AC table saw motor only seeing 90V across the inputs?

[gct]

I bought an old Craftsman table saw (circa 1982) a couple months back and today the motor stopped turning, instead it started just sitting there making an electrical buzz. I've got it apart and all the connections and stuff looked good, so I broke out the multimeter and measured the input voltage, which came out as 90V, which is well outside of the +/- 10% envelope you usually hear for operating these motors. It's a capacitor start motor, could this indicate a bad capacitor? What else could cause this low voltage?

 

[Guest123]

You say you measured the input voltage....was this at the supply terminals???

If you're only getting 90V at the supply terminals then the problem is external to the machine (unlikely as nothing else would work either).

Sounds like your start capacitor is faulty, you could try switching the saw on as usual, then spin the blade yourself, it should then speed up as normal (if it is the capacitor).

Other checks...make sure nothing is jammed up, all connections are sound, brushes are sound, belts aren't over tight etc etc.

 

[benji]

oooo mind your fingers


benji

 

[gct]

You say you measured the input voltage....was this at the supply terminals???

If you're only getting 90V at the supply terminals then the problem is external to the machine (unlikely as nothing else would work either).

Sounds like your start capacitor is faulty, you could try switching the saw on as usual, then spin the blade yourself, it should then speed up as normal (if it is the capacitor).

Yes I measured it at the supply terminals, but the voltage at the wall is solid right at 120, so I don't know if it's a problem there. I tried turning on the power and manually spinning the rotor but it didn't get it started. If the voltage is low I don't know that I would expect it to. I can buy a spare capacitor for $10, so maybe I'll just give it a shot, if that doesn't work it's probably something I can't fix anyhow.

 

[Guest123]

So you say you have 120V at the wall socket...I assume then that it's a 110V tool???

Sounds like a problem between the wall socket and the machine if you are losing 30V between the plug and the machine.

Have you checked the plug itself (assuming it's plugged in and not hard wired) and the machine's flex???

 

[Sintra]

I am taking it that the op is in the US 120v 60Hz.

 

[gct]

Hi sorry for the delay, yes I'm in the US so 120/60Hz at the wall. I've tried the motor on three different circuits now and same results, 80-90V on the input terminals. I took off the cable to test that it wasn't bad somehow and connected straight to the wall I see 120V when the leads aren't across the load, so something in the motor is causing a voltage drop...

Edit: think perhaps some of my outlets aren't properly grounded (I rent or they'd be fixed), moved to an outlet I know is good and now I'm seeing 110V, with like 7 volts between neutral and ground...

 

[Des 56]

Does the motor have a centrifugal switch

 

[gct]

I got it working again, I think it was a combination of bad household wiring and the centrifugal switch was flakey