Motor Overload Capacity above Base Speed

[DickDV]

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.

 

[buzzp]

I believe all of this is talked about in the NEMA standard MG-1(? not sure of the number on the end). This is a design standard which addresses how a motor should operate under normal and adverse conditions.

 

[Marke]

Hi DickDV

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

 

[mc5w]

A 60 Hertz 230/460 volts motor when connected for 230 volts can be operated as a 460 volt 120 Hertz motor provided that the motor is an inverter duty motor that is rated for higher frequency operation. You would need a 460 volta drive with at least twice the nameplate horsepower of the motor and well as mechanical components that can take the extra speed and power.

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.

 

[DickDV]

mc5w, I believe you have misunderstood my question. It pertains to the short-term overload torque capabilities of an induction motor, not the continuous torque capabilities.

Anyone else know of any reference material on this obscure subject?

 

[CWHart]

The motor requires constant volts/hz to maintain the stated torque at any given frequency. When the voltage stops rising, the flux in the rotor drops off as frequency increases. The motor load capacity is based on the product of the flux in the rotor and the flux in the stator. Stator flux is determined by the stator current. Stator current cannot rise to maintain torque as the rotor flux drops off above base speed, so the capacity of the motor to provide additional HP above base speed is limited to the current capacity of the stator.

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://www.ab.com/drives/techpapers/DriveMotorBasics01.pdf

 

[electricpete]

I agree with most above.

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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[mc5w]

However, if a motor is subject to deliberate overloading on a duty cycle or occasional basis, the variable frequency drive would still need to be oversized so that you do not trip out the drive or chronically overheat the power MOSFETs.

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.

 

[DickDV]

Thanks everyone. Electricpete, do you have a written source for your data. Have you seen this same material in written form?

I'd like to document this on my laptop for reference when doing training.

Anyone ever seen this overspeed data in a formal document?