What capacitor for this motor?

[Deadeye]

240V single phase motor (plate picture attached) - buzzes loudly on turn on but does not run.
There's a AC220V 60uF (+/-25%) capacitor wired to it separately (a lash up), which I'm guessing has failed/is the problem.

I've ordered a new one (identical) but am concerned that the current one may well not match the original spec?!

60uF sounds a bit high for a 0.75HP motor from what I can gather around the interweb. I don't have a multi-meter with capacitance, so any advise?

Many thanks

John

 

[Sideways]

One type of single phase motor has a capacitor permanantly wired in (psc = permanent split capacitor) . 20uF for an 1100W motor would be right, so 60uF sounds waay too much for a 750W PSC motor.
Another type (cap start, or cap start, cap run) has a bigger capacitor connected for only a couple of seconds while starting. 60uF sounds big but I've seen lathes with a 150uF start cap so maybe not. That type of motor normally has a centrifugal switch in it to disconnect the start cap when the motor is up to speed.
Cap start usually goes with cap run. Ie, 2 capacitors, one bigger, one much smaller, and the switch swaps between them. But a motor will run off a single winding once spinning so some cap start motors don't have a second cap.
There are tables on the web of typical cap sizes for start and for PSC. They get bigger as the horsepower increases.

HTH

 

[Deadeye]

One type of single phase motor has a capacitor permanantly wired in (psc = permanent split capacitor) . 20uF for an 1100W motor would be right, so 60uF sounds waay too much for a 750W PSC motor.
Another type (cap start, or cap start, cap run) has a bigger capacitor connected for only a couple of seconds while starting. 60uF sounds big but I've seen lathes with a 150uF start cap so maybe not. That type of motor normally has a centrifugal switch in it to disconnect the start cap when the motor is up to speed.
Cap start usually goes with cap run. Ie, 2 capacitors, one bigger, one much smaller, and the switch swaps between them. But a motor will run off a single winding once spinning so some cap start motors don't have a second cap.
There are tables on the web of typical cap sizes for start and for PSC. They get bigger as the horsepower increases.

HTH

Thanks Sideways. This is a general motor (5 wire feed) and had the capacitor wired off in a separate box. Google suggests the cap might be from a pressure washer!

It's a single cap motor, I'm pretty sure - there's no mechanical switch to remove it from the circuit.

I just wondered of there was a rule of thumb - they're only a tenner, but I might end up trying a lot of variants!

 

[Sideways]

Easy to answer that.
You have a 3/4 hp motor
4 pole / 1400 rpm ish
PSC type
Looking that up in a helpful document published by capacitor manufacturer Kemet and shared on the Farnell website, we are told that a good choice of capacitor for your motor should be 20uF.

Make sure the voltage rating is ample. Since it only costs pennies more and a capacitor working comfortably inside it's voltage range might last longer, I like to see motor capacitors rated above 400V.

For other sizes of 240V PSC single phase AC motors, 2 or 4 pole alike, the recommnded are as follows:

0.5HP 6uF
0.75HP 20uF
1HP 25uF
1.5HP 32uF
2HP 40uF
3HP 60uF

You'll notice I said 20uF for an 1100W motor in an earlier post. That was original factory fit capacitor on an 1100W PSC motor in a Kity bandsaw I think it was, where the Kemet table would have said use 32uF.

Don't get hung up on it. These are approximations based on maths and generalised. Motors are fairly tolerant of the exact value and since we can't trust that the 60uF on your motor is the original value as designed by the manufacturer, these general rules should be fine to get your motor running again.

 

[MorrisWoodman12]

As sideways say don't get hung up with specific values. Capacitors come in preferred values. 32uF normally isn't a PV but 33uF is and that will be fine. The next PFs are 27 and 39. 27 may be a little small but 39 will be OK. Likewise remember that 240VAC is the RMS value and may peak +10% giving 373Vpk (240+10% × root 2) hence a 400V cap.