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

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

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

(OP)
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

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

(OP)
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

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

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

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

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.

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

Quote:

(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!

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

Quote (GrimesFrank)


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.

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

(OP)
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

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

Quote:

- 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?

Quote:

- 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)

Quote:

- 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.

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

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
www.edison.co.in

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

(OP)
electricpete:

Quote:

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

Quote:

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.

Quote:

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.

Quote:

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

Quote:

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

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

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

Muthu
www.edison.co.in

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

(OP)
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

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

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
.

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

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

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

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: Motor torque oscillations during DOL start 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?

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

(OP)
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

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

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

Muthu
www.edison.co.in

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

(OP)
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

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

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

What do you mean by re-used?

Muthu
www.edison.co.in

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

(OP)
Re-used meaning that frames, etal are being sourced by non-N.A. workshops are stripping old motors for parts and recycling for new purchases.

Posting an operational video of motor coupling to hear harmonic and see speed.
I hope it is able to be played.

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

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

And Toshiba put their name to it? If yes, it's clearly a malpractice.

Muthu
www.edison.co.in

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

Quote (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?

Quote (GrimesFrank)

1. 56A vs 7.5ish A = 750%. We don't run at nameplate FLC....very much underloaded when in the LP condition
The running current is irrelevant. The thumbrule is that locked rotor current is 5-7 times nameplate full load amps.

Ordinarily if the motor is not drawing locked rotor current I might expect a power supply problem. But that doesn't fit anything else. You measured the voltage. There was no current unbalance (which rules out voltage unbalnce).

And then there's that noise in the video.... I think it's around 480hz. If you're in the US that is 8 times line frequency.... a very peculiar frequency to show up. I certainly wouldn't expect that for voltage unbalance or low voltage.

You mentioned crawling and edison mentioned slot combination, I wonder if that type of noise is expected for that. The fact that it started rapidly and jumped right to that speed where it remained... that really seems to be exactly what would be expected from crawling.

Of course crawling is built into the design (slot combination). There's nothing to suggest it would get worse over time after installation. Degradation over time of course brings to mind rotor bar/circuit degradation, but for rotor degradation I'd expect a slow start from the beginning, not a rapid start and then lock in at that speed.

I wonder if there is something about the sequence of the events that is tricking us into thinking that it's degradation when it's really just random or dependent on starting conditions. Or is it the case that once this problem occurs on a given motor, it keeps on occuring on that motor during every start?

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

Since the 'new' motor runs fine for a few days/months and then starts to crawl and it was built with crappy re-used parts, I think the 'designer' put the original rotor bars too close to the crawling zone and a few rotor bars failed over time thereby putting it in the crawl zone.

Once we have the stator & rotor slots and rotor photos, the picture could become clearer.

Muthu
www.edison.co.in

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

May be an issue related to power supply and/or cable and/or contact connections + variable load start torque request.
At low start, voltage dip to 500V and motor torque may remain below needed start torque requested by load.

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

Today, our repair shop received the following crappy chinese 16 KW, 6 pole high slip (13.5% slip, probably NEMA D motor) Al die cast rotor with six bars broken clear of the end ring. High slip means higher rotor losses and higher rotor temperatures. Melted Al sprayed over the stator winding and killed it too.

Customer complaint was the motor was running slow and taking high current and then tripped on OL. That rotor is toast.







Muthu
www.edison.co.in

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

I fully expect Toshiba will take a standard 1800rpm or 3600rpm motor from their robot factory and then strip the winding to re-wind it into this dual speed motor. That's what other manufacturers do. I've been in the local shop that modifies Marathon motors. The TECO motor that I mentioned before certainly appeared to be done that way. It was wound to a lower rpm than their standard catalog offerings.

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

The "bad" rotor/stator slot combinations (which combinations cause crawling, among other things) depend on the number of poles.

One implication of that is that a 2-speed motor has more bad slot combinations than a one speed motor (so it might be a bit more challenging for designer to select a good combination and meet his other constraints)

Another implication is that it's a big problem if they don't carefully consider the number of rotor/stator slots when rewinding an existing motor for a new speed or for multiple speeds. I gather that was Lionel's point. It's a good point, I'm just spelling it out in detail in case it wasn't obvious to op.

