Compressor Starting Relays and Capacitors) Current Relays and Overloads

 

Hermetic Compressor Terminal Boxes

• Example 1: Mounted current relay (not plugged onto compressor terminals).
  • Found in larger hermetic and semi-hermetic compressors.
  • Requires correct positional mounting indicated by an up arrow or decal.
  • Incorrect mounting can cause contacts to remain closed, leading to overload cycling and potential damage.
• Example 2: Plug-on type current relay.
  • Plugs onto S and R terminals.
  • Overload protector in the common leg, with compressor terminals under the relay.
  • Includes a start capacitor for additional starting torque.

Capacitor Start Induction Run Motor

• Start Capacitor: Adds starting torque, especially needed with thermostatic expansion valves due to high head pressure at startup.
• Wiring Diagram:
  • Current relay with LMNS terminals and normally open contacts.
  • Coil in series with run winding.
  • Start capacitor in series with start winding and relay contacts.
  • Locked rotor amperage energizes coil, closes contacts, powers start winding via start capacitor.
  • At ~80% speed, contacts open, de-energizing start winding and start capacitor.

Overload Protectors

• Example: Current relay plugged onto start and run terminals with an overload protector.
• Placement: Typically in common or neutral leg to protect against high amp draw.
• Wiring:
  • Neutral to overload to common terminal.
  • Line voltage to terminal 1 or L.
  • Jumper wire between terminals 1 and 2 can be removed for start capacitor wiring.

External Two-Wire Overloads

• Components: Bimetal disk and heater.
• Operation:
  • Increased motor amperage heats overload heater and bimetal disk.
  • High amperage warps bimetal disk, opening contacts between terminals 1 and 2, breaking power to compressor motor.
  • Bimetal disk cools and closes, restarting motor.
  • Prone to rapid cycling; repeated cycles can cause the overload to fail open after ~80 cycles.

Views of Overload Protectors

• Three-Wire and Two-Wire Overloads:
  • Bimetal disk in the center.
  • Heater across terminals 2 and 3.
  • Power goes out to compressor.
  • Heater wired in series with the motor, experiencing same current draw.