Overcurrent effect for motors
Overcurrent effect for motors
(OP)
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!
AND,
HAPPY NEW YEAR!





RE: Overcurrent effect for motors
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.
RE: Overcurrent effect for motors
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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RE: Overcurrent effect for motors
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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RE: Overcurrent effect for motors
Zlatkodo
RE: Overcurrent effect for motors
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
www.edison.co.in
RE: Overcurrent effect for motors
RE: Overcurrent effect for motors
RE: Overcurrent effect for motors
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
RE: Overcurrent effect for motors
One answer for Elmotor; it's S1 duty.
We will try to mount a temperature sensor.