WR motor failure 2

[pierrec]

Hi:

We had a wound rotor failure while commissionning a new
slip energy recovery drive. Arcing occured in the brush
slip ring area. I wonder if anyone could explain what might
have happened. Here is the story:

The 500HP/4KV motor drives a pump. It can be run either
with the drive in variable speed mode, or in fixed speed mode. In fixed speed mode a resistor banks and three contactors are used to start the motor and bring it up to
speed. Rotor disconnects and a mechanical interlock make it
impossible to have both systems on at the same time.

The guys doing the commissionning needed to have the drive
connected to the motor while it was starting with the resistors. This allowed the drive to measure voltages and log data in order to calibrate drive parameters.

They defeated the mechanical interlock and wired a temporary switch to initiate the resistors contactors sequence. Normally, this sequence starts with closing the HV breaker and stops as soon as the breaker opens.

They then closed the HV breaker and hit the switch for about 10 seconds. The first step will typically last about 15 seconds so they never went beyond that. When they turned the switch to off (which did not open the HV breaker) the motor winded down a bit and then... BOOM - there was arcing around the rings.

The motor name plate states a 1090 V rotor voltage.

Any explanation would be appreciated.

 

[Shortstub]

The clssic "ohnosec" error. Bypassing interlock gave us Chernoble!

 

[pierrec]

Hi:

Actually I have more information. The pump pushes liquid into a vertical pipe and there is no check valve. IE when the motor stops it starts to spin in the other direction for a few seconds (long enough to empty the vertical pipe).

The motor is about 20 years old.

So I figure the rotor voltage must have gone well above 1090 V and probably beyond the isolation capability of a 20 year old motor.

Does this make any sense?

 

[UKpete]

pierrec I'm not sure I follow this - you have a wound rotor induction motor which can be run either:
1. fixed speed mode, in the old fashioned way with the stator connected to the 4kV line via a HV breaker, and the rotor winding connected to external resistances to start (ultimately the brushes all connected together when up to speed), or
2. variable speed mode - please describe exactly what this is.

You say there was arcing "around" the rings, I can't see how this can happen on a slip ring. If the voltage on the rotor was too high there could be arcing "between" the rings. If the rotor current was too high I could understand that there might be sparking at the trailing edge of the brush, but all the way around?

If the HV breaker did not open, i.e. the 4kV was still connected to the stator, then if the speed is low then the slip is very high so the rotor voltage will be high also. If you run with the HV connected but the rotor open-circuit, this could cause a flashover between the rings.

 

[pierrec]

Hi UKpete:

The variable speed mode is implemented with a slip energy recovery drive that feeds the energy back into the 4 KV primary.

Arcing occurred in the general area of the rings. I did not see the damage myself but arcing between the rings is most probable.

Those who witness the failure report that the arcing occurred after the shaft started to rotate in the wrong direction when the vertical pipe emptied itself (with 4 KV still applied on the stator).

Faraday's law says that voltage is proportional to speed of magnetic flux (and number of turns). So if voltage is 1090 V when the shaft is idle do I correctly conclude that rotor voltage went above that level with counter rotation?

 

[Marke]

I would not expect the voltage to rise with counter rotation, the frequency of the rotor voltage would change, but the magnitude would stay the same.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GusD]

Hi Pierrec

I am glad that somebody else has slip[ring flashovers.Just kidding.We use many WRMs ,mostly 1250 HP,and a 16 step start with resistor grids.
Ours is an Outdoor installation and these motors are used to power the large conveyors.Our Winter Temp.goes from 40 C
below to 35 C in the Summer .Pretty harsh environment.What does this have to do with slipring flashover,you ask?
In my 20 years of dealing with these motors,I would say the environment plays a large part on the slipring flashes.Some Stats.In 20 years we lost 5 stator windings for several reasons.In the same period we probably had 100 rotors sliprings that flashed over and either burnt the rotor winding or just damaged the rings quite severely.
Most imporatant stat.90% of these failures happened during the months of Nov to March.The other 10 % any time of the year.I wish I could tell you that we have now found the cure for this problem.(Note:we have done extensive testing on this problem,by us in house house or contracting out)

I have no sure cure, but we did implement some controls that minimized this problem considerably.
1-We make sure that ANTI-CONDheaters are working properly.
2-Slipring compartments are free of carbon dust.
3-On the control system we alllow a 1 sec time delay when dropping out the Resistor Shorting Contactor.

