Refrigerator Motor Protection

The most common cause of motor failure is overheating. This condition is created when a motor exceeds its normal operating current flow. The result can be either a breakdown of the motor winding insulation and a short circuit, or a winding burn-out. For this reason, overload protection is provided in the form of a current and temperature sensitive control which will open the circuit before any damage to the motor can occur.

The following types of control are used: fuses, circuit breakers, bimetal switches and thermistors. Fuses and circuit breakers are located remotely and are normally required to protect the circuit conductors.

Bimetal switches are accepted as an overload protection for most hermetic and semi-hermetic motor compressors (Figures 42 and 43).


A thermistor is a solid state semiconductor which heats up as current is passed through it. As the temperature of the material of which it is made increases, a greater resistance to current flow is created and under overload conditions the current flow very quickly almost ceases, thereby stopping the compressor. Most thermistors are made of lithium chloride or coated barium titanate. The resistance changes approximately 6 per cent of each degree C.

Thermistors that have a PTC are connected in series with the windings and prevent current flow when the temperature of the motor increases.

The negative temperature coefficient (NTC) device is a small module which is embedded in the motor windings. Internal overload protectors perform the same function as the external and thermistor types (Figure 44).


Motor overheating has various causes. Motor compressors used for refrigeration duty are designed to start when there is an equalizing of pressures within the compressor. An increased starting load such as a high head pressure can cause overheating and excess current to be drawn. A shortage of refrigerant can also result in overheating of the motor windings, since the motors rely on returning suction vapour to assist in the cooling of the windings.

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