Stator Winding Damage 3

[noel0589]

Hi all,

For a 50HP 200V 3 phase wye-delta ac induction motor with class F insulation and FLA 148Amps and 1.0 service factor and 80 starts/hour rating (elevator duty):
Is it possible to calculate insulation life/winding life/damage if motor is run occasionally above rated load for current of up to 160?
Thanks for any input.

 

[Skogsgurra]

Theoretically, yes - perhaps (there is a rule saying that insulation life is halfed when you increase temperature eight to ten centigrades). IRL no.

It all depends on what "occasionally" means. If your motor protection is OK and "occasionally" means once in a day or so, then I wouldnt be worried at all.

The insulation isn't being "eaten" each time you do this. The rule about half live applies if you are running continuously with elevated temperature. And you do this very seldom, so if there is an influence on insulation life I would say that it is barely noticable. But if you make it a habit to overload the motor most of the time it will surely shorten the motor's life.

 

[UKpete]

Bearing in mind that a machine of that size will have a thermal time constant of several minutes or more, you might get a better idea of duty by calculating the RMS current over say a 10 minute period. You need to include the starting current+duration (and take into account the star/delta switching) in the calculation. For example see:

http://www.motionsystemdesign.com/full_story.php?WID=7049

If your RMS current over the 10 minute worst case period is below the rated current, obviously there is nothing to worry about; but it is above - then it's as per skogsgurra.

A secondary consideration is the variation between motors from the same batch. It is not uncommon for motor winding temperatures to vary by 10°C or more after running at the same duty, so some motors will fare better than others.

 

[electricpete]

I agree with skogsgurra in general an occasional 160 / 148 = 8% overload of a 50hp motor is not likely to be a problem in itself.

It is more important to understand the duty cycle and effects of repeated starting.

I believe there are some significant shortfallings in the link posted above

http://www.motionsystemdesign.com/full_story.php?WID=7049

Look at the first example of rms current calculation that they provided:
Starting or acceleration current = 10 A for 0.2 sec
Operational or running current = 1.5 A for 0.1 sec
Deceleration current = 8.5 A for 0.2 sec
Idle time = 0.8 sec

They calculate an RMS current of 5.16A based on the entire interval 0.1+0.1+0.2+0.8 seconds. This is the effective current used as the basis for thermal evaluation.
I1 = sqrt((10^2*0.2 +1.5^2*0.1 +8.5^2*0.2)/(0.2+0.1+0.2+0.8))=5.16

The logic for this approach is as UKPete explained that the average power input is related to the rms current and presumably this is a good enough approximation if the motor time constant is longer than the duty cycle.

Ordinarily I agree with that approach IF we are talking about a motor running under varying loads but always running. In the particular example posted in the link, the motor is NOT running thoughout the duty cycle but in fact idle more than 50% of the time! We have taken credit for the reduced (0) heat input during this idel period but we have not penalized for the reduced cooling during the period when the motor is idle. It amounts to an assumption that the idle motor dissipates as much heat (for given deltaT) as a running motor. It is a very lopsided calculation and a very dubious non-conservative result.

To envelope the true equivalent heating current we can do the above non-conservative calculation I1 over the whole cycle 0.1+0.1+0.2+0.8 seconds..... AND an additional overly conservative calculation I2 over the running period only while running. This overly -conservative calculaiton equates to an assumption that no heat is added or dissipated during the idle period which is overly conservative (there is some heat dissipated).

The conservative calculation:
I2 = +sqrt((10^2*0.2 +1.5^2*0.1 +8.5^2*0.2)/(0.2+0.1+0.2)) =8.3

Actual rms current for calculation of effective heating would lie somewhere in between
I1 < Ieffective < I2
5.16 < Ieffective < 8.3

Another laughable thing about the link is their treatment of thermal time constant.
Thermal time constant is the prodcut of specific heat capacity and thermal resistance. Comparing the running and idle conditions, the heat capacity is the same but thermal resistance is an order of magnitude higher during idle conditions than during running conditions..... therefore thermal time constant is an order of magnitude higher for idel conditions than running conditions. But if you look at the calculation they use the same time constant for both idle and runnning conditions!