As for the interlock you people jumpered out,it is always a risky step.Did it play a part on the flashover,maybe yes or maybe ia just a side show.
Good Luck.If you ever correct these problem,please let us in on it.

Thanks

Pierre -I wish I new more about this problem.These failures are the Aquilles heels of these drives and they are our bread and butter.Moving a lot of Tarsands.

GusD

 

[jraef]

If I read your sequence correctly, it sounds to me as though you open circuited the rotor. Not good on WR motors, especially not at 4kV! You can short the windings, you can add or subtract resistance, but NEVER go open circuit. Your slip recovery system technicians should have known that.



Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion: Generically, voltage in the rotor Vr is proportional to:
Vr~4.44 x frequency x turns x flux
Therefore, the frequency increased due to rotor rotation in opposite direction.

 

[electricpete]

My vote: As the motor slows down (slip increases), both the rotor voltage frequency AND rotor voltage magnitude will increase.
v ~dPhi/dt ~ Slip*Phi

 

[UKpete]

pierrec
you are correct.

With the stator connected there will be a field within the machine rotating at synchronous speed. If the rotor rotates at the same speed & direction you have the no-slip condition and the rotor induced voltage is zero. If the rotor speed is reduced the slip increases and so does the rotor voltage as per electricpete equation. If the rotor is reversed the rotor voltage will go on increasing so the worst case condition for generating a flashover at the slip rings is at maximum reverse speed.

As jraef points out, you should not operate with the rotor open circuit whilst the stator is still connected. It sounds as though this is what happened - opening the rotor circuit will cause the motor to slow as you described (as there will be zero torque) and the rotor voltage will start increasing.

 

[pierrec]

Thanks for the replies.

The motor is an indoor installation where dust and humidity are not a problem. I know there was preventive maintenance done in the past but cannot tell precisely when it was done last.

The motor is rated at 600 RPM. From past observation I do not think it went above 300 RPM in reverse rotation but I do not know for a fact.

I assume there was enough carbon dust in there to reduce the safety margin considerably. If indeed reverse rotation peaked at 300 RPM does it mean the rotor voltage also peaked at 1090 * 900 / 600 = 1635 V ?

 

[UKpete]

Assuming the rotor gives 1090V when stationary, you are correct.

 

[UKpete]

GusD - I was interested to hear about your rotor failures. DC traction motors also operate in those sort of environments and many are open-ventilated, and usually without too many problems.

If your failures are truly initiated on the slip rings (possibly sometimes this is consequential damage after a rotor winding failure?), traction motors had similar problems on the commutator endring. The solution was to fit PTFE sleeves.

Motor armatures in that environment are best given a VPI treatment after rewinding rather than a resin dip, but I can understand why a rewind shop probably won't do this - you need a jig to ensure that the slip rings aren't covered in resin in the VPI tank. Maybe it's worth looking for a more specialized rewinder.

 

[GusD]

Hi UKpete

A little more info on our WRMotors.As for our traction motors on the Heavy Haulers trucks they are going the way
of the dynossaurs.Most wheel motors that we have now are AC motors.We still have some older trucks with DC wheel motors and they are not without their problems.Open ventilated wouldn't make it in our environment.
They also operate on lower voltages than our WRMs.
As for our WRMs ,we still use 65 -1250 hp motors in our plants.Even on our new conveyor installations, we just recycle the old work horses.They were not without problems but all in all,they did the job over these many years.
Most of the rotor failures were slipring flashover and no problems with the rotor windings.Some were rotor of course.
As for the VPI treatments,we always made sure that all the rotor rewinds were VPI.In addition ,we make sure that winding to slipring connections were taken apart and inspected throroughly everytime.The original motors had a problems with this connections ,reason why we inspected it.
We didn't have just a single shop repairind these motors .We supply a WRM motor repair spec to all of the shops in our area and this spec has fairly tight tolerances.
The Manufacturer that sold us all of these motors, didn't seem to have a better idea on the fix than we did.Many of this repairs were warranty work.To ilustrate a point,
4 or 5 years ago, I had 3 Engs. from the Factory in Europe,
spending about a week in our plant, to verify our maintenance practices ,control circuits,predictive program and looking from every angle to see how they escape from costly repairs under warranty.
UKpete, if you notice on my previous post,the stats for Winter failures versus other seasons is quite significant.Other than extreme cold -40 C,there are no large differences otherwise.We also have many other induction motors that don't experience any problems.
When we started making sure that Anti-cond Heaters were always working,sliprings always free from dust,the failures became few and far between.
The motor manufacturer likes to say, that we were the experts on these motors,because of the history and long association with this drives.Experts or not,we could never explain some of these failures very well.

