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

 

[roydm]

I think it would be very unusual to pass that much current through the brushes on a continuous basis, are you sure there is not some means of shorting out the slip-rings when up to speed. A poor short might account for the low current you see.

 

[jraef]

In what we can assume is a Woubd Rotor Induction Motor, there are usually various steps of resistance added to the rotor circuit that change the torque capability of the motor and thus the speed under specific loads. As a general rule you start it with high resistance / slow speed and slowly step up the speed by dropping out resistance, often with contactors in MV motors. If the original starting sequence of operations has been lost over the decades and someone is just switching it on at the resistance that will provide 80% load, that would put a lot of stress on the slip ring brushes and burn them out. The imbalance you are seeing in the Rotor current may also be indicating damage to a slip ring as well, and/or a step contactor too. If you have what's called a Liquid Resistance (liquid rheostat) control system, your electrolyte may be wrong and needs servicing. You need someone who fully understands WRIMs to go through the motor, controls and operating sequence.

It might also be time to consider dragging this into the modern age, because generally large motors like this are intimately tied to the bottom line of whatever your company does, so having it operate reliably means everyone stays employed[pre][/pre]. If your operating speed never changes now, consider using a solid state soft starter for the stator and that fixed resistance value you already have. If the speed needs to vary to follow some sort of process signal, you can use a VFD but at 11kV that's going to be a budget buster.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[dArsonval]

What attention (if any) has been given to the slip rings? With all these burning brushes,
the slip rings would likely be in less than optimum condition. Including the brush holders.

Photographs of the slip rings, brush holders and the failed brushes provide very useful details.

John

 

[Feg]

Hi Guys

Thanks for initial replies.

This motor has been working since Jan 2000, its a slip ring motor started by the same liquid resistance starter and the sequence of starting has not changed over the years. Its a fixed speed all the time. its starts very infrequently, it runs normally for weeks/months at a time.

I have attached some photos, one attachment shows the damage after failure, the other show's the motor in operation with one brush wearing more than the rest, the brushes were changed after this was found.
The slip ring assembly was repaired, skimmed with new brushes fitted and returned to service within 3 months it had the same failure.

F.E.G

 

[edison123]

How are you measuring the rotor amps, which is of slip frequency?

Are you using the brush grade recommended by the OEM?

Muthu

http://www.edison.co.in

 

[Feg]

Hi Muthu,

we recently fitted CT's to measure the motor amps, the OEM no longer supplies the brushes for this motor but we are using the same grade brush as the original and they have been working well up to this problem.

F.E.G

 

[dArsonval]

Thank you for posting a photo. In viewing the picture, there are obvious signs of insulator deterioration
between the right and center brush sets. Note the discolored (darker) insulators.
There is also a sign of intense heat along the edge of where the slip ring is mated with its insulating hub.
Note the dull look of the metal spacing rod and its lost varnished sheen in comparison to the exposed
portion of the rod further to the left.

You’re not going to want to read this.

The entire slip ring assembly pressed on the rotor shaft must be removed from the apparatus
and completely cleaned/washed, dried out and re-varnished, or totally replaced.
Obviously the brush holder assembly would need the insulators replaced, or inspected for
their electrical integrity as well.

As clean as it looks, it’s difficult to comprehend potential tracking between the
three rings, “but” it does… and likely is.

One opinion based on one photograph.

Keep us posted.

John

 

[roydm]

It does have a lot of brushes but I still can't see 1375 Amps 24/7
And the very low rotor amps running is trying to tell us something
Excessive wear on those brushes

Could there be something wrong with the liquid resistor, I am not familiar with the liquid type.

 

[LionelHutz]

Assuming your current reading is before the shorting device, the rotor current should be approximately 1100A when the motor is running at full speed and 80% load. Your current reading of 70-80A is indicating that thee rotor current is bypassing the external rotor circuit, possibly by a short between phases in the brush holder assembly or the slip ring assembly.

 

[waross]

Do you not have a shorting contactor to short the brushes when the motor is up to speed?


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

 

[edison123]

Hi Feg

You use CT's on the rotor circuit? Your read rotor currents are then suspect.

Possible reasons for brush wear out.

1. Wrong grade

2. Inappropriate spring pressure

3. Loose connection from slipring to rotor winding lead

4. Hard dirt (like cement particles)

5. Excessive temperatures

Have you tried measuring the brush temperatures during running with a thermal camera or a non-contact temperature meter?


