2 Speed 2 Wind Motor Won't Cold Start After 9 Months in Full Operation 2

[GrimesFrank]

Could use some 3rd Party Expertise please.
New motor replacement for 30yr application (Circ Pump LP/HP); 60 Hz, 575V, 13.3/40 HP, 1200/3600 RPM, 17.5/37.4 FLC, Wye Motor
- In 30yr nary an electrical issue; only bearing failures.
- New motors run for ~9 months then 1200 RPM won't cold start; gets to ~100 RPM before O/L kills. Loud electrical noise (screech not hum)
- Can run with bump from 3600 RPM. No loud noise just normal starting harmonic.
- Cold Start has balanced Voltage/Current as measured by Baker/Meggar Explorer Dynamic @ motor (490 V RMS ph-ph and 45A RMS ph)
- When bump started; Starting voltage dips to 305 V RMS ph-ph but and SS(s) @ 340 V RMS ph-ph.
- When bump started; Starting current maxs at 34A RMS ph and SS(s) @ 2.4A RMS ph

I have a bit more data from the Explorer but I don't want to influence opinion. My rudimentary reading has formed an opinion but would like to hear from all.
What are your suspicions, can we expect 3600 RPM to fail too, what physically would you inspect and finally are there specific tests you would perform to confirm suspicion?

Thanks.

Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[GrimesFrank]

Going ask question to move topic along;
Can bump/rundown starting of motor overcome effect of Crawling (7th Order Harmonic)?
Can the Crawling effect come into play after limited use say if a degradation/damage to the rotor occurs?
Can the Crawling effect disappear after manipulation/re-execute of motor terminations?

Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[jraef]

Kind of sounds like a crack in a rotor bar or end ring connection. The higher torque from the numbbump at high speed gets hastpast the worst effects.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

Sounds like a rotor problem, assuming it worked well from the start.

If it never started right, then I'd say it is a bad stator-rotor slot/bar combination. Ran into one of those not too long ago and the motor would cog away at a low speed and not accelerate further until it usually jumped to full speed.

 

[waross]

(screech not hum)

Are there any marks on the rotor or stator indicating that the rotor is dragging on the stator during heavy starting torque?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zlatkodo]

New motors run for ~9 months then 1200 RPM won't cold start

Does "new motor" mean a really new motor with the original winding of the manufacturer or a recently repaired motor with a new winding?
I ask because the problem you mention is often caused by incorrect repair.
Otherwise, it's very likely that something with the rotor is wrong.
One more thing.
There are 6/2 pole motors with only one winding instead of two separate, which must have a special way of starting.
Have you tried starting the motor at high speed and then switching to low speed?
Who is the manufacturer? Attach a nameplate photo.

 

[GrimesFrank]

Reply for all;
- This is a "new" purchase Toshiba motor. Did Toshiba re-use previous casings, etc. Unknown but likely.
- This is confirmed a 2 winding motor; 2 independent wye set-ups.
- It does run on "rundown" from a high speed start.
- That timing of this is what is confusing us. Installation/testing works fine. We operate for ~6-9 months
(Every day scheme: LS for 6h LS, 6h HS, 6h LS, 6h HS) and then it decides to not start in LS anymore.
- We have not pulled the rotor out yet to examine but that is upcoming.....will look for indications. Are you say look for "scorching" or physical scoring?

Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[electricpete]

- Cold Start has balanced Voltage/Current as measured by Baker/Meggar Explorer Dynamic @ motor (490 V RMS ph-ph and 45A RMS ph)

Where are you measuring voltage? Is it downstream of all contactors?
Do the currents appear balanced during start if you neglect the decaying dc components?

- That timing of this is what is confusing us. Installation/testing works fine. We operate for ~6-9 months
(Every day scheme: LS for 6h LS, 6h HS, 6h LS, 6h HS) and then it decides to not start in LS anymore.

How many times has this cycle repeated? (i.e. how many times have you replaced a motor and then seen a problem 6-9 months later)

- When bump started; Starting voltage dips to 305 V RMS ph-ph but and SS(s) @ 340 V RMS ph-ph.

Hmmm. Do you allow it to get to full fast speed and then start in slow as it's coasting down? How long is it off before you start in slow? To my thinking if it was above 1200rpm when attempting start in slow then (other than a potentially out-of-phase transition if not deenergized long enough) the motor should act like a generator briefly as it decelerates its inertia to 1200 rpm. Although it may still be consuming reactive power (vars) and still cause a voltage drop even while briefly acting as a real power generator. That would be interesting to hear more about.