Thank you UKpete and Everybody else for the help.

GusD

 

[UKpete]

GusD I guess you've done enough! it does sound like a damp + carbon dust around the slip ring problem. I mentioned the PTFE sleeve because that was the problem they were having on the traction motors - these were rail motors by the way, typically 50-250hp.

The locomotive motors were a lot larger and needed to be force-ventilated and had filtered air, but on nearly all the rapid transit cars they were self-ventilated, with a centrifugal fan mounted on the armature at the opposite end to the commutator. Outside air was drawn in unfiltered from under the train at the commutator end and it all went through the motor - occasionally you would find leaves and other rubbish drawn in, but in general it was pretty clean inside, not much carbon dust. I guess the ventilation air just carried it all through and out. We don't have cold over here but we have plenty of wet, and I'm suprised there weren't more problems.

The PTFE sleeve was a shrink fit on the end of the commutator vee-ring, i.e. the outboard end of the commutator. This didn't attract carbon dust like the old fibre-glass sleeve. I would imagine that slip ring machines also use PTFE sleeves between the rings to prevent flashover.

 

[pierrec]

Hi:

The drive people confirm the rotor voltage went up around 1500-1600 V. However, I am now concerned by something else:

They apparently did the same test a couple of times for short durations of 4-5 sec before the last 10 sec test that leaded to the flashover.

Given that the rotor current is in the range of 400 A when the heavy duty resistor bank contactor opens could a high voltage transient have damaged something in the rotor insulation making it weaker at 1600 V ?

Preventive maintenance is done every three months after about 1000 hours of operation. Is the carbon dust theory still holding up?

 

[jbartos]

Suggestion and elaboration to pierrec (Computer) Apr 27, 2003 marked ///\\I assume there was enough carbon dust in there to reduce the safety margin considerably. If indeed reverse rotation peaked at 300 RPM does it mean the rotor voltage also peaked at 1090 * 900 / 600 = 1635 V ?
///Assume that
Turns=constant1=T1=T2
Flux=constant2=F1=F2
Vr1~4.44 x frequency1 x T1 x F1
Vr2~4.44 x frequency2 x T2 x F2
Vr2/Vr1~frequency1/frequency2
or
Vr2~Vr1 x frequency1/frequency2
or
1635V=1090V x 900RPM/600RPM\\

 

[jbartos]

I beg your pardon, correction
Vr2/Vr1~frequency2/frequency1
or
Vr2~Vr1 x frequency2/frequency1
or
1635V=1090V x 900RPM/600RPM\\

 

[GusD]

Hi Pierreec

You mention that the drive people told you that the actual rotor voltage was 1500 or1600 v.
The nameplate on your motor shows rotor voltage at 1090 volts.How did they get 1500/1600 volts?
I am curious to know if it in fact goes up to about 1600v,what is limiting the rotor from going higher.
During our investigations on our WRM problems ,we actually had someone from a repair shop claiming that the rotors when stalled with a full load could actually have 5 to 10 times the rated voltage.To prove or disprove that theory, we had these people come to our facility and using a very fast waveform recorder, to be able to catch the fast high Voltage spike when motor was stalled with a full load.
I was actually hoping that we could measure something like that,unfortunetly we could never capture any voltage spike that high.Or even higher than rated voltage.
Pierre,a combination of carbon build up,(not necessarily a lot of build),with the right humidity and temperature can definitely cause flashovers.
Thank you


GusD