Any one familiar with ABB Drives in Torque Control and DTC?

Hello All,

This is not a PLC question. I deeply apologize for that but I am looking for help from great members like DickDV and others. I am new to this forum.

I work in a plant where we have winders and un-winders machinery. Each of those are built with ABB ASC800 drive powering 1 HP, 1725 RPM motor connected to 60:1 reducer. The drives are set in Torque Control macro and DTC function is activated. We run in torque control mode most of the time but sometimes we run in speed control mode. There is a switch connected between point X22.3 and X22.7 (I am assuming you can get a copy of ASC800 wiring diagram). If the switch, on in run on torque mode and if its off, it will run on speed control mode.

The problem we are having and its only on torque mode: at some points when the load speed gets too low (load speed gets too low when the material diameter gets larger) (load speed will be around 3 RPM at load side of the reducer and around 180 RPM at the motor side) the load will start to jerk. By that I mean it will not run smooth like when it faster. The reel will stop for less than a second and fast up again.

We have plenty of those winders and un-winders but we have the problem only on few of them. All of them were built using same exact mechanical parts and the drives were set the same way. Any advice? I would appreciate any help and reply.

Thank you

http://www.abb.com/product/seitp322/42c57f5d7a8393d8c1256df9003e9b74.aspx

Take a look at the above article.

"Higher Torque Quality at Low Speed
If smooth running free from jerks at low speeds is to be obtained, the magnetization of the armature core laminations must be uniform. The built-in magnetic polarization, a normal characteristic of electrical steel results in jerky running at low speeds, if this cannot be suppressed."

It appears to be in the motor design.
Hope this helps,
Garry
http://www.accautomation.ca

Thank you Garry for replying and thank you for your efforts to help.

Switching to DC motor will not be an option. BTW, I am using Marathon Y378 motor. This motor was designed for variable speed control and up to 20:1 constant torque to speed ratio. I think its more a drive issue.

Anybody had this issue with ASC800?

One potential drawback of DTC (vs FOC) when run without an external feedback sensor (encoderless operation) is that because of what it takes to make DTC work, it is more subject to variations in the motor stator resistance. That resistance can then change with temperature. So what can happen is that at low speeds, some motors have a hard time cooling themselves, the stator temperature rises, and the DTC modeling becomes unstable.

You might be able to overcome this by adding an external encoder and doing closed loop control. I don't know this part from experience, it came up in a conversation I had with an ABB person once long after I solved my issue. I actually solved my problem by using an externally cooled (blown) motor.

it looks like you are running the motor to slow
most VFD's running below 6 Hz will cog and when load is large it gets worse
a large dia. winder will be both heavily loaded and running at a very slow speed(Hertz)
the best solution would be to change the gear box to allow the motor to at a higher speed with a full winder. You can run the motor a up to 120 Hz when it winder is empty that should give you some room to play with.

Where is the drive getting the torque command from? Are you sure there isn't something going on at the other end of the line that is causing this? What is the actual torque signal showing during these jerky movements? What does the torque command show? This application should be no problem for an ACS800 and Marathon microMax motor, assuming you are not exceeding any ratings.

Thank you all for your input.

I still have the problem. I am sure adding encoder will fix the problem but that will be costly since I need to add encoder input card to my drives. What drives me crazy is I have more than 90% of them run flawlessly without encoder and under the same exact conditions.
We are using the drive reference button to set the torque and it will stay constant during the jerking but current will change of course. We usually set the torque reference at 18-25% when the load is not heavy. And once the load get heavier, we increase the torque reference to 30-40%.The ones that need higher torque reference are the ones show the jerking problem.

Still sounds like the motor heating issue to me. If some work and others don't, it may be an ambient condition that seems insignificant to you, but makes a difference to the motor cooling. Look for blocked or obstructed air flows, radiant heat from other equipment, broken cooling fans on the motor etc. might even be a bad bearing and the heat of the hearing is transmitting to the windings. A vibration analysis might find something too.

It's not the issue of the drive clogging at low speeds. That happens with simpler V/Hz drives, but not with Vector drives or DTC as ABB uses. The drive is perfectly capable of making the motor run smoothly at any speed, even approaching zero speed. Instability at different winding temperatures however is a potential drawback of the DTC method.

Keep in mind that when in torque mode, the smoothness of the movement depends greatly on the smoothness of load. It wouldn't hurt to do a full motor ID run on these again to see if that helps.

I agree that this should be an easy job for an ACS800 in DTC mode. The first thing I would check is the mechanical smoothness of the winder at slow speeds. If that is nice ad smooth, I would rerun the motor model but do it on the motor when it is fully warmed up. I have had a few test cell applications where warming the motor to operating temperature has resulted in a better motor model and better torque/speed management. I'd change the motor out before I would even think about an encoder. The ACS800 can generally manage smooth rotation at less than 1rpm so something else is causing the jerking.

Please let us know what you find.

I than you are looking at this from the wrong side
at the slow speed where the instability starts the VFD is only running at 6 HZ very few if any can run at 6 HZ or below without an encoder and in Flux Vector Mode.
below 6 Hz you will never get it run stable the simplest solution would be to change the VFD out with a good Flux Vector one with an encoder and at that you still need to get a motor that can handle that low speed at full load without overheating
I have run a Flus Vector VFD down below 0.5 HZ running smooth enough to run a bead of weld that looks better then any hand weld. the gear box is just fine its just the wrong ratio

Gary S, you are clearly not familiar with the performance capabilities of the best sensorless vector drives, including the ABB ACS800. The3se drives can operate an induction motor down to zero speed and develop full nameplate torque at those low speeds as well. No encoder is needed. Now, if the motor shaft has to remain absolutely motionless at zero speed (no twitching) or if the speed error has to be non-cumulative, then an encoder is necessary, otherwise not.

Ditto to what DickDV said.

Thank you all for your helpful posts. I did full ID run on all of them and that fixed the problem on some of them. I found bad bearing and tight chain on two of them. They worked fine after replacing these parts. The rest of the winders (2) became smoother with time. I tried everything but they kept jerking. They tended to jerk when we set the reference at higher values. So we reduced the reference setting. After 2 weeks of operation, they became smoother.

Thank you all for your help.