Determining v/f profile

[abfer]

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.

 

[itsmoked]

At a low enough frequency you will probably get a form of cogging where because you only have a few motor poles you get uneven torque hence "rough"

 

[UKpete]

As well as the cogging torque referred to by itsmoked, you also have torque ripple due to current harmonics in the winding. The winding inductance effectively forms a low pass filter. At the higher speeds i.e. higher frequencies, the inductive reactance of the winding will be higher and this will inherently give an improved more sinusoidal current by filtering some of the higher frequency current harmonics.

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.

 

[alexit]

What is motor maximum frequency?

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.

 

[CJCPE]

Another thing to keep in mind is that the synchronous speed at 1-2 Hz is 30-60 RPM for a 4 pole motor. The full load slip speed would be in that range. At no load, the motor will run at 30-60 RPM. At full load, the motor will be stalled. That is another reason that a more sophisticated control scheme is needed.

 

[abfer]

Motor is 50Hz. I didn't load the motor. It's unloaded. What my be the sophisticated control to make it run smoothly at low speeds? As i said before, i kept frequency constant and varied output voltage but couldn't achieve a smooth operation. I think UKpete's thesis is right. And at low speed voltage source inverter may not suffice. May be one method to solve this problem is using current source inverter.

 

[Skogsgurra]

Do you have PWM or block switching?

You can not run at low speeds without PWM.

Gunnar Englund

http://www.gke.org

 

[CJCPE]

The more sophisticated control needed should probably be some type of vector control. At a minimum, the control strategy needs to be PWM. Current source is probably not the answer. If the load is very steady, it might be possible to get good results by trial and error adjustment.

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)

 

[abfer]

I'm using PWM. If smooth operation is possible i want to enable V/Hz operation if not i'll continue to vector.

 

[electricuwe]

The effect you are observing is also related to deadtime, I skipped that in my answer to your other thread because I didn't know that you are working with frequencies that low.

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.

 

[electricuwe]

One source where you will find basic information on that effect is: Mohan, Undeland, Robbins: Power Electronics which is a popular university textbook in power electronics and should be easy to get

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.