Sorry, I am not meaning to be picky. Any calculation has assumptions, simplifications and weaknesses which can be improved. This particular link is definitely one of them.

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[electricpete]

I guess the magnitude of the error from neglecting reduced cooling at shutdown will depend on the motor.

If it is a TENV, probably the error is small. Limited to the fact that internal circulation is reduced so end turns have less cooling... but still very effective cooling from stator winding to the frame.

If it is TEFC, then bigger error. When motor is idle we lose both internal and external fan action.

I think the the biggest error would be for open drip proof motors.

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[UKpete]

epete, I agree that the analysis isn't accurate for TEFC. There are probably better links out there too (I didn't put much effort into looking).

The basis of my reply was the method of rating open self-ventilated traction motors (i.e. the cooling varies with speed as for TEFC). But comparison with a TEFC motor isn't properly valid because I overlooked the fact that although these are rated for a particular duty by calculating the RMS current for the whole journey, the average speed is also taken into account i.e. the motor continuous rating is defined at the average speed of the motor, not the maximum speed as for TEFC, hence the reduced cooling effect is allowed for.

But there are some mitigating factors: should the motor temperature rise be elevated due to low speed / poor cooling because it is TEFC, then the heat transfer coefficient (frame to ambient) is higher than it would otherwise be at that speed because it is proportional to delta T i.e frame minus ambient temperature. This is also the case when the motor is up to maximum speed - the htc of a hot motor will be higher than in continuous running at max speed because of the larger delta T hence there is a tendancy to "shift" a backlog of heat and limit the period of over-temperature (to what degree depends on a lot of things). The other factor is that the iron/bearing/windage losses reduce with speed also. So the RMS current is by far the predominant factor determining the motor temperature over the speed range, even though it does give an optimistic answer.

 

[UKpete]

Oops, slipped up again. It's not the heat transfer coefficient that increases with delta T (temperature difference) of course, but it is the overall heat transfer. So the overall effect is as I stated.

 

[electricpete]

All of my criticism in my first message are minimized if the motor is TENV.

I still believe if it is TEFC or ODP, the error of assuming full cooling capacity during off periods is very significant, particularly in this example where off period is more than half the time. I base this primarily on my experience with large ODP motors. Thermal time constant upon startup is signficantly smaller (faster) than thermal time constant upon shutdown, even accounting for the startup current surge. As mentioned above for a given motor (given heat capacity), thermal time constant is directly proportional to thermal resistance. If I get a chance I will post some plots tomorrow.

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[electricpete]

The smallest motor that we have continuous temperature monitoring on is a 300HP TEFC vertical 460vac squirrel cage induction motor.

Based on temperature data recorded by our plant computer, the time constant (tau) for temperature change following a start is 2 - 3 hours (hourly computed time constant remains in this range for 11 hours which leads me to dimiss starting current as an important factor). Load was constant during the running period. The time constant for temperature change upon stop was 7 - 10 hours.

tau_running / tau_stopped = [C * Rrunning] / [C* Rsecured] = Rrunning/Rsecured ~ (1/3)
where
tau = time constant
C = thermal heat capacity (constant regardless of run or stop)
R = thermal resistance... inversely proportional to cooling effectiveness.

I conclude the thermal resistance while stopped is 3x the thermal resistance while running and the cooling effectivenss while stopped is (1/3) of the cooling effectiveness while running. If I were to try to model the motor during on/off cycles, I would be closer to correct by assuming no heat transfer during off period than assuming full heat transfer during off period.

Sorry to beat a dead horse. I just wanted to explain where I was coming from.

btw UKPete - I definitely enjoy all your posts and learn a lot. Definitely not meaning to criticize you or your comments. Just bringing out another aspect.

