Slow speed motors

Alan Case

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Apr 2002
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After observing DC motors (approx 4kw) run at a smooth 10 rpms, I am wondering whether an AC motor and a good drive will be able to give the same performance. Usage: Goss printing press.

Regards Alan Case
 
I think you can.

I have seen tests (not real application), where a motor with encoder was controlled by PF700S and the speed was basically zero. Also, that very same VFD can control servomotors.
So I cannot see any probelm.
The problem is, that normally that kind of configuration (good AC motor and high-end VFD) price exceeds the price of servosystem.

BR
Lauri
 
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With a gear box, I saw one running at 10 minutes per rev.
Its not the speed that causes trouble, but the operating range. If all you want is 10rpm, day in, day out, I would use a standard motor with a high ratio gear box. If you want to creep along at 10 rpm, then shoot up to 3000, then I would use an SEW (or similar) brushless DC servo motor.

Doug
 
The motor speed is to be 10 rpms up to 1500 rpms. Cooling is not a problem. I am just wondering whether a standard squirrel cage motor with a quality VFD can get the same control nowadays that was once only achievable with a DC drive. Alan
 
probably an "inverter-duty" motor would be a better choice, rather than a standard squirrel cage motor. overheating, and a certain percentage of torque loss would be 2 important factors to keep in mind.
 
DC and AC are not the same animal!

Remember the basic AC motor requirements. The nameplate values of HP, current, rpm etc. are at full load. If you operate the motor at different speeds, the nameplate values change and the motor is no longer the one described on the nameplate. You can get full horsepower out of a DC motor at 10rpm but not from an equivalent size AC induction motor.
 
I have had good results with AC drives at low speed, using sensorless vector you can get full torque down to about 1.5 Hz. The speed control is more accurate using encoder feed back. In fact I have used an AC drive with encoder feedback to give me full torque at 0 Hz. This was holding a washing machine drum at a fixed position with an uneven load. At below about 50% speed you need extra ventilation without de-rating, although some motor manufacturers reckon that their motors don't need forced vent. A gearbox will allow a smaller motor for the torque needed.
 
Full hp out of a DC motor at 10rpm! I guess not! (Getting out my calculator) Let's see, torque and speed are inversely proportional at constant hp so, if the motor is nameplated for 1750 rpm, the torque at 10 rpm would have to be 1750/10 times the design torque at full speed. I think the magnetics, the shaft, the bearings, and probably other things like the motor mounting feet would have left the scene of the accident (broken) long before we got to 175 times nameplate torque. Mercy sakes!

Using a Flux Vector AC Drive (that would be with an encoder) or the best sensorless vector drives (no encoder) you can produce full nameplate TORQUE down to essentially zero speed. That is the same performance as a shunt or compound wound DC motor.

So, Alan, the short answer is YES. You can get full nameplate torque out of a commodity AC induction motor at 10rpm. Cooling is an issue but you state that you've got that part covered. As in DC, loop tuning is very important at that speed.

There is one curious benefit to using AC over DC at these speeds and it becomes significant in polishing and buffing applications particularly. At such low speeds, a DC motor will have a rotational speed disturbance that reflects the passage of each commutator bar under the brushes. AC rotation is smoother. In polishing applications, this slight disturbance pattern appears in the polishing pattern and is usually a problem since polishing is normally to be pattern-free. Using an AC motor, in my experience, cured that problem--no more buffing pattern in the polished surface.
 
Thanks Dick. So at very slow speeds a properly set up AC system can be smoother than a DC setup. We are finding parts and/or repair work on DC drives and motors are becoming cost prohibitive and are seriously looking at retrofitting AC units. Regards Alan Case
 
Q for DickDV:

In "the old days" we used to experience cogging at motor speeds below 10 Hz or there abouts. This was never a problem for my applications, but it might be for others. Your reply indicates that this is no longer a problem. What have the drive manufacturers done to get rid of this?
 
DickDV said:
Using a Flux Vector AC Drive (that would be with an encoder) or the best sensorless vector drives (no encoder) you can produce full nameplate TORQUE down to essentially zero speed. That is the same performance as a shunt or compound wound DC motor.

As you might guess we run at very slow speeds, however havent had any luck with the Sensorless vector drives. Actually we tried it on one project and after a few days of trying to get the "cogging" (as Tom called it) out we installed the encoder feedback...problem solved. As I understand it ABB 1st came out with this feature, and as of a few years ago it didnt work very well at very low speeds.

Alan, Go for feedback...and Yes is the answer to your question.
 
clogging

In "the old days"
Tom I believe DickDv is going to tell you about IGBT`S and because of their speed clogging is not the problem it used to be. Ron has an good coarse on his site all enterested in motors and drives should study (not just read). I see a lot of answers on drive and motor questions that maynot be 100% right (some from teachers). Alot of things have changed in motors and drives in the last few years. I know you stay on top of things,but here`s a site where others can learn if they want to. http://www.sea.siemens.com/step/default.html

:site:
This coarse explains about clogging at very low speeds also!
Some drives are 40 to 1, 100 to 1, 1000 to 1. 10 to 1500 rpms 1500 divided by 1000 equal 1.5 rpm this means any where between 1500 and 1.5 rpm the drive will have control, but i would still have to see this to believe it.All though some one who realy new motors and drives most likely could. I believe this would be better with feed back.
 
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Alan
You said you are going to retrofit an AC where you have an existing DC.
Make sure you check the HP. caculations because the DC systems would let you run in current limit @ 200% for a short period of time in web winding aplications. AC systems are not that forgiving.
The AC stuff is a lot better now than a few years ago. I too have winders running in the low rpm range and do use the encoder feed back.
Good luck with your project.
Lynn
 
Tom, the cogging we used to see at low speeds was due to a poorly formed AC current waveform, especially from the old six-step drives but to a much lower degree on the early PWM drives running carrier frequencies below 1KHz.

Smooth rotation at very low speed today is due primarily to the higher carrier frequencies made possible by IGBT's just as tracy mentioned above.

I have hydraulic pump test stands running at full torque at 1-5 rpm without cogging on sensorless (Direct Torque Control) systems. As a rule, the above posters are correct, most sensorless drives will not handle these super low speeds well and an encoder is necessary.

And, my special humble apologies to randylud for jumping all over his well-intended post above. I had just gotten off an unusually nasty call from my boss and was not much inclined to be charitable. Shame on me! Keep posting, randy, we need you.
 

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