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Why Does the Start Capacitor Overheat?

high voltage hardware - isolators and capacitors image by Roman Ivaschenko from

A start capacitor refers to an electrical capacitor designed to alter current amounts that are sent to windings in an electric motor to create a rotating magnetic field. A start capacitor can overheat and suffer damage due to a number of causes.


Start capacitors work at voltage classifications that typically range from 370 to 440 V (volts). A sudden surge of voltage, called overvoltage, can cause a buildup of pressure in the sealed aluminum that makes up the outer casing of the capacitor, which can lead to overheating damage to the capacitor’s internal parts and possibly cause the capacitor to explode.

Sticking Relay

As relays open and close, electric sparks, called arcs, can cause the contacts to stick together. A sticking relay forces the capacitor to stay in the circuit for a longer period of time. The excess buildup of electricity can ultimately cause the capacitor to overheat.

Compressor Cycling

Compressors, or mechanical devices that compress gas, can run in rapid cycles that may cause a capacitor to overheat. Start capacitors that cycle above 20 starts, or cycles per hour, may be subject to overheating.

Start Capacitor Is Bad

When the air blows warmer than it should or the normal hum of your machine begins to sputter and clunk, it can lead to worrying questions. It not only turns it on, which is the woosh sound you may hear when the air conditioning system kicks on, but it also keeps the machine purring while it sends cool air throughout the home. This taxes the capacitor and ages it rather quickly. This is also why the capacitor tends to go bad right when you need cool air the most. Summer makes the machine work double time. To test for electric motor capacitor problems, you can use a long stick to push on the unit’s fan if it isn’t on. If the blades spin undeterred from the slight push, you have a start capacitor failure.

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