Motor Overload Capacity above Base Speed
Motor Overload Capacity above Base Speed
(OP)
I have seen it claimed that induction motor overload capacity falls as the square of the overspeed while the motor continuous torque rating falls as an inverse of the overspeed. Using these rules, a motor with 200% overload capacity at base speed would have no overload capacity at double speed.
Can someone direct me to written material on this subject---white papers, manufacturers application data, etc.
I do a lot of application work using motors above base speed and I need to understand this subject better.
Thanks in advance.
Can someone direct me to written material on this subject---white papers, manufacturers application data, etc.
I do a lot of application work using motors above base speed and I need to understand this subject better.
Thanks in advance.





RE: Motor Overload Capacity above Base Speed
RE: Motor Overload Capacity above Base Speed
My understanding is that the torque is directly proportional to the flux in the iron. As the motor is driven above full speed, the drive is voltage limited to the supply and so the flux in the iron drops. This causes a drop off in torque. The increasing motor speed and reducing torque give a contant power rating above rated speed at rated line voltage. This should not affect the overload capacity that I am aware of provided that we can ignore the harmonics that appear in the output waveform at overspeed.
The only other consideration is that there may be a drop off in flux due to the effect of the leakage reactance at the higher frequency.
I am curious also.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Overload Capacity above Base Speed
Larger inverter duty motors cannot take 120 Hertz operation, only say 80 Hertz maximum. You need to look up the data for the motor and all of your mechanical components.
RE: Motor Overload Capacity above Base Speed
Anyone else know of any reference material on this obscure subject?
RE: Motor Overload Capacity above Base Speed
This subject is very well covered for DC motors and reviewing the torque curves and constant horsepower region of operation for DC machines will effectively explain AC machine operation. There are minor operational differences in the curves between AC and DC, but not much.
There are always sources, try this for a explaination that includes some AC motor Variable Frequency operation. http://w
RE: Motor Overload Capacity above Base Speed
My summary is
Above rated:
V (volts) constant.
F (hz) ~ N (speed).
Flux ~ V/hz ~ 1/N
Peak overload torque available ~ Tpeakoverload~ Flux^2 ~ 1/N^2
Continuous power rating is a thermal limit and should stay roughly the same. P constant.
To keep within the continuouspower rating, we have to decrease continuous torque approximately by 1/N. ie. Tcontinuous ~ P (const) / N ~ 1/N
If we operated at the constant rated power and increase speed above rated, since Tcontinuous ~ 1/N and Tpeakoverload ~ 1/N^2, the margin between Tcontinuous and Tpeakoverload decreases. i.e. less overload capability as a fraction of continuous.
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RE: Motor Overload Capacity above Base Speed
Also, when an induction motor is running off of a VFD the acceleration torque and breakdown torque are not as great as when operating off of sinusoidal alternating current from the mains. What a VFD puts out is a train of square waves that "average" to an approximation of sinusoidal 3-phase power. The "alternating current" that a VFD puts out is rather synthetic.
Also, the service factor of a motor that is connected to a VFD needs to be multiplied by 90% because the VFD generates extra heat in the motor.
Deliberate overloading of a motor that is on a VFD is therefore a perilous venture. You probably would be better off to buy a bigger motor. Keep in mind that some makes of VFDs can do reduced voltage running of an oversized motor which cuts down on magnetizing current, eddy current losses, and hysteresis losses. That is, by cutting the applied voltage, you can make a 10 HP motor behave as a 5 HP motor when the motor is loafing.
RE: Motor Overload Capacity above Base Speed
I'd like to document this on my laptop for reference when doing training.
Anyone ever seen this overspeed data in a formal document?