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11kV Motor failing to start
2

11kV Motor failing to start

11kV Motor failing to start

(OP)
We have a 11kV, GEC, 6.5MW motor which is connected via a gearbox and pinion to a ball mill. The motor is soft started by a LRS.

About a year and a half ago, April and again now yesterday the mill has gone for a start and has sat there not turning. It has sat there pulling 1800A until the protection eventually triped.

In all cases the motor and LRS has been inspected with nothing found. The mill was then inched by a seperate inching motor with no apparent mechanical windup or resistance. The mill was able to be restart after the inspection each time.
The LRS conductivity was measured at 14mS as compared to a design value of 12.7mS.
We have also had the motor reciently tested by a motor test company with no obvious abnormailties.

We are at a loss to what could be the cause of this intermitant fail to starts and any suggestions would be greatly appriciated.

RE: 11kV Motor failing to start

For a slipring motor, you're pulling almost 6 times the FLA at the start, which is quite high. Suggest you recheck the connections from the rotor to the LRS (brushes, springs, cable terminations at both ends).

Muthu
www.edison.co.in

RE: 11kV Motor failing to start

Quote:

The mill was able to be restart after the inspection each time.
.
Are you saying that the motor started every time after "inching"? If so that is the normal procedure, especially when mill has sat idle for a while.  

Rafiq Bulsara
http://www.srengineersct.com

RE: 11kV Motor failing to start

GEC Motors and Generators are nowadays Converteam, well since about 2003. You could try your local Converteam rep to get them to check out the motor and starter based on the original project. Where is the motor installed?

RE: 11kV Motor failing to start

Muthu's suggestion makes sense to me.  If you had intermittent open on one of 3 phases of rotor, I think you would develop no torque (like single phasing stator of a 3-phase induction motor).  
Maybe rotating it is enough to reseat a brush.

Less likely, I have seen sleeve bearing machines intermittently bind up upon starting if they have been sitting stationary for awhile.  But breaking them loose was not easy.
 

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

RE: 11kV Motor failing to start

I have had to roll 3000HP motors before starting. Sleeve bearings dry out in as little as 12 hours. If you have sleeve bearings I would roll the motor with the turning gear before each start. Leave it on the turning gear until the turning gear amperage drops and stabilizes.  

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

RE: 11kV Motor failing to start

(OP)
The rotor connections have been tested with the results attached, there is some small variences but we believe they are within tolerance. We havent done an internal tank inspection as yet. I have attached the multilin waveform capture which captues the end of the last event, it shows stator phases and differential CT's. I do note that there is differential current and indeed the last trip was on the differential current not over current.

The mill was installed 2002, the motor was replaced around 2 years ago. From the history ive herd this problem has not occured before than.

There is 13 brushes per phase so to me it seems unlikley that all of the brush connections would go bad.

The mill has to be inched if is the mill has been stationary for longer than around an hour. This is mainly so that the ore and ball charge to not become 'frozen', if the mill is started with a frozen load and the frozen load sticks together until it is on the top of the mill then falls it will cause severe damage to the mill. Indeed in this case the mill was inched up until around 2 minutes before the mill was started.

I did forget to mention in my first post that this has occured after shutdowns or long downtimes (6 hours).
One of the theorys floating around is that the sodium carbonate is settling out of solution and so the motor tries to be started with a less conductive solution and thus does not develop sufficient torque. The circulation pump is run for an hour or so before starting under this assumption. Personally i dont think this is probably it as sodium carbonate is quite soulible and i have not herd of it coming out of solution before. has any one else herd of that type of senario?

RE: 11kV Motor failing to start

So, part of what you are looking at is: what changed.

One thing to think about is voltage.  Does your incoming voltage vary?

It appears you have about a 20% droop in stator voltage during the start.  Is that normal for this motor?  

How many poles (what is synch speed)? It occurs to me if more than 2-pole, we could do a d-q transform and look at the oscillations. Line frequency oscillations would be expected/normal.  Oscillations at line frequency times number of poles pairs would indicate unbalance in the rotor circuit. The calculation is worthless if 2-pole motor, so I'd like to know before I try.

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

RE: 11kV Motor failing to start

Also the traces show a stator current that decays away very slowly after the motor trips.   That is odd. Is it a delta-wound motor with CT's monitoring inside the delta (that would seem unusual).  Otherwise I can't immediately see why we expect slowly decaying stator current indication after the trip.

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

RE: 11kV Motor failing to start

If the  problem seems to occur after long shutdown, another possibilty is that the required mechanical torque is higher during that time for some reason... either something about your driven equipment/process, or else less likely something about sleeeve bearings (Bill mentioned drying out in a matter of 12 hours... our problems came after months of sitting idle).  

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

RE: 11kV Motor failing to start

Attached I have shown some traces in a more readable format.

