View Full Version : Troubleshooting AC single phase motor

08-15-2009, 02:06 AM
My brother's got a table saw with a Baldor single phase motor that quit working and just hums when powered up.
I went over there and checked the wiring, which varied from what the nameplate said. So I rewired it to match 220 high voltage, which is 2-3-8 and power to 1 and 4-5. Now it works but runs backwards, and of course the backwards part was discovered after I left.

I don't remember if the nameplate said anything about reversing (didn't take a pic of it,) but after a bit of searching how to reverse it, it looks like 8 and 5 are probably the starter and need to get reversed.
If I reverse those then it's back to how it was originally wired. Note that it DID work for quite some time before it developed the hum-no-start problem. But for some reason reversing the starting coil and starting in the opposite direction avoided the problem.

So what is the likely cause of the starting problem? Bad capacitor?


08-15-2009, 02:47 AM
If the motor is open there is sawdust in the contacts of the centrifugal starter switch. If it is a closed frame motor most likely a bad cap.

08-15-2009, 03:42 AM
Or when you went into the wiring you disturbed a poor connection and it became good again. Those spade terminals sometimes lose the connection between the wire and the terminal itself. When you move the wires around you can make it 'good' again.

Most likely causes- bad contact on starter switch, or bad cap. Less likely would be an internittent start winding.

08-15-2009, 08:24 AM
Switch 5 and 8,that should reverse it.If he's happy running it on 220 leave it be.If not rewire for 110.

Like others said,check the cent. switch,they do stick especially in a dusty enviroment.

08-15-2009, 11:53 AM
It's a TEFC motor so likely (I'm hoping!) it was just a loose wire. That would perfectly explain the failure, and why it worked after reversing it.

I'll have him reverse the start coil (5 & 8) and see if that works.

If it were a bad capacitor, would it start only in one direction? I doubt it, but I'm not an expert.

Thanks for the info!


08-15-2009, 07:55 PM
If the cap was responsible, it wouldn't matter which direction it was wired for, it still wouldn't start. Very seldom would a cap go bad, then be good again for awhile. They dry out and at some point become ineffective, then they're toast. Sometimes they develop localized internal hot spots, which can cause them to explode. I haven't seen that happen, but I've heard of it.

What I have seen is the cap being replaced because it's suspected bad, and not because it is bad. Most problems like that are contact issues, and a common one from my experience is anywhere a connector is attached to a wire. Generally that's a crimp-on, which is a mechanical squeeze which gets looser over time. Once loose a bit, the connection gets worse, which makes it worse still-

Any discoloration of a connector is a reason to replace that connector, and clean up the lug that it goes on. It's pretty common to take pliars and squeeze the thing tighter, but that's a stopgap, not a real fix.