DC motor rotor failure

[elinBG]

What is the cause of this failure?

Thank you!

http://www.eldvigateli.com

 

[elinBG]

sorry, now should be ok?

http://www.eldvigateli.com

 

[dArsonval]

The photo shown indicates the motor has not yet been fully dismantled to adequately inspect the
probable cause(s) of the commutator bar edge burning. Many charts published on the Internet
provide insight as to what causes this condition.
Random Google Image URL here for example:

https://www.google.com/search?q=com...r4DADQ&ved=0CEoQsAQ&biw=1456&bih=777&dpr=1.15

Understanding the bar edge burning is a "guess" until the apparatus is fully dismantled,
and the various aspects of the motor are tested/inspected.
There is likely a failed coil group on the armature. How, or why the coil group (may have) failed
is another question to ask once the armature can be fully viewed/inspected.
Additional photos of the actual failed component if they can be made available will
be useful in answering this post.

Always enjoying the forum,

John

 

[elinBG]

Hello,

Thank you for your answer and the information!
When I searched as you told me I found that the possible problems are 2. Please check the link bellow:

http://www.gulfelectroquip.com/technical.php?technicalname=commutator

I am confused which of them is my case (please if somebody have the knowlage/information to share it)

The first option is:
BAR EDGE BURNING: Results from poor commutation. Check 1.) that the brush grade has adequate voltage drop, 2.) that the brushes are properly set on neutral, and 3.) that the interpole strength is correct.

It looks like my case because the burning is only in the edges of the slots.
But on this picture the edge burning is at all the bars, in my case it is 1:4 in every fourth bar.

The second option is:

SLOT BAR MARKING: Results from a fault in the armature windings. The pattern relates to the number of conductors per slot.

It looks like my case because the burning is in pattern simillar to the burning of my comutator.
But on this picture the bars are whole burned, in my case they are "edge burning".

Thank you for your opinions!

Regards,
Teodor

http://www.eldvigateli.com

 

[electricpete]

I think that's why John suggested armature fault. My simplistic thought: some circuits will draw more or less current (giving the pattern every 4th bar possibly). Changes associated with the fault might push the brush riding over those circuit bars to unfavorable operating condition which results in burning…either unfavorable voltage drop (due to change in current) or some disruption of the timing of commutation.

John and others here know a lot more about dc motors than me, I can guarantee. Wise advice to not jump to conclusions without careful investigation.

Next logical step might be bar to bar resistance checks to see if armature winding checks out.


=====================================
(2B)+(2B)' ?

 

[dArsonval]

Bar Edge Burning and Slot Bar Marking are somewhat "cousin failures".

Pin pointing the true cause of Bar Edge Burning is problematic because more than one
reason or scenario can create the same/identical visual appearance on the commutator.

In studying the original posted photo again... I'm leaning toward a new "armchair" theory
of "High Mica" in the commutator slots causing [brush hop].

Why?

Because the white appearance within the commutator slots... to the left,
and right... via the camera angle capturing the original photo posted...
denotes mica material not fully removed during a previous repair process.

Also, the center [brush set] ~~burning pattern~~ is slightly different in
overlapping a second commutator bar... "possibly" authenticating high mica as the culprit.

Direct Current Motors with all their nuances' are hearty machines.
I'll hazard another guess that the motor depicted in this post is possibly still running despite it being labeled a "failure".

John

 

[waross]

I think that the primary issue is an armature fault causing the intermittent marking. That fault may be upsetting the commutation, leading to edge burning as a secondary effect.
Possible check.
I would use a variation of an electrical setup procedure.
I will describe the whole procedure as it may be helpful when you set up the machine after repairs but you will not need the first part of the procedure at this point.
Step 1;
A> Disconnect one or both leads from the brush-gear.
B> Connect an AC voltmeter across the brush-gear.
C> Disconnect the field supply conductors.
D> Apply approximately 120 Volts AC to the field.
E> Shift the brush-gear until the nul point is found. (Zero volts or the center of the movement that gives zero volts.)
You may use this procedure to adjust the brush-gear if it has been moved during repairs. You may have to do a fine adjustment when the machine is running under load but this will give a setting that is generally safe to start.
To check for armature faults:
A> Disconnect one or both leads from the brush-gear.
B> Connect an AC voltmeter across the brush-gear.
C> Disconnect the field supply conductors.
D> Apply approximately 120 Volts AC to the field.
E> Turn the machine shaft and watch the voltmeter.
The voltmeter should read zero or close to zero. DON'T adjust the brush-gear on a machine that has been commutating well.
If the voltage at the brushes varies as the armature is rotated this is a dependable indication of an armature fault.
Let us know what you find.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Motorwinder]

Could be interpole polarity.

Mark and shift brush holder ring out of neutral point. Take a 12 volt battery charger and flash A1 and A2. The armature should "kick" in the same direction that you shifted the brush ring. If it kicks in the opposite rotation, switch the brush holder leads.

