Finding A New Compressor Capacitor

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There are lots of comments on sites and blogs about finding a new compressor capacitor.

Before we get to helping with that, let’s just talk a bit about what the compressor motor capacitor does.

In the real world, often the power supply to an air compressor is marginal.

What Is Marginal Compressor Power Supply?

Typically, when we are talking about marginal power supply, we are referring to the home or small workshop air compressor, plugged into a wall socket, often with a 15 amp breaker.

Think of the power cord to a compressor as a water hose. The diameter of the hose and the length of that hose from the source of the water, along with the amount of water that is available at that source, all affect the flow of water through the hose to the faucet. Then, if what you need the water for demands a greater supply of water than can flow through that hose, you won’t have enough water to do the job.

The wire to the compressor is like that. The breaker that supplies the power to the wall socket may be a long way from where the socket is. The size of the wire from the breaker to the wall socket affects the power supply. The size of the breaker (for example, a 20 amp versus a 15 amp breaker) affects the amount of power to the socket. The number of other devices on the same circuit as the compressor, that may be on and pulling power, that affects the power getting to the wall socket.

That and the fact that there is barely enough power in a 120 volt power supply to start a motor means that getting the motor to start at all is difficult.

The start capacitor helps.

How many capacitors on a compressor motor?

The capacitor is inside the round "bump" on the side of this electric compressor motor -
The capacitor is inside the round “bump” on the side of this electric compressor motor.

The typical air compressor motor has one or two capacitors. The Dayton motor shown above has one.

This is what a typical compressor motor capacitor looks like, once you get the capacitor cover off.

compressor motor start capacitor

And, here’s more information about capacitors

Find a new compressor motor capacitor

Now that we have a better understanding of what the capacitor is, how do we find a replacement?

Assuming that your existing capacitor hasn’t heated up to the point where the stencil on the side of it is no longer legible, the markings should show a voltage or voltage range.

The markings should also show a microfarad rating, a number of range of numbers usually followed by µF ,  the symbol for microfarad.

The existing capacitor will have connected to the motor circuit somehow, and that’s typically a spade type electrical connector.

A spade connector that connects the motor capacitor to the compressor motor wiring may look something like this one:

Typical spade-terminal -

To find a replacement capacitor for your compressor motor first:

  • find any one on line that has the same microfarad rating – the exact number or a range that includes the rating of the old one
  • find one that has the same voltage rating as the old
  • find one that has the same connectors, or plan on soldering the motor wires to the connector on the capacitor
  • make sure the capacitor you found that fits all the parameters above also fits on your compressor

I can’t read the capacitor label

Well, that’s a bit more of a problem.

If this is the case, you need to get all of the specifications of your compressor motor from the motor information plate, and using them as part of your search criteria, Google search for a capacitor for such and such an electric motor.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)