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WR motor failure
2

WR motor failure

WR motor failure

(OP)
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.
 

RE: WR motor failure

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

RE: WR motor failure

(OP)
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?

RE: WR motor failure

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.

RE: WR motor failure

(OP)
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?
 

RE: WR motor failure

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

RE: WR motor failure

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

RE: WR motor failure

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

RE: WR motor failure

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.

RE: WR motor failure

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

RE: WR motor failure

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.

RE: WR motor failure

(OP)
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 ?


 

RE: WR motor failure

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

RE: WR motor failure

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.

RE: WR motor failure

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

RE: WR motor failure

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.

RE: WR motor failure

(OP)
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?



 

RE: WR motor failure

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\\\

RE: WR motor failure

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

RE: WR motor failure

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

RE: WR motor failure

Suggestion: Many types of dusts of organic origin can also cause flashovers.

RE: WR motor failure

It seems to me that rotor voltage proportional to slip is a good approximation if we are comparing two cases where rotor is open-circuited in both cases.

If instead the motor is overloaded as I think is Gus' case, then there will be high current through the stator series leakage reactance would tend to decrease the rotor voltage below the proportional-to-slip.

Back to pierre's problem, I would think we might expect even slightly higher than proportional (higher than 1635V=1090V x 900RPM/600RPM) if the 1090v is based upon full load current through primary reactance, while only magnetizing current flows through reactance in the the 300rpm reverse rotation, open-circuited rotor configuration.

RE: WR motor failure

(OP)
Hi:

Humidity and environmental dust are definetly not the root cause.

I learned that similar motors are designed for 200% rotor voltage plus 1000 volts. In my case that means 3.2 KV isolation.

Normally the motor is powered off by opening the HV breaker.
This cuts the power to both stator and rotor. In my case, the rotor first stage contactor was opened with power still applied to the stator. The motor winded down and then went into reverse rotation at 300 RPM when the flashover occurred.

When a coil is disconnected a transient voltage can be calculated from:

V = L * dI/dt

I is equal to about 400 A. t is in the range of a few ms.
I do not know what the rotor inductance is (yet) but I suspect the resulting transient voltage can be large and exceed 3.2 KV.

Can 4 or five of these transients damage the insulation?



RE: WR motor failure

pierrec - I would say it is a possibility.  There may have been some low energy flashover/tracking at the instant of opening the rotor circuit, although a good proportion of the energy will be dissipated at the opening contacts which were breaking 400A.

But it sounds like it is too late to say for sure, as you have now had a big flashover which has destroyed the evidence on the slip-ring insulation.

RE: WR motor failure

We have said that clearly the final failure did not occur at time of breaking of rotor circuit, but the damage may have occurred earlier as a result of spikes from breaking the rotor ciruit.

One would think that such spikes could attack the rotor turn or ground insulation.  But doesn't appear from what we know the failure initiated there.  I don't believe that surges will typically initiate carbon tracking across surfaces in the area of slip ring which would leave it weakened.  I believe normally it takes current flowing over time to establish that kind of track.

Just my opinion.  I could be wrong of course...

RE: WR motor failure

(OP)
Hi:

I saw the motor yesterday. It is vertically installed. The lower ring looks good. The arcing occurred between the two upper rings. The middle one is all blacked out and there are grooves in it up to 1/8" deep. 60 degrees clockwise from the brushes there is a black spot on the middle ring that looks like the foot print of a brush.

Something else, the motor protection relay did not open the HV breaker. A human did manage to hit a mushroom about 5 seconds after the beginning of the event. The stator is OK I'm told.

This motor will be repaired quickly. The application is critical and enough pumps must be available. They are now down to three pumps and *cannot* risk to operate with only two. I will keep you posted if I can learn anything from the repair shop.

RE: WR motor failure

HI Pierrc

I Hope you find some answers to your flashover problem.
It is a plesure just to read through the posts and admire the amount of knowledge and expertize that reveals itself by all who answered these  not so easy questions.We all benefit from this knowledge bank.Thank you all for that.
I have a couple of points that I like to clear up regarding some of my own flashover problems.