Slides 2-4 are voltage and current for phases A, B, C.  Current lags voltage as expected (I assume the data included was phase to neutral voltages... labeling was ambiguous but magnitudes and phases match expected phase to neutral voltages).

Apparently where the trace begins at t=-0.25 sec, the motor has already been energized for some period (the dc transient is gone).

At around t = +0.05, the motor trips as evidenced by decreas in current and increase in voltage (by about 20%). The peculiarity previously mentioned is highlighted in slide 1.... phases A and C don't stop immediately but decay to zero in equal/opposite fashion.  I don't know what that's all about.

Slide 5 is the d-q variables.  I set the frequency to 50.07.  There is also a shift in the entire time axis of 0.0556 seconds for eveyrthing which lines up phase A with a cosine function and creates the expected pattern: Vq is positive dc while Vd is zero.

Id (the component in quadrature with applied voltage) is much higher than Iq (the component in phase with applied voltage) as expected since the load is very inductive.  The traces are very bouncy ... I think in part because the sample rate is very coarse (600hz) and perhaps I have not recreated the exact frequency perfectly (although 50.07 is the best match).  

What we see at the beginning of the waveform is line frequency (50hz) oscillation in Id and Iq.  That is a normal feature after energization (it corresponds to the dc offset in the phase currents, but paradoxically disappears much lter than the phase currents in simulations).  There is also a growing 100hz component that starts out barely perceptible at -0.15 seconds, and dominates the waveform just before trip at 0.05 seconds.  I'm sure there can be lots of causes of that which I'm not aware of, but the one thing I was looking for was "pole pass frequency" which for a locked rotor condition would be poles/2 * line frequency.  If this is a 4-pole motor, this could be indication of a rotor-side imbalance which is growing substantially during this period.  

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

RE: 11kV Motor failing to start

Just to avoid confusion, I should mention that the time scale of slides 1,5 and 6 is shifted upward by 0.0556.

So the trip happens just after 0.05 seconds in slides 2-4, but it happens just after 0.010 seconds in slides 1, 5 and 6.
 

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

RE: 11kV Motor failing to start

Actually, 2*LF is pole pass frequency for a locked rotor condition regardless of number of poles.

Fp = 2*s*LF
For s=1, Fp = 2*LF

Another way to say it
Fp = p *(fsync - fmechanical)= p * (fsync-0) = p*fsync = p*(2*LF/p) = 2*LF

The frequency indicating rotor circuit unbalance in a locked-rotor machine is 2*LF (i.e. 100hz) for any number of poles
 

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

RE: 11kV Motor failing to start

Although I am certainly at a loss to explain why unbalance didn't seem severe at the beginning, and why it would be growing over time.

Take it for what it's worth... analysis for your consideration.  You get what you pay for winky smile

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

RE: 11kV Motor failing to start

Then again there is the differential current and associated trip. I don't need to tell anyone that on the surface that suggests a stator fault.

Attached I have added the differential currents and ground currents plotted as slide 7.  They don't appear anything near sinusoidal or periodic with line frequency.

We combine this with the observation of the decay of A and C phases after the trip, it builds a puzzle. Actual stator fault or some kind of malfunction in the CT circuit?

Perhaps you can describe the stator windings (wye or delta) and the location of the CT's.

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

RE: 11kV Motor failing to start

Whoops. Attached is the revised powerpoint with differential and ground currents added as slide 7.  (Note there is a different scale for A phase than the others as indicated in the legend on right side of plot).

Also you said you had tested rotor and attached some results... but I didn't see that in the attachment (just the fault recording).

Seems like some electrical testing of stator and rotor and perhaps CT circuits are all in order.

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

RE: 11kV Motor failing to start

(OP)
Thanks for your posting electricpete, you have obioulsy put alot of time and effort into your above postings
 
its a 6-pole machine.
 
20% does seem exsessive to me also. I just did a quick simulation through PTW and at 5 times FLA it predicts 21% voltage drop for our system. For a normal start (2*FLAish) i certianly would'nt expect 20% voltage drop.
 
Its a star-wound motor with CT's on the line and star side of the stator winding. There is rotor CT's at the LRS but unfortunately they only feed into our DCS and thus their trend is only in minutes and does not help. If the fault were to happen more predictable we would set up our data recorder to capture this, but we use it so often its hard to leave it in the one spot for months.
I agree with your comment electric pete in resgards to the slow decay of the motor current, i am at a loss to explain this also.
 
As for the recording the multilin relay has only a limited amount of waveform capture. The motor is energised for approx 5 seconds before the current finally builds up enough to trip the motor. The multilin only records the final section of the fault.
 
The motor was tested by a test company in july with the results attached (including LRS tests which i failed to attached earlier!). The test company we use dont have any answers for us and dont seem too concerned, which does not help us!

