Run Capacitor Size

Discussion in 'General Electronics Chat' started by david37g, Dec 5, 2012.

  1. david37g

    Thread Starter New Member

    Dec 5, 2012
    I have a Ajax (out of business) 7.5hp, 230v, 39.5 fla, 1730 rpm single phase electric motor. The run capacitor blew and the motor would start and then blow the breaker. The numbers on the capacitor can not be read. I replaced it with one of the same physical size 25uf, 370/440v and the motor runs. I am concerned about motor damage based on internet posts about wrong size capacitors causing too high amps or low power. The nla with this capacitor is 13.6a. I have tried adding 5uf in parallel, and get 12.5nla, 10uf added in parallel gives 11.0nla and 15uf added in parallel gives 10.0 nla.

    I also tried adding 5uf and 10uf to a pressure washer with a 15uf run capacitor. The run amps dropped with each addition in parallel

    I have read some post in HVAC forums where they change run capacitors until they get the lowest amps. These tests were under load conditions. This does not seem to work?
    Are there any other ways to determine the right run capacitor size?
  2. david37g

    Thread Starter New Member

    Dec 5, 2012
    I found an Ajax motor description for a 5hp single phase that uses 2ea 15uf run capacitors in parallel(gives 30uf total). Decided to use a 35uf capacitor. The motor runs under load. My loaded amps is 20A.
  3. bmoore_17

    New Member

    Dec 9, 2012
    Check the data plate on the side of the motor. It should give you the correct mfd. rating for that motor. You could damage the motor if the capacitor is to small or to large, although you can be 10-20% off in either direction without hurting the motor. The run cap. limits the current in the start winding and causes a phase shift for the motor to start and also run more efficiently.
  4. Duane P Wetick

    Senior Member

    Apr 23, 2009
    What you are doing with these capacitors is correcting the running power factor(s)...Adding capacitors will improve the power factor (less power drawn from the source) while maintaining the loaded running amperes of the motor.
    You will not harm the motor by doing this...just use less power from your source. The FLA figure (39.5) that you give is the current the motor will draw (un-corrected) from the source when the motor is running at the rated (7.5HP) load.

    Cheers, DPW [Everything has limitations...and I hate limitations.]