Troubleshooting Motors : Start Capacitors

 Introduction to Start Capacitors

• Start capacitors are used in many split-phase motors, especially on compressors.
• Testing a start capacitor is similar to testing a run capacitor, using a capacitor tester to measure microfarads.

Testing Start Capacitors

• Start capacitors have a specified range (e.g., 145-175 microfarads for a 160 microfarad capacitor) with a ±10% tolerance.
• Before testing, turn off the power and discharge the capacitor using a proper capacitor discharge tool (avoid using screwdrivers to prevent arc flash or shock).
• Disconnect or isolate the capacitor from the circuit before testing.
• A measured capacitance within the specified range indicates a good capacitor.

Visual Inspections and Precautions

• Always discharge the capacitor before testing to remove any stored charge.
• If the start capacitor has a bleed resistor, consider disconnecting it to avoid inaccurate readings.
• Visual signs of failure: perforated or blown fusible plug, top blown off, or evidence of explosion within the control panel.
• These visual signs indicate the capacitor is faulty and doesn’t require further testing.

Replacing Start Capacitors

• Replace with a capacitor within the same range and with an equal to or greater voltage rating.
• Universal replacement start capacitors are available for emergency use but are expensive. Replace with the proper OEM capacitor as soon as possible.
• Always replace the starting relay when replacing a faulty start capacitor, as relay defects can cause capacitor failure by keeping contacts closed too long.