Motor Problem

[fRED mCnEILL]

I have a Rockwell table saw with a 3 phase motor running off a static phase converter and the motor has quit.When turned on it rotates very slowly and then stops. The switch is an electronic switch and when the motor quits the switch flashes inside its enclosure (it has a clear plastic enclosure).

Any ideas for me.

Thanks
Fred

[fRED mCnEILL]

Anybody have a suggestion??

[David L Morse]

A static phase converter usually consists of a start capacitor, a contactor and a time delay circuit. With the symptoms you describe there's a good chance that the start capacitor is defective.

That assumes that the saw worked properly at one time. If it has never started then the start capacitor could simply be the wrong value.

[Mike Heidrick]

Ive seen exactly that on an ideler motor for a rpc. Test the phase converter for sure. The contactor coil on the rpc was wrong and there was no start circuit. So guessing you have a bad pootential relay, contactor, or start cap/s arre bad. Pretty hard to kill a 3ph motor.

[Brice Rogers]

I would first check out the motor with an ohm meter. On all three legs (with nothing else connected) I would expect each phase to be very close to each other in resistance. If so, you can rule out the motor and move on to more likely culprits.

Caveat: If you want to troubleshoot your problem, you need to be extra careful that you don't electrocute yourself. Wearing rubber gloves may be a good idea. If you don't have confidence/experiencing working with potentially lethal voltages, then perhaps you should have someone else do it.

The capacitor on the static inverter is a likely culprit. You can often do a rough check-out of the capacitor: First, with everything disconnected from the power, check to make sure the cap is discharged with a voltmeter. If you read a voltage, then discharge the cap. Next, physically examine the cap has any bulges, which is an indication that it has failed. This may sound a little stupid, but it isn't unusual for a failed cap to show this. Then set the ohmmeter for a high resistance scale like 2M and check the "resistance" of the cap. You should see some initial conductivity (the cap is charging) but the resistance reading should continuously increase. After you do this, you may want to discharge the cap and do it again. A good cap will show an initial conductivity (low resistance) which drops (showing a higher resistance). If it shows a continuous medium level resistance, then it is "leaky" and should be replaced.

If the caps are okay, then the problem is likely to be the electronic switch. I'm not sure what the circuit is, but perhaps you could bypass it.

[fRED mCnEILL]

"I would first check out the motor with an ohm meter. On all three legs (with nothing else connected) I would expect each phase to be very close to each other in resistance. If so, you can rule out the motor and move on to more likely culprits."
Thanks Brice.
By "with nothing else connected" are you meaning it should be disconnected from a power source. I often do electrical work but not with motors or resistance and do have an ohm meter..

[Don Jarvie]

Look over at OWWM. Their WIKI page is very good. Lots of 3 phase stuff there.

[Dennis Ford]

"I would first check out the motor with an ohm meter. On all three legs (with nothing else connected) I would expect each phase to be very close to each other in resistance. If so, you can rule out the motor and move on to more likely culprits."
Thanks Brice.
By "with nothing else connected" are you meaning it should be disconnected from a power source. I often do electrical work but not with motors or resistance and do have an ohm meter..

Definitely need to have the power Completely OFF while testing the motor. The easiest way is to measure resistance at the contactor (on the terminals going to the motor). If you find a problem, do it again at the motor with the leads disconnected to eliminate wiring problems.

[Mike Schuch]

A static phase converter usually consists of a start capacitor, a contactor and a time delay circuit. With the symptoms you describe there's a good chance that the start capacitor is defective.

That assumes that the saw worked properly at one time. If it has never started then the start capacitor could simply be the wrong value.

I agree, it sounds exactly like the start capacitor is bad. If not then possibly the relay is stuck open for some reason but much more likely the start capacitor.

3ph motors are much more simplistic than a single phase motors because there is no starting mechanism required on a 3ph motor (other than the static phase converter you are using to start the motor on 1ph). Because of the simplicity 3ph motors are usually very robust.