Troubleshooting Motors : Checking Motor Overloads

 Familiarization with Control Box:

• Important to understand the control box components.
• Primary Component: Motor overload.

Example: Semi-Hermetic Single Phase Split-Phase Compressor:

1. Motor Connections:
   • Designations for start, run, and common terminals.
   • Power to the compressor passes through the overload, connected to the common terminal.
2. Overload Function:
   • Normally closed contacts.
   • Opens when temperature is too high.

Hermetic Compressor Terminal Box:

• Similar components to semi-hermetic but with minor differences.
• Components: Compressor terminals, overload protector, current relay, start capacitor.
• Checking the Overload:
  • Use an ohmmeter.
  • Zero ohms = good overload (closed).
  • Measurable resistance = bad overload (should not have resistance).

Single Phase Compressor with Open Internal Overload:

• Overload protects against excessive current and internal temperature.
• Testing:
  • Place ohmmeter leads across terminals (S-C and R-C).
  • Open Reading (infinite resistance): Confirms overload is open.
  • Good Reading: Indicates overload is functioning properly.

Three-Phase Compressor Motor with External Overload:

• Less Common: External overloads are not frequently seen but can occur.
• Connections: Power from contactor connects to motor windings via L1, L2, L3.
• Overload Function:
  • In series with motor windings.
  • Opens to break power to motor windings if conditions require.

Procedure for Checking Overloads:

1. Use Ohmmeter:
   • Set multimeter to ohms function.
2. Testing Motor Windings:
   • Ensure overload is closed before testing motor windings.
   • Zero Resistance: Indicates shorted winding or closed relay contacts (good overload).
   • Measurable Resistance: Indicates potential issues with overload protector.