What Does A Compressor Capacitor Do?

Many air compressor motors have capacitors on them. What does a compressor capacitor do?

Electric motors on air compressors, and motors for other devices, pull a great deal of power when they start. This is known as inrush, and quite often, the inrush demands of a compressor motor will exceed the capacity of the air circuit and the result may be a compressor that will not start or a blown fuse or breaker in the compressor supply circuit.

To quote a supplier of capacitors “A start capacitor is used to briefly shift phase on a start winding in a single phase electric motor to create an increase in torque. Start capacitors possess a very large capacitance value for their size and voltage rating.”

air compressor motor capacitorIn other words, a start capacitor – one of which is shown in the photo, discharges its power very quickly during the compressor motor start phase to help get the motor running.

There is another type of capacitor sometimes found on an air compressor motor, and that’s a run capacitor. The run capacitor modulates the current and voltage to the compressor motors run windings, and is considered a continuous duty device.

If either capacitor fails, your air compressor will not start or will not run properly.

What does a failed capacitor look like?

When a motor capacitor fails, it sometimes exudes the contents onto the outside of the capacitor housing, like the contents overheated and melted. The capacitor may have ruptured completely. It may have a blister on its side that looks like a heat blister.

If the air compressor is hard to start, or will not stay running properly, then a capacitor failure is suspect.

How to test a capacitor?

You need to be able to use a multi-meter in order to test to see if your compressor motor capacitor is working.

Once you have that figured out, there are many “how to” videos on line that show you how to test your compressor motor capacitor. The following is one of the best as far as we’re concerned.

A functioning capacitor is critical to the operation of your air compressor.

Some motors have one start capacitor, others have a start and run capacitor, and others have a combination start / run capacitor.

The authors of the video embedded above would be pleased to help you identify what capacitor you need for your air compressor motor.

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4 years ago

50 mfd is the original capacitor is there any problem if i change it to higher mfd like 55 mfd..this capacitor for air cond.compressor 2 tons.

Rick Hammer
5 years ago

What causes the compressor to run slow for awhile then trip a fuse….a/c delco 15 gal..model 841150..2hp

5 years ago

I a steele 3.5 hp peak 8 gallon twin tank air compressor and it wont start. When i plugged it in it tried to start but then stopped my pressure switch looks fine. The run compasitor dosent look bad but idk what it could be i know there is a burn spot on rhe plug