motor start and run capacitor question

Discussion in 'General Electronics Chat' started by jgmo, Jun 9, 2012.

  1. jgmo

    Thread Starter New Member

    Jun 9, 2012
    First, let me say I know just enough about electricity to be dangerous. I am a retired diesel tech that has been forced back into "making ends meet" due to inflation. I have a strong background in automotive electronics but am a bit weak when it comes to alternating current. Much of my current work involves ac motors and I try to read all I can to keep myself safe and to not screw up the customer's equipment.

    I recently had a job repairing an automatic gate opener for a client's storage yard. The electric motor that ran the gate would not start. I removed the motor and used the capacitor test function on my Fluke 87a multimeter to determine that the capacitor attached to the motor was open. And, here's where I ran into a problem that I'm hoping someone on this forum might be able to explain.

    The capacitor was labeled 65 microfarad +/- 5%, 240vac. Ok, I'll just run down to the air conditioner supply I do business with and buy another. NOT! This capacitor is not available anywhere that I can find including the OEM of the gate controller. All info that I read on motor capacitors also leaves me confused.

    Start capacitors are typically rated at 125, 165, 250 and 330 volts while run capacitors are typically 370 and 440 volts. So, my cap's voltage is in the start column.

    But, start capacitors usually have a wide spread or range of capacitance like 65-77 mfd while run caps are very specifically rated with a tolerance usually given as 5 or 6%. So, that puts my cap in the run category.

    Of course, silly me, I want to know why. No one locally, including severaly motor repair shops and even the controller manufacturer themselves seem to be able to answer my question as to what this capacitor is actually called and why it is so different from most other applications I have come across. The gate control people said to call it a run capacitor and that they can't get it anymore so they use one that is 60mfd and 250 volt instead. That is what they sold me and it works but I would like to know more.

    Any input appreciated and thanks for reading.
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Usually a motor with only one capacitor, the capacitor is a start capacitor. The value is not critical as long as it is close to the original one. In your case the 65mfd is close enough to the 60mfd that you bought it will be OK.

    The voltage is more important. Start capacitors should be ~1.5 times the line voltage of the motor. I'm assuming your motor is running on 115-120 volts so the 250V rating is fine. Not all motor capacitor makers use a standard voltage for some reason. But as long as the voltage is as least 1.5 times more your alright.
  3. #12


    Nov 30, 2010

    The capacitors with a wide range on their label are, "dry electrolyte" capacitors. They feel light weight and can only survive for a few seconds under full load. That's why they are used as start capacitors. When the motor fails and won't start, the dry electrolyte caps announce their death with large clouds of white smoke. The starting torque is dependent on the value of this capacitor. For instance, I installed a 5 horse motor on my table saw and it was starting so hard that it would jerk the arbor out of alignment. I changed the start cap to 25% of the original value and that solved my problems. Lower than 25% caused "not starting" problems. Point is: the size of the start cap is negotiable.

    The heavy caps are "oil filled run capacitors". You can connect them straight across the power line and leave them connected for a week, and nothing will be damaged. The problem with them is that they don't measure properly when you use a tiny DC voltage from a meter. You measure them by connecting them to the proper AC voltage and measure the current they allow. Then use math to calculate their size. A capacitor in the "run" position is fairly picky about being the right microfarads...not negotiable. When you can't buy the right size, you buy smaller caps and wire them in parallel.

    Gotcha started?
  4. jgmo

    Thread Starter New Member

    Jun 9, 2012
    #12 and shortbus, I want to thank you for your responses.

    I didn't know anyone had answered until just now. Is there no auto notification on this forum?

    Anywho, prior to reading your responses, I ran across an old friend that has been in the electric motor selling business for many years. I posed my question to him and he said "hmmmm, I've seen that someplace before"!

    And off he went to search through his stacks of catalogs and brochures.

    What he came up with was reference material for a line of capacitors that are called "Start/Run" caps.

    Ok! There you go! But, there wasn't much in the way of description or specs other than that.

    He also told me that the voltage rating was unimportant as long as it was the same or HIGHER in value than the original. He stated that his shop and service trucks are pretty much stocked with the 440v and 600v version of any given capacitance rating as they are "good to go" for almost any application. He said the voltage rating was a function of what the cap could take, not what it puts out and that a cap rated at a lower voltage is simply a less expensively constructed cap.

    I did open the old cap and, even though I considered it to be fairly light weight, it had SOME oil in it. Not much to be sure but some.

    As I read your explanations, some of this seems to be in conflict with what you both said. I'm not losing any sleep over it but there does seem to be a bit of confusion in the industry when it comes to these things and, I must say, I'm still confused too. But, then again, THAT is a pretty normal condition for me.

  5. #12


    Nov 30, 2010
    I think there is a "subscribe" in the "User CP" at the top of the page. Poke around in the controls provided for each user. 370 volt caps reside in my truck because I almost never work on anything over 240VAC.
  6. jgmo

    Thread Starter New Member

    Jun 9, 2012
    Yep, that's where it is. I should already know this stuff but I'm pretty dense when it comes to computing.
    Thanks for straightening me out.
  7. RichieO

    New Member

    Aug 1, 2012
    Hi Guys, I am new here, and have a problem that hope I someone can help me sort...
    I have a 0.25hp single phase 230v ac pump motor, the start capacitor is burned out, the details on it are not readable, but were not readable before it burned, as it was painted over by the manufacturer, so I need a guide as to what microfarad to use, that would be safe and not detrimental to the motor...
  8. PackratKing

    Well-Known Member

    Jul 13, 2008
    220 need to start your own thread with this and original concern ......
    jgmo may have his problem solved, and you may not get much more traffic on this particular thread. I'll rattle moderators cage ...
    Last edited: Aug 1, 2012
  9. #12


    Nov 30, 2010
    Apparently they sleep in on Wednesdays. I rattled them 4 hours ago.
  10. jgmo

    Thread Starter New Member

    Jun 9, 2012
    Hi Richie,
    I agree with PackratKing, you'll get more and best responses starting a new thread. And, PackratKing, you are correct...I solved my problem.

    But, Richie, while I was looking into my dilemma, I did come across a formula that was an answer to a question similar to yours. I cannot vouch for it's validity though. Just throwing it out there since I have it.

    The formula for unknown start cap given was:
    2650 X FLA/VAC
    2650 multiplied by full load amps and divided by supply voltage.

    Again, I'm not convinced this is a valid formula but it might generate some useful discussion on the subject.

    Good luck.