motor stopping rapidly during a fault
motor stopping rapidly during a fault
(OP)
We had a 500hp hermetically-sealed centrifugal chiller motor trip on instantaneous current 15 seconds after starting.
Normally I would think instantaneous trip is due to electrical fault, not mechanical origin, however...
Personnel present reported that the machine stopped very abruptly <1 seconds after the trip, whereas it normally takes 10-15 seconds to coast down during normal shutdown.
It occurred to me maybe there is a higher different load torque imposed by the machine at the time of trip during shutdown. When I talked to the chiller mechanical engineer, he said the torque should be roughly the same.
Is it possible that certain fault currents can act similar to dynamic braking to stop motor quickly?
(I kind of doubt it... suspect other simpler explanation... just wanted to ask about this possibility)
Normally I would think instantaneous trip is due to electrical fault, not mechanical origin, however...
Personnel present reported that the machine stopped very abruptly <1 seconds after the trip, whereas it normally takes 10-15 seconds to coast down during normal shutdown.
It occurred to me maybe there is a higher different load torque imposed by the machine at the time of trip during shutdown. When I talked to the chiller mechanical engineer, he said the torque should be roughly the same.
Is it possible that certain fault currents can act similar to dynamic braking to stop motor quickly?
(I kind of doubt it... suspect other simpler explanation... just wanted to ask about this possibility)





RE: motor stopping rapidly during a fault
An external fault that didn't open a device would make the motor decelerate very quickly, because it would be supplying power into the fault.
If the motor starter or short-circuit device opened under a fault, there would then be no current flow to create any kind of braking. Unless there was a fault in front of the opened device, or some other path for current, like capacitors or resistors?
RE: motor stopping rapidly during a fault
Could it be due to some high back-pressure condition on the compressor??
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
Well, there is more information. Let me start again from the beginning.
While starting the motor following chiller maintenance, the motor ran for 15 seconds and then tripped (all three phases instantaneous). At the moment of the trip there was also a lound bang heard, and the normal decelleration whine was not heard.
Motor was meggered. The term box smelled, one pair of leads was open, all three leads had ground. Motor is definitely bad. But not the end of the story....
Because of the abnormal de-acceleration, we disassembled to the point where we could gain access to the shaft. It is a screw-in connection between motor and compressor shafts. When motor transmits torque to the compressor it acts to tighten that screw connection. It was found totally unscrewed.
Compressor was verified to be capable of rotating freely. We did not get a chance to check free by-hand rotation of the motor yet based on our limited access.
Motor has not been determinated in years, so there is no chance it was connected backwards.
So we have two firm symptoms: motor bad, coupling unscrewed. In addition a band at the time of trip, and lack of decelleration noise.
Under what scenario do those symptoms fit togehter? I have some ideas and we are continuing to troubleshoot but I'm interested to hear your thoughts.
RE: motor stopping rapidly during a fault
"in addition a bang at the time of trip..."
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
That gentleman was pretty adamant that a very common cause of shafts unscrewing is momentary interruption and restoration of power (bus transfer/reclose). Worst case in the neighborhood 3-10 cycles or 0.5-1.5* open circuit time constant. We all know that can result in large transient torques capable of breaking shafts. It makes sense they might momentarily be reverse.
IMHO, that reclosure scenario is irrelevant for us. It applies most directly to people fed from the distribution circuits where there are reclosures. We live in a power plant connected directly to 345kv breaker-and-a-half switchyard via transformers with no auto reclose or transfer on any of those breakers.
Here is my theory: Something got caught in the air gap between rotor and stator and caused both (1) – rotor to stop suddently and (2) – stator winding damage. Maybe that something was a rotor bar coming out of it's slot and catching on the stator.
When the rotor stopped abruptly (1), the tendency of the compressor inertia to keep the compressor spinning caused the unscrewing action. When the stator winding got damaged (2) there was instantaneous trip all three phases and severe damage indicated by open circuit measured later.
There has been no further inspection done beyond what I described above. But we have made the decision to rewind the motor and rebuild the compressor (possible compressor damage in many of the scenario's…. tough to rule out without disassembling). Next step is to build the rigging for removal which actually will take over a week due to resources. I don't expect to have any more info until then…. including no ability to check motor shaft for binding.
So plan is set, motor will be rewound, no further problem to solve until we see actual physical evidence. In the meantime it is still interesting to speculate. Does my scenario make sense? Other scenario's?