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

Lionel

Per OP, the motor worked fine first in both speeds. So bad slot combination, which I am sure Toshiba knows, was not an issue. A bad rotor with broken bars could result in that bad slot combination resulting deteriorating performance as times goes on.

pete

The number of bad slot combinations for cogging and crawling are the same for all the poles since the rules/formulas are the same for all poles.

Muthu
www.edison.co.in

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

Quote (edison)

bad slot combination, which I am sure Toshiba knows, was not an issue.
Every manufacturer makes mistakes from time to time.
I acknowledge there was more to your statement, which is discussed below...

Quote (edison)


Per OP, the motor worked fine first in both speeds. .... A bad rotor with broken bars could result in that bad slot combination resulting deteriorating performance as times goes on.
It was acknowledged by myself and others above that deterioration is a feature of rotor bar circuit degradation and not slot combination. Nevertheless the total collection of symptoms presented do not seem match any of the scenario's exactly so one of the reported symptoms may be incorrect (which is why my post 11 May 23 22:29 questioned the op about the details suggesting it is a degrading phenomenon).

The following factors steer my personal opinion away from rotor bar circuit degradation and towards rotor / stator slot combination causing crawling
  • 1. It appears from the video that speed initially rapidly accelerates to a few hundred rpm and then appears to stay steady at that speed... with rock-solid audio pitch. That seems to be what we expect for crawling from rotor/stator slot combination, but imo that is not what we expect for rotor bar circuit degradation
  • 2. The motor cold starts fine in fast speed but does not cold start in slow speed. That's exactly what we expect for crawling (since the particular rotor/stator slot combinations that cause crawling DO depend on speed or poles, more below), but this is not expected for rotor bar degradation (fast and slow speed share the same rotor, and starting in fast speed is generally more challenging).
  • 3. The motor starts fine in slow speed during coastdown but not in slow speed from cold start. That's exactly what we expect for crawling if the coastdown does not drop below the crawlng speed, but it's not particularly what we expect for rotor bar circuit degradation.
  • 4. No abnormal symptoms reported for steady state running with a load. The lack of symptoms while running loaded is exactly what we expect for rotor/stator slot combination resulting in crawling, but not for rotor bar circuit degradation. (op seems very familiar with condition monitoring and I certainly assume he would have mentioned any running anomalies that were observed, but I guess at this point it's appropriate to double check with op: are there is any pulsing in noise, vibration or current indication while running under load? Are there any elevated pole pass sidebands around running speed in vibration and around line frequency in current?)
To be clear that's just my opinion, but I don't think anyone can guarantee a diagnosis from what we have been presented since none of the scenarios seems to explain all the facts (so everyone is entitled to their own opinion on this particular question and it's difficult to proclaim any one opinion as right or wrong)

Quote (edison)

pete

The number of bad slot combinations for cogging and crawling are the same for all the poles since the rules/formulas are the same for all poles.
I believe you are mistaken.
See figure below from EPRI 5036 Volume 17 (a very useful reference imo and also available for free).
Each row corresponds to a given number of poles P given in the left hand column.
The right hand column "C" represents "cusps" which are identified elsewhere as cusps in the torque speed curve that may dip below zero (I interpret that as crawling)
The entries in that column C are not all the same, they change depending on number of poles. Relevant to the current discussion, the table entries in column C for 2 and 6 poles are different.

for op - The figure also tells us in advance what you might be looking for (when you find out stator/rotor slots) to cause cusps/crawling for the 6-pole winding. Difference in rotor/stator slot combination of 5 in either direction, or else rotor bars exceeds stator slots by 30 (rotor bars exceed stator slots by 12 would also be problematic, but it would be problematic for both speeds). I would recommend you make that count on at least one of these failed-start motors.. either ask your repair shop to do it, or else ask your own maintenance staff to do it




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

Quote (edison)

A bad rotor with broken bars could result in that bad slot combination resulting deteriorating performance as times goes on.
I don't rule out there are more than one thing going on.

SCENARIO 1 - Rotor Degradation first, harmonic crawling 2nd. I think that's what edison was suggesting. Let's say we have a 6 pole motor where the table shows us a bad combination is R-S=12 and it is built with R-S=13. If one bar manages to open circuit, that would not give the same results as the table combination of R-S=12. The table combinations are based on spatial harmonics which depend on angular spacing between bars which would be unchanged (total number of bars is used as a proxy for spacing between bars). But certainly for one or more open bars, the rotor spatial harmonic profile would change in very complicated ways so I can't rule out that it might somehow create the condition for crawling although I have not heard that once reported in the literature (so call me skeptical).

