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.

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.

5 Replies to “HVAC Motor-start relays”

  1. 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…

  2. Very interesting. Thank you.
    Hoping you can help me.
    I have a refigerator that has stopped working.
    I’m sure it is the PTC (2 Pin) with external Thyristor but the manufacture hasnt got the parts to replace.
    Can I just get any PTC or is it critical to get the exact same?
    Any help would be much appreciated.
    Thank you.

Leave a Reply

Your email address will not be published. Required fields are marked *