Vector & Servo Motor
Vector & Servo Motor
(OP)
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





RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
What is your application?
RE: Vector & Servo Motor
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!
RE: Vector & Servo Motor
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?
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
The encoder with analog sine/cos outputs and following multiplication up to 8,192 maybe suggested as position feedback.
RE: Vector & Servo Motor
For interest sake, why is your application needing 2HP torque at zero rpms.
RE: Vector & Servo Motor
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?
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
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.
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
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
RE: Vector & Servo Motor
RE: Vector & Servo Motor
RE: Vector & Servo Motor
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!