Thermal Relay

A thermal relay is commonly known as a hotwire relay. It is available in at least two different basic designs and is supplied by several manufacturers. All thermal relays operate on the theory that electrical energy can be turned into heat energy and that, when the temperature of a metal is increased, the metal expands. Thermal relays, like current and voltage relays, operate the starting winding circuit. In addition, the thermal relay controls the running winding circuit, if for any reason the circuit draws excessive current.

The device consists of a specially calibrated wire made from a material with high oxidation resistance and two sets of contacts, all of which are integrally attached to form the relay. Figure 14-37 illustrates a typical thermal relay motor starting circuit. The contacts are controlled by the hot wire, either through the use of heat-absorbing

Figure 14-37.-A typical thermal relay motor starting circuit.

bimetallic metal strips, or by its expansion of the hot wire, depending on the design of the relay.

OVERLOAD PROTECTORS

Essentially, an overload protector is a heat sensitive device much like a circuit breaker. When current in the circuit increases above normal, the added current heats a bimetallic strip that bends and opens a pair of contacts. The opening of the contacts disconnects the motor-running circuit

and the motor stops. This prevents damage to the compressor motor when troubles occur, such as a defective starting relay, an open starting capacitor, or high-head pressure. Figure 14-38 shows a typical bimetallic disk-type overload protector. This overload protector is connected in the common line and mounted on the compressor motor shell.

CAPACITORS

In hermetic refrigeration and air-conditioning work, capacitors are

Figure 14-38.-Bimetal disk-type overload.

identified in two groups: start capacitors and run capacitors. These may be identified further as dry capacitors that are used for intermittent operations (start capacitors) and electrolytic capacitors that are used for continuous operation (run capacitors).

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