Shop practices for unloaded motor run

[electricpete]

1 - Is there typically any motor protection (overload, ground fault) applied during a shop unloaded motor run? The particular setup in question has a rotary converter for "soft start" voltage ramp-up. Let's say that during voltage ramp-up the motor emitted smoke, was manually secured and copper beads and hard ground found. No automatic protection trip occured. Would you say the protection was lacking?

2 - What are typical precautions to ensure motor insulation is not moist during unloaded run (to avoid energization of moist winding causing failure). Is megger test always required or only when moisture is suspect? Would it be acceptable to load test a motor until 3pm one day, allow to sit overnight with no space heaters in indoor environment (not humid) and perform no-load test 7am the next morning without any testing (under the assumption that the winding has remained warm)

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[aolalde]

Hi Electripete.

The protection criteria for a test facility could be considered different that one applied to a single motor, due to the diversity of motor sizes tested.
However basic ground and short circuit relays should be installed for property and human protection.

A good trained electrician will never take for granted a good insulation condition of a machine before it is energized. As a minimum precaution the insulation resistance per IEEE STD 43 should be satisfactory, before attempting to energize a machine.

 

[acmotorengineer]

I agree with aolalde - all incoming motors coming into our shop are checked before running whatever the reason for repair, even failed motors.

Good insulation systems should be able to safely start even in a moisture rich, overnight, enviornment.

However, I've had failed motors pass a "meggar" test.

The way I look at it, the no load test is just another insulation test. If it failed, it would have failed anyway, and maybe after the expense of installing the motor

 

[electricpete]

Correction - megger test was performed and results >4,000 megaohms.

Do you consider space heaters unnecessary in an indoor warehouse environment (non-air conditioned) as long as megger test passes? Or not?

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[electricpete]

Here is a description of the sequence of events:

==============================
Load test on Tuesday.

First thing Wed morning, the unit was uncoupled from gearbox in preparation for uncoupled run.

Motor casing was noted to be warm (from previous day's run).

A winding insulation test (megger) was then performed (at 1000 VDC), with satisfactory results (>4K MegOhms); the winding resistance was found to be 408 milliOhms for all three phases, with ambient temperature recorded at 27.6°C

"Power was then applied to the motor, starting at 0 volts and increasing until current was at approximately 95 Amps and volts were approximately 1000 VAC, where the voltage remained for approximately 30 seconds until the unit reached operating RPM (890); at that point, the current had dropped to approximately 25 Amps. Repair Shop personnel then began raising voltage towards normal operating levels (4000 VAC). When the voltage reached approximately 3000 Volts, and current was approximately 30 Amps, it appeared that the unit began sparking from the lower end (pulley end) of the unit. The Repair Shop personnel immediately secured power to the unit."
===========================

One thing I have to check is what temperature does 408 milliohms correspond to.

I am not trying to blame the shop, but it is a big coincidence that the motor failed at the shop when it had previously operated ok at the plant for years... so I have to check all the angles.

One more aspect I'd like to explore is the starting method:

It's reduced voltage (but not reduced frequency) starting using a rotary converter.

One thing that occurs to me is if the voltage is increased too slowly, the motor may stall at a point where it cannot overcome friction and windage to get to full speed. I don't think it would stall at 1kv though. 1KV is 25% voltage which should give 12.5% torque. Assuming full-voltage breakdown torque is 200% of rated torque, then 1kv breakdwown torque should be 200%*12.5% = 25%. That should be able to overcome losses up to 25% of full load which I'm sure is much greater than friction and windage. So, it seems like 1kv is a good voltage but I should be interstested in how fast they increased from 0kv to 1kv? Does that sound right?

I don't know much about the rotary converter. I think it may be a wound rotor motor hooked up for variable voltage. Is there anything about the power supply or startup that might cause the motor to fail? Any checks that should be performed on the converter (I'm thinking at least a check for balanced voltage if not already done).

I'd be interested in any comments on how the power supply or startup might cause a failure.

I'll have a chance to visit the shop and see the motor on Monday.



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[mc5w]

At 25% voltage you get 0.0625% torque corresponding to a starting torque of approximately 12.5% of full load torque. Even with no load attached to the motor the motor would have an extremely hard time starting.

30 seconds at 25% voltage is an eternity from a motor's standpoint. Normally, when soft starting at full frequency the voltage ramp from zero to say 70% voltage is supposed to be very rapid to avoid cooking the motor.

 

[electricpete]

Thanks mc5w.

I was thinking that breakdown torque would be relevant in terms of whether the motor can get up to speed.

If we assume at least 200% breakdown torque at full voltage, then we have (1/4)^2 * 200% = 25% torque at 25% voltage. This should be enough to overcome friction and windage equivalent to 25% of full load. I believe typical friction and windage are in the neighborhood 10% or less.

From this standpoint, I don't see any problem with the 30 seconds at 25% voltage during an unloaded start.

But I have been wrong before. Am I wrong again?

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[electricpete]

I guess the other issue is whether the motor torque speed curve comes close to the friction and windage curve at any point during startup which would slow the acceleration.

I'll see if I can get a copy of the torque speed curve to see the lowest motor torque during startup.

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[edison123]

pete,

In the cash strapped days of our firm's start up in 70's, my father had rigged up 400 / 50 V step down trafo and used it to start LV motors (415 V). Almost all of them got up to speed at which point he used throw in 415 V. Now a days we use stepless auto trafo and the motors used to get up to speed at around 15 to 20% voltage.

Coming to your motor failing in the repair shop, I have had it happen in some cases where the overhauled motor passing megger, PI, surge etc. would go belly up on no-load run.

The worst I had was a 415 V generator which was overhauled, tested and passed the no-load run. The client commissioned two days later when it suffered serious interturn short and failed.

I still don't know what happened.

 

[electricpete]

Hi Kumar. Yes, it could be just one of those things.

Going back to whether 1kv is ok, I feel pretty confident it would be OK to start a 4kv unloaded motor at 1kv, based on the following:

The factory data for the motor shows total losses at full load are approx 5% of full load. Of those friction and windage are less than 10% of those losses, amounting to 2100 watts (0.35% of 590,000 watts full load output).

So the friction and windage amounts to a load equivalent to 0.35% (0.00035) of full. The torque capability is reduced to 12.5% (0.125) which is still an order of magnitude higher than 0.35%.

Based on the above, my main focus will be how fast did we get to 1kv.

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[stardelta]

Pete?
Any comments from the repair company? It would be interesting to hear what they have to say about the failure although my guess is they will claim to have tested hundreds of similar motors in this way and never had a problem. I am speculating and thinking maybe something has entered the casing or become disloged internally during removal and transport, then earthed the winding during the run up, especially with you saying the problem seems to be in the lower half of the machine. I have seen similar occurances many times when fitters/electricians accidently drop nuts and washers from the terminal block into the machine and choose to say nothing, always with disasterous results. However as I am sure you are aware, careful examination of the winding will give some indication as to the root cause.