start/run capacitor motor=do i need the start capacitor?

Discussion in 'General Electronics Chat' started by david101, Jul 29, 2010.

  1. david101

    Thread Starter New Member

    Jul 29, 2010
    hi guys, motor specs: ~10amp, 115vac 72s/20r mfd 480v capacitors. so ill get to the point :p i want to try and run it with only the run capacitor. today i tried it and when i set everything up the motor turned slowly then the circuit breaker went off. now i think there could be 2 reasons for this, 1 being in my title (no start capacitor--the motor wont be under load (only its own) while starting) but i also used low gauge (22 i think) wire to just test it because the motor is on a plug which i don't want to cut just yet, so i used the low gauge just for testing. so my question :p would the motor turning slowly then blowing the breaker be caused by the no start capacitor, incorrect wiring (im gonna try thick gauge tomorrow) or both? is there a way to only use the run capacitor? it'll save me a few bucks on a low budget project :p thanks in advance, david
  2. mcgyvr

    Senior Member

    Oct 15, 2009
    Yes you typically need the start capacitor.. Without one the motor will just sit there and hum and not "start". And please use the correct wire gauge and properly rated circuit breaker to avoid a fire.
  3. SgtWookie


    Jul 17, 2007
    If you had a way to spin the motor in the correct direction at nearly it's operating speed before applying power, then you might get away with not using a start capacitor.

    However, if the power failed for a few moments (allowing the motor to stop) and then came back on, or you partially stalled it, your motor would rapidly overheat.

    Don't try to directly connect something like an electric drill to the motor's shaft. It will likely "get away" from you and cause a hazardous situation.
    Last edited: Aug 1, 2010
  4. david101

    Thread Starter New Member

    Jul 29, 2010
    thanks for the replies guys. i ended up hooking up fatter wire and same thing happened. so to see whether it was cause of the missing start cap i ended up winding string on the shaft, pulling and getting it up to speed and then plugging it in, and it works pretty good. so i guess ill go out and grab a start cap now that i know the motor is a good one. i do have one question thought if anyone wants to answer it--this motor was made for an OEM manufacturer so there isn't really any diagrams on bodine's site (i'm writing an email now). i wired it according to a photo apparently emailed to another guy from bodine form a similar motor withsame colours etc. motor runs fine when its up to speed so i doubt wiring is a problem..but my question is it looks like this motor doesnt have a centrifugal switch. looks like in the schematic, both caps are in parallel with eachother and the start cap just has a switch. would i be able to use a regular switch and flick it off really quick at start up (like a momentary switch) or should i use soemthing fancy to measure the current and let it turn off like that? i figure long as i'm attentive and turn it off quickly it should be ok, but i'm not completely sure hah.

    thanks, david

    Active Member

    May 15, 2009
    There is a very good discription of single phase motors and the start and run capacitors in the section at the top of this page under AC, Find motors under Tesslar etc.
    A start capacitor is exactly that. It strarts the motor in one direction and then "falls out of the circuit" It does not use current during running and there is no need to switch it out of circuit.
    A good rule of thumb is 25micro farad per horse power. Make sure the voltage is in spec. (at least double the operating voltage)
  6. amadan00

    New Member

    Aug 7, 2015
  7. amadan00

    New Member

    Aug 7, 2015

    Can you confirm that 25uf per horse power is enough as a rule of thumb . I have a 5Hp with a missing start capacitor and thought it may be more
  8. MaxHeadRoom

    AAC Fanatic!

    Jul 18, 2013
    Poster has not been seen for 4yrs, legacy thread.