MED Voltage Motor - Rotor amps 1

[Feg]

Hi Eng-Tips

looking for some help on MV Motor, the nameplate gives the following information

2900Kw, 991 Rpm, 11000 V Y, 991 rpm, 50 hz .085 cos, Rotor 1375 A

This motor has been working fine for 15 + years I'm told, I have only started working in this plant.

In the last 2 years, the motor has started burning brushes on one phase and not always been the same phase.
We looked at the trends and the motor seems to be working at 80% of its capacity, but the rotor amps is very low just 70-80 amps per phase. We do see one phase with approx 10-15% higher reading tot he others.
On this trend we also see the vibration rises from normal 1.2 m/s to 2.6-2.9 m/s which is still quite low. The motor will then trip and the brush will be burned out.

I would like to try and ask your opinion as to why you think the rotor amps are so low and to see why you think the brushes are burning.

Regards
F.E.G

 

[electricpete]

The rightmost pair of slip rings (which has overheated pigtails) is sunk much deeper into the brushholder. Is that a result of wear or something else?

1.2 m/sec would be very high vibration velocity. I assume you meant 1.2 mm/sec.

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(2B)+(2B)' ?

 

[Feg]

The wear was even across all the slip rings, we know that the motor and slip rings perform well for most of the time as we see no damage until the failure happens so the problem arises quite quickly
the vibration was a typo its 1.2mm sec

F.E.G

 

[waross]

Have you checked the external circuit for hot spots and or loose connections?
Are the contacts in the shorting contacter in good condition?
Have you considered a ductor test of the rotor to determine if there is a failing internal connection to one slip ring?
A poor connection to the slip ring may cause excess heating of the slip ring under heavy loads.

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

 

[electricpete]

The wear was even across all the slip rings

Thanks. Is it correct to conclude that the right brushes were heavily worn? (or is there some other reason they are sitting deeper in the brushholders).

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

 

[Feg]

yes, its when it failed it had heavy wear on that one ring. But 3 days prior when we inspected it all rings were good with even wear, so we are trying to establish why it is failing on one ring. This is the original motor the spare went in and it has the same failure but on a different ring and the original is back in now.

 

[fiaz1404]

1: I think it is due to mixing of carbon brush grade; below I will try to create a scenario.

In case if we replace some worn brushes with grade other than original. Suppose new brushes have high conductivity comparing to original brushes, then these new brushes will carry more current than other brushes. Wear rate will increase on these brushes due to excessive current and finally in very short time brushes will reach to their minimum length. Now at this moment, burden of those worn brushes will be shifted to remaining brushes and current density on remaining brushes will increased. In result, after some period all brushes would be looked heated-up and consequently worn-out.

2: I think your current measurement seems not ok; you cannot use normal CT's to measure low frequency rotor currents.

 

[waross]

What is this motor driving? Is the operator able to control the loading on the motor? Where is the motor mounted? Is the environment clean or dirty? Are there periods when there is dirt or contaminants in the air? Is the motor installed in such a way the air circulation is impeded in any way?

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

 

[electricpete]

So let me summarize. Please comment if I've said something wrong:

1. Occasionally something happens where vibration goes up, motor trips, and brushes are found worn and overheated.
2. Those worn brushes are found in a different phase each time.
3. But (at least from latest photo), the two brushes within a given phase have similar wear?
4. After the failure, brushes replaced but nothing done to slip ring?
​

If I have it right that's tough to figure.
Overheating may be a consequence of severe brush wear which extends spring to the point that contact pressure is lost and it creates the heat. But in that case, what would cause the severe brush wear to begin with.
Items 2 and 3 don't seem very consistent with a mechanical cause.
Electrical cause on rotor doesn't seem likely to shift phase.

Maybe something to do with external shorting...if there are multiple contacts involved and all of them have some erratic behavior. Just a shot in the dark, not sure. Maybe try some resistance measurements from the brushes with brushes lifted off the slip rings and the external circuit in the shorted/running configuration (with motor not running though!).

Photos of slip rings or the other side of the brushes might possibly have some clues.
Also if there is a spectrum associated with this increased vib, analysis of which frequencies increase may give some clues (does it suggest a stator unbalance 2*LF or a rotor unbalance which I think might have pole pass characteristics, or something else). For that matter, any unusual frequencies in the normal vib? Also if you notice the vib going up you may be able to grab some three phase current measurements on stator or rotor if there are safely accessible measurement points and methods. Also take a close look at your maintenance procedures, spring tension checks, condition of slip rings, also the potential contamination issues mentioned by John / D'Arsonval


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

 

[wroggent]

I know next to nothing about brushed motors or brushes:

Wouldn't imbalanced current on the three rotor poles cause vibration?

Higher current on one pole is caused by lower resistance in that pole. Lower resistance could be due to:

Shorted turns in a winding?
Shorter lead length to the shorting point?
Higher brush contact pressure compared to the other poles?


I've heard that brushes have a negative coefficient of resistivity within certain temperature ranges (a property of carbon). Something I've been wondering: to avoid thermal runaway, is it necessary to have a certain minimum resistance in the shorting circuit on each pole such that the resistance of the brush is not a significant portion of the total resistance in each pole? (i.e. varying brush resistance does not significantly influence rotor current) If the shorting resistance in each pole was approximately equal but more importantly, too small, would thermal runaway be possible? In such a case, I would expect the pole to burn out to be the one that had the lowest contact resistance which is I'd guess would be most dominantly determined by the initial spring tension.

Is it typical that the brushes are in shorting circuit on a WRIM?

 

[panter]

We have similar 6kV motor and sometimes we performed simply test to see if exists some problem with external shorting , contacts and resistor .
We give low voltage * 3 x 380 Volts* on stator windings ant then simulate motor start with contactor and timers beetween every steps .
Then you should measure current on every stator and rotor phase with amper clamps without big risk because motor did not run .
In several last steps, stator and rotor currents are a bit larger so that you need to pay attention to this when selecting the power wire .
IF is current at every step of all phases is same and ballansed you did not have problem with rotor , contactors , start resistor and your problem is on the other side .
Good luck

 

[edison123]

panter

The rotor will turn even at 380 V if the rotor circuit is closed with or without resistor.

Muthu

http://www.edison.co.in

 

[waross]

11,000 Volts down to 380 Volts?
Uncoupled the rotor will turn.
We don't know what this motor or the motor that panter tested were driving.
A lot of loads will not turn with 3.5% of rated voltage applied.

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

 

[panter]

It is 6kV, 3,55 MW motor with more than 4 Tons rotor weight on sleeve bearings .
It is coupled with pump and 380 V is low voltage to turn rotor during test .
Good luck !

 

[waross]

Let's look in the other direction. An unbalanced supply may cause heavy rotor current.
Have you checked the incoming supply for balanced phase to phase voltages.
Check first with the motor off-line and again with the motor running.
A running induction motor acts as an induction generator and will try to correct unbalances in the supply.
This may cause circulating currents and excess rotor heating.
My experience with unbalance issues has been with squirrel cage motors, synchronous alternators and transformer banks.
Given that unbalanced loading has caused rotor heating and damage in alternators, and unbalances have caused unbalanced currents in motors and rotor heating and in the case of transformer banks, caused one transformer to run much hotter than the second overheated transformer, it is possible that unbalanced supply voltages may cause heavier current in one brush.


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