You've got that Explorer, so it would be interesting to see waveforms from this transient (start from fast first and then slow) for curiosity. But even more relevant to the original problem, it would be helpful to see waveforms for a successful direct start from off to slow, and waveforms for an unsuccessful direct start attempt from off to slow resulting in a trip.

===

Several people have commented potential rotor degradation which certainly makes sense as a time-based damage mechanism that can eventually result in unsuccessful start. Is there any condition monitoring performed capable of detecting this? As a first screening, look for pole pass sidebands around running speed in vibration using a high-res spectrum. If pole pass sidebands are not present in vibratoon of sufficient resolution, then I'd rule out rotor problems. If those sidebands are present then rotor bar problem is likely but not confirmed.... more confirmation can be obtained using current signature looking for the magnitude of pole pass sidebands around line frequency.

There may be clues about rotor problems in the starting record. The most obvious being longer acceleration time if you are trending that. I've heard some claim that the particular wiggles in the starting waveform might tell you something about the rotor, but I'd be careful about that because there can be a variety of wiggles even in a normal start.

 

[edison123]

At 13.3/40 HP, it's more likely a cheap ass Al die cast rotor and one or more Al bars have developed internal cracks. A growler test on rotor will identify the cracked bars.

Muthu

http://www.edison.co.in

 

[GrimesFrank]

electricpete:

Where are you measuring voltage? Is it downstream of all contactors?
Do the currents appear balanced during start if you neglect the decaying dc components?

- Voltage is at the Terminal JB next to the motor as we didn't want to unwrap the lug terminal connection.
- Contactors are over 1000 ft away upstream.
- Currents do appear balanced

How many times has this cycle repeated? (i.e. how many times have you replaced a motor and then seen a problem 6-9 months later)

We've had (2) instances of "failure" so far; we have (2) other motor changes of the same application where operation has continued without issue.

Do you allow it to get to full fast speed and then start in slow as it's coasting down? How long is it off before you start in slow?

I pulsed it on HS (1 sec) then switched over to low speed (LS); I don't think it got above 1200 RPM in that time (no tach at that time).....but I likely did get above 1/7th of 1200 RPM.

As a first screening, look for pole pass sidebands around running speed in vibration using a high-res spectrum. If pole pass sidebands are not present in vibratoon of sufficient resolution, then I'd rule out rotor problems.

Understood and agreed; thanks.

edison123

At 13.3/40 HP, it's more likely a cheap ass Al die cast rotor and one or more Al bars have developed internal cracks.

For the amount we paid for these motors the rotors should be made of the purest ore, welded with gold (sic)



Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[edison123]

If it's only 9 months, claim the warranty.

Muthu

http://www.edison.co.in

 

[GrimesFrank]

I did these charts quickly so I may have messed the conversions.
Here are Torque, Current and Voltage taken at the JB Terminals next to the Motor during a loaded (not uncoupled) start.
(1) ZERO RPM Start
(2) HS Bump Start


Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[electricpete]

Can you provide a longer timeframe view of (1)

Also..... I just counted 10 cycles of voltage in 1000 milliseconds so.... 10hz?!?
EDIT "Time Scale Count = 0.1665 msec" explains it. 1000 units is 1000*0.166msec = 0.166 seconds. 10 cycles in 0.166seconds is 60hz.

 

[GrimesFrank]

Can you provide a longer timeframe view of (1)

Here it is until it blew the O/L.

Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[wcaseyharman]

Do you have a chart of all 3 phase currents for the failed start? That pulsing torque is very intriguing.

 

[electricpete]

So the current went almost immediately to 80Apk = 56Arms (~ 3 x FLA) and stayed there for 6.4*0.167sec ~ 1 seconds and then it tripped on overload.

A few things don't seem quite right.

1. I'd assume the 56A must be your locked rotor current but 3 x FLA seems a little low to be locked rotor current. Voltage is above nameplate. Is there a KVA code assigned to the low speed winding or other indication of expected locked rotor / starting current in slow speed?

2. I wouldn't think one second in that is long enough to trip. Had the motor been cycled recently before this failed attempt to start?

3. If it is locked rotor current I would expect to start to see current decrease in most cases by one second so, yeah I guess that's a problem.

Torque ripples seem of secondary importance to me, but I'll give an attempt to try to interpret them.

The steady state torque ripples appear to be at line frequency. Unless it's 2-pole motor I'd say electrical in origin.

