You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

Reply
 
Thread Tools Display Modes
Old September 15th, 2020, 04:20 PM   #1
norunningintheplant
Member
United States

norunningintheplant is offline
 
Join Date: Aug 2020
Location: Washington
Posts: 6
Induction motor speed running away

I have an auger powered by an induction motor under flux vector torque control driven by a PowerFlex755. According to my log, the command velocity was ramping up at a reasonable rate while the actual velocity was lagging behind. As soon as the command velocity began ramping down again, the actual motor velocity ran away up to the drive speed limit and shut down. I have included a a screencap of the log showing the velocities. https://imgur.com/a/MWtlVRm

How/why would this happen? PID instability? I'm at a loss. Thanks

Last edited by norunningintheplant; September 15th, 2020 at 04:22 PM. Reason: Improved clarity
  Reply With Quote
Old September 15th, 2020, 05:03 PM   #2
James Mcquade
Member
United States

James Mcquade is offline
 
Join Date: Oct 2007
Location: Tennessee
Posts: 2,895
please explain your application a little better.
how is the speed set in the vfd?
what controls the speed?
how long do you want to ramp up / down the auger?
james
  Reply With Quote
Old September 15th, 2020, 05:58 PM   #3
norunningintheplant
Member
United States

norunningintheplant is offline
 
Join Date: Aug 2020
Location: Washington
Posts: 6
Hi James. Thanks for the response. The speed reference is sent via Ethernet from a ControlLogix processor and is geared to a pump speed. I am ok with the ramp times not quite matching up, but my real concern is that the command and actual accelerations move in opposite directions as soon as the drive is commanded to slow to a stop. If you take a look at the linked plot, the feedback speed (red trace) is increasing while the speed reference (blue trace) is decreasing.
  Reply With Quote
Old September 15th, 2020, 06:20 PM   #4
ndzied1
Lifetime Supporting Member
United States

ndzied1 is online now
 
ndzied1's Avatar
 
Join Date: Aug 2002
Location: Chicago, Illinois
Posts: 2,528
Is the drive in velocity mode or torque mode? If you are in torque mode and the resistance goes away, your motor will behave exactly as you described.
__________________
nOrM
======================
nOrM=Norman Dziedzic Jr.
I've never been to China but my phone has.
  Reply With Quote
Old September 15th, 2020, 06:25 PM   #5
Gene Bond
Member
United States

Gene Bond is offline
 
Join Date: May 2004
Location: Indianapolis, IN
Posts: 458
Is it possible the load is over-driving the motor? It could be you need some braking torque, and without a DB resistor, there's nowhere for the energy to go.

The drive would go into bus regulation mode, and try to scrub the excess voltage by speeding the motor up.
  Reply With Quote
Old September 15th, 2020, 06:37 PM   #6
norunningintheplant
Member
United States

norunningintheplant is offline
 
Join Date: Aug 2020
Location: Washington
Posts: 6
Quote:
Originally Posted by ndzied1 View Post
Is the drive in velocity mode or torque mode? If you are in torque mode and the resistance goes away, your motor will behave exactly as you described.
The drive is in torque mode. It suppose it is possible, but unlikely, that the resistance decreases. The motor turns an auger that pushes a powder into a tank.

The odd part is that the motor feedback acceleration increases at the exact moment that the command velocity begins to decrease (when a pump switches off).
  Reply With Quote
Old September 15th, 2020, 08:35 PM   #7
OkiePC
Lifetime Supporting Member
United States

OkiePC is offline
 
OkiePC's Avatar
 
Join Date: Mar 2005
Location: ENE of Nowhere Oklahoma
Posts: 10,686
If the VFD is in torque mode, then you are providing a torque reference, not a velocity command, right?

It has been a few years (like 12) since I worked with VFDs in torque mode and runaway velocity was something that could be complex to prevent in some applications.

