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PostPosted: Mon Jun 08, 2009 9:09 pm 
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Location: Resume Speed, Kansas
I got my Dewalt DW871 Chop Saw too hot a while back cutting some heavy material and the brushes started sparking, rpms dropped, smelled bad - you know the drill (Dang I shouldn't have done that!)... Anyway, pulled it apart and dressed/sanded the brushes and the armature. Put it back together, it's a little better but still sparking and won't come up to full RPM. New one is about $200, is it worth trying to get a new field coil / armature or should I put it down?

Anyone know how to test those (field coil, etc.)?

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PostPosted: Tue Jun 09, 2009 9:22 am 
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Location: Edmonton Alberta Canada
You need to test the armature on a growler, to do a proper test. But if you take a multimeter and set it to check continuity, all the commutator bars (the segmented copper strips where the brushes run) all should have continuity to themselves but nothing else. This is no a complete test but it will show a short. The field if it has 4 wires will be built in 2 halves, each having two wires on them. There should be continuity between the wires on one coil winding but not across the two. If it has 2 wires there should be continuity between the wires but to no other metal on the coil.

Hope this makes sense, if not take them to a Dewalt factory service center and they can test them in very easily.

Hope this helps
Corey


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PostPosted: Tue Jun 09, 2009 9:51 am 
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Thanks Corey - it's been a LONG time since I played with an electric motor. I'll try some of that and see what I can learn. If it's just one of those parts it's like $60 to fix it in parts, but much more than that and I'd probably be better off just buying a new one.

When they spark like that on the brushes is that typically an indication of a short somewhere?

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PostPosted: Tue Jun 09, 2009 1:17 pm 
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If the motor is sparking at the brushes its a short in either the armature or Field. Usually the armature goes first, then if the motor is allowed to run with a shorted armature the field coil will also fail.

If both are burned then you probably will save more money getting a new one.


Corey :smt001


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PostPosted: Fri Jun 12, 2009 9:41 am 
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It should spark at the brushes as current passes, it probably sparks more now probably due to the brushes not getting a good contact, try new brushes as they are cheap, we use to due the same thing back in our slot car days. As we new the owner, after hours we would turn the power way up on the track and sometimes damage our motors due to this.

We use to clean the armiture at high speed and a pencil eraser or 1000 grit sand paper to smooth it out, then add new brushes.

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PostPosted: Fri Jun 12, 2009 9:47 am 
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I did the "home" repair job on the armature and brushes, it helped a little but still doesn't want to come up to full RPM and is down on power. I'm pretty sure I hurt it. Haven't had a chance to check the resistance and all yet.

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PostPosted: Fri Jun 12, 2009 5:01 pm 
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Sorry to hear that John, we use to run them for some time ands they would pick up speed, try taping or Zip tie it so it stays running for a while and see if it wears in, if not your worth a new tool.

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