Causes of frequent burning of induction motor(400kw) slip ring? 1

[bengalayjr86]

Dear All,

Im new to this forum, but I want to solicit ideas from you about my current study.

I am currently assign to investigate the frequent burning of Ball Mill motor slip ring.

Induction Motor Specification:
400KW output, 870rpm
3300V, 60hz, 8 poles

This motor drives an equipment called "Ball Mill".

My initial finding is that, maybe the motor experienced over-loading. The only available information for me is the incoming feed rate and discharge rate in terms of Tons/hr of ball mill.

My questions;

1) How will I much this info (tons/hr) to the rating of the motor so that I can say that the Ball mill is overloaded? Or with the given specifications of motor, how can I convert it to capacity in terms of TPH so that I can compare it with the feedrate of the Ball Mill?

2) Aside from overloading, what else could be the cause of overheating or burning?

Your responses will really help me find the root cause of this problems....

Thanks,

BenGalay
Philippines

 

[Drivesrock]

I think a good place to start is to know what does your repair shop report as probable causes for the motor failures? Any photographs?

If you google motor failures you will sure find some documents and photos to compare.

Do you have any recordings of motor current?

How many starts per hour - perhaps the motor has been overloaded in that way - ie too many or too close together?

Do you have many harmonic loads at 3.3kV or significant voltage distortion?

What is ambient temperature, altitude..?

In summary, I think we need some more info to start narrowing the area of investigation.

 

[waross]

The first, second and third most common causes are sticking brushes.
Next check for loose connections on the brush holders.
Then investigate the grade of brushes.
For a quick check on possible overloads with no other information, check the rated running current of the motor against the setting of the overload devices. I would suspect that if the overloads are functioning at up to 125% of rated current then the brushes will easily carry the current.
I would expect frequent tripping or stator heating before brush failure with overloads.
If your overload protection is up above 150% of rated current the machine may well be overloaded.

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

 

[electricpete]

I don’t work with slip ring machines much. But I’ll chime in some thoughts...

fwiw, I agree with Bill’s comments – not likely a simple overload since stator sees similar overload ratio as the rotor (just like a transformer).

Just to brainstorm some other things that can go wrong with brushes: wrong brush tension, wrong shape/contour, out-of-round slip rings, contaminants.

Do you have any photo’s of the damage to the slip rings... also photos of brushes?
Does it occur all the way around or just one location?
Do the brushes spark during operation?
Is it a new machine? New symptom?


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

 

[waross]

Is the mill running wet or dry? If this is a dry mill then dust in the brush holders is a high probability.

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

 

[bengalayjr86]

Thanks everyone for your positive reply.
We experienced again Slip ring failure yesterday, Dec 14, 2011. Please see attached pictures.
We gathered some information prior to the failure. We monitored the following;
• Stator Windings Temp: 96°C
• Primary Current: 86A
• Secondary Current: a-306A, b-290A, c-302A
• Slip Ring temp: u-160°C, v-156°C,w-142°C
• Ambient Temp- 40°C
Induction Motor Nameplate:
Vrated: 3.3kV
Irated: 92A
Outputrated: 400kW
Sec Voltage: 740V
Sec Current: 325A

Specification of Brush:
RC53 (ringdoffs)- Metal Graphite
Resistivity(uΩm) : 1.30
Density(g/cm³): 3.2
Flexural Strength,MPa: 30
Hardness Rockwell B: B4HR10/60
Current Density, A/cm²: 12
Peripheral Speed: 35

 

[LionelHutz]

I've been told that high humidity causes problems with brushes.

 

[electricpete]

Thanks for all that info.

I have a dumb question....

The slip ring on left is dull copper color ( looks similar to ours.)
The center and right slip ring have shiny glazed/chrome color.

Which is normal ?


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

 

[electricpete]

The shiny chrome color is result of brush material transferring to the rings?

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

 

[burnt2x]

Methinks you might have missed to post an important data here. You gave us the rated amp per brush area but we also need to know the dimensions of each of the twin-brush per slip ring. Please check if your amp/cm2 is enough to carry the rotor current (amp rotor must be < than the amp/cm2 X total cm2 of the twin brush.)

Should you find the results leaning into the brushes being overloaded, search for a better type of brush that gives you a larger capacity margin than what you are presently using (i.e. a different type of brush material, hence a bigger amp per cm2).

As mentioned, cleanliness of the slip rings affects the inefficiency of the slip ring brushes. If you have a wet milling process, chances are slurry spills get into the slip area and can be the source of problems.

If you can't possibly find another type of brush, you just have to lessen the amount of balls loaded in the ball mill cylinder. Note that the weight of the milling balls are far more heavier than the material load being crushed. Easing the total amount of balls greatly lessens the load on the ball mill motor. The rule of thumb is to have the mill half filled with grinding media (balls).

Also check the critical speed to see if you are not over-driving. Operating at a lower speed (some rotor resistors in-circuit) can possibly reduce the rotor current and could help your brushes.

Glad to be of help.

 

[electrolitic]

It is just a suggestion. After you have solved the brushes/rings
problems I would suggest ring temperature monitoring using infra red noncontact sensors (Raytek CI series/analog output).
Normally ring box has enough space to install them.
Supervisory system can provide trend grahps, alarms, etc.
Sometimes the stator current monitoring is not enough to provide complete wound rotor motor protection.
Regards