capacitor voltage

[KevinW]

I have a 120vac motor and I'm measuring 8 vac across the two leads going to the start capacitor with the capacitor removed. Should I have 120vac going to the capacitor?

 

[danadak]

Yes. the start switch typically NC as it opens under centrifugal force.

But that means the L1, L2 on/off switch to the motor must also be closed.


Regards, Dana.

 

[KevinW]

Thanks Dana.

 

[KevinW]

I believe I have a millisecond of 120 vac on start up but it happens so quick it was difficult to detect.

 

[rjenkinsgb]

If the motor is working normally, the capacitor is only in circuit for a very brief time as the motor starts to spin up.

Once it reaches an adequate speed to run without the start capacitor, the centrifugal "start switch" within the motor should open and disconnect the cap.

(Different motors use various start methods, but as the drawing for that includes the capacitor switch within the motor itself, it is most likely a centrifugal one).

 

[Nigel Goodwin]

I believe I have a millisecond of 120 vac on start up but it happens so quick it was difficult to detect.

It should certainly be a LOT longer than 1 millisecond - at 50Hz mains you'd need 20mS to even get one full cycle of mains.

Does the motor work?, if it does then nothing is wrong.

 

[KevinW]

I'm waiting on a new capacitor, the motor hums but will start by spinning the shaft.
The centrifugal switch works fine as I can see it open and close.

 

[Nigel Goodwin]

I'm waiting on a new capacitor, the motor hums but will start by spinning the shaft.
The centrifugal switch works fine as I can see it open and close.

Will it's dead simple to check - just measure the AC voltage across the capacitor - while the motor isn't spinning. The meter should read full mains if the capacitor is duff, and the switch is OK. I'm rather concerned that you only got voltage for a millisecond or so?. I'm presuming you've measured across the capacitor on ohms?, and it's not S/C?.

 

[KevinW]

The cap isn't shorted Nigel, it reads .7 esr.
My new cap will be here today.
This is an old Leeland .33hp motor.
The motor isn't humming any longer, now it is running without the start capacitor but the voltage spikes so quick on the digital meter that I can't read it before it drops off due to the internal switch contact opening.

 

[KevinW]

Found the problem inside, wire disconnected on start switch and the loose wire had tape on the connector.
I think this motor had been running without the start winding.