How Capacitors Fail

Why this matters

A capacitor stores and releases electrical energy. In motor circuits it gives the kick that gets a motor spinning and the phase shift that keeps it running efficiently. On control boards it smooths power. It is a low-cost part, but a weak one drags down everything around it: a motor that hums and will not start, a compressor that trips on overload, a board that resets at random. Capacitors are one of the most common single-part failures in the field, and they fade gradually long before they die, so a meter beats a guess every time.

The two jobs and the two types

• Run capacitors stay in the circuit the whole time the motor runs. They are sized in microfarads (the unit of capacitance, written uF or MFD) and stamped with a voltage rating. A weak run cap makes a motor draw more current, run hot, and lose efficiency.
• Start capacitors kick in only at startup for a second or two, then drop out. They have a much higher microfarad value. A bad one means the motor hums but will not break free.
• Board-level electrolytic capacitors smooth and filter power on control boards. When they dry out, the board behaves erratically.

The failure modes

• Capacitance drift (the slow fade). The most common. The cap loses microfarads over time. A run cap rated for a value but now reading well below it cannot do its job. The motor still tries to run but pulls high amps, overheats, and shortens its own life. This failure is invisible without a meter because the cap looks fine.
• Open (dead). The cap reads zero or near-zero capacitance. The motor will not start at all or starts only with a manual spin.
• Shorted. The cap reads a dead short. It usually trips a breaker or blows a fuse, and the motor does nothing.
• Bulging or venting. A swollen top, a domed can, or a popped relief vent means the cap cooked itself. Heat and over-voltage are the usual causes. A bulged cap is a clear visual condemn, but a flat cap can still be weak inside.
• Leaking. Oil residue around the base of a metal-can cap, or a crusty deposit on a board electrolytic, means the dielectric is breaking down.

How to test one

Always discharge a capacitor before you touch the terminals. A charged cap holds a real jolt. Bridge the terminals through a resistor or an insulated screwdriver with an insulated handle, then verify zero volts with the meter.

1. Read microfarads. Use a meter with a capacitance function. Disconnect at least one lead so the circuit does not skew the reading. Compare the reading to the value stamped on the cap. A run cap should read within a tight tolerance of its rating (commonly within plus or minus six percent). A reading meaningfully below rated means replace it, even if everything looks fine.
2. Check for short. On the resistance setting, a good cap shows a brief charge curve then climbs toward open. A steady near-zero reading means it is shorted.
3. Eyeball it. Any bulge, vent, leak, or scorched terminal is a condemn on sight.

Warning signs in the field

• A motor that hums but will not start, or needs a hand spin to break free.
• A motor or compressor that starts hard, runs hot, or trips on overload after a while.
• A control board that resets, freezes, or behaves randomly (dried board electrolytics).
• A breaker that trips the instant the load engages (shorted cap).
• A visibly swollen, leaking, or scorched capacitor can.

What to do about it

Match the replacement on both numbers: microfarad value and voltage rating. You can go up on voltage rating safely; never go down. Match the microfarads to the nameplate, not to whatever was last installed, because a previous tech may have grabbed the wrong one. A dual run capacitor (one can serving two motors) must match both stamped values. If a new cap fails fast, the cap is the victim: look for a hot location, a failing motor pulling high amps, or over-voltage feeding it.

References

• Manufacturer capacitor specifications (microfarad value, voltage rating, tolerance)
• NFPA 70 (National Electrical Code) for circuit overcurrent protection
• Trade-standard practice for safe capacitor discharge and capacitance measurement
• See related: how switches and relays fail; the baseline reading you should always take