RE: motor stopping rapidly during a fault
All standard equipment is screwed in a manner that is tightened by normal forward motor torque. In order for torque associated with the load to unscrew the connection, the load torque must push in the normal rotation direction, not the reverse rotation direction.
An experiment to prove it's true? Hold a comb in front of you in both hands.
Comb represents shaft.
Right hand represents motor.
Left hand represents load/pump.
"forward" direction is comb rotating away from you on top.
"reverse" directon is comb rotating toward you on top.
Now try applying forward torque with the right hand (motor) and restraint with the left.
Compare to applying reverse torque with the left hand (pump) and restraint with the right.
The comb deforms the same either way. If there were threaded connection in the middle it would tighten either way.
Now apply forward torque with the left/pump and restraint with the right. The comb deforms opposite direction (opposite from normal operation torque applied with motor/right). Likewise when you apply reverse torque with the right/motor, the comb deforms in a direction opposite normal.
I think the last scenario is what we had. Motor bound up possibly due to rotor bar catching on stator and applied reverse torque causing unscrewing. If it was just the bearings binding, there would be no reason for a simultaneous electrical fault on the motor.
Sorry to go at length into the unscrewing thing. We talked about it a lot today and it's very fresh in my mind. I think most people would jump to the wrong conclusion based upon intuition initially.... i.e. assume that pump needs to push reverse to cause unscrewing. That is not the case. The pump in danger of unscrewing is the one which has higher external pressure applied to it's suction than it's discharge.
RE: motor stopping rapidly during a fault
There are larger than 500hp hermetic-refrigeration units, but they can make an expensive mess with a stator electrical failure in refrigerant. Seems like putting a lot of eggs in one basket.
I can’t add much advice to the situation, but if there are any sort of warrantee issues with the equipment, I would want to see one of your guys present when the rebuilder first breaks it open. Something mechanically seized associated with short-order winding-insulation failure is a strong possibility.
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
I struggle with the question of whether a fault at the motor leads or any point inside the motor would cause rapid deceleration.
I am skeptical of that scenario. But I would be interested to hear any more thoughts on it or whether anyone has seen that effect in action.
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
Folowing your last lead regarding the abrupt stop being caused from possible rotor bar problems.The rewind shop should be able to have a very good idea of what the problem was once they take it apart.
Are you thinking of overhauling compressor because of the
motor problem or is it time for an overhaul?
I am curious to know what the motor shop comes up with.
Regards
GusD
GusD
RE: motor stopping rapidly during a fault
So I go though the thought excercize which I think is interesting. Entering a guess is free, and if the motor tear-down proves you right, then you have bragging rights to say you knew it all along
Compressor was overhauled 10 years ago but that is the most recent of any of our 8 compressors (20 year cycle)... so it is not exactly due for overhaul.
Compressor teardown is based upon strong recommendation from Y*rk and a group decision-making process from the people in the room at the time. One aspect is that if my scenario holds true, the axial movement of motor or compressor must have been at least 1/4" to unscrew. Normally motor is free to move axially but not clear if it would be during the jammed rotor bar or other scenario's. If the compressor moved more than 20 or 30 thousandth of an inch axially, it's thrust bearing is probably damaged.
We have no easy way of checking the thrust bearing. Don't have motor available to run to check the compressor. Waiting for motor rewind to find out that the compressor is not good would set the schedule too far back. With the extent of disassembly necessary to inspect the thrust bearings you might as well do an overhaul. Also there is a degree of uncertainty do we really understand the problem and will we understand it much better in the near future.
RE: motor stopping rapidly during a fault
Did the motor bearing(s) seize causing abrupt stop ?
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
If the bearings had seized at the time of motor start, then definitely overcurrent trip will occur due to almost locked rotor condition. If the windings were old and if the stator current were high, then stator winding could have been damaged even leading to earth fault. Was the overcurrent relay calibrated in the recent time to check its operation ?
RE: motor stopping rapidly during a fault
The fact that we found the motor actually bad by megger (ground) and continuity check(open) confirms there was an actual winding fault.
RE: motor stopping rapidly during a fault
Seems to me that if you had a solid fault between two of the motor leads or windings, you'd essentially be feeding the same voltage to two windings, and therefore about the same current too, leading to a high level of zero-sequence current. Wouldn't that tend to brake the motor? That would also give you instantaneous flags on at least two phases.
