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Shorted 2hp Table Saw Motor

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Forum topic by hobie690 posted 10-08-2012 07:56 PM 1118 views 0 times favorited 17 replies Add to Favorites Watch
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hobie690

10 posts in 808 days


10-08-2012 07:56 PM

Hello,

I have a contractor’s saw made by General.

It appears that the motor wires that attach to the hot and neutral are shorted together, as read with an ohm meter.

I’m thinking about digging into it, but does anyone have any tips? I’ve taken the capacitors out, and it still reads as a dead short.

Has this happened to anyone else out there?

Thanks!

Tim


17 replies so far

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crank49

3524 posts in 1722 days


#1 posted 10-08-2012 08:56 PM

What kind of meter you using?
How many ohms does it read?
Are there any readings between any leads and frame ground?

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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IsaacH

128 posts in 848 days


#2 posted 10-08-2012 09:23 PM

Most AC motors will read close to a dead short when checked across the phases. The resistance comes from “inductive reactance” when current is applied. When theres no current, theres no (or little) resistance. If you contact the manufacturer of the motor, they can give you the specific ohm range for the windings. It all depends on the length and diameter of copper wire in the windings.

What is the saw doing when you try to run it? Tripping breaker, just not running, is the magic smoke coming out of the wires?

If your tripping a breaker, you may have an overloaded circuit, assuming its not on a dedicated and/or 220v circuit. A single phase 120v 2hp motor draws almost 20A all by its lonesome. A 220V will draw half that.

-- Isaac- Decatur, GA - "Your woodworking....NOT machining parts for NASA!!!"

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hobie690

10 posts in 808 days


#3 posted 10-08-2012 09:28 PM

The saw tripped the breaker the instant it was plugged in. As far as I know the start button wasn’t engaged. It threw a very big scary spark the minute it was plugged in.

One I calmed down from the incident, I went back out there and plugged everything in with the breaker off. The second I turned it on something made it trip immediately again.

As for the earlier question about the meter, I’m using a digital multimeter. I’ll have to check again, but I believe I had it set at 200 ohms and it read zero. I can try it at some higher ranges.

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IsaacH

128 posts in 848 days


#4 posted 10-08-2012 11:43 PM

If you saw sparks and the motor wasn’t on, then it wasn’t the motor. Check the cord thoroughly. If you have the switch off, you should read infinite ohms (open circuit) from flat blade to flat blade. If that checks out ok, check any wiring you can find on the saw between the switch and motor. If the sparks came from the wall, after ensuring the breaker is off, pull the outlet out and make sure it wasn’t a short on the outlet. Sometimes bare wires can be almost touching and the force required to push the plug in is just enough to move the outlet enough to let the wires touch.

should get you started….Ive got faith in ya!

-- Isaac- Decatur, GA - "Your woodworking....NOT machining parts for NASA!!!"

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hobie690

10 posts in 808 days


#5 posted 10-08-2012 11:50 PM

Flat blade to flat blade (hot to neutral) it reads 0 ohms. I figured it was a cord, too, so I opened up the service access where the cord attaches, disconnected the cord, and measured between the leads that the hot and neutral attach to, and again, it measured 0 ohms. I now have the motor indoors and removed from the saw and get the same measurements.

I measured a similar motor on my drill press from hot to neutral and I get 16 ohms.

I’m pretty certain at this point that it’s not the cord or the outlet, but am at a loss for whether or not there’s anything additional that’s serviceable by me.

I’m trying to keep the faith!

View cutworm's profile

cutworm

1066 posts in 1545 days


#6 posted 10-08-2012 11:57 PM

Easiest thing to do is replace the cord with a new one. Based on what you said it is most likely in the cord or plug. Unhook it on the other end and measure the resistance again. It should be open.

-- Steve - "Never Give Up"

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crank49

3524 posts in 1722 days


#7 posted 10-09-2012 12:05 AM

At this point, based on what you said, re-checking and comparing to the other motor, I’m inclined to think your motor is fried. It could still be the capacitor but I think you said you disconnected it. Without the capacitor, if the moror was not shorted out, it should hum for a few seconds befor tripping the breaker. One obscure, but possible malfunction is for the centrifugal switch that switches the capacitor in and out of the circuit is bad. But, otherwise you might have to take it to a motor shop for a rewind.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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hobie690

10 posts in 808 days


#8 posted 10-09-2012 12:18 AM

Is that centrifugal switch normally open or normally closed? Michael, do you know? Right now I think it’s closed.

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hobie690

10 posts in 808 days


#9 posted 10-09-2012 12:20 AM

I just answered my own question. It’s normally closed.

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hobie690

10 posts in 808 days


#10 posted 10-09-2012 12:21 AM

Oh, and I remain convinced it’s not the switch the cord or otherwise. I feel like I’m running out of user serviceable items.

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hobie690

10 posts in 808 days


#11 posted 10-09-2012 02:32 AM

OK…so now I’m no longer convinced. The cord connectors are suspect. We’ll see what tomorrow brings!

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IsaacH

128 posts in 848 days


#12 posted 10-09-2012 06:21 PM

You should not read 0 ohms on the cord with the saw off. you should read infinite ohms….if you read 0 ohms, then your short is in the cord.

-- Isaac- Decatur, GA - "Your woodworking....NOT machining parts for NASA!!!"

View EEngineer's profile

EEngineer

906 posts in 2365 days


#13 posted 10-09-2012 09:12 PM

The saw tripped the breaker the instant it was plugged in. As far as I know the start button wasn’t engaged.

Time to debug this thoroughly. Even if the motor is shorted, if the switch is not engaged, then it shouldn’t trip the breaker until you throw the switch. You gotta check everything from the plug to the motor. I personally don’t think this is a motor problem (yet).

General table saws I have seen (only in pictures) all look like they have a magnetic switch on them. I would start there.

-- "Find out what you cannot do and then go do it!"

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MT_Stringer

2114 posts in 1982 days


#14 posted 10-10-2012 12:08 AM

If the saw’s switch wasn’t on, the problem has to be upstream. Right? Receptacle or cord.

Did you try another appliance or light in the receptacle to see if it is OK?
If so, then the power cord could be the culprit.

Also, did this problem just suddenly happen from one use to the next?
Good luck.

-- Handcrafted by Mike Henderson - Channelview, Texas

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hobie690

10 posts in 808 days


#15 posted 10-10-2012 04:53 AM

All is well! Motor is fine; it was a bad cord. Now if only I wouldn’t have taken the motor off. Getting it back on and getting the belt to line up properly is pain!

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