HVAC Motor-start relays


Relays are a necessary part of many control and pilot-light circuits. They are similar in design to contactors, but are generally lighter in construction so they carry smaller currents.

Compressors used for household refrigerators, freezers, dehumidifiers, vending machines, and water coolers have the capacitor-start, induction-run type of motor. This type of compressor may have a circuit that resembles Fig. 14-2. When the compressor is turned on by the thermostat demanding action, the relay is closed and the start winding is in the circuit. Once the motor comes up to about 75 percent of rated speed, there is enough current flow through the relay coil to cause it to energize, and it pulls the contacts of the relay open, thereby taking the start capacitor and start winding out of the circuit. This allows the motor to run with one winding as designed.

compressor motor HVAC Motor start relays

Figure 14-3 shows the current type of relay. This is generally used with small refrigeration compressors up to 3/4 hp. Figure 14-4 shows the potential type of relay. This is generally used with large commercial air conditioning compressors up to 5 hp.

Protection of the motor against prolonged overload is accomplished by time limit overload relays. They are operative during the starting period and running period. Relay action is delayed long enough to take care of the heavy starting currents and momentary overloads without tripping.


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  1. simple but explain the whole idea of potential relay, great

  2. Somewhat backwards. At startup, the motor is essentially stalled, so the current through the run winding is high enough to energize the relay, which CLOSES, energizing the start winding through the capacitor…

  3. Scratch that (my description is for a current relay), but your diagram is flawed. Coil goes between C and S…


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