Is the motor wound star? If so, then a three phase fault would seem to be an even more effective brake and would indicate inst trip on all three phase flags.
If it's wound delta, it seems a bit more unlikely that you'd show overcurrent on all three flags -- possibly if a three-phase fault occurred in the middle of the windings.
A mechanical cause located within the motor windings would seem probable, as an instantaneous three-phase fault would otherwise be rather unlikely.
Am I missing the point? If you take a three-phase motor and run all three leads to a single-phase source, won't it tend to brake the motor?
RE: motor stopping rapidly during a fault
You may be right. None of those conclusions are obvious to me. Why would zero sequence act like a brake.
I guess I can analyse the simplest of possibilities.
Short all three phases together. The motor acts like induction generator feeds the fault. In the absence of any substantial resistance in the circuit, fault current limited primarily by winding inductance. As resistance in the circuit appraoches zero, real power dissipated in resistance is zero, no slowing effect on the motor. I think as we increase resistance the max power transfer to that resistance would occur when resistance equals magnitude of inductive reactance. That is based upon steady state ac… not sure how it applies in this transient scenario.
Arc in non-linear. There is energy dissipated with the arc and the damage that is sometimes seen… but usually we assume most of that energy comes from the power system which supplies a constant voltage. But a machine could act a little different. Voltage in the machine decays in proportion to product of flux and speed which are both decaying. As speed approaches zero, voltage approaches zero, and the slowing torque would decrease. If anything we might perhaps an exponentially decaying type behavior.
That’s about as far as I can get thinking about the theory. Maybe someone can take it farther?
The simple fact is that I have never heard of machine coming to an abrupt start in the presence of any kind of electrical fault (other than perhaps out-of-phase reclosure or phase reversal mentioned). That’s what makes me skeptical. But interested to hear more.
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
Does the motor turn free? Guess this is a stupid question, but could the windings burn into the rotor causing a mechanical drag? Phase to phase shorts seem to get violent.
I didn’t read all 20 some post (sorry.) You mentioned something about breaking. Could the problem be something within your starter? I’m guessing a soft-start or VFD. In some cases with my deepwell pumps, if a power company has an auto-recloser that operates faster than a starter relay can drop out we get some very strange and devastating torque loading. It will normally break shafts for us when we get out of phase with the power system.
D23
RE: motor stopping rapidly during a fault
DOL start. In our power plant environment, we are not subject to auto-reclose. (an advantage of direction connection to transmission system vs distribution system).
Do not have access yet to check motor rotating by hand, but we will be able to check that soon.
RE: motor stopping rapidly during a fault
may be I am confused. Does your motor have instantaneous overcurrent or inverse time delay overcurrent protection ?
In any case, once the power is lost (assuming your OC relays tripped the breaker), there is no question of any current (retarding or otherwise) flowing thru the winding to stop the machine abrutply.
My bet would still be on siezed bearing(s).
RE: motor stopping rapidly during a fault
We tripped on instantaneous.
Even after the power is removed, if fault were for example in the term box, the motor would continue to feed it like an induction generator, producing the counter-torque described above. I still think this generator effect does not result in an abrupt stop.
RE: motor stopping rapidly during a fault
since induction generator works at speeds above synchronous speed, I do not understand how you could get induction generator effect on an unpowered motor.
RE: motor stopping rapidly during a fault
There is initially a flux before the power was removed.
Then we apply a short circuit to the terminals and immediately open the breaker.
That flux decays away in according to the motor "short circuit time constant" defined in NEMA MG-1.
While the flux remains, there is a voltage induced at the terminals proportional to speed times flux. That voltage produces a current flow which will create power dissipation in the resistive portion of the fault and winding. The power comes from slowing down of the machine.
That's the scenario which leads me to believe there may be some countertorque. (If it's incorrect let me know.) Still I have never heard of a machine coming to an abrupt stop due to a fault.
RE: motor stopping rapidly during a fault
I think bearings are gone,
so rotor may go to resonans
frequency and in certain moment hit stator.
It may be in X or Y axes.
Then you have "bang",
instant decceleration and
protection trip on all 3 phases.
Thanks,
Vladimir.
RE: motor stopping rapidly during a fault
Well, DC is used to brake motors -- wouldn't zero-sequence AC do about the same thing? Won't the rotor try to match the rotation speed of the stator field, which with zero-sequence would be zero rotation speed?
