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Author Topic: Can someone educate me on 110 VAC motor starting systems?  (Read 1652 times)
N1AUP
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Posts: 15




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« on: April 25, 2012, 04:25:05 AM »

I rescued a Dewalt radial arm saw from the junk pile.  It's in very nice shape considering, but needs some TLC.  When I plug the unit in, I have to hand spin the blade to get the motor to start.  This suggests that the motor start capacitor is shot.  Removing same from the unit, the cap fell apart in my hands.

The cap is marked 86 - 103 mf, 110 VAC.  Looking inside the broken cap, there is a 1 watt resistor across the terminals too.  Finally, there is a relay mounted near the cap (which I assume, switches out the cap when the motor starts).

Looking around the web, I found this unit.

http://www.epoolshop.com/runcapacitor88-108mfd125v1-7/16x2-3/4.aspx

It has a range of 88 - 108 mf at 125 volts.

How close in value does the replacement have to be?  Will this work?  What are the important specs that I need to consider?

Second, can I assume that the cut out relay is 110 VAC, and can I replace it with a generic equivalent, or might this item be something special?

Called Dewalt, but they need a model off the saw, and that model plate is long gone.

Thanks
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AD4U
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Posts: 2156




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« Reply #1 on: April 25, 2012, 05:38:32 AM »

The cap you found should work just fine.  Instead of ordering one off the internet, most any motor shop in your neighborhood should have one.

I do not know what the 1 watt resistor does.  What value is it?  Seems I remember this being a "quenching" resistor in some very old applications.  I do not see where the resistor is needed.  The saw probably will run OK with it or without it.

As for the relay, I also do not know.  Most capacitor start motors have an internal centrifugal switch that switches the cap in and out of the circuit.  Maybe the centrifugal switch inside the motor switches the relay on and off which switches the cap in and out of the circuit as needed.

Sorry I cannot help any more.

Dick  AD4U
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G3RZP
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Posts: 4442




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« Reply #2 on: April 25, 2012, 06:00:02 AM »

The resistor is usually a discharge resistor so the cap doesn't sit there woith a lot of volts on it. But the value makes it sound like an electrolytic, which shouldn't really need a discharge resistor, although it's a nice thing to have.
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N7NBB
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Posts: 381


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« Reply #3 on: April 25, 2012, 06:55:23 AM »

I agree with AD4U.... Bright and early some morning (Mondays  might be best) take a dozen doughnuts into your local motor repair shop. Ask them if you could pick their brains for a minute. Ask the same questions you've asked here.  Don't take a lot of their time. Just ask, receive, and leave. You may even score some "free" parts (in exchange for your yummy doughnuts).  Can't beat one on one, face to face elmering as opposed to internet queries.
Good Luck restoring your saw.
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KA1MDA
Member

Posts: 543




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« Reply #4 on: April 25, 2012, 07:49:23 AM »

Relay could be part of an electrical motor brake. Many saws have a brake circuit which causes rapid deceleration when the run switch is released. If I remember right, some electrical brake circuits short motor windings, while others inject DC voltage to the motor.

Tom, KA1MDA
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K8AXW
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Posts: 3722




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« Reply #5 on: April 25, 2012, 04:34:43 PM »

Your friendly ACE Hardware stocks motor start capacitors.  Our local ACE can even check them to see if they're bad.
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