Running an induction machine beyond rated frequency
Running an induction machine beyond rated frequency
(OP)
Hello,
I have a customer who wants us to set his VFD max speed to 90Hz on a 60 Hz 1 Hp motor. He wants to know what will happen. According to induction motor equations, the current will go down, and so the horsepower will also go down. Also, there may be an issue of vibration at this increased speed. Other than that, I can see no problem in running the motor at 90 Hz. Am I missing something?
thanks
I have a customer who wants us to set his VFD max speed to 90Hz on a 60 Hz 1 Hp motor. He wants to know what will happen. According to induction motor equations, the current will go down, and so the horsepower will also go down. Also, there may be an issue of vibration at this increased speed. Other than that, I can see no problem in running the motor at 90 Hz. Am I missing something?
thanks





RE: Running an induction machine beyond rated frequency
Horsepowers will not go down. At least not as long as you do not go beyond peak torque (then the motor stalls). And current will not go down either - it is a question of magnetizing current and load combined - so you can expect to see constant or more current with respect to the current drawn at 60 Hz.
If it is a four pole motor (1800 RPM type) you will probably not have any issues with balancing or bearings. A 3600 type motor CAN have problems with bearings and also balancing. It depends, you have to find that out from the manufacturer.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Running an induction machine beyond rated frequency
RE: Running an induction machine beyond rated frequency
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Running an induction machine beyond rated frequency
Take Gunnar's word for it. He has done this before.
Search this site. There have been numerous discussions on running VFD driven motors over-speed. The next step is to go over speed and over voltage simultaneously.
Bill
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"Why not the best?"
Jimmy Carter
RE: Running an induction machine beyond rated frequency
Wrong. At higher frequency, the VFD will maintain the V/Hz and increase the voltage by 1.5 times (if it is available in the input).
So, the motor at higher speed with higher voltage, will have higher capacity.
But whether it can be run at that speed depends on the driven load, balancing, a capacity to withstand centrifugal forces (square of the speed) and the bearings.
Muthu
www.edison.co.in
RE: Running an induction machine beyond rated frequency
RE: Running an induction machine beyond rated frequency
If limit voltage to 460vac in the range from base speed to 150% of base:
Steady state Power Limit (a thermal limit) stays roughly constant
Corresponding steady state Torque at limit varies as 1/speed
Peak torque capability varies as 1/speed^2
The margin between steady state and peak torque decreases
If ramp voltage proportional to speed in the range from base speed to 150% of base
Steady state Power Limit (a thermal limit) varies as speed^1
Corresponding steady state torque limit is constant.
Peak torque capability stays constant
The margin between steady state and peak torque stays the same
Working with reactances dosn't get you far as Bill explained. At no load and starting conditions the reactances control the current. But near full load as an approximation your can forget the reactances and focus on the R2/s element in the equivalent circuit that controls current... basically draws enough power to satisfy the mechanical load.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Running an induction machine beyond rated frequency
As far as mechanical effects of 150% speed, I think it needs to be verified within the capability of the individual motor based on namepalte info and associated specifications. My memory is standard NEMA motors are rated for 125% speed, VFD rated motors for 150% speed. Maybe Bill, Gunnar or others can comment.
Also seems like this subject comes up periodically should be a FAQ by Gunnar or someone.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Running an induction machine beyond rated frequency
Over voltage; Reconnect the 460V motor for 230V. Now you can extend the V/Hz ratio to 200% voltage and 200% frequency with a 480V supply.
Bill
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"Why not the best?"
Jimmy Carter
RE: Running an induction machine beyond rated frequency
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Running an induction machine beyond rated frequency
RE: Running an induction machine beyond rated frequency
That beer gets sticky when it dries...
David Castor
www.cvoes.com
RE: Running an induction machine beyond rated frequency