Running an induction machine beyond rated frequency 1

[eeprom]

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

 

[Skogsgurra]

I did that a couple of hours ago, only it was a 50 Hz motor that I took up to 100 Hz.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[eeprom]

If the frequency goes up by 1.5 times, then the reactance of the motor goes up the same. This means that the windings have a greater impedance than they have at 60 Hz. Why wouldn't the current go down? And why wouldn't the power be reduced? Same voltage with less current means less power.

 

[Skogsgurra]

It is obvious that you know this better than me. I let someone else answer that question. (Yes, I feel grumpy today).

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Yes you are correct. The reactance of the winding will be greater and the magnetizing current may go down. But, winding characteristics are just part of the story. The reactance of the rotor is reflected back through the windings. It has a large influence and varies as the slip frequency, which varies as the load.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

"Same voltage with less current means less power."

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

http://www.edison.co.in

 

[eeprom]

I thought that it would be smart to limit the voltage to the motor's rated voltage, 480. Plus, the VFD bus voltage is usually around 670V, which wouldn't make it to the required 720V. So the VFD scales voltage from 0 to 480 corresponding to a frequency of 0 to 90 Hz. But if that weren't the case, you would be correct. The result would be a 1 Hp motor will 50% more speed and 50% less torque. But, aside from the mechanical wear from vibration, I cannot see any additional wear on the motor. I think it is actually easier on the motor due to a lesser stator current.

 

[electricpete]

To summarize the two options, with added discussion of peak (breakdown) torque:

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.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

By the way I never heard of the option of increasing voltage beyond nameplate except on this forum (from gunnar I think). It is not discussed in the NEMA VSD application guide. But seems like a reasonable approach for low voltage vfd motors where there is apparently some margin in insulation rating.

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.

 

[waross]

If the manufacturer uses identical frames and bearings for both 100 RPM and 3600 RPM, the main issue may be lubrication intervals.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Sorry, I've got a sticky "8" key. 100 should be 1800.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[elmotor]

I would say that with 1 HP motor, identical bearings are mainly used for both 1800 and 3600 RPM motors not lubrication interval is not the issue as bearings for this size of motors are greased for life and there is possibility to re-grease the bearings.

 

[dpc]

Sorry, I've got a sticky "8" key.

That beer gets sticky when it dries...

David Castor

http://www.cvoes.com

 

[eeprom]

Update, the motor's been running for a couple days at 90 Hz. The no load current is about 30% less than it was at 60 Hz. The full load current is about the same, as been stated a couple times in this thread. There is no noticeable vibration. Thanks for you help.