Line frequency torque ripples immediately after start are not unexpected. I'm not all that familiar but I spent awhile simulating things like that using standard textbook transient models a few years back. Here is an old thread that talked about it which is not necessarily a good reference (I'm sure there are better) but I link it because it's what I'm familiar with: thread237-284708 The first link within the op of that thread is broken, but you can see the referenced presentation in the attachment here. Slide 26 shows 60hz torque oscillations predicted during start. Slide 27 shows that an explanation which is not necessarily intuitive but way more intuitive in d-q coordinates than in 3-phase coordinates. For this choice of conventions, the direct axis stator current is close to dc an the quad axis stator current has a dc and a decaying 60hz component. So the interaction of the decaying 60hz component from the quad axis and the dc from the direct axis gives decaying 60hz torque. For the particular motor parameters chosen (it was a very large motor with data posted by milovan in one of the threads linked from that thread), you can see the torque oscillation dies down by about a second. I think it generally dies down faster for small motors (but I could be mistaken)

But the confusing part to me is you also have apparent line frequency torque oscillations persisting in steady state in the successful start after jog. You reported no current unbalance. And if you did have unbalance I think that should give twice line frequency torque oscillations rather than one times line frequency torque oscillations (twice line frequency ripple due to interaction of forward and reverse rotating fields in unbalanced conditions). Maybe it could be simply minor errors in measurement accuracy that propogate through the alogrithm as a line frequency torque ripple (although again at first thought I'd think that would resemble and unbalance which would give twice line frequency ripple). Tough to say why you have those electrical-looking ripples persisting at steady state after successful start.

Out of curiosity what is the motor speed (3600, 1800 etc)? Is the driven pump centrifugal pump or positive displacement? Is there any belt or gear between pump and motor?

 

[GrimesFrank]

Update to all
- Mid April replaced motor with stock; same issue uncoupled (crawling speed/loud harmonic) then when coupled to load (LP Centr Pump direct coupling drive) it picked right up to sych no problem with no harmonic noise.
- Yesterday replaced motor had been shutdown for system maintenance and now exhibits same behaviour (crawling/loud) coupled.

Sorry eletricpete for not responding earlier
1. 56A vs 7.5ish A = 750%. We don't run at nameplate FLC....very much underloaded when in the LP condition.
2. I apologize I cut the time off once reached steady state (to provide detail of transient area); it goes on in the end state for 15s before trip.
3. WRT to the torque ripples the high speed stator is a 2 pole to run at 3600 RPM; could there be an influence there?

New questions/comments
- We can install these motors in the same application (4 of 12 replaced) and they will work (the first one installed is continuing for 2 years now) and others will fail in 6 months and now possible < 1 month. The failed all behave as presented (Balanced FLC until trip, crawling at 150 to 200 RPM, large harmonic sound); therefore can we agree there is a degradation going on?
- If there is a degradation it has to have initiating event for it to occur so soon. I am leaning towards motor fabrication methodology. We have a theory our OEM (Local not Corporate) didn't really know how to produce a 2-Winding motor and that either the LS stator or dual rotor is not appropriate for its use. Leaning toward rotor not being balanced properly between 2 stator use (wrong number of bars, maybe easily cracked, etc.) I keep seeing rotor bar harmonic from our Baker Explorer 3000 and our motor specialist keeps saying to not believe it; too many false positives.
- Will be taking rotor / stator frame apart soon for forensics; bringing in a retired Reliance/Baldor factory engineer to consult on local Toshiba's construction methodology.
- Developing a solution right now open for comment; size a VFD to ramp start the stator until we find a new motor perhaps. Think a soft start will not achieve what we want....need to ramp Hz with volts to ensure rolling start since we have shown rolling starts work on these stators.

If you have any more questions/suggestions keep them coming; I will update the team



Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[edison123]

Post the no. of stator and rotor slots (and photos of the rotor, if possible).

Muthu

http://www.edison.co.in

 

[GrimesFrank]

edison123
Will do when we have it splayed out in the shop; unfortunately too many irons in the fire at the moment.

Question for motor construction knowledgeable people;
Is it common for a N.A. OEM to source all their "structural" parts (frame, rotor shafts, etc.) re-used from Asia and just wind and test before shipment? I have been hearing rumours about this local factory that concern me.

Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[edison123]

"Contract manufacturing" = bean counter mba mantra in corporate world.

What do you mean by re-used?

Muthu

http://www.edison.co.in