DC Motor commutator marking

[sparkatola]

Hi Folks,

I have dc motor that has a commutator with a shadow type mark on every 3rd bar. I feel the motor shop that rebuilt the motor is being less than honest with me and that there may be an internal winding or wiring issue. Anyone with any insight? Thank you.

Spark

 

[waross]

One thing that you may try:
Energize the shunt field with AC, the same voltage as the DC supply is safe.
Measure the voltage across the brushes.
It should be zero or close to it.
Rotate the armature slowly and observe the voltage at the brushes. There may be a slight voltage ripple as the brushes cross from one commutator bar to the next, but an armature winding unbalance will show up as a voltage discontinuity or increase.
This test is commonly used to find the brush neutral point before putting a machine in service. The brushes are shifted to the point of zero voltage.
That is a safe starting location for the brushes. The brushes may be adjusted with the machine in service, maybe not.

The test may also be used as an in-situ growler test.
Winding opens and shorts that are normally detected with a growler, will show up on this test as a varying voltage across the brushes.
We used this test once on an armature that would develop an internal fault when heated up in service but would not develop a fault when heated in an oven.
The rewind shop was reluctant to rewind an armature when they could not duplicate the fault in the shop.
We performed this test, and described it to the rewinder.
He agreed that we did have a faulted armature and went ahead with the repair.
You may also try increasing the brush pressure one notch on the end ring of brushes and reducing the brush pressure one notch on the next ring of brushes and see if the pattern changes on either row.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

sparkola: could be very slightly off neutral (particularly if the armature was rewound recently), coupled with a less-than-full equalization. My first inclination would be that there are only two equalizers per slot, with three coils (and therefore three bars). The two "unblemished" bars have equalizers, while the third "marked" bar does not. The reason I think you may be ever so slightly off neutral is that the marking is starting from the edge of the bar - and looks like a bit of a burn in the filming.

That being said - regularly spaced marking that does not match either the pole count or the winding pitch (the distance between the top leg in one slot to the bottom leg in the other slot) can be a sign of a small irregularity in the armature winding - usually in the form of slight differences in resistance from one circuit to the next caused by hand-forming the armature coil shape.

Converting energy to motion for more than half a century

 

[dArsonval]

Edge burning is a fairly common "text book" D.C. Motor issue.

This PDF published directly from a carbon brush manufacturer is authentic:

https://www.helwigcarbon.com/wp-con...comm-condition-guide-broch-ta4-hi-res-web.pdf

As Gr8blu suggests, the neutral setting likely needs tweaked a tad and the burning edge should/will be reduced or eliminated.

John

 

[waross]

You can avoid several pages of technical explanations with one short sentence.
"Shift the brushes to cover the sparks".


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[sparkatola]

Thanks Guys. There is really useful information there. I'll check it out.

Spark