RE: 11kV Motor failing to start

5xFLA at start indicates that the motor is no longer a slipring motor => SS liquid has become fully conductive or the shorting contactor is closed.

Muthu
www.edison.co.in

RE: 11kV Motor failing to start

Is it possible the motor could have been energized with the LRS not in the start position? I'm thinking part way through the travel so the resistance is reduced a fair bit compared to where it should be on a start. If you decrease the rotor resistance too much then you will really decrease the locked rotor torque.


 

RE: 11kV Motor failing to start

For any scenario we come up with, we can come up with a counter question that seems to contradict it.  If the problem is on the rotor side,  then why did we get a stator differential trip?  Perhaps we might say CT saturation from higher than expected currents... but why does the A phase differential current seem to grow over time?  And why do decaying A and C currents remaining after the trip like that?

Or if we say the problem is on the stator, how do we end up with balanced currents?  And still can't quite explain that A and C behavior after the trip.... seems to require a loop which includes A and C phase CT's.   Can't even explain that with phase to phase fault... unless the CT's which are used to provide phase current are the ones on the neutral side (vs wye?).

Looking again at A-phase differential current, we can say it has a very definite component of current at 2*50hz = 100hz througout.  What on stator side would cause that??? Nothing on the stator side that I can think of... Would seem to point toward rotor side I think but just another piece of the puzzle.

Was the electrical testing done after the differential trip or before?
Has another successful start occurred after the differential trip?
Is this a new installation, or has operated fine for awhile and then just started acting up (I assume the latter from some of your comments, but just wanted to verify).
 

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

RE: 11kV Motor failing to start

"unless the CT's which are used to provide phase current are the ones on the neutral side (vs wye?)"
should have been
"unless the CT's which are used to provide phase current are the ones on the neutral side (vs line?)"
i.e. does the phase current indication that you provided come from the neutral side CT's or the line side CT's?

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

RE: 11kV Motor failing to start

Another thing that doesn't add up... the large differential current in phase A needs a return path either as differential current in another phase or as ground current... the other phase differential indications and the ground current indication show much lower magnitude than the A phase differential current.

The ground current does not resemble the sum of the three phase currents. How is the ground current signal developed?   And from which end of the winding (neutral or line)?
 

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

RE: 11kV Motor failing to start

At the beginning of normal start, total (internal plus external) R2 <= |X1 + X2|  (that is how we intend to set it up, right?)
|Znormal_start| = sqrt(2*(X1+X2))^2 = sqrt(2) * X1+X2

With resistors fully shorted out, R2 >=0.
Zshorted_minimum = X1 + X2   

So the difference in starting stator current is at most sqrt(2)=1.4 if we have all or none of external resistinace in place, right?  But we have somewhere around 2.5 difference.  I think (although haven't confirmed) that the difference can be higher when there is an unbalance on the rotor side (?).  Certainly the presence of 100hz in the Id as well as the A-phase differential current seems to suggest a side rotor unbalance.   

If you haven't had a tester in since the differential trip, I'd think it is time to get him back in for thorough check of everything: stator, rotor, rotor resistors/controls, possibly stator CT's. One scenario is that you've had a lingering rotor-side problem which causes intermittent failure to start (either due to intermittent nature of rotor side fault, or due to fault which places starting torque just on the brink of where it needs to be to breakaway the machine, so it doesn't stop it every time)... which eventually took it's toll and led to a serious stator fault.
 

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

RE: 11kV Motor failing to start

I would suggest that the two most probable causes may be Lionel's suggestion that the LRS may not have been in starting position and dry bearings.
An hour for the pump seems excessive. I would expect that minutes may suffice.
How long will the motor be inched? We needed 20 or 30 turns to lubricate the bearings. If inching is very slow, it may require more turns.

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

RE: 11kV Motor failing to start

(OP)
The motor was tested after the first two failed started but before the last one we had a few days ago. The plant was installed around 2002, this motor was changed out around 2 years ago. We have had three failed starts, but other than that we havent had any other problems with this motor.

Im not a 100% sure how long it is inched, but around 30-40mins seems around the number, which is around 2 revolutions of the mill i believe. That would mean around 200 revolution of the motor. It seems like if there was some bearing bind or resistance we would see it in the inching motor which is i think 150kW (its the weekend so i cant check!). You can see the current on the motor change with the charge moving so its quite responsive to load. In saying that the inching current is not displayed so unless it tripped no one would notice high current draw.

Its funny you made that comment Lionel because this LRS is unique to our LRS fleet in that it is the only one where the LRS electrodes stay at the bottom and only return to the start when the motor stops. On its own that should not cause any problems but it does hint that there may be a gremlin hiding somewhere there.