Electropete.=You mention that probably we had an overload.
Some failures ocurred while starting empty conveyors.
As for your belief that you need current for carbon to track, I agree.Obviously we have to have a combination of Voltage and current.Highly conductive mediums,high Humidity and a little carbon can be very explosive when voltage stressed.

To JBartos'.=Your formula for the peak voltage
1635V=1090V x 900RPM/600RPM\\\.Is it correct to assume that if the 600 rpm is reduced to 200,we could have 3x 1635v ? Is it right ?
 
We did experienced a lot of rotor "shorting contactor"
failures, before we added a 1.5 sec delay to open whenever
the system tripped with heavy loads.This time feature did eliminate the contactor problems.
IN our systems we cannot have a rotation reversal
condition.

Thank you Gents

GusD

RE: WR motor failure

Suggestion to pierrec (Computer) Apr 30, 2003 marked ///\\\

When a coil is disconnected a transient voltage can be calculated from:

V = L * dI/dt

///Initial conditions also have to be considered, since the inductor has stored energy inside with certain voltage entering the initial conditions.\\\

RE: WR motor failure

One does not open the circuit on a current transformer, and the wound rotor motor is operated like a transformer, where the secondary or armature or rotor changes values as the resistance increases or decreases the amount of current, which it see's, thereby increasing and decreasing the speed and torque.

RE: WR motor failure

I have to give farkel a star here.
I think we've all seen CTs after being open circuit. The wound rotor is the similiar in that it is low resistance(impedance?)and relies on the external resistors (short links) to provide voltage regulation. Open circuit it and there is no current flow ===>voltage rising vey high. I suspect if the rotor is reversed the transformer effect will be increased and the voltage will be increased more.

Now if I'm wrong we could confirm it,-- if someone finds a circuit for a "salt bath" starter. ie the one where the plates are lowered into a conductive solution. My memory says the unit we worked on had a resitor bank to give the max ohms for the rotor and then was progressive lowered into the solution until shorted at max speed. The unit I recall was supplied by Seimens
The resistors were to stop open cct on the rotor

Regards
Don

RE: WR motor failure

Sorry to be picky, but IMHO, an open-circuited ct secondary is not a good analogy.

Yes, there is v=L*di/dt in both cases. The similarity stops there.  

First: a ct steps up the voltage from primary to secondary by 10:1 or more. No such stepup ratio in a w/r motor... I'm guessing effective stator and rotor turns ratio is not too far from 1.

Second: The primary circuit of a ct in a power system acts like a current source.  Specifically the series impedance of the power system load is much higher than the impedance presented by the ct (ct secondary ckt impedance is generally low and is further reduced by turns ratio squared when seen on primary).  The power system load impedance controls the primary current, and primary current will flow regardless of what we do on the secondary ckt.  There is nothing comparable in a motor.

RE: WR motor failure

Suggestion: Wound rotors have certain voltage rating, e.g. medium voltage wound motor has low voltage rotor winding rating. The rotor is not exactly the current source. However, its voltage is changing during the motor startups. The rotor could be referred to as a current varied voltage source.

RE: WR motor failure

Hi again.Pierrec.

In a previous post I reffered to our WRMotors and its problems,that we had  introduced a TDelay of 1.5 to 2 sec before we open the shorting contactor.
This time factor allows the stored energy in the rotor to dissipate as heat thru the resistor grids .
Our original control systems did not have this TDelay  feature, causing many failures of the shorting contactors.
Possibly other failures as well.  

GusD

RE: WR motor failure

Suggestion to the previous posting. Apparently the failing contactors used for shorting are not adequately rated.

RE: WR motor failure

Hi JBartos

As for the shorting contactors rating I don't think that was a problem .We were still using the same contactors sizes until 2 or 3 years ago.Of late, all of the new WRM
Rotor stepping contactors are of the Vacuum Bottle types .
The shorting contactor failures stopped many years ago,long before we started using the newer Vac contactors.
JBartos, It is true that if the contactors were larger ,we would have minimized our failures.

Thanks JB

GusD

RE: WR motor failure

Suggestion to the previous posting. It is necessary to know more about the time delaying. Evidently, if the time delay allowed for the rotor energy dissipation in the wound rotor motor starting resistors then the current shorted by the contactors will be smaller.

RE: WR motor failure

I am sure that the frequency of starts has an appreciable impact on contactor life.

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