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[electricpete]

Below is the data for start and stop in 5 columns, defined as follows:
1 = Date/Time
2 = Winding Temperature
3 =+LN((WindingTemp-183)/(71.5-183)) for start
3 =+LN((WindingTemp-71.5)/(188.3-71.5)) for stop
4 = rate of change of #3 per hour based on this row and previous row.
5 = 1/#4 = estimated time constant in hours

=======START========
2/19/03 7:00 71.54 0.00
2/19/03 8:00 71.63 0.00 0.00 -1286.75
2/19/03 9:00 79.46 -0.07 -0.07 -13.71
2/19/03 10:00 110.39 -0.43 -0.35 -2.82
2/19/03 11:00 130.86 -0.76 -0.33 -3.02
2/19/03 12:00 151.04 -1.25 -0.49 -2.04
2/19/03 13:00 161.57 -1.65 -0.40 -2.50
2/19/03 14:00 169.12 -2.08 -0.43 -2.30
2/19/03 15:00 173.81 -2.50 -0.41 -2.42
2/19/03 16:00 177.96 -3.10 -0.60 -1.66
2/19/03 17:00 179.12 -3.36 -0.26 -3.85
2/19/03 18:00 180.27 -3.71 -0.35 -2.84
2/19/03 19:00 181.42 -4.26 -0.55 -1.82
2/19/03 20:00 182.58 -5.57 -1.31 -0.76
more steady state data deleted here
=======STOP========
2/23/03 10:00 188.32 0.00
2/23/03 11:00 152.03 -0.37 -0.37 -2.69
2/23/03 12:00 141.63 -0.51 -0.14 -7.23
2/23/03 13:00 131.42 -0.67 -0.16 -6.35
2/23/03 14:00 125.05 -0.78 -0.11 -8.90
2/23/03 15:00 118.69 -0.91 -0.13 -7.90
2/23/03 16:00 112.32 -1.05 -0.14 -6.90
2/23/03 17:00 108.39 -1.15 -0.10 -9.89
2/23/03 18:00 104.81 -1.25 -0.10 -9.78
2/23/03 19:00 101.22 -1.37 -0.11 -8.78
2/23/03 20:00 97.63 -1.50 -0.13 -7.77
2/23/03 21:00 95.05 -1.60 -0.10 -9.61
2/23/03 22:00 93.31 -1.68 -0.08 -13.00
2/23/03 23:00 91.56 -1.76 -0.08 -12.00
2/24/03 0:00 89.82 -1.85 -0.09 -10.99
2/24/03 1:00 88.07 -1.95 -0.10 -9.99
2/24/03 2:00 86.33 -2.06 -0.11 -8.99
2/24/03 3:00 84.58 -2.19 -0.13 -7.99
2/24/03 4:00 83.79 -2.25 -0.06 -15.97
2/24/03 5:00 83.13 -2.31 -0.06 -18.07
2/24/03 6:00 82.47 -2.37 -0.06 -17.07
2/24/03 7:00 81.81 -2.43 -0.06 -16.06
2/24/03 8:00 81.14 -2.49 -0.07 -15.06
2/24/03 9:00 80.48 -2.57 -0.07 -14.06
2/24/03 10:00 79.82 -2.64 -0.08 -13.06

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[electricpete]

Since I rambled a little bit above I want to emphasize one point:

* The purpose of the excercize above was not to determine the time constants themselves... it was to estimate cooling capacity while shutdown (calculations based on time constants.). The conclusion for this particular motor was that the cooling effectivenss while stopped is (1/3) of the cooling effectiveness while running.

