Compressor Starting Relays and Capacitors) Potential Relays

 

Differences from Current Relays

• Potential relays use back EMF (electromotive force) instead of current to operate.
• Terminals:
  • Active terminals: 1, 2, 5.
  • Dummy terminals: any others.
• Normally closed contacts in series with start winding and possibly start capacitor.
• Coil placed between terminals 2 and 5.

Usage and Characteristics

• Used on single-phase compressors up to 5 horsepower.
• Normally closed contacts minimize arcing by starting closed.
• Back EMF is generated voltage by rotor turning against stator.

Operation of Potential Relays

• Back EMF or secondary voltage energizes the relay coil.
• Pickup voltage: sum of applied voltage and back EMF, higher than the applied voltage.
• Contacts open with pickup voltage, breaking power to start winding and start capacitor.
• Back EMF keeps contacts open while the motor runs.

Wiring

• Terminal Designations:
  • 5: common.
  • 2: start winding.
  • 1: run winding and possibly a capacitor.
• Mnemonic: 521 - "5 goes to common, 2 goes to start, 1 goes to run."
• When contacts open, power to start winding and start capacitor is broken.

Internal View

• Exposed potential relay shows terminal designations, coil, and normally closed contacts.
• Contacts start in the closed position, providing power to the start winding at startup.
• Upon reaching pickup voltage, the coil energizes, pulling contacts open.

Diagram and Function

• Capacitor start induction run motor with potential relay:
  • Normally closed contacts allow power to flow to start winding and start capacitor.
  • Motor starts turning, generating back EMF.
  • Pickup voltage opens contacts, removing start winding and start capacitor from the circuit.
  • Run winding remains in the circuit, back EMF keeps coil energized.