Prelude to scenarios 2 and 3: The next two scenarios require us to pre-suppose that the crawling based on factory combination is intermittent based on starting conditions. That doesn't sound unreasonable to me. It might not sound intuitive to you that a motor can successfully start passing through a cusp where motor torque dips to/below zero, BUT the torque speed curve represents a quasi-static model of the motor which neglects certain dynamic behavior. When we consider the full dynamic model of induction motor it is well known an induction motor can actually overshoot sync speed during an unloaded DOL start as I discussed here thread237-248895: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed That is a behavior which defies the intuition you might have based soley on the quasi-static torque speed curve, and yet it accurately describes motor behavior including the dynamic electromagnetic effects (currents and torques are a function of not just speed, but also recent history). I think it's logical that similar behavior can propel the motor to successful start past a zero/negative cusp of the torque speed curve IF conditions are favorable (some combination of low load torque, low load inertia, high voltage). Alternatively maybe the steady-state torque anomaly doesn't go below zero but goes very low, so that successful start depends on load torque below that value at that speed. Either way during the successful starts it might slowed passing through the crawling speed but never be noticed until it actually gets hung up there. So it's possible this factory-initiated condition only exhibits intermittently. That leads to two more scenarios:

SCENARIO 2 - Slot-combination-based crawling (no rotor degradation). The failed starts only occur intermittently as discussed above. Once it occurs you call the motor failed and move on to swap the motor. That's why I'm particularly interested to know how many more unsuccessful starts you have after the first unsuccessful start and trying to nail down whether there is repeatable change in behavior after a certain point in time.

SCENARIO 3 - Slot-combination-based crawling first, rotor degradation 2nd. Harmonic crawling occurs intermittently as discussed above. When it occurs the first time, it resembles a locked rotor condition which damages the rotor and makes subsequent successful starts less likely. If there is truly a change in behavior over time this could be the mechanism. Although the fact that the motor tripped after only one second in the posted waveform makes it seem less likely that rotor could be damaged by a single failed start unless it is a really bad rotor.

so in summary I think they're all somewhat unlikely. Scenario 1 and 3 are unlikely because they require rotor degradation but we see no sign of rotor degradation during running. Further we see no signs of rotor degradation based on successful starting after coast and successful start in fast speed. Scenario 2 is unlikely to the extent the phenomenon described by op is changing over time (rather than just intermittent depending on start conditions).... I haven't nailed that down and 2 is still my personal favorite unless/until op clarifies the extent to which it is repeatable after first "failure"

So with everything somewhat unlikely, what is the action. I see op already plans to inspect the motor with expert assistance and edison already suggested count slots and inspect rotor. So I'm in agreement with all that, and I have no different actions to suggest. Unless maybeyou can somehow measure speed during failed start to check if it is close to 1200rpm/7.... like maybe high frame-rate video of start for slow-motion playback at a known rate. I see you reported speed at 150 to 200rpm... I'm curious how did you determine that.

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

Quote (Misattributed to Einstein)


The definition of insanity is doing the same thing over and over again and expecting a different result.
I hope that you have found a source for replacement motors other than Toshiba.
A number of very smart and experienced engineers are not agreeing on a definitive source of the issue.
In plants that I have seen, production was king.
Management was more concerned with results (eg: reliable production with no down time) than with details.
Have you considered stepping away from the problem by replacing failed motors with a different brand?
Or
A seat of the pants solution:
I remember a two speed machine in a mill that used two motors belted together.
A large motor for high speed and a smaller motor for slow speed.
Select a 1200 RPM motor built on the same frame as 3600 RPM motors.
Belt that 1:1 to the existing large motor.
Use a reversing contactor to select one or the other motors.
Your present motor is 13.5/40 HP.
You may not need the full 13.5 HP on slow speed.
The solution won't be pretty but it may work and will appeal to the bean counters who just look at the numbers and not at the actual machinery.
Then you will have all the time in the world to investigate the Toshiba problem.

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

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

pete

I have literally posted the example of Scenario 1 of rotor bars degradation affecting the motor speed and performance, which happens often, regardless of you having not read any literature on it or not.

