Burned out new 340 hp motor 2

[pbcd]

Installed a newly rewound 340hp motor on our air cond. chiller unit about a month ago. Ran chiller for about two weeks. Encounter no problems. Today, I switched on the chiller and after about 4 min. of running, the motor burnt out.

Question: The chiller is designed with a solid state starter. Could the starter be the cause of the burn out? I don't want have the motor rewound and have it burn out again.

 

[itsmoked]

Wow that is painful!!

Seems to me you need some serious protection installed.

Single phasing is a common killer of refrigeration motors.
It can also be caused by faulty overload safeties, loose connections, bad contacts, faulty semiconductors in a soft starter. Could also be a bad rewind. Water.

Why did you have to replace the motor the first time?

Others will have further ideas.


Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[Artisi]

What does motor "burnt-out" mean.
Please describe the damage - ie. 1, 2 or 3 phases destroyed, what type of failure etc.

Why didn't the protection gear catch it ?? unless itwas a major winding fault.
But need more info.

Naresuan University
Phitsanulok
Thailand

 

[pbcd]

Thank you for your reply.

The SS starter has numerous safeties such as half phase, phase loss, phase rotation, overload, SCR temp, OOL (?) and several others that does not come to mind.

Single phasing and overload should have been caught by the starter's safeties. I just installed the motor and had no loose connections. "faulty semiconductors in a soft starter", wouldn't you have to lose both half phases in the same phase and at the same time (since the half phase safety did not fault) for the SS starter to cause single phasing? Wouldn't moisture have been an issue in the previously years when the winding insulation was very poor? The motor was rewound since it grossly failed the megging.

 

[stardelta]

You need to involve the company who carried out the rewind. Ask them to examine it and report back to you, it could well be due to an error on their behalf.

 

[electricpete]

The possibilities are wide open. Inspection is a great start. If possible send someone independent to look along with the motor shop (trust but verify).

Just guessing from very limited inof, it smells like a moisture problem to me. You had previous bad meggers on the old motor. Failed shortly after start. Did you ever figure out the cause. Outdoor motor? What enclosure type? What space heaters?

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[petronila]

Hello pbcd,

If this is a chiller´s motor I think is a special motor called hermetic, this kind of motors are mounted hanged on the chiller(are not a external motor´s) this motor are not fan ventilated and are cooled by the same refrigerant, due to the motor´s design are smallest than standard motors.

If the above is your case, ask to the shop if they was made a VPI Process to the stator winding is not this is the failure cause.

Due to the motor´s small size, this kind of motors experiment more mechanicall stress by magnetic forces and is necessary to add strenght to the windings, take acount too the shop need to use hermetic materials in order to wound the stator.

Regards

Petronila

 

[rbulsara]

What was the reason for original replacement?

 

[waross]

rbulsara

The motor was rewound since it grossly failed the megging.

pbcd;
Re petronila's post;
Is this a semi hermetic or an external motor?
yours

 

[jraef]

To address your question about the SS starter causing the damage, you are correct in thinking that you must have SCR failures in 2 phases in order to have an off-state current path through the motor windings. Almost all SS soft starters include shorted SCR detection, but many only prevent restart. You must have a "Shunt Trip" feature that will open an up stream device such as a shunt trip on the circuit breaker in order to prevent single phase power flow damage to the motor.

In your case however, since you were able to start it prior to it failing, this was most likely NOT the case, otherwise the SS starter would have locked you out from starting at the very least (there is no way of knowing if an SCR is shrted when running, and it makes no difference anyway). The only possibility is if someone disabled the Shorted SCR protection feature and it did not have Shunt Trip protection. Post your SS starter brand and model and I can tell you for sure what type of protection is has and how to tell if it was disabled.

Once your motor is at full speed, the SS starter is really no different that any other starter, in fact it most likely has a bypass contactor anyway. I seriously doubt it had any role in the motor damage.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[pbcd]

Additional info:

"burnt-out" - motor sparked and smoked

Motor shop will open motor and inspect in a few days. Not sure how conclusive the findings will be.

This is a York chiller with an open motor (external) and is located in a machinery space. No space heaters - Hawaii.

SS starter also made by York. Will obtain mod #.

No bypass contactors are used.

jraef: I was under the impression that in order to single phase, both SCRs would have to fail on the "same" phase?

Also posted my question in the Solid State forum.

Thanks guys!

 

[petronila]

Hello pbcd,

The failure most be analysed in detail by the repair shop,the electric motor´s failures are consecuence of many factors: electrical, mechanical, enviromental, missalignment and mounting failures,power quality,etc. For it the shop most be conclusive with the root failure cause sending a complete technical report including all mechanical fits at shaft and bearing housings,stator loop test, bearings diagnostic and supporting all with pictures.

The most failure types are ilustrated in detail by the most OEM or repair shop and this pictures will give you a forensic idea. This link could help you:

http://www.firstelectricmotor.com

(Look for motor´s failure).

Regards
Petronila

 

[LionelHutz]

I can't see how shorted SCR's would let you turn-on the chiller and then have the motor fail 4 minutes later. That really doesn't make sense. If there are shorted SCR's you pretty much know it when you turn on the main breaker.

