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How To Test Motor Run Capacitors While The System Is Running

Posted by dekielectronics on May 21st, 2020

When it comes to testing a run capacitor, many technicians pull off the leads and use the capacitance settings on their meters to test them. If you check capacitors constantly as part of your regular testing and maintenance tasks, then you should test them when they’re running. It’s one of the best ways to confirm whether the capacitor is performing properly on not. The reading that you get when you switch off the unit isn’t as reliable as the one you get when the same remains on. Here you will learn the steps that you need to follow to test the run capacitors.

1. You need to start by measuring the amperage of just the start wire. It will be the wire between the Motor Run Capacitors and the compressors. If you’re dealing with a 4-wire fan motor, then it’s usually the all-brown wire and not the one that has white stripes. For compressors with dual capacitors, you have to choose the wire that goes to the HERM terminal.

2. Once you get the amperage readings from the start wire, you need to multiply it by 2652. Some experts say that you should do it with 2650, 2653, and 2654, but the most accurate and reliable results come from multiplying it by 2652.

3. Now, you have to measure the voltage across the capacitor. The compressors that remain between HERM and C, the results acquired will run across the start and run terminals on the motor.

4. You have the divide the total of the start wire amps times 2652 by the voltage results that you get from the last action. This value is the capacitance.

5. Don’t forget to read the nameplate MFD on the capacitors and compare the same to the actual readings that you get. Most capacitors allow for 6% of positive or negative tolerance. If it remains out of that range, then you should consider replacing the existing capacitors. You must always go through the calculations several times before you get in touch with Motor Run Capacitors Manufacturers and sellers.

6. Repeat the entire process on all the run capacitors and you will have assurance whether all of them are functioning properly under loads or not.

7. You also have to keep in mind that the existing capacitors may not be the right capacitors for the machine. Probably the previous technicians replaced the motor or compressor or maybe they used the wrong size.


Now that you know the steps that you must follow to get an appropriate reading, you should get to work as soon as possible. However, it does seem a bit more complicated than what you did before. You just disconnected the capacitors and measured the values directly. So, why should you go through all the trouble? You have to bear with it if you want to test the capacitors while they perform.

Also See: Run Capacitors, Start Wire, Motor Run, Technicians Pull, Run, Capacitors, Wire

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