Muthu

http://www.edison.co.in

 

[waross]

A long time ago, and the details are hazy, I had to load test a new generator for a week to satisfy the supplier that the piston rings were well seated to avoid future possible warranty claims if the engine started using oil.
I used six 60 gallon plastic barrels and 12 truck leaf springs for electrodes. With a clamp meter on the lead, I poured salt into the water until I saw an acceptable current. I was running about 200 Amps per barrel, 400 Amps per phase.
What's the point of this?
I boiled off about 600 Gallons per day of water. I made multiple trips to the site every day to refill the barrels.
In the mornings the barrels would be close to empty.
Please tell me that you are not leaving the liquid rheostat in the circuit while the motor is running at speed.

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

 

[LionelHutz]

OK, I missed the CT comment. When the motor is at full speed the rotor frequency is really low so a standard CT won't measure the current. You'd have to use a hall effect sensor type of device to measure the current with the motor at full speed. Using standard CT's would be why your current reading is so low.

I would suspect the quality of your brushes has changed.

 

[waross]

Has there been any RTV silicon sealant used in the vicinity of the brushes recently? The fumes from RTV silicon are a known brush killer.

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

 

[Feg]

Waross you may be on to something. We sent the brushes away to be analysed and they found a trace of a type of silicone/rubber compound we don't know where this came from.
The Elec guys tell me they checked that the contractors are working properly and the starter is out of the system when the motor is up to speed.
I'm a mech guy so I would like to clarify the following, I would have thought that when the motor is up to speed there would be no volts/amps in the rotor is this right ? We are seeing approx 70 amps with the CTs which we only fitted recently, as they only ever trended the stator amps prior to the problems. What we are doing now is monitoring it with the CTs and when it gets to 250 amps it trips, this is actually protecting the motor now and we know this as we have had 8 months now trouble free but we have had 3 trips which is a problem for us. The 250amps was just a guess in case you wonder where we came up with that.

They fitted rope type CTs from Powertekuk RCTRMS-3ph/1500/4/500 (4-20 mA)

Edison, we got a thermal check done and it's shows one phase a lot hotter than the others I just don't have it to hand as I'm away from site. The area is clear with no deposits of dirt, the grade of the brush is the same as the OEM according to SCHUNK and new brushes and springs were fitted after failure.

Thanks for input so far guys

 

[roydm]

Edison 123
Good point, the CTs won't measure the slip frequency I missed that.
Is there a correct way of doing that?
I recall we had a clipon that was nothing more than a magnetic circuit with a changeable moving iron movement, Hall effect nowdays I guess.

 

[edison123]

Bill

"Please tell me that you are not leaving the liquid rheostat in the circuit while the motor is running at speed."

Once the LRS is shorted (either via contactors or a shorting ring at the end of the travel of the LRS), not much current will be flowing through the liquid. The case you are describing is where the salt water was used as a load 24x7.

Feg

"I would have thought that when the motor is up to speed there would be no volts/amps in the rotor is this right"

No, the rotor will have the rated rotor current at full load at the rated speed.

Again, you cannot use CT's to measure low frequency rotor currents. And rotor currents are not measured or used for motor protection in normal applications.

A complete revamp of your sliprings (not just surface machining) is needed.

Muthu

http://www.edison.co.in

 

[dArsonval]

The Silicone Theory would be a good one if all three slip rings were being equally attacked by the corrosion causing [out gasses] non-electrical grade silicone
exhibits during its curing process (which takes years).

The photo of the slip ring area of the motor clearly shows one phase is affected. Not all three.

(Silicone -out gasses- would not choose just one phase; it would attack all copper bearing surfaces.)

The slip ring area of the motor is tracking “potential” which needs to be corrected.
And the only way that can happen is by removing the apparatus from service, and fixing it.

The absence of visual dirt or debris is meaningless. Conducting carbon from past brush changes can be/is likely
painted over from previous attention to the apparatus.

John

 

[LionelHutz]

I would have thought that when the motor is up to speed there would be no volts/amps in the rotor is this right ?

Easy approximations for the rotor of a slip ring motor.
- current is proportional to load
- voltage is inversely proportional to synchronous speed.
- frequency is inversely proportional to synchronous speed.

So, at full speed and full load you should see a low rotor frequency, a low rotor voltage and rated rotor current.