VFD limitation

[electrolitic]

Sirs
I have heard from some designers that Tmax/Tnom must be limited at 1,8 times if motors are driven by GFS in order to get stable operation.
Is this true?
If yes, why?
Thanks

 

[Skogsgurra]

That is probably because a normal motor has a peak torque somewhere around 2 times nominal torque. If you allow more torque than that, the motor will stall. But don't take 1.8 as a universally valid constant. It depends on the motor's carachteristics.

Gunnar Englund

http://www.gke.org

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

 

[LionelHutz]

What is a GFS? Is that VFD?

The max torque that can be produces is the torque you get when you reach the rated maximum current of the VFD. In other words, start at rated current and full speed on the speed vs torque/current curves and slide back in speed until you reach rated VFD current. Then, pick the torque from the torque curve at this speed and that is max motor torque possible with that VFD. Try to pull more torque and the VFD will either trip on over current or it will begin to current limit (and torque).

 

[DickDV]

But, LionelHutz, you can deliberately oversize the drive to take the motor all the way to breakdown torque. Then, the motor becomes the torque limiter, not the drive.

You should program the drive to limit current or torque or both right at peak or slightly below peak or you could get the above mentioned stalling.

For NEMA B motors, this peak is usually about 2.2 times nominal.

 

[LionelHutz]

Yes, that is true. I just didn't spell that out. I guess I assumed that if you looked at the curves and found the max drive current rating was at a point where you didn't get enough torque that you would know to investigate getting a drive with a higher max current rating.

My point is that you very likely can not install a drive that matches the motor size and get the breakdown torque from the motor. The maximum torque will be limited by the drive. Oversize the drive and you can get up to the breakdown torque, well close to it anyways.