Measuring commutation efficiency

[BobM3]

I'd like to know some quantitative ways to measure how well our motors are commutating. Right now we look at the sparking but it's a judgment call. I've read that some have measured the voltage profile across the thickness of the brush. That seems like a good measurement but how do you do that? How about measuring bar voltages via a slip ring? I would think measuring RF noise might be another way to go.

 

[waross]

How about some dimensions.
Brushes; width, thickness number per brush holder?
Diameter of the comutator
HP.
RPM
Voltage.
Can you split a brush in three or four pieces?
Then reasemble the brush with varnished cambric or some other insulating medium between the pieces.
Now you can connect three or four leads and get your voltage readings.
The significance of the readings will be a higher voltage at one side than the other.
We used to "Shift the brushes to cover the sparks"
You would probably want to shift the brushes in the direction of the higher voltage.
Are you having commutation problems or is this a search for more understanding? Nothing wrong with that, but if your commutation is OK and you're trying to increase your understanding we needn't bother you with a lot of information on setting brushes that you probably already know.
yours

 

[BobM3]

Yes, we are having commutation problems. The armature reaction for this motor is significant and messes up the commutation. The commutator is 1.25" dia. The existing brush is .25" thk and .75" wide (1 brush/holder). The motor is a universal with 115 vac input. At rated load it puts out 1.3 hp at 10,800 RPM.

I think I could mill down 4 brushes and assemble them as one.

 

[Skogsgurra]

I think that you should stop dreaming about good commutation in a universal motor. At least as long as you are feeding it raw AC.

One way out COULD be (I don't say IS) to feed the motor with rectified and smoothed voltage. There is one problem: the motor will see about 40 percent overvoltage - you may need to reduce the input voltage.

Anyhow, trying to apply DC motor techniques on a universal motor is futile - at best. Measuring commutating voltage profile on such a motor is not a very clever thing to do.

Gunnar Englund

http://www.gke.org

 

[waross]

I concur with skogsgurra
I would suspect a partial winding failure in either the armature or the field.
Check that the brushes are not sticking. They must be able to slide freely. Weak brush springs will also cause sparking.

If a winding is failing then D.C. may extend the life but probably the cheapest in the long run is to replace the armature and field or replace the whole machine.
We have a shop in town with an asortment of armatures and fields for the popular brands of hand tools. The price is much cheaper than a rewind.

Splitting a brush is only appropriate on large machines with multiple brushes per holder. You have to have enough brushes left to carry the load.
yours