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[vanstoja]

Assuming Class F insulation rating temperature of 155C exists at FLA of 148 and winding hotspot temperature rise increases as the square of the current overload ratio, then at 160 amps, the temperature overload is about 17% and hotspot temperature reaches 181C which is 26+ degrees C over insulation rating. For halving life with each 10C above insulation rating temperature, the life ratio becomes 0.163 for the lifetime fraction spent at the cited overload condition. If overload exists for 10, 20 and 30% of total life, then overall reductions of insulation life are about 8,17 and 25%, respectively. Is it known that FLA corresponds to insulation rating temperature or did the motor manufacturer hedge his bet by calling FLA the 90% insulation rating winding hotspot temperature? A 10% insulation temperature margin was employed by one of our motor manufacturers and such a practice may be prevalent among motor designers. If so, the above estimate is conservative. The 80 starts per hour seems to be a real challenge to guaranteed insulation life considering starting inrush currents up to 6 times FLA for short time periods. How do motor designers handle insulation lifetime calculations for many-start motors like those in elevator service?

 

[UKpete]

vanstoja, I am not aware of the number of starts being limited by any standards (NEMA or IEC), except for large machines where the starting time is significant. I doubt that any small machine designer (say less than 100hp) takes any interest in the number of starts, it would be up to the experience of the specifier to allow for it if they have an application where frequent starts are required. I guess that is also what service factors are for in NEMA territory. I am also confident that a considerable safety margin is allowed on winding temperature, possibly about 20degC. This assumes the machine is working at maximum ambient, which it frequently isn't anyway. They have to allow a big margin partly because of the variation between machines of the same batch.

epete, I do agree with your analysis. I hadn't been aware of the variation of time constant with cooling but it figures. Incidentally, you are fortunate to work with real machines (and not a computer jockey like me), I don't any more and rely on what I read here mostly to stay involved.

noel, assuming you are still with us, just stick with skogsgurra's advice!

 

[Jensgisla]

The Montsinger rule: "...thermal life of insulation is halved for each increase of 10°C in the exposure temperatrue" is a thumb rule and perhaps a bit to simple for this task. I would reocomend to use Arrhenius model:

L=B exp[phi/(kT)]

where L is life, B is a experimental constant, phi is activation energy of thermal deterioration (in eV), k is Boltzmann constant and T is hot spot temperature (in Kelvin). (For Class F insulated motor my reference suggests B = 6,92E-9 and phi = 1.05)

The sweet part of using this Arrhenius model is that you can calculate equivalent running time at rated temperature! Use this equation:

tr = ti exp[B((1/Tr)-(1/Ti))]

where tr is equivalent time at reference temp, ti is real time at real temp, Tr is reference temp and Ti is real temp.
Now you can calculate the running times at any temp to a equivalent running time at a reference temp (e.g. at rated temp!)

Note: this only counts for the temperature deterioration of the insulation system. Other aging factors are excluded.

Reference: E. L. Brancato, "Esitmation of Lifetime Expectanceis of Motors," IEEE Electrical Insulation Magazine, Vol. 8, No. 3, May/June 1992.

 

[Jensgisla]

Sorry, there is a error in the second equation. The correct equation is:

tr = ti exp[(phi/k)((1/Tr)-(1/Ti))]

 

[UKpete]

While we're on the subject, I don't think that the often-quoted "life halved for every 10°C increase in temperature" (which some say only applies above the rated temperature, others say 10 to 20°C and so on) should be taken too literally either.

According to Krempel (section on ageing, they also mention Monsinger):

http://www.krempelasia.com/Chapter0.htm#Ageing

- mechanical ageing with temperature is more severe than deterioration of dielectric strength, and breakdown usually occurs because of insulation becoming brittle and failing mechanically quickly followed by electrical failure as a result. As we all know, no two machines are the same and weak spots do occur, e.g. where the winding comes out of the stator core. So even with the same insulation materials a well designed and carefully manufactured machine will probably be more able to withstand overload than one that has been thrown together cheaply. Again, the onus is on the specifier.

 

[electricpete]

Here are NEMA recommendations for allowable number of starts for design B motors. I'm not sure whether it applies to elevator duty motors.

http://www.joliet-equipment.com/allowable_starts.htm

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