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How do I find Capacitor rating to start / run Single phase motor

  1. Feb 10, 2007 #1
    I am trying to refurbish a 1.1KW single phase motor (1445 RPM) but both start and run capacitors are missing.

    How can I calculate what capacitors are required.

    Motor is Rated 7.2 A (220 VAC)

    Look forward to your help.

  2. jcsd
  3. Feb 10, 2007 #2


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    Call an industrial supplier and give them the make/model of the motor. They will set you up. Do you know why these items are missing? There is usually a good reason for it. Something else is wrong with the motor and the 2 capacitors were salvaged?
  4. Feb 10, 2007 #3
    Thanks for your reply, which unfortunately is not a lot of help in my circumstances.

    I am in Malawi, Central Africa, and "industrial suppliers" are very thin on the ground. Being in Malawi is also the most likely explanation for the capacitors being missing - in Africa if you don't use it you lose it! It is of course possible that the is a problem with the motor, but I have done a few basic tests and I think it is ok. Obviously replacing the capacitors was the best place to start.

    Is there not some way to calculate the likely ratings so I at least have somewhere to start with trial and error?

    Many thanks.
  5. Feb 10, 2007 #4


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    I'm sure there is a way to calculate it, or a typical set of guidlines for it. However, I wouldn't know what they are. I DO know that the values will be similar for simlar types of motors of a given horsepower rating and voltage. You do realize that you don't need a starting capacitor to do a few basic tests? If you have the running capacitor installed and just leave the starting capacitor out, NOT bypassed, just leave the terminals unhooked, apply power to the motor and manually spin the motor in either direction. The motor should spin up. Of course you haven't tested the starting winding, but you might look into wiring a bunch of light bulbs in parallel with each other but in series with the starting winding (in place of the capacitor). If it does start on it's own, it will not have much starting torque, but it should prove that the starting winding is good if it does spin up on its own.
  6. Feb 10, 2007 #5


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    Just a note. It's probably not the case here, but I have seen motors where the start windings (and capacitor) get disconnected form the circuit once the armature speed is high enough. The motor has a centrifugal switch on one of its shaft ends.

    After some google searching; looks like for a 230vac motor, the start caps are 16-22uF.

  7. Feb 10, 2007 #6


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    If the motor has a starting capacitor as well as a running capacitor then I am sure that the starting capacitor is switched out after a certain RPM. I assumed that was a known.
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