Vector & Servo Motor 1

[toutiao]

Dear Folks,

I'm looking for a motor solution that can:

1. Provide high power (2 hp) / torque at low or 0 rpm;
2. Rotate stably and accurately at very low (10~80) rpm;
3. Relatively low cost;

I read through the FAQ article of vector and servo motor. But I'm still not sure which one fit best for my application.

Thanks in advance!
Toutiao

 

[toutiao]

One more thought: is DTC (Direct Torque Control) motor better suited for such application?

How populor is it to use DTC in industrial applications?

Toutiao

 

[itsmoked]

AC?
DC?
Continuous?
Can't use any kind of gear reduction like timing belts?
Slosyn motors run at 72 RPM have lots of torque but aren't variable. Do you need variable speed control?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

HP is an expression of torque at a given speed. You can't have "2HP/torque" without expressing the speed that you want that HP at. If for instance it is 80RPM, 2HP is 131 ft-lbs, but at 10RPM, 2HP is 1050 ft-lbs! Please restate your question in the proper terms, i.e. what torque do you need at what speed.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems."

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[ozmosis]

Toutiao
What is your application?

 

[toutiao]

The application is a radar type of gyro mechanism (20,000 lbs) that tracks the sun. The pan/tilt rpm ranges from 10~80 rpms in normal operation condition and the rpms changes constantly, although very slowly. Under hostile weather conditions, the rpm goes to around 2,000, mainly to move the gyro in a predefined "safe" position.

Yes, we do use a gear reduction system. A huge one, actually. About 18,000:1.

We are not sure if we use DC or AC. We are leaning toward AC, since it is more cost effective and consumes less power (correct me if I'm wrong).

I'm still trying to find the torque requirement. I'll update as soon as those numbers are available.

Thanks, folks!

 

[toutiao]

Latest Updates:

We require about 20 in-lbs at startup (stall torque) and 4 in-lbs during regular operation (nominal 30 rpm).

The fundamental question is:
1. Can we find an AC servo motor to generate this much torque at stall / 0 rpm and and operate steadily at lower rpm (such as 10 rpm stably, instead of goes to 10.2 rpm, then 9.8 rpm, etc. etc...).
2. If no AC servo motor can do this, is vectro drive AC motor capable of the requirements?

 

[cswilson]

You will have no problem finding an AC servo motor that can generate these torque levels. It took me one minute in a servomotor catalog to find motors rated to 1000 in-lbs continuous stall torque, and still capable of 2000 rpm.

Smoothness at low speed (i.e. low velocity ripple) tends to be much more of a system issue, involving the mechanics, feedback resolution, and the control algorithm. If you have an 18,000:1 gear reduction, I think that will be the dominant element in this regard -- even if the motor has significant velocity variability, I doubt much of that would make it through the gearbox.

I expect you will want very high feedback resolution. At 1/6 rps, you will need many thousands of feedback counts per revolution to get even 1 count per servo cycle.

Also, choose a motor with low or no cogging torque, as cogging will cause major velocity ripple at low speeds.

Curt Wilson
Delta Tau Data Systems

 

[blacksea]

Pay attention that "cswilson" told about AC servo motor (with sine or vector commutation). Such type of motor have significantly better velocity smoothness for very low speed vs Flux Vector Acontrolled C induction motor.
The encoder with analog sine/cos outputs and following multiplication up to 8,192 maybe suggested as position feedback.

 

[Bestboy]

'Provide high power (2 hp) / torque at low or 0 rpm'
For interest sake, why is your application needing 2HP torque at zero rpms.

 

[toutiao]

Thanks for all your replies!

I'm a little confused. I read from FAQs that vector drive can provide high torque at low speed and run stably at low rpm (such as crane applications). But blacksea mentioned "AC servo motor (with sine or vector commutation) ... have significantly better velocity smoothness for very low speed vs Flux Vector Acontrolled C induction motor".

Where can I find a more detailed (with equations, i.e.) break down of the vector drive?

 

[cswilson]

Fundamentally, vector control of an AC induction motor makes it behave like an AC servo motor, which does permit high torque at low speeds and stable low-speed operation. Remember, though, that these FAQs are usually comparing vector control to open-loop volts/hertz operation of an induction motor, which doesn't have these capabilities.

I actually don't think there will be a huge difference in low-speed velocity smoothness between the two types of motors, other things being equal (and especially as seen through an 18,000:1 gear reduction).

BUT -- Typically you will only see vector controlled induction motors at higher power levels than what you require, where permanent-magnet servo motors get too expensive or just not available. Typically, the crossover point is near 10HP. I just don't see an induction motor as appropriate for your application. To me, your application looks like a classic one for a servo motor.

Curt Wilson
Delta Tau Data Systems

 

[toutiao]

Dear Curt,

Thanks for the info. I'm fully convinced that AC servo is capable of our type of work.

I noticed that you work for delta tau. I actually used PAMC II PC for my master's thesis back in 1999. I took a traning in Delta Tau LAX. I was working with Steve Melici (?) and Ali.

It's a pleasure meeting you all.

Many Thanks Again!
Jing Zhang

 

[blacksea]

One remark to last "cswilson" posting:
for correct comparison with low speed behaviors AC induction motor with flux control must have an encoder with the same resolution. And if for AC servo motor an encoder is build-in feature than for AC induction a encoder must be attached externally.

 

[jraef]

I totally agree with Curt; at this power requirement a servo is better suited. An AC motor, encoder and full vector drive is going to be more expensive, bigger and all the parts are likely to come from disparate vendors, where a servo system can be put together from one source.

I maybe wouldn't have agreed with the 10HP crossover point, I thought 5HP, but I bow to Curt's experience on that issue. I have never used a servo over 5HP (equivalent) or a full vector drive under 5HP. Not by investigation and choice, it just happened that way, so I don't know for sure.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[cswilson]

jraef:

Ten years ago, I would have used 5HP for the crossover point, and I almost wrote 5-10HP above. However, in recent years, I have (unscientifically) noted a trend toward using larger servo motors, I think due to the improvement in rare-earth magnet materials, so I get the sense that the boundary has been moving up in the power range.

Curt Wilson
Delta Tau Data Systems

 

[magneticmaster]

Exactly what speed at what torque? And what is your available voltage?

 

[PH927]

I just finished working with 3 different manufacturers on two bids for servo applications, and this sound very doable for any of them. Full torque throughout the speed range was a common feature among them all. Also if repeatability in positioning is important you may want to look at putting the encoder after the gear box to avoid dealing with the backlash. Using an absolute encoder will stop you from having to use a homing routine when you lose power.

 

[toutiao]

Thank you for all your postings, folks!

I was out of town so I couldn't keep track with your updates.

If I have new questions, I'll get back with a new topic thread.

Many thanks again!