How to test a capacitor.

How to test a capacitor.

Posted by Gerd F. Bauer II on Jan 26th 2022

Capacitors aid in the starting and or running of AC electric motors, over time they do degrade and will need to be replaced. In this article we will show you how to test a capacitor with a multi-meter. Multi-meters are very useful tools to measure voltage, current, resistance, and of course capacitance.

  • First we should understand what a capacitor does, a capacitor holds electrical energy.
  • A start capacitor aids in starting of the motor by briefly increasing the starting torque.
  • A run capacitor is used to energize the auxiliary coil to create a magnetic field while the motor is running.

Some motors will only have a start capacitor some motors will have a start and a run capacitor.  The MAG-9000 Sharpener has a start capacitor and the MAG-8000 has a run and start capacitor. Capacitance is measured in microfarads the Greek symbol of uF. For example one of the MAG-8000 capacitors is 40 microfarad. To measure the capacitance take your multi-meter and turn the dial to the symbol shown below.

Microfarad Unit Symbol
Capacitance Symbol


Make sure you make note if your multi-meter requires you to push a "function button" to make capacitance option active. Some have yellow symbols, then you have to push a yellow button to make the function active that the dial is turned to.

Place red probe of the multi-meter on one post of the capacitor and black probe on the other post. It does not matter which probe you use on the posts – they just need to be on opposing posts. On the multi-meter display you will see the microfarad amount.The label on the capacitor will have the microfarad spec and the tolerance. For example one of the MAG-8000 capacitors is 40 microfarad with a +/- 6%.

Watch the video below to watch the process.

If you need a new capacitor see below:

MAG-9000 Sharpener has two capacitor options, for BALDOR 9000-31B and for LEESON 9000-31L

MAG-8000 Sharpener uses two capacitors 8000-25a and 8000-25b