Armature burning in AC motor

intermittant wiring short when the motor is spinning ?

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At this point I'm at a loss so I suppose that anything is possible. Lenny

klem kedidelhopper Inscribed thus:

Check interference suppression capacitors for o/c.

If only a couple commutator slugs are burned, that is a sure sign of a shorted turn in the associated armature winding. If there are no particular slugs that are burned, then I'm thinking the field winding has a shorted turn. What happens in this case is the field is now out of phase with the AC line and armature current, so the armature sees a lot higher line voltage at the wrong part of the AC cycle. Does it seem to run at the wrong speed, take a different amount of time to spin up, etc?

(I'm assuming this is a universal motor.)

Jon

You need to use a DCR bridge or Kelvin bridge meter and slowly rotate the rotor looking for significant differences in turn windings on the poles.

Also, check brush alignment.

Jamie

Does the planer have a dynamic braking feature? If so, I'll bet that's what's causing the burning. I have a Ryobi cordless drill that has dynamic braking, and after running the drill, when the trigger is released, there is some major sparking at the commutator & brushes. I haven't seen anybody offer a reasonable fix that can be proven to work and not disrupt something else in the circuit. I know that arc suppressors and/or Tranzorbs might be viable fixes, but I haven't seen anything definitive on it. A pointer to more info on arc suppression for dynamically braked motors would be appreciated. Google turns up lots of patent applications, but I haven't found anything that substantiates the claims.

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The motor is out of the unit. There are no brakes involved. Lenny

If you have a universal motor (most likely), then I suspect a problem in the field.

If you inspect the motor's wiring, you should see one of the leads going to the field and back out and then to a brush, or, the motor allows the wires out side for you to wire it this way.

It's common for these types of motors to short in the field there by, heating up faster than it should and sparking a lot on the brushes. Also, you may notice lack of power.

The field could look and check good on a mega but still have shorted turns in it. You need to know the correct ohms to determine that.

Jamie

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I would never be able to determine correct resistance. How about the run current? If the brushes are sparking should I expect to see a substantial increase in line current? On second thought the run current would probably vary depending on load, so I don't really know how much information I could get from that either. I really don't want to give up on this thing but a new motor is more than 2/3 the cost of replacement so it's starting to look that way. I just wish that there was a definitive way to determine exactly what the problem is. Lenny

a lot of those motors spark anyway. That is the nature of them. They don't last very long! that is, the brushes don't last long in them.

When I say spark as in normal, I am only talking about a slight visual spark on the corners. You shouldn't be seeing blacken sparks with in a couple of run times though.

How good is the bearings in this motor?

Jamie

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The bearings are fine, and as far as the sparking goes, I know what you mean about a "slight" spark at the brush contact points. I've seen that before. This however is like a a white hot spark mostly at one brush only which is strange, and quite large too. You can even hear it burning over the motor noise. Lenny

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Inspect the leads going in, if the arching brush is the one that gets connected to the field inside, you have a short in the field. Could be to case. This assumes that you have a 2 wire motor of course. I wonder what would happen if you removed the ground wire from the motor and make sure the case does not come in contact with ground? You could also place a DMM between the case and ground to test for leakage when operating.

jamie

The usual causes for excessive sparking at the brushes

and commutator when the brushes are new are;

- weakened brush springs which aren't holding the brushes firmly enough against the commutator,

- or the brushes contact surfaces do not conform fully to the curvature of the commutator.

Well the brushes and springs are new and the brushes come pre curved for the commutator so I don't think that is the problem. I'm going out of town for two weeks but when I get back I'm going to take another look at which brush it is that is excessively burning and I'll add it to the thread. Thanks everyone for all the suggestions. Lenny

Try running the motor on 12 to 24 VDC to seat the new brushes. I used to polish the armature with a gray ink eraser while running the motor this way, when I repaired vacuum cleaner motors.

The new brushes are pretty well shaped to the arc of the commutator but I guess it doesn't hurt to try. You can do this to a 120VAC motor without hurting it though? Lenny

I rebuilt hundreds of motors with no problems. A variable power supply is great. Most of the motors ran the right speed between 6 & 16 volts. The wide gray eraser is used to polish the surface and reduce arcing. Polish the armature, and let the motor run at the reduced speed for a while to seat the brushes. Use just enough pressure to just slightly slow the motor. This will prevent the eraser from getting hot & melting or burning. I used to refer to the process as a 'Briarhopper Armature Lathe'. I refused to show anyone how I took a nasty, worn armature and made them look brand new, but they kept coming back for rebuilt motors for their used vacuum cleaner business. The best part was that they would give me hundreds of motors that 'Aren't worth fixing', then buy them back. ;-)

Even though the curve is close, they aren't a great match until they are run for a while.

There used to be a specific abrasive 'stone' used for that process and it was very effective, especially where a high power tool had to be ready for use immediately (seating the brushes that is).

I heard about them about 30 years ago, but cound never find anyone with them in stock, so I came up with the other method. I would get catalogs of motor parts that listed them, but they were always on backorder.