Maximum Speed for AC Induction Motors
Maximum Speed for AC Induction Motors
(OP)
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?
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?





RE: Maximum Speed for AC Induction Motors
RE: Maximum Speed for AC Induction Motors
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.
RE: Maximum Speed for AC Induction Motors
RE: Maximum Speed for AC Induction Motors
These guidelines also presuppose cast aluminum rotors. If wound or fabricated rotors, its time to call the factory.
RE: Maximum Speed for AC Induction Motors
RE: Maximum Speed for AC Induction Motors
RE: Maximum Speed for AC Induction Motors
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.
RE: Maximum Speed for AC Induction Motors
Maybe you would be so kind as to steer me to the Table you refer to.
Thanks, DickDV
RE: Maximum Speed for AC Induction Motors
RE: Maximum Speed for AC Induction Motors
"Venditori de oleum-vipera non vigere excordis populi"
RE: Maximum Speed for AC Induction Motors
Sorry - wrong document. Try this:
http://www.nema.org/stds/acadjustable.cfm
dpc
RE: Maximum Speed for AC Induction Motors
RE: Maximum Speed for AC Induction Motors
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)
RE: Maximum Speed for AC Induction Motors
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.
RE: Maximum Speed for AC Induction Motors
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)
RE: Maximum Speed for AC Induction Motors
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.
RE: Maximum Speed for AC Induction Motors
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.