capacitor-start motors

hello all

how tolerant are electric motors to capacitor ratings? (ie, if i change the cap with a different (bigger) one?)

having trouble with an electric motor.. about 2hp (electric winch) and it is having difficulty starting under load. but, once started (with no load) it has alot of lifting power. (i'm not too light and it picked me right up off the floor... only AFTER it started spinning) ... if i load the motor (hold the cable) it just sits there and buzzes... let it go till the cable moves, hold on, and it picks me right up.

so, trying a little experiment, i changed the capacitor. it has ~50uF 400V cap and i put in a ~130uF 230V cap (its all i had around, pulled it off of a drill press)

with the new capacitor it starts fine under load. picks me right up off the ground.

but now, when i switch directions (lowering), the motor makes an AWEFUL sound.

i then put both caps in parallel and starting under load is fine, but more noise. put the caps in series, wont start under load, less (very little) noise.

so... noise and starting torque is related to the size of my capacitor. (i'm guessing thats why its called a capacitor start)


  1. can i use this motor safely just by changing to a larger capacitor? (which size should i use)

  1. why does it make that terrible noise? and only in one direction?

  2. is this a sign that i just need a bigger motor? (even though once started this 2HP is plenty strong?)



Reply to
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"tony" wrote in news:yLeob.383493$R32.12695090

Before doing anything else, I would confirm that the centrifigual switch that engages the capacitor has not failed open.

Reply to
Charly Coughran

Are you sure it's a capacitor START motor. 50 uF is about the right size for consumer grade capacitor RUN motor. In these there is no centrifugal switch and the capacitor is in circuit all the time. They produce something like their rated power when up to speed but have dead lousy starting torque.

Your terrible noise is the result of using too much run capacitor on a motor thats very lightly loaded.

If it's this type of motor adding the 130uF starting capacitor is an OK fix. However it needs to be brought in by a push button or a starting relay so that it is automatically disconnected once the motor is up to speed.


Reply to

Jim pentagrid sez: "...Are you sure it's a capacitor START motor. 50 uF is about the

With dead lousy starting torque, isn't it unlikely a capacitor-run motor would be used in a winch - assuming, of course, the winch has no clutch?

Bob Swinney

Reply to
Bob Swinney

i will admit that this is somewhat of a homebrew winch but its driving a gearbox (50:1)

i think i better understand whats happening. perhaps it *is* a run capacitor? i will try wiring it in with a relay that cuts out after start.

is there a middle ground? say a 20% larger capacitor i could use.. so that i dont have to take it out of the circuit after starting, and, at the same time, not do any damage?

any more info, however, on why it makes so much noise with the higher capacitance.. and in only one direction, at that, would be much appreciated.

thanks again


Reply to

Tony, when we put together my phase converter, we found that using too much capacitance made the idler motor buzz. Too little, and it wouldn't start. Somewhere in the middle, it runs fine and doesn't buzz. Since this is purely start capacitance across one leg, it would seem to apply.

I have an old Baldor grinder (single phase) that has run caps permanently in the circuit. So it is done sometimes. These caps are oil-filled. You would likely know if you bought an electrolytic or run cap, the difference in price is huge (run caps are a lot more moola).

You can *never* leave an electrolytic in the circuit. If you need to do that, use an oil-filled run cap.

Do you shop at Grainger? They have all this stuff including potential relays, which are what you need to cut out a start cap.


t> i will admit that this is somewhat of a homebrew winch

Reply to
Grant Erwin

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