Troubleshooting Motors : Run Capacitors

 Introduction to Run Capacitors

• Troubleshooting single-phase motors often involves checking run capacitors.
• Symptoms of capacitor issues: motor hums, draws high current (lock rotor amperage), and trips on overload.

Initial Steps

• First, check the incoming voltage to ensure it's within the 5-10% tolerance.
• If voltage is correct, check the run capacitor.
• If the motor is seized (not rotating), it may indicate a lubrication issue, requiring motor replacement.

Testing Run Capacitors

• Use a capacitor tester or a digital multimeter with capacitance measurement.
• The capacitance should be within 10% of the capacitor’s rated value, unless specified otherwise (e.g., newer dry type capacitors with ±5% tolerance).
• Discharge the capacitor before testing to avoid electrical shock or arc flash.
• Disconnect one side of the capacitor to isolate it for accurate measurement.

Example

• For a capacitor rated at 25 microfarads (±6% tolerance) with a voltage rating of 440V AC:
  • Acceptable capacitance range: 23.5 to 26.5 microfarads.
  • A measurement of 26 microfarads indicates the capacitor is good.

Visual Inspections

• Check for physical signs of capacitor failure, such as bulging or oil leakage.
• If these signs are present, replace the capacitor without further testing.

Wiring Multiple Capacitors

• Parallel Wiring: Add the capacitance values directly (e.g., two 10µF capacitors in parallel result in 20µF). The effective voltage is the lower of the two capacitors.
• Series Wiring: The total capacitance is lower than the smallest capacitor. Use the formula: (Cap1 × Cap2) / (Cap1 + Cap2).

Temporary Replacement Capacitors

• Multi-rated replacement run capacitors offer various capacitance combinations but are expensive and intended for temporary use.
• Replace with the correct OEM capacitor as soon as possible after the emergency situation.