Anyone know how to test a electric motor armature/field coil

Fabrication, Welding, Shop Tools, Tips & Techniques

Moderator: John_Heard

Post Reply
Message
Author
User avatar
John_Heard
Site Admin
Posts: 5734
Joined: Thu May 12, 2005 11:20 am
Location: Resume Speed, Kansas
Contact:

Anyone know how to test a electric motor armature/field coil

#1 Post by John_Heard » Mon Jun 08, 2009 9:09 pm

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.)?

User avatar
CoMax Racing
Posts: 169
Joined: Sat Oct 01, 2005 4:40 pm
Location: Edmonton Alberta Canada

Re: Anyone know how to test a electric motor armature/field coil

#2 Post by CoMax Racing » Tue Jun 09, 2009 9:22 am

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

User avatar
John_Heard
Site Admin
Posts: 5734
Joined: Thu May 12, 2005 11:20 am
Location: Resume Speed, Kansas
Contact:

Re: Anyone know how to test a electric motor armature/field coil

#3 Post by John_Heard » Tue Jun 09, 2009 9:51 am

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?

User avatar
CoMax Racing
Posts: 169
Joined: Sat Oct 01, 2005 4:40 pm
Location: Edmonton Alberta Canada

Re: Anyone know how to test a electric motor armature/field coil

#4 Post by CoMax Racing » Tue Jun 09, 2009 1:17 pm

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

User avatar
dadnova
Posts: 296
Joined: Sat Dec 16, 2006 8:57 am
Location: Nuevo, California

Re: Anyone know how to test a electric motor armature/field coil

#5 Post by dadnova » Fri Jun 12, 2009 9:41 am

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.
Gotta plan, spend it before she can, and go as fast as you can.

User avatar
John_Heard
Site Admin
Posts: 5734
Joined: Thu May 12, 2005 11:20 am
Location: Resume Speed, Kansas
Contact:

Re: Anyone know how to test a electric motor armature/field coil

#6 Post by John_Heard » Fri Jun 12, 2009 9:47 am

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.

User avatar
dadnova
Posts: 296
Joined: Sat Dec 16, 2006 8:57 am
Location: Nuevo, California

Re: Anyone know how to test a electric motor armature/field coil

#7 Post by dadnova » Fri Jun 12, 2009 5:01 pm

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.
Gotta plan, spend it before she can, and go as fast as you can.

Post Reply

Who is online

Users browsing this forum: No registered users and 5 guests