Compound miter saw won't work on circuit with a GFCI

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Old 01-29-08, 08:50 PM
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Compound miter saw won't work on circuit with a GFCI

Hi all,

I just rewired the outlets in my kitchen with new 12 guage wire, 20amp breaker and made the first outlet in the series a GFCI outlet. The outlets work fine EXCEPT the other day I plugged my saw in the kitchen, tried to fire it up, and the breaker tripped. I repeated this three times.

Is this expected (somewhat) since the power tool is drawing a lot of power, or is something wrong if it's that sensitive?
 
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Old 01-29-08, 10:18 PM
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When you say "the breaker tripped," do you mean the circuit breaker in the panel, or are you referring to the GFCI outlet tripping? If it's the circuit breaker, it could be an overload if you've got some other heavy load on the same circuit. A miter saw by itself should not trip a 20A breaker. A GFCI receptacle will not trip due to an overload. There's probably a defect in the saw if it's tripping the GFCI.
 
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Old 01-29-08, 10:49 PM
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Many miter saws have a blade brake which basically short the windings after the trigger is released. I'd guess that could be causing the problem if it is the GFCI not the breaker.
 
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Old 01-29-08, 11:04 PM
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Many miter saws have a blade brake which basically short the windings after the trigger is released. I'd guess that could be causing the problem if it is the GFCI not the breaker.

"Double-tap' the trigger on a saw with a blade brake, and you will trip the GFCI , EVERY TIME.....
 
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Old 01-30-08, 04:57 AM
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I don't think they are actually shorting the windings as much as the current is going through a half wave rectifier, changing the motor to a generator in reverse, and when the blade stops, ceases becoming a generator or a motor. Had to change the rectifier on an old radial arm saw once. Turn it off and it ran forever.
 
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Old 01-30-08, 07:08 AM
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Interesting discussion...

The breaker itself is tripping. The GFCI tripped once but the second two times it was the breaker.

The only other item plugged into this circuit at the time was a coffee maker and it wasn't operating.

When I plugged the saw into a different 20a circuit, it didn't trip the breaker (which is why I thought it had to do with the GFCI).

I don't know if my saw has a blade brake or not. However, the trip occurs when I first grab the trigger, not when I release it (which is when the brake would become active).

Any other thoughts?
 

Last edited by Chrisatunc; 01-30-08 at 08:36 AM.
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Old 01-30-08, 07:20 AM
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Equipment with a motor uses a high start up current. This start up current is tripping the breaker.

The breaker is becoming weaker and/or the length of the run comes into play and/or the number of connections on the circuit before the location where you have the saw plugged in is coming into play.

In other words, this does not sound like a problem. Try a different circuit, try swapping breakers, try a different location on the same circuit.
 
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Old 01-30-08, 10:34 AM
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dynamic motor brakes

Originally Posted by chandler View Post
I don't think they are actually shorting the windings as much as the current is going through a half wave rectifier, changing the motor to a generator in reverse, and when the blade stops, ceases becoming a generator or a motor. Had to change the rectifier on an old radial arm saw once. Turn it off and it ran forever.
A brushed DC (permanent magnet) motor can be braked by simply attaching the motor wires together or thru a high power resistor (if current needs to be limited). Using a relay is typical. There would be no line connection here.

On the other hand, an AC induction motor typically uses a half wave rectifier (diode, maybe a SCR) to apply a pulsing DC current thru the stator, using line power. (the trick here is stopping the braking current when the rotor stops!)
In both cases, the motor windings dissapate the rotating energy as heat.
 

Last edited by telecom guy; 01-30-08 at 10:49 AM. Reason: fix universal to PM
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Old 01-30-08, 06:38 PM
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Isn't the braking power stopped by the lack of rotation of the rotor? No rotation, no pulses,no power. Interesting on the DC motors. Not up on that, until now. Thanks.
 
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Old 01-31-08, 07:53 AM
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Originally Posted by chandler View Post
Isn't the braking power stopped by the lack of rotation of the rotor? No rotation, no pulses,no power. Interesting on the DC motors. Not up on that, until now. Thanks.
On a AC induction motor, there is no permanent field induced flux, so braking requires an external power source. Using a DC current effectively turns the stator into a fixed electromagnet, with no "rotating field".
I haven't done this with my chop saw, but it would be interesting to see if the blade keeps spinning if the line cord was disconnected with the saw running. If my theory is correct, it will spin much longer, than when in normal operation, using the switch to shut it down.
 
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Old 01-31-08, 01:28 PM
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WOW!! All this high tech talk makes my head hurt!
I haven't beena member very long, but I can see some GOOD help available. THANKS!
The more you learn;the less you find you know.
 
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Old 01-31-08, 03:20 PM
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Mike, did you steal that from Yogi Berra?
 
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