and reliable but they can fail in several ways. A broken wire in a motor winding can prevent the motor from running. It can also fail due to an internal short circuit where one or more motor winding conductors short together or short to ground. Any one of these problems justifies the replacement of a hermetic compressor. 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 hermetic and semi-hermetic
compressors have some type of internal overload. Single-phase compressors have internal overloads called line breaks (Figure SP-9-6) that sense both motor current and winding temperature. If the overload opens, it breaks the current path and shuts down the motor. Some single-phase and three-phase hermetic compressors have an internal thermostat that senses winding temperature only. This thermostat is wired in series with other components in the low-voltage control circuit and will open if temperature limits are exceeded. This in turn opens the control circuit, deenergizing the compressor contactor.
Should a line break overload or internal thermostat fail in the open position, compressor replacement is required. Before replacing a compressor because of an open winding or internal overload, always make sure that the compressor 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 of 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 indoor thermostat switch to OFF immediately. The time between the customer’s call and the service call visit is usually long enough for the motor to cool down.