I think a full inspection of the LRS may be in order. Check all of the proxies, connections and other gear inside and outside the tank. A change of LRS liquid would probably rid the myth onsite here that the sodium carbonate is falling out of solution as well.

RE: 11kV Motor failing to start

Dear all,
I've experienced a similar incident with a 6MW-6kV-6p wound rotor motor before.
We received that motor with some damages in the rotor bar connections due to some explosions.
Our customer also reported of some failed starts, happened randomly in a long time period before motor breakdown. After each failed starts, they'd checked the motor&LRS carefully but found nothing wrong. And the next starts were all OK, until the breakdown at the last start.
You all know that rotor windings mainly are wave type, consist of rotor bars formed and connected at two ends which we called rotor fins (see attached).
Examining that rotor, we found that there were some kind of arc appeared between some adjacent fins, which belonged to two different phases of rotor. It means there was a short circuit between, for example, phase A and phase B of rotor winding.
And we presumed that before, sometimes an electric arc also appeared at that point at starting time, but not to the level of causing any destruction to the winding. Just caused the motor a fail start.
To conjure an arc between two adjacent rotor fins (belong to different phases), there are some conditions:
-There are some damages inside the fin insulation.
-The moisture and/or dust buil-up in the space between the fins reach a certain level.
-Voltage and, especially current, high enough to create a strong magnetic field that could ionize the air and trigger an arc.
And when an arc appears, it connects two phases of rotor, and causing stator current unbalance, high starting current, rotor at a stand-still, etc. After its disappearance, everything might return to normal, and testing of the motor found nothing wrong. And it doesn't extinguished abruptly after CB cutout, so due to some inductions there's still some residual current inside stator winding.
And my advice for you is that you should take your motor to a qualified workshop as soon as possible for rotor winding check-up.
 

RE: 11kV Motor failing to start

Good comments.

I have been thinking about the way that IA and IC are equal/opposite decaying to zero.. and IB was zero at the time of termination.

I have described a model of a very simple model of a CT here:
thread238-217182: ability of clamp-on probe to see exponentially-decaying dc current

Attached I have applied the same model to 2 different scenario's of current dropping suddently to zero: one scenario is that we have a dc current that drops to zero, the 2nd scenario is that we have a sinusoid dropping to zero.  In both cases, you can see that the the result is a CT output which jumps to the opposite side of zero and then decays away.

The whole calculation is done with Laplace Transforms, so if you are into that you should be able to follow it.

But if not, you can still scan the 4 graphs to get the big picture of the calculation.  
Graph 1 - Primary DC input which drops to 0 at t=0.01
Graph 2 - Secondary output for the input described in graph 1
Graph 3 - Primary AC cos input which drops to 0 at t=20.004 seconds (not a natural current zero)
Graph 4 - Secondary output for the input described in graph 3

If anyone is interested, I tried adding a secondary inductance into the model, but it had no effect on this overshoot when L2<<Lm, so I took it back out to keep the model simple.

The bottom line, I am thinking this equal/opposite decaying Ia and Ic might be an expected result of chopping a sinusoid at somehwere other than it's natural current zero.

Then again, I am still unable to explain the differential current (unless there is an actual stator fault).

We don't have digital relays, and I have never had the fortune to monitor current during motor trip (plenty of starts, but don't know when the trip will happen), so I have never seen this. I am curious if anyone else has seen this as a normal expected CT response, or for that matter anyone has comments on the analysis attached.

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

RE: 11kV Motor failing to start

There was an error in what you saw displayed. I changed the paraemeters R2 and L2 but didn't re-execute the plot, so the last plot you saw didn't match the R2/L2 parameters.

Attached is a revised version, freshly executed the whole thing so all plots are updated correctly.

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

RE: 11kV Motor failing to start

Now a different subject.

I presume that some of the external rotor resistance is decreasing during the time period of the waveform you provided, correct?

I am not sure if it makes a huge difference, but my intuition is that what we are seeing is an open circuit in one phase of the rotor circuit (rather than shorted external resistor in one phase).  The reason is that the Iq current shows increasing 2*LF = 100hz over time. That suggests an increase in imbalance over time (which would correspond to initial open circuit at inifnite impedance... other phase resistances get farther and farther away from that as the external resistance decreases) Rather than an initial short circuit on one phase external resistor (which corresponds to low initial resistance on defective phase, other resistors get closer and closer to that as the other external resistances decrease).


If I get energetic, I will try to do a numerical simulation of one open phase of a wye-connected rotor, to see if it acts like your waveforms.

The other much less likely alternative to explain apparent increasing rotor imbalance might be that you have shorted rotor turns and they are continuing to short more and more during the short... but doesn't seem likely that you would continue to operate long in that scenario.

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

RE: 11kV Motor failing to start

How about a position switch on the LRS and a latching relay to indicate if a start was attempted with the LRS electrodes in the wrong position?

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

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