In general, If you're close to the motor you pretty much know it if a soft-starter is acting up. The motor will sound alful and likely won't accelerate or accelerate really badly.

As the others have posted, get the motor analyzed first to see why it failed then investigate the starter if it's necessary.

 

[itsmoked]

You could have one SCR fail and still damage the motor after it was running with a non-bypass starter. Something like it's comming loose from its heat sink or a fan failure. But I admit it would probably be rare. I'm not sure if a controlled contactor or CB was in the system to drop out the motor if the SS cried foul.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

No space heaters on a large motor? Doesn't sound right. What difference does it make if you're in Hawaii?

If this is an open motor (vs TEFC) I would look very very closely at moisture. If TEFC, you may subtract one very.

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[waross]

If a weather system moves in with warm damp air, and a large motor has been sitting overnight you have a good chance of internal condensation. The motor rotor and stator and windings will be cool from the temperature drop at night. If a sea breeze moves in a mass of warm air with high relative humidity the dew point will typically be quite close to the ambient temperature. It is probable that the inner metalic mass of the motor will be cooler than the dew point of the incoming weather pattern. This will result in instant internal condensation.
Relative humudity is more important than temperature in regards to condensation.
About six months ago I got a flight out to a Caribean island. The plant operator reported that the strip heaters had failed on a 1.6 MW diesel generator. The utility general manager understood the possible consequences. I flew out in the morning, and spent an hour or so repairing a loose connection on one terminal of a strip heater. The heat corrosion had burned through one of the wires. I repaired everything and spent the afternoon sightseeing and visiting old aquantances. I had a good supper and a good hotel room. Flew home in the morning.
What electricpete said.
Hawaii is probably more at risk for condensation issues than most of Canada and the Northern two thirds of the U.S.
Machine rooms in Canada and the US tend to be enclosed and heated.
Machine rooms in the tropics tend to have wide open windows and doors and lots of flow through ventilation. Not always, but often.
respectfully

 

[jraef]

SCRs fail shorted (barring the unlikely occurrence of what itsmoked mentioned). So there is no difference in any phase between one SCR being shorted or both of them, in fact that is somewhat rare because as soon as one shorts, the other one has zero potential across it. One shorted phase has no current return path (assuming there is no ground fault) so there is no damage to the motor. If you get one SCR shorted in Phase A, and another in Phase B, then current can flow from A to B through the motor windings, and the only restriction to flow is the small amount of winging resistance. It's exactly the same as welding 2 contacts in a contactor. That means the motor can burn up when it is NOT running, hence the shunt trip feature of some soft starters.

The reason I doubted that the SS starter was your cause, is that the shorted SCR detection (which works by looking for a lack of a voltage drop across the SCR stack), would have prevented you from restarting that chiller had there been even one shorted SCR, let alone 2. The York SSS does have shorted SCR detection by the way, but not shunt trip. You are correct that they don't have a bypass contactor because they use a water cooled heat sink (shudder).

While I really think it wisest for you to have your motor professionally analyzed, here is a decent primmer on common modes of winding damage to see if it is anything obvious.
EASA winding damage failures

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[LionelHutz]

jraef, the only exception is the brick SCR's that can fail open if they blow apart. Usually, the smaller brick devices are the only ones that do this. Obviously, this type of failure is very apparent when looking at the starter. It's a very major fault to blow a puck type SCR out of the heatsink leaving the ciruit open but it does happen every once in a while.

The gate drive circuit could also have an issue and not fire one or more of the SCR's. This leads to at least 1/2 of a phase missing. Once again, this is a condition that is very apparent if you try to start the motor with the failure. If the motor is running it should stall or current imbalance bad enough that the protection picks-up.

 

[jraef]

LionelHutz,
Agreed, however remember this was a 340HP motor; bricks don't go that high. I have yet to see a completely evaporated hockey puck. I've seen the sides blown out but they were still conductive. Maybe I'm just lucky!

You're also right about the gate failure possibility, however rare that would be. It's highly unlikely that it would start with a failed gate, but if it failed while running that would be another story because the unbalance and DC component in the windings would be very high, leading to rapid overheating that may not be picked up by the Overload Relay fast enough. I hadn't thought that through because I'm a firm believer in bypass contactors, in which case you never know if a gate failed while running. Still, 4 minutes to failure? That's extremely fast for a motor that size.

It still seems like an unrelated motor failure to me.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Marke]

I have come across soft starters where there has been an intermeittent in the firing circuit. This has cuased one SCR to intermittently not conduct.
The result of this is one phase running half wave and effectively injecting DC into the motor windings. The high increase in current results in sever vibration in the motor and the breaker on the line side trips. I have come across a batch of faulty H Puck SCRs where the gate became intermittant when the puck was up to temperature. The starter would start fine (except for high inertia starts and then play up after a period of operation. I had to heat the SCRs up to prove that the gate was internally intermittant. - never lost a motor due to this, always tripped the starter on imbalance, or tripped the breaker.

The standard shorted SCR failure would not cause an issue with the motor while running. If the SCR failed before the start, the motor either would not start due to the protection, or would give an obvious very rough start and probably trip some protection somewhere.

Best regards,

Mark Empson

http://www.lmphotonics.com