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Lynn Livingston
Apprentece
USA
5 Posts |
 Posted - Apr 15 2007 : 1:07:45 PM
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My DeWalt TS has a univ ac brush motor. It is 4 yrs old and has been lightly used. While using it recently, I started smelling a "brush fire" and looked to find that the brushes were arcing way too much.
I pulled the motor and tore it down to find that the brushes were not overly worn, but they and the commutator pads were scorched badly.
I turned the commutator clean on my lathe and polished down to 800 grit. I also cleaned between the pads and installed new brushes. Everything looks brand new and the brushes appear to have good cross section contact with the pads.
However, upon starting the motor, the over-arching is still present. In fact, it looks like someone is welding in there! I don't know much about motors (or electronics), it seems obvious to me that too much voltage is dropping across the brushes causing excess current flow through them?
I have looked at the rotor and ohmed through the pads. I don't know what I'm looking for, but I didn't get any readings on a pad that were obviously out compared to the others.
Likewise the field windings look good (no arc marks or spots) and both coils ohm the same and none are grounded to the case. I'm stumped. The brush clocking is not adjustable on this motor.
Any ideas or adice would be appreciated. BTW, a new motor has to be bought in pieces, and would end up costing more than half the TS cost (500.00) when all the pieces are added up!
Thanks in advance!
Lynn Livingston
East Texas
(Edit...Move to Electronics Repair) |
Edited by - Aaron Cake on Apr 16 2007 09:41:37 AM |
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cyclopsitis
Nobel Prize Winner
Canada
732 Posts |
Posted - Apr 18 2007 : 2:19:54 PM
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The arcing can happen on DC or universal motors (those with commutator) for another reason other then there is too much current going through the windings. I mentioned on another post about the natural (neutral) plane (can't remember what word it is right now). A motor is a funny thing as the motor is power by the Ac it creates a magnetic field and as the rotor turns the same magnetic field caused by the field windings induce a voltage into the rotor windings (called CEMF counter, electro motive force). Anyway what that means is that when the brushes pass from one commutator section to another the voltage is as low as possible this also minimizes the amount of arching because rotor windings have fairly low resistance (ie. less then 30 ohms).
There should be an adjustment screw to rotate the brushes into the best position to minimize this.
So that might be the problem... Other then that I'm not sure what it would be.
Hope that helped
Ken
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Lynn Livingston
Apprentece
USA
5 Posts |
Posted - Apr 18 2007 : 10:38:33 PM
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quote:
Originally posted by cyclopsitis
There should be an adjustment screw to rotate the brushes into the best position to minimize this.
So that might be the problem... Other then that I'm not sure what it would be.
Hope that helped
Ken
Ken, the brush clocking is non-adjustable. The brush housings are replaceable, but not clockable. The brush housings fit into a notch cast into the motor's casing. Thanks for replying Ken.
Lynn
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cyclopsitis
Nobel Prize Winner
Canada
732 Posts |
Posted - Apr 20 2007 : 6:04:15 PM
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crappy! I'm not sure what would fix it then. Can you measure the inrush current? The inrush current should be no more then about 5 to 7 times the running current. So a 2A motor should have no more in-rush current then 14A (for household use that would put it under the instantaneous peak current trip for a home breaker). I would try that incase the resistance readings are not quite right.
K
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Lynn Livingston
Apprentece
USA
5 Posts |
Posted - Apr 27 2007 : 12:31:32 PM
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Well, I have not been able to find a way to measure the in-rush current. None of my meters can handle that much amperage. I wonder if could measure the voltage across the brushes? Could I determine from that how much current is being used?
Also, I can borrow a clamp-on meter that I think can handle the current from an electrician friend. I just don't know how to use it, but maybe I can get it to measure the in-rush current.
I'll let you know if I'm successful.
Thanks again,
Lynn |
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cyclopsitis
Nobel Prize Winner
Canada
732 Posts |
Posted - Apr 27 2007 : 8:47:41 PM
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I was assuming you had a clamp on style meter LOL. They are really simple to use, you just clamp it on around the hot wire and plug it into a meter that is meant to understand the current probe and it will display it on the digital display. I only assumed you had one because I sometimes measure currents of 4000 to 10,000 A and in that case you'll never find a meter able to read that unless you use a CT (current transformer). So the clamp on is your best bet. I hope it works out for you!
I am going to try and get ahold of one of my motor friends for you. I'll ask him what the problem could be (he taught me all that I know about motors so I think with his experience he would know a solution off the top of his head). I'll need the spec. on your motor though. Like the type of rotor, amps, volts, RPM, No. of poles, type of motor ect. If you could put up a few pics that would be great too!
Good luck
K |
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Lynn Livingston
Apprentece
USA
5 Posts |
Posted - Apr 28 2007 : 1:35:15 PM
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I will take a few pics Monday. We've got the grandkids this weekend and nothing (hobby-wise) is gonna' get done!
I will also define the motor a little better. I'll have to tear it back down also Monday.
Thanks again for the replies and effort. Have a great weekend!
Lynn |
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Lynn Livingston
Apprentece
USA
5 Posts |
Posted - May 02 2007 : 2:16:50 PM
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Well, my Mavica isn't up to the challenge of taking good, detailed pictures in the shed. So, I have a JPEG crop of the breakdown manual and it shows all the motor parts. Although the motor carries no tags or nameplates, my original sales brochure specifies "a 15.0 amp high torque" motor. That's all I've been able to find on the motor.
Here's the parts breakdown:
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mrenjan
Apprentece
India
19 Posts |
Posted - May 14 2007 : 07:30:23 AM
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| Since the motor is already quite old, did you try putting some lubication oil in the bearings. This should reduce the friction and hence improve performance. The bearing may be just a rollor bearing or a brass ring. There will be two nozzles in the front side and back side to drop oil into the motor. |
Renjan |
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Topic |
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Univ AC Motor Problem
