Maximum Speed for AC Induction Motors 1

[roryrobb]

100 hp Reliance IEEE 841 design 1200 rpm (6 pole) motor.
Running on PWM VFD. Is there a standard for frequencies over 60 hz. or a maximum speed rating for motors.

I know on larger motors peripheral speeds should not exceed 10000 ft/min but what about a guestimated 13" rotor core diameter?

 

[Skogsgurra]

Reliance is building a new searchable motor page. I looked up general purpose motors 100 HP 1200 RPM. There are many variables - so have a look for yourself on

http://www.reliance.com/mtr/prfchr.htm

 

[vanstoja]

I don't know of any limits or guidelines for operation of AC induction motors on variable frequency drives at frequencies above the motor rating frequency. The most likely limiting frequency would be determined by motor winding insulation overheating which could halve motor life for every 10C temperature rise above insulation rated temperature. It would depend on the kind of load the motor is driving. Assuming a centrifugal pump load, the motor's shaft horsepower output would increase as the cube of the speed above rated, and assuming a winding temperature rise increase proportional to current squared times winding conductor resistance,I^2*R then windind temperature would go up as the 6th power of the speed increase. Thus, a 10% increase in rated frequency from 60 to 66Hz would roughly raise the winding insulation hotspot temperature 77% above the 60Hz full load current temperature. Do you have the needed margin to insulation rated temperature?
Other non-electrical concerns would be overstressing the rotorshaft or rubbing in the airgap due to increased rotor deflection at higher speeds. The driven load may have other limitations on running speed as well.

 

[Skogsgurra]

Yes, there are limits. One is the maximum mechanical speed. Often surprisingly low and sometimes surprisingly high. It is about critical speed, centrifugal forces, balancing and bearing life. I have run a standard two-pole motor at 7000 RPM with good results. But only after checking with the manufacturer first.

 

[DickDV]

I will take a NEMA frame below 360 to 90Hz, 360 to 440 to 80Hz (does not include 2 pole motors) without asking any questions and without any trouble. 2 pole motors will go about half that amount of overspeed in each frame.

These guidelines also presuppose cast aluminum rotors. If wound or fabricated rotors, its time to call the factory.

 

[aolalde]

See standard NEMA MG-1 Part 30. Table 30-1 "Maximum safe operating speeds for typical standard motors used on adjustable speed power supplies with direct drive".

 

[DickDV]

aolalde, do you have that NEMA MG-1 text in downloadable form or know where I can get it electronically?

 

[dpc]

http://www.nema.org/stds/ics7-1.cfm#download

This should be a link to a free download of NEMA ICS 7-1 which is an application guide for "AC Adjustable Speed Drive Systems. Table 5-1 gives specific limits for operation above 60 Hz, depending on motor hp, enclosure and synch speed.

This should be a good starting point.

 

[DickDV]

dpc, thanks for the link but, unless I'm going blind, I don't see any table 5-1. In fact there are only 4 chapters in the document.

Maybe you would be so kind as to steer me to the Table you refer to.

Thanks, DickDV

 

[vanstoja]

Can't somebody just summarize or paraphrase the standards for speed limits in the above cited NEMA documents? Most of us don't carry motor or other engineering standards around in our pocket and would never pay the outrageous asking prices to get them for personal use. The above cited internet connection requires an affadavit to get at whatever information is there and appears to be not worth the effort. Some of these interminable threads would be better served (and beneficially attenuated) by presentation of best-known or even competing, contradictory statements of purportedly factual data/information from those who have access to it together with a citation of the precise source of the data for those who may require a lookup/search for proof of authenticity.

 

[jraef]

The only "rule of thumb" I have heard and adhere to, is that the mechanical speed limit of any manufacturers design, without consultation directly with them, is the 2 pole speed. So in your case roryrob, your 6 pole motor could be taken up to 3600RPM, which would mean 180Hz. Any more than that and bearings / balancing issues come into play, where I would think each motor mfgr will be slightly different. You should contact Reliance tech support and ask them.

"Venditori de oleum-vipera non vigere excordis populi"

 

[dpc]

DickDV,

Sorry - wrong document. Try this:

http://www.nema.org/stds/acadjustable.cfm

dpc

 

[DickDV]

thx, dpc. I got it now. Good info!

 

[bigbillnky]

roryrobb
Goodmorning. I'll see if I can give you a little info that may help. When you change the volts to hertz ratio of a motor by increasing the frequency above the design ratio, the motor is in a state called field weakening. In this state, motor torque is greatly affected. The formula for calculating the reduction in torque is
(Rated frequency/Extended frequency)Squared.
This will give you the percentage of reduced torque. Since torque decreases, and speed increases, horsepower remains the same. Hope this has helped.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[DickDV]

Ummmmm, no, bigbillnky, the torque is reduced by the ratio of overspeed, not overspeed squared.

Check the hp formula hp=torque x rpm/5250. The rpm isn't squared so the torque reduction isn't either to keep hp constant.

 

[bigbillnky]

DickDV
I actually believe you have read something into the post I made. The formula I stated will give the reduction percentage of motor torque when the motor is operated in field weakening(increasing frequency without a proportional increase in voltage). A 60 hertz motor ran at 120 hertz will produce only 25% of rated torque at 120 hertz. (60 divided by 120 equals .5) and .5 squared equals .25, which is 25% of rated torque. I did not mention RPM in the original post. Please reread.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[Skogsgurra]

I think that you are a little bit off topic, guys. The maximum speed is a mechanical thing - bearings, balance, centrifugal forces ond so on. If the motor can reach that speed or not is another question. It can - mostly.

Regarding the torque vs frequency discussion: It is the peak torque that goes down with frequency squared above base speed. Since cooling is improved at higher speeds it may look like the motor can produce a torque that is inversely proportional to frequency (or RPM) without overheating, but it can only do so if maximum torque (which goes down with f squared) still can make the motor rotate. Above a certain frequency it will simply stall.

Do not confuse above base speed AC induction with DC field weakening (where the constant power law is correct). They are two completely different things.

 

[DickDV]

Gentlemen, it is a well established fact that, when an AC induction motor is operated above its base frequency, it enters a constant horsepower zone for at least some amount of overfrequency. In this range, the torque reduces by the inverse of the overfrequency ratio, not the overfrequency ratio squared.

In the example bigbillnky cites, the torque at 120 Hz will not be 25% of basespeed torque but 50%.

Many motors cannot maintain constant horsepower that far above base speed but start to drop horsepower at some overfrequency point. That is due to the torque falling off faster than the overfrequency ratio. This occurs before 120 hz on many motors not designed for inverter service.

Skogs, your comment about peak overload falling by the square of the overfrequency ratio is interesting. I had observed that in the field a few times but had never heard anyone confirm it. Thanks.