Or is my whole concept off, if you put single-phase power to all three phases of a 3-phase motor, will it just tend to run like a 1-phase motor?
RE: motor stopping rapidly during a fault
I like your experiment scenario... except for a slight flaw .. presumes experimenter has hair.
RE: motor stopping rapidly during a fault
Sounds like you have a number of scenarios, I like the rotor bar possibility and a visual inspection will help to qualify this one.
As a suggestion, I would promote tha potential for the breakdown to have been stimulated by a momentary disconnection on the supply. We know that this can cause a major current and torque transient. The torque transient could very easily have been a negative torque and the forces around the terminations due to the current transient could have stimulated an insulation breakdown as well.
How about this suggestion, one or more switching activities causing momentary loss of supply, auto reclose effect causing major negative torque transient and current transient causing insulation breakdown. Torque transient braeks the thread bonding allowing the motor to disengage from the load and the insulation breakdown causes an effective short on the stator windings which would act like an electrical brake. This could also be accompanied by a thrown rotor bar as well. I would seriously consider a supply interruption being the initial stimulus.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: motor stopping rapidly during a fault
My guess is that when you open the motor up you will find a piece of the end-casting/fan of the rotor wedged in the air gap and in the end turns of the stator winding. Here's my sequence of events/reasoning:
1. Motor gets up to speed in less than 15 seconds,
2. A piece of the rotor fails (stuff happens),
3. Rotor is instantly unbalanced and contacts the stator causing the loud bang,
4. The flying piece(s) finds the end turns, tearing into them and/or the laminations,
5. Some end turns are torn apart, some are shorted together, some go to ground,
6. The instantaneous relays open because of all the fault current flowing,
7. The motor doesn't decelerate because it can't turn due to the pieces that are now wedged between the rotor and the stator,
8. The compressor rotor unscrewed (luckily) because it had inertia from the 15 second runup.
The scenario of an external electrical cause is unlikely unless the motor controller opened and reclosed within 3-5 cycles (improbable based upon the design and age of the switchgear).
RE: motor stopping rapidly during a fault
Most people are connected to the distribution system which is subject to reclose.
RE: motor stopping rapidly during a fault
Checking out the compressor (mechanical) is duely warranted because some mechanical misalignment that may have caused rotor to sag. There may be a number of reasons. Hermatically sealed motor do not permit periodic checking of rotor gaps or bearing like a stand alone motor so you have no warning.
RE: motor stopping rapidly during a fault
Motor sat idle for 6 weeks with no refrigerant applied in a covered but non-air-conditioned building... until we were able to pull it out and look at it in shop today.
Observations;
No sign of any rub between rotor and stator.
No sign of any bearing problem.
Motor meggered 10,000 megaohms. Motor has only three leads... => could not easily separate phases for separate test.
Connection end was black with soot.
Identified 4 different coils on connection end that had apparent fault from top coil to iron, right at the point where coils exit the slot. Shop is determining which phases they belong to and what position with respect to line.
End-turn lacing and blocking in-tact. No cracking. No sign of any excessive movement had occurred.
Massive amounts of rust on rotor and stator core and stator frame. Green corrosion adjacent to four fault areas.
Analysis:
Why did stator fail simultaneously at 4 different locations? Possible scenario: slug of water in the refrigerant. Hit the motor shortly after start causing multiple faults. (note motor tripped 15 seconds after start... instantaneous on all three phases).
What was the course of all the corrosion: may have been long term moisture presence. But not likely that corrosion buildup due to long term moisture caused this simultaneous fault at all locations. More likely scenario is that the corrosion occcured over the last 6 weeks.
Why did the coupling unscrew?
We have ruled out mechanical binding of motor.
As explained above I firmly disbelieve our power system is subject to intermittent interruption which can cause transeint torque.
The only explanation left is the one proposed by Dan and others... large fault current created large torque on motor acting as a generator. We know we had high fault current due to instantaneous trip on all 3 phases... although not high enough to leave any evidence of coil movement. I am reluctant to embrace this explanation, but all the others have been ruled out.
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
4kv system hi-R grounded thru 240-ohm giving max ground fault current of 10A.
For all three phases to trip we must have had phase to phase. I will have to think about where that may have occurred.
And if ground fault was the first fault and later escalated to phase-to-phase... why didn't the ground fault trip at all during the entire event? Ground fault is identified as 51G so I think there is some time delay.
I will review the settings and provide some more info if I can.
