Compressor Starting Relays and Capacitors) Intro to Starting Relays

 

Measuring Windings

• Measure resistance:
  • S to C: start winding.
  • R to C: run winding.
  • Start to run: sum of both windings.
• Open or infinite reading indicates an open winding.

Role of Starting Relays

• Starting relays remove the start winding from the circuit once the motor reaches 75-80% speed.
• Necessary to power both windings initially.
• Start winding must be removed to prevent overheating and damage.

Motor Windings Layout

• Layout shows windings between R and C.
• Start winding is parallel to the run winding.
• Start winding is smaller and not designed for full line current.
• Removed after startup to avoid damage.

Types of Starting Relays

• Hermetic compressors use starting relays; open drive motors use a centrifugal switch.
• Types of starting relays: current relay, potential relay, PTC thermistor (solid state).
• Function: send power to start winding, then break power once up to speed.

Starting Relay in Circuit

• Motor terminal configuration: run (R), start (S), common (C).
• Example: 120V motor.
• Starting relay in series with start winding.
• Relay breaks power to start winding at ~80% speed.

Capacitors for Starting Torque

• Start capacitor: additional starting torque, especially with TXV metering devices.
• Pressures that don't equalize need start capacitors for higher load start.
• Fixed metering devices typically don't need start capacitors.

Split Phase Motors

• Construction: run winding in the stator, start winding out of phase.
• Adding capacitors and relays can improve starting torque and efficiency.
• Types:
  • Resistance start induction run: basic with start relay.
  • Capacitor start induction run: improved starting torque for high load start.
  • Capacitor start capacitor run: highest starting torque and running efficiency.

PSC Motors

• Permanent split capacitor (PSC) motors: moderate starting torque.
• Run capacitor in series with start winding.
• Run capacitor limits current, improves efficiency, and protects start winding.

Common Starting Relays

• Current relay: plugs onto terminal configuration of hermetic compressors.
• Potential relay.
• PTC thermistor (solid state relay).