And no, it's not a challenge at all for a winding designer to pick the correct slot combination for 2 speeds or even 3 speeds. I often convert speeds (the above rotor being the most recent example of conversion from 6 pole to 4 pole) and I faced zero issues with any of the conversions.

Muthu
www.edison.co.in

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

Quote (edison)

pete

I have literally posted the example of Scenario 1 of rotor bars degradation affecting the motor speed and performance, which happens often, regardless of you having not read any literature on it or not.
Good grief. Yes, I'm well aware rotor bar degradation affects motor speed and performance as I'm pretty sure everyone reading this thread is. You're focusing on scenario 1, so let's look at what it is that you seem to have skipped over:

Quote (electricpete)

SCENARIO 1 - Rotor Degradation first, harmonic crawling 2nd
Your posted rotor has nothing to do with harmonic crawling. Harmonic crawling is the scenario where the torque speed curve has a cusp associated with spatial harmonics. It is not the same as simple rotor degradation which just lowers the whole torque speed curve. I have enumerated things that don't fit simple rotor degradation and one of them is the video which shows evidence of harmonic crawling rather than rotor degradation (rapid initial speed increase followed by sudden speed plateau at approx 1/7 sync speed... the most common crawling speed). Another is that it appears to only affect low speed (expected for harmonic crawling, not for rotor degradation). Scenario 1 would be where opening of a rotor bar shifts the spatial harmonics in such a way as to create spatial-harmonic-induced crawling (that is the type of crawling that would create a torque torque cusp and only affect one speed). I have not heard of that and I'll assume you haven't either.

Quote (edison)


And no, it's not a challenge at all for a winding designer to pick the correct slot combination for 2 speeds or even 3 speeds. I often convert speeds (the above rotor being the most recent example of conversion from 6 pole to 4 pole) and I faced zero issues with any of the conversions.
Are you sticking with your previous claim that problematic rotor/stator slot combinations do not depend on speed (15 May 23 14:06), or have you revised your position on that?

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

I was wrong to suggest the crawling speed would necessarily be exactly 1/7th of sync speed in the case of 7th spatial harmonic crawling. It would be exactly 1/7th of sync speed in the case of synchronous cusps, but it would be somewhere above 1/7th of sync speed in the case of asynchronous cusps associated with 7th harmonic
https://www.eeeguide.com/crawling-and-cogging-in-i...

I had mused about the possibility of dynamic electromagnetic effects allowing the motor to accelerate past the torque anomaly as one possible explanation for behavior that I postulated was intermittent/nonreproducible (I postulated it was intermittent to come up with an alternative to degrading). If that particular scenario is at all plausible, it seems it would be more plausible for the synchronous cusp, where the torque anomaly only exists in a very narrow band of speed and so a very narrow window of time during acceleration.

I hope op will feed back his findings because this is an interesting one. If it does turn out to be a slot combination crawling problem, that's something few of us ever get to see.

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

(OP)
Thank-you all. I took a much needed vacation to refresh my mind from this.
Before I reply/go through your valued responses; an update:
  • As was stated by me earlier the motor was replaced with a fresh identical. I have come back to find out it has "intermittently" failed to cold start. Was told it failed to achieve min flow/start (no one present to view in field) then re-attempted and started on 2nd pass. Operations are now gun-shy and have temp procedure in place to perform pump duty swaps in HS ONLY and then revert to LS until solved. No cold LS starts...no issues. Production pressure. ;)
  • <br>
  • Motor expert has not been to site yet NOR motor pulled apart even to inspect stator/rotor. Trying to decontam motor I am told to ship to rewind shop for inspection. Not likely successful once it goes in containment it can't leave station. Frustrating.
  • <br>
  • Motor project is rescoping. Looking to source either a VFD to make this HS winding do dual duty OR (my choice) match new non-Toshiba motor/VFD set to cover duty. 2 speed motors are so passé. ;)
Now onto your posts;
  • I have been in the edison123 camp from the beginning on this; see my original posts. The "designer" tried to balance the LS/HS winding slot combinations; leaning heavily to the edge of LS acceptability and if the rotor end tacking is poor the cold starting torque degraded it to pop (not as bad as edison123's photos though ;)) Great slot combo table electricpete to use when we get to finally look at this thing. All the info posted here has been quite helpful in evaluating this.
  • <br>
  • If we have installed a motor that works from first the behaviour is...it works, then occasionally misses a start but goes on the re-go, then mis-start frequency ticks up slightly, then no joy at all. Previously as stated this was over a 6-9 month period but for this install we have been in for a month and it is exhibiting the behaviour already. Also remember we have 2 other instances of this motor (same duty; different Bus) that have never given us a problem for 2 years.
  • <br>
  • For electricpete: Shaft speed was measured with a hand held laser tachometer so accuracy is what it is.
  • <br>
  • For waross: Like those ideas; I tell our interns we're farm tractor engineers...make what works and then justify it. ;) In this case we have some time and for some reason this time a budget to fix it. Likely Capital Finance folks are feeling a pinch.
  • <br>
I will keep you all posted as info comes in; we only cool irradiated fuel with these pumps, nothing too important. ;)

