Start capacitor

jbfan said:

I have a bad start cap on an air compressor.
The old cap has a nice shiny spot where the size was. All I can see is the voltage.
Grainger found what might be the right size, and I have one coming.

The question is what if it is not the right size? What can I expect if it is too small or too large?

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if the cap is metal cased and around 4mfd
it is not a start cap

it is a run cap ...... exact size matters a great deal

as mentioned performance will decrease if the size is wrong
and the size of the case is not a good indicator of mfd size

start caps are nonmetal cased and in the hundreds of mfd
larger mfd is better, higher voltage is better

run caps are always metal cased and less than a hundred mfd
larger or smaller mfd is bad, higher voltage is better

a higher voltage rating on a cap simply means the insulation is better, it will last longer
always get higher voltage if possible

this is where i buy caps

micromind said:

Something else may be wrong........

That motor is cap start-cap-run. The metal cap is always in series with the start winding, hence the 'cap-run'. The other 2 caps are connected in parallel (in order to get a higher µF rating. They are also in parallel with the run cap during starting then the centrifugal switch disconnects them.

If the run cap failed shorted (fairly rare but it does happen) then the motor will not turn but will draw tons of current. If it failed open (they usually do) then the motor will start as usual but it'll run at reduced HP.

If the run cap failed open and the motor won't start and draws a ton of current, most likely the centrifugal switch is stuck open and the start caps are not in the circuit.

Click to expand...

A little confusing here. A starting cap is in series with the starting coil.

Starting caps are quite often but not always the biggest ones and my experiences is often in parallel for motors that size. They are always electrolytic types which are notorious for short life (8-10years). As stated they are often 100-500 microfarads. Run caps are usually under 100. There is no formula for caps. In the shop they vary the run cap for lowest no load amps (ideally zero). They vary the start cap for max torque at stall or to give about a 35 degree phase angle advance. So if you have a scope with an amp clamp and know what you are doing it’s easy to pick one. Centrifugal switches are common but not the only way.Potential relays or just timers are the other two. The start coil only energized a second or two to give the motor a kick in the right direction and help boost starting torque (which is crap on single phase). After that it becomes a drag (load) and sucks high amps so must be disconnected before the start cap explodes or the start coil burns up.

Single phase motors are actually harder than three phase.