Determining v/f profile
Determining v/f profile
(OP)
I'm trying to drive a motor via a self made inverter. It's a small motor 360W, 230V, delta connected. I measured stator phase resistance as 135 ohm. It has no name plate on it but it's rated phase current is somewhat between 0.7-0.9A rms i think. The book which i look says, about determining the minimum voltage or boosting voltage, that
Vmin=Rs*Israted
But i couldn't be able to run it smoothly at low speeds; 1-2Hz. I kept the output frequency constant and varied the output voltage to make it smoothly revolve. At a point the torque was max but still it didn't revolve smoothly. At a bit higher frequencies it revolves smoothly.
What i ask is, can it run smoothly at low frequencies? and if there is a source about this subject that explains the matter detailed, it will be very good.
Vmin=Rs*Israted
But i couldn't be able to run it smoothly at low speeds; 1-2Hz. I kept the output frequency constant and varied the output voltage to make it smoothly revolve. At a point the torque was max but still it didn't revolve smoothly. At a bit higher frequencies it revolves smoothly.
What i ask is, can it run smoothly at low frequencies? and if there is a source about this subject that explains the matter detailed, it will be very good.





RE: Determining v/f profile
RE: Determining v/f profile
At low frequency you may be operating with a simple 3-phase square wave, this won't give you the smoothly rotating field you need.
So I guess what I'm saying is that you need a more sophisticated drive scheme for very low speed operation. Or maybe a filter on the input lines (though that will affect higher speed operation).
disclaimer: I am not a drive expert so I could be wrong, but I have read about them.
RE: Determining v/f profile
Place motor on dyno and operate at 0.3*maxhz. Adjust voltage until output is 360W at 0.9A. You should have good V/Hz value now. To confirm do same test at 0.6*maxhz. If you get very different values motor might have knee points (where V/Hz value changes.) Usually these are described in the motor data as speeds below max with different voltage.
When operate motors too slow, sometimes drive must boost voltage to operate smoothly, many VFD have settings for boost and knee points.
RE: Determining v/f profile
RE: Determining v/f profile
RE: Determining v/f profile
You can not run at low speeds without PWM.
Gunnar Englund
www.gke.org
RE: Determining v/f profile
References:
FAQ area on ENG-TIPS FORUMS:
What is Vector Control in a VFD? FAQ237-1062
http://www.eng-tips.com/faqs.cfm?fid=1062
How does Vector Control compare to Servo Control? FAQ237-1063
http://www.eng-tips.com/faqs.cfm?fid=1063
NEMA Site:
Application Guide for AC Adjustable Speed Drive Systems
http://www.nema.org/stds/acadjustable.cfm (free download)
RE: Determining v/f profile
RE: Determining v/f profile
The problem is that during deadtime the output voltage is not determined by the PWM signal, but by the direction of the output current. This introduces an rectangular error voltage which is superposed to the modulated signal.
Reducing deadtime or switching frequncy will help, but to get really rid of the problem you need more sophisticated type of control, but there are approaches more simple than vector control. I worked on that topic about 15 years ago and I will post a suitable reference next time I visit eng-tips.
RE: Determining v/f profile
For more detailed information search fpr papers in the
IEEE Transactions on Indutrial Applications
IEEE Transactions on Power Electronics
IEEE Transaction on Industrial Electronics
or the correponding conferences
During the last 20 years there are at least 20 papers available on that effect and countermeasures.