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

Quote (OP)

Also remember we have 2 other instances of this motor (same duty; different Bus) that have never given us a problem for 2 years.
How is your bus voltage?
Low voltage may be exacerbating a bad design.

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

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

(OP)

Quote:

How is your bus voltage?
Low voltage may be exacerbating a bad design.
We have had failures from different pumps on different Busses. Each time we measure the Bus Voltage it is as near to 600V as I think you can be.
We had (1) other path we were chasing but it reached a dead end as the voltage at the motors, when working, was well enough to start this thing. From MCC contactors to motor goes from #4 AWG (500 ft) to #12 AWG (1000 ft on mobile flexible cable carrier) to #4 AWG (65 ft). We originally thought that the #12 was causing too high a volt drop on a new high efficiency motor to start but we measure and we get what you see above in the charts, we even swapped them for spare #6 AWG on a failed motor and still couldn't start it.

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

Quote (2 speed motors are so passé.)


Yup, their 15 minutes were up years ago. Especially, with the advent of sophisticated VFD's, which more than anything else, provide smooth acceleration with no inrush current spikes and related mechanical shocks to the system, I doubt OEM's will make 2 speed/3 speed motors any more. I used to rewind many 2 speed motors from sugar industry (notoriously conservative when it comes to adapting to new technology) till nineties and almost all of them have been replaced with drives. Sure VFD has its own drawbacks, but then which system is perfect?

Hope you get the chance to tear down this joke of a motor and look at its entrails.

Muthu
www.edison.co.in

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

Below is the original good rotor of an unused spare motor received today.

I think the rotor bars started failing due to high slip (high rotor loss) and frequent starts (high inrush currents).


Muthu
www.edison.co.in

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

Quote:

I have come back to find out it has "intermittently" failed to cold start.
If you revisit my discussion 16 May 23 14:53, the only thing the slot combination could not explain was the initial report that this seemed to be a degrading condition. So this new info reinforces the opinions I expressed 16 May 23 14:53. Namely it's a slot combination-induced spatial harmonic problem (scenario 2). It may or may not be accompanied by rotor degradation before (scenario 1) or after (scenario 3). In order I'd rank most likely scenario 2, then less likely 3, then least likely 1.

If it were a simple degrading rotor, you would have other symptoms like pulsating noise and vibration while carrying load. The speed would be lower than expected while carrying load. You probably would have more problems starting in high speed than low speed (And of course as discussed we expect slot related problems to affect only one speed, unlike simple rotor bar degradation). During start it would not jump immediately to 150-200rpm and then stay there (it would be slow from the get-go... it might end up stalling wherever it met the load torque but it would be quite a coincidence to stop at 150-200rpm speed band... where right in the middle is 171rpm =1200/7 which is the most commonly mentioned frequency for spatial-harmonic cusps).

Quote (electricpete 17 May 23 14:30)


[to edison]Are you sticking with your previous claim that problematic rotor/stator slot combinations do not depend on speed (15 May 23 14:06), or have you revised your position on that?
EDISON - It's interesting that you keep posting your random rotor photos, but you ignore the question directed specifically to you. Let's recall you "corrected" me in this matter. It would be common courtesy to me to admit if you were mistaken on that "correction". It would also be common courtesy to the entire thread audience to acknowledge when/whether a contradiction that you introduced into the thread has been resolved. It is not a random piece of trivia of no consequence to the thread ( It could explain why the problem only occurs in slow, and it might also be relevant to explaining how the problem arose to begin with as Lionel suggested in rewinding an existing motor for multiple speeds without considering slot combinations)



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