Overcurrent effect for motors 3

[parlitu]

A motor of 45 kW from a ventilation unit, (nominal current 82A) due to a bigger load, function actually at 95A-measured current. The motor is cool down by a big air volume of 15C degrease. What are the odds that this motor to be burn? All the cold air can compensate the overcurrent effect? I think that is more probably to have a shortcircuit in one windinng then between windings (corect me if I'm wrong), and this scenario is a bed one, because the winding rezistance will decrease, but not enough, to made the overcurrent protection to act immediately (the overcurrent protection have been rise to 95A temporarly due to some constraint- not a good ideea but necessary).

AND,
HAPPY NEW YEAR!

 

[TurbineGen]

I would be more concerned with the rotor. While the stator may get adequate cooling, the rotor is likely not. Once the rotor fails, the stator will quickly follow.

What is the service factor of this motor? If it has a 1.15 SF then you might be ok so long as the motor remains under it's maximum rated temperature. If it has a Service factor of 1, then I don't think that motor is going to last too much longer.

In the event of a short, your overcurrent protection should catch it immediately. However, I agree that stepping it up just to run a motor harder is usually a bad idea.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[jraef]

parlitu,
As a general rule, motor life is directly related to temperature, but more specifically, temperature rise. As a rule, the winding insulation is based on a "normal" ambient of 40C, then there are different "Classes" of insulation that describe the overall rise above that which a motor will tolerate without loss of life expectancy. So without knowing what your Class rating is, there is no way to tell for absolute sure. Certainly, 15C air flow will help, but TurbineGen makes an excellent point about the rotor. Rotor temperature ratings tend to follow the stator insulation class they are used in, so depending on the type of motor enclosure design you have, that may not help. For example, if you have a TENV motor (Totally Enclosed Non Ventilated) with class A insulation, that 15C air stream may be insignificant because the rotor will never see it. In addition, a "rule of thumb" is that motor life expectancy decreases by 50% for every 10degC rise above the rated maximum temperature. So if you have Class B insulation, the max temperature is 40C, + 80C rise, + 10C hot spot, for a maximum motor temperature of 130degC. If you have no sensors embedded in the rotor to tell you what it's temperature is and that cool air never gets to it, you are taking a big chance in shortening the life of it.


TurbineGen,
Typically if someone describes their motors in kW rather than HP, they are not using NEMA rated motors and therefore they have no "Service Factor" as we know it. IEC motors tend to be the equivalent of what we would consider 1.0SF, especially those used in ventilation systems.


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[electricpete]

I am not sure where the concern about rotor temperature is coming from.

I agree that knowing more about the motor including insulation class and temperature rise and service factor would help people give better guesses.

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[zlatkodo]

It is difficult to say anything when we do not know even all the details from nameplate and other important informations (no-load current, history of maintenance, etc).
Zlatkodo

 

[edison123]

First of, the motor current is not dictated by the stator winding resistance. So, a low ambient means lower stator and rotor resistances and hence lower stator & rotor copper loss and lower temperature rise but not higher stator current.

Ultimately, the rotor heat has to be dissipated via the stator frame, especially in a totally enclosed motor. So, a colder ambient will definitely reduce the rotor temperature also.

As for this motor capable of taking overload, given the lower ambient, it is a possibility. What is the motor temp class (normally stamped in the nameplate) ? Are there winding RTD's (remote possibility) to measure the winding temperature ? What is the stator frame temperature before and after the overload ?

Muthu

http://www.edison.co.in

 

[parlitu]

Isolation class - F. According to manufacturer, this motor can work to a temperature of 100C. The air flux is cooling down only external frame. Protection- IP55 according to IEC Publication 34-5 (but I don't think that counts). Also according to manufacturer, this motors can work with 108% from nominal power at 30C; for 15C manufacturer did not provide any data. We dont have the possibility to measure winding temperature, but we want to install one in order to evaluate the situation.

 

[elmotor]

It seems that the manufacturer would like to stay on safe side, giving this 108 % at 30C. I mean that the temperature rise will be appr. 90K in case that at nominal load it is 80K, for what I have some doubts. Usually this size of 4-pole motor have temperature rise well below 80K, depending on manufacturers and therefore have margin, even 5...10K. Using your 95A, it means that there is 15% overload and temperature rise would be slightly above 100K, using the same assumption as before. Thus the temperature would 130C, but if you mount some sensors at the drive end, you will find the hot spot ie. 5...10C more. Having F -class insulation system, your would not burn, but the lifetime is reduced. By the way, is the duty S1, continuous duty ?

 

[Marke]

If we assume that the motor is rated for full load at 40 degrees C, and a temperature rise of 80 degrees C, then when we operate that motor with a higher load so that the current increases from 82A to 96A, the copper losses in the motor will increase by 34%.
If we assume that the copper loss contributes to 60% of the temperature rise, then the total losses in the motor will increase by 20.4%, so the temperature rise would be in the order of 80 x 1.2 = 96 degrees.
With this simplified model, if you could guarantee an ambient temperature 16 degrees lower than 40 degrees C, then the absolute temperature on the windings should not be to far from the absolute temperature at the ratings and the motor should not be very compromised.
Hotspots can be exaggerated and so increased safety margins should be applied.

Best regards,
Mark

Mark Empson
L M Photonics Ltd

 

[parlitu]

Thank you all for response;
One answer for Elmotor; it's S1 duty.
We will try to mount a temperature sensor.