 

[rhatcher]

I suspect that the motor has been overloaded. Notice the irregular appearance of silver material on the commutator riser. This is solder and is evidence of the commutator being overheated to the point of the solder melting and running out of the riser joints. Also, notice in the bottom right of the picture that there appears to be solder splatter on the face of the shunt fields. This is solder that has melted and been thrown out of the riser by centrifugal force.

 

[Skogsgurra]

No patina. Indicates that the collector has been turned, for some reason.
That, in its turn, indicates that there was a problem before and that that problem is still there. Overload (rhatcher) seems possible. I would take the motor to the shop and have a good look at the rotor. Also check excitation and armature current when running.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ozsalty]

What colour spark do you have at commutator....Green usually indicates an open circuit coil,blue ususally indicates a shorted coil....

 

[petronila]

Hello elinBG

We can have manny sources for this problem, so is important more information about the motor perfomance.

If the mottor is performing without high armature current and with excesive sparking the causes coul be:

1. Someone replace the brushes?

- Wrong Electrical adjusment (neutral point offset)
- Wrong brush grade
- Light brush pressure

2. Others?

- Vibration
- High Mica
- Contamination

3. Machine is not working proper drawning high armature current?
- Armature winding fault

Pleae verify brush spring pressure, brush grade, neutral point, vibration levels, room contamination(also inlet air taked from extrenalfan) and mica height.

If all this point are find, verify if armature winding is shorted.


Regards

Carlos

 

[elinBG]

Hello everybody!

Thank you for all your answers!

I am not replying yet, because I havent contacted the persons who are working with the motor yet.

As soon as they answer to my (your) questions I will post all the info for the motor here.

Hope that you will be here again...

Regards!

http://www.eldvigateli.com

 

[elinBG]

Hello again!

Today I talked with the guys who use the motor.
Exactly as @John said "the motor is still running despite it being labeled a "failure".|!

The motor is just getting very hot >80oC after 1-2 hours of work, that is "the only reason" why it was bringed in the repair shop.
Good commutation with minimum sparking between the brushes, no noise, vibration or anything else.

I am ready to answer every your question to find the cause of the problem and I will keep you informed with everything what is happening with it.

Thank you in advance for your help!

http://www.eldvigateli.com

 

[nic00b]

Hello elinBG,

Have you carried out a drop test on said armature yet?

 

[elinBG]

No, I have not.
Isnt the growler test the same?

http://www.eldvigateli.com

 

[elinBG]

Hello again!

We did all the things you suggested in this post, even more - setting the neutral, the brush-holder height, the mica, ordered original brushes from the manufacturer, dynamic balancing and etc...

The motor worked fine for a month or this is what was told to us
Yesterday they called saying that the motor failed.

The excitation winding shorted to ground.
The brushes were unequally weared and one of them is even missing.

Please check the photos and tell me what do you think

http://www.eldvigateli.com

 

[Skogsgurra]

Did you ever dismantle the machine?

12 Jan 14 11:38
No patina. Indicates that the collector has been turned, for some reason.
That, in its turn, indicates that there was a problem before and that that problem is still there. Overload (rhatcher) seems possible. I would take the motor to the shop and have a good look at the rotor. Also check excitation and armature current when running.

Molten tin indicated overload already in the January 8 picture. What is the protection set at? What is the namplate current of the machine? Sufficient air? Air flow switch?

"Pattern reading" is a popular pass-time. But measurements and examination is better. Is there excessive ripple in the armature current?


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dArsonval]

Brushes, and commutator wear is quite obvious in all the photos related to this post.

[The commutator photo shown in the original post with this thread appears to have
been TIG Welded eliminating any molten tin/slash solder splatter theory.]

In looking at all the photos posted, and looking at them again...

I'm going to Call-a-Bluff, and claim the brushes were never replaced.

I'm also guessing... the End User opted to go with the carbon brush situation...
hoping for the best, and ended up with what we're reading about here.

The heavy groove in the commutator surface nearest the riser is from the brush length reaching its useful life,
being severed, and the brush shunt lead serving as the brush connection until it quit.

As for the shunt winding 'going-to-ground'?
That's unrelated to the brush wear shown in the photos, and another issue.

Always Enjoying this Forum,

John

 

[elinBG]

Thank you for your answers!

dArsonval,
If I understand correct your english you doubt that the brushes were replaced?
If so the brushes were replaced before 1 month with original brushes sent from the manufacturer (firstly the motor was with EG283 then with EG571)

"molten tin/slash solder splatter theory" ? Do you mean that the commutator is not welded correctly and this can cause the problem?


Let me clear it little bit more...
The rotor was rewinded somewhere before and the motor was delivered to us in the condition from the first photos, saying that it is getting too hot.
Then it worked for a month when we received it in the conditon from the second portion of photos.

Actually my question are:

Is it possible this to be caused by wrong rotor/armature connection or rewinding?
If so how to know it? If the winding is wrong is it possible to find the correct winding diagram?