motor stopping rapidly during a fault 16

[electricpete]

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)

 

[electricpete]

Our instantaneous relays are set above LRC to avoid tripping during starting. Therefore, we would not expect them to trip as a result of locked rotor conditions. We would expect our time overcurrent relays to trip if there were locked rotor conditions, but they did not.

The fact that we found the motor actually bad by megger (ground) and continuity check(open) confirms there was an actual winding fault.

 

[peebee]

I'm not quite catching the point of confusion, pete (although I admittedly know precious little about motor guts).

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?

 

[electricpete]

Thx peebee. It is connected in ungrounded wye (star).

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.

 

[jbartos]

Suggestion: The negative sequence or the direction of motor rotation (reversal, plugging), which is about the same thing, could cause the motor to stop abruptly.

 

[DanDel]

A motor feeding a fault will develop counter-torque to the direction of rotation, the same as a generator would. Either a generator or a motor feeding a fault will quickly decelerate.

 

[d23]

Pete:

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

 

[electricpete]

Thx d23.

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.

 

[edison123]

pete,

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).

 

[electricpete]

Our motor has instantaneous and time overcurrent.
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.

 

[edison123]

pete,

since induction generator works at speeds above synchronous speed, I do not understand how you could get induction generator effect on an unpowered motor.

 

[electricpete]

Let's assume the scenario:
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 &quot;short circuit time constant&quot; 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.

 

[plc123]

Hi,
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 &quot;bang&quot;,
instant decceleration and
protection trip on all 3 phases.

Thanks,
Vladimir.

 

[peebee]

Re: &quot;Why would zero sequence act like a brake.&quot;

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?

 

[Shortstub]

Electricpete,
I like your experiment scenario... except for a slight flaw .. presumes experimenter has hair.

 

[Marke]

Hi Pete

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

 

[RAMConsult]

Hi Pete,

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).

 

[electricpete]

Thanks Ramc. I agree with your scenario. Based upon familiarity with our power system which is connected direct (via transformers) to 345kv breaker-and-a-half transmission switchyard with no auto-transfer equipment between (only manual make before break transfers), there is simply no possibility of interrupt/reclose scenario.

Most people are connected to the distribution system which is subject to reclose.

 

[rbulsara]

plc123 and ramconsult's scenarios are most likely. This is clearly a case of mechanical fauilure internal to the motor leading to rotor coming in contact with the stator leading to rapid decceleration or bang, failing stator and rotor windings and instantaneous 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.

 

[electricpete]

UPDATE:

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.

 

[electricpete]

I'm sorry... motor meggered at 10,000 ohms (not megaohms). Clearly there was severe fault from the inpseciton.