RE: motor stopping rapidly during a fault
For all three phases to trip on instantaneous we must have had phase to phase
RE: motor stopping rapidly during a fault
1. When was the last time the refrigerant quality was checked?
2. Was this the first start after any type of maintenance on the refrigerant circuit?
3. Is it possible to determine the direction of refrigerant flow; i.e., is it motor first then compressor or vice versa?
One possible scenario is the slug of water knocked off two birds with one stone- it caused an end-turn to end-turn flashover then traveled to the compressor where its incompressability caused the rotor to stall. The stalled rotor would have forced a kickback through the shaft causing the motor rotor to instantly reverse and unscrew from the compressor shaft. The problem I have with this scenario is I would expect to find plenty of internal damage in both the motor and compressor, yet you say that both shafts turned freely. Suggestion- have a sample of the green corrosion analyzed, it may yield a clue as to the source of the contamination, also watch as the windings are removed to see if there was a long term build up moisture in the slots. Please keep us advised.
RE: motor stopping rapidly during a fault
green corrosion I am fairly common is copper oxide from copper and occurred after the fault. This seems well supported by the fact that the green corrossion is localized only at the fault.
We also have red rust corrosion accross the entire stator core, rotor core, and stator frame which I assume is something like iron oxide.
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
Green copper-oxide formation may have been a result of the original fault. Often refrigerants will form hydrogen fluoride and hydrogen chloride in arc presence. Six weeks of air exposure may have introduced moisture to form hydrofluoric and hydrochloric acids, which may have reacted with winding enamel.
RE: motor stopping rapidly during a fault
It seems to me that this can occur (and pete's experience backs this up), but I am still not sure of the exact mechanism.
RE: motor stopping rapidly during a fault
I take it that the chiller being discussed is a low pressure R11 machine. Has a sample of the refrigerant been taken and sent out for analysis? Moisture content, total acid number and high boiling residue are 3 things that may indicate ongoing issues such as low side leaks (vacuum drawing in moisture laden air) or excessive oil in refrigerant.
Total acid number will indicate a long term moisture problem.
High boiling residue...has someone been adding oil to this machine due to "oil loss"? Oil is noncompressible and seems more likely than a slug of water.
RE: motor stopping rapidly during a fault
No sample was drawn after the fault. By the time we inspected and started thinking about moisture scenario refrigerant was gone and opportunity was lost.
RE: motor stopping rapidly during a fault
high boiling residue is the percentage of (usually oil) liquid left from a weight of refrigerant analyzed by a lab set up to do this. If you evaporate all the refrigerant off during a test like this you would have 0% HBR. Usually depending on how well loaded the machine is you might find 3-5% HBR. A heavily oil foulded machine will have poor boiling action, poor heat transfer and if severe enough will carry over waves of oil rich refrigerant. Look in the sight glass if you have one. Badly fouled refrigerant looks like dark beer.
RE: motor stopping rapidly during a fault
RE: motor stopping rapidly during a fault
In this case, since it involves a piece of equipment I am more than fairly familar with, I feel compelled to ask the following as well as interject some thoughts.
1- It seems to me that someone failed to mention that this unit evidently has one of the old forced commutation VSD's on it called the "Turbo-Modulator".
2- When applied to a "Mono-Shell" or "Screwed-Compressor" unit, an additional device was installed @ the output called a "Vacuum-Switch".
During a catastrophic Fault, the output SCR's were "ALL" fired Which would result in almost immediate stopping of the motor thru some serious braking. To "Prevent" the compressor shaft(s) from unscrewing at any 1 the 4 screwed components which "Can & Will" result in extreme damage to the compressor, a Vacuum Switch was installed to disconnect the motor from the Drive in <1 milli-sec. or 1/10 of a Hz.
It is apparrent that the 1000volts that is stored in two capacitors and triggered to dump the vacuum switch was never applied, for whatever reason.
However....if this unit does not have the "Turbo-Modulator" installed as I stated, then my entire post is a mute point.
RE: motor stopping rapidly during a fault
Motor is turned on and off by cycling a breaker which applies 4kv direct to the motor. No turbo-modulator, no vacuum switch in the circuit.
I am not much familiar with compressors and certainly not familiar with the features you describe. For our unit, normal motor torque tightens the screwed coupling. Sudden motor stopping from dynamic braking or other anything else would tend to unscrew the unit to my way of thinking.
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