Maybe modern drives have better controls, but back in the olden days, we had to switch the VFD to velocity mode in order to truly control speed and to do that we had to briefly disable the drive, then re enable it.
__________________
You can choose a ready guide in some celestial voice. If you choose not to decide you still have made a choice.
  Reply With Quote
Old September 16th, 2020, 01:55 AM   #8
GaryS
Member
United States

GaryS is offline
 
GaryS's Avatar
 
Join Date: Aug 2003
Location: Lancaster Pa.
Posts: 1,382
A few thoughts on this
Fist you never said what the application is.
From your description and your screen cap I would guess that it a web tensioning application.
Normally with a torque control application you set the max speed of the vfd and control the allowed torque (Torque Command or torque limit). The torque limit is usually received in the vfd as the channel 0 or the main analog input
In your case you have the max speed set as the default max 60 hertz
And it looks like the vfd receives the torque limit through the Ethernet command.
In the start up with very low torque command it slowly ramps up to the desired torque and the motor speed will go up to meet the web speed. The motor speed will be whatever it needs to maintain the web speed. Keep I mind if your web speed drops down, increases or even stops the motor speed will also follow the web speed but torque will remain constant as long as it in torque limit.
You say when you stop the vfd the speed goes up and the torque goes down. That would be explained if the web broke or ran out and then you try to ramp down the torque on the motor. While the load (tension on the web ) is now at zero but the torque command is at some level ramping down, the vfd will try to output the torque you ask for but it has nothing to work with so the output hertz / motor speed will go up trying to give you what you ask for. Your actual speed command is the max hertz you set in the vfd
I generally like to look at the line speed usually from the main drive and set the max speed / hertz to just above the level that way when the tension on the web drops the speed will only increase a small amount until you can get control or shut it down.
In your case I would recommend that you set both the accel and dccel ramp time in the vfd to 0. The ramp time in the vfd is for speed command only it has no effect on the torque command. In this case it is hurting you.
You are controlling the ramp of the torque limit with the PID control so the ramp in built in to the PID control.
keep in mind that a torque command in a vfd is not a true command, it is a torque limit, if the torque limit is set high but the actual load / tension on the web is low the vfd will increase the output hertz until the torque limit is reached.
You need to quickly shut down the vfd or set the torque limit to 0 when the web breaks or when the load drops quickly.
To help explain this try to run the vfd without the wed / load the motor speed / vfd output hertz will increase to the max limit on the vfd. You will not have any speed control or torque control
A good vfd should be able to maintain 100% torque at 0 speed and have full control over the motor. You donít need a Flux Vector vfd to do torque control they are designed for accurate speed control or even positioning.
I have used them to replace servo drives without any problems
  Reply With Quote
Old September 16th, 2020, 12:05 PM   #9
norunningintheplant
Member
United States

norunningintheplant is offline
 
Join Date: Aug 2020
Location: Washington
Posts: 6
Quote:
Originally Posted by Gene Bond View Post
Is it possible the load is over-driving the motor? It could be you need some braking torque, and without a DB resistor, there's nowhere for the energy to go.
I don't expect it to, but I will look into it.


Quote:
Originally Posted by OkiePC View Post
If the VFD is in torque mode, then you are providing a torque reference, not a velocity command, right?
All of the logic is built in velocity and it sends a jog (MAJ) command with a velocity reference, but it must be getting converted to a torque somewhere. I can monitor the commanded torque on the drive and see that it fluctuates wildly, but the velocity feedback usually tracks pretty close to command velocity in the logic. To be fair, I don't quite understand torque mode or why it is being used in this application.


Quote:
Originally Posted by GaryS View Post
A few thoughts on this
Fist you never said what the application is.
From your description and your screen cap I would guess that it a web tensioning application.
The motor turns an auger that pushes a powder through a track into a tank. The VFD speed is geared to the speed of a pump in the tank. The VFD is commanded to stop when the tank is full.
  Reply With Quote
Old September 16th, 2020, 01:49 PM   #10
GaryS
Member
United States

GaryS is offline
 
GaryS's Avatar
 
Join Date: Aug 2003
Location: Lancaster Pa.
Posts: 1,382
if that's the case why are you trying to control torque you should be controlling speed
as simple speed follower setup feed speed follows the pump speed. i don't see any advantage to torque control here
  Reply With Quote
Old September 16th, 2020, 03:11 PM   #11
thingstodo
Member
Canada

thingstodo is offline
 
Join Date: Feb 2012
Location: Saskatoon
Posts: 377
Quote:
Originally Posted by norunningintheplant View Post
I have an auger powered by an induction motor under flux vector torque control driven by a PowerFlex755. According to my log, the command velocity was ramping up at a reasonable rate while the actual velocity was lagging behind. As soon as the command velocity began ramping down again, the actual motor velocity ran away up to the drive speed limit and shut down. I have included a a screencap of the log showing the velocities. https://imgur.com/a/MWtlVRm

How/why would this happen? PID instability? I'm at a loss. Thanks
Looking at your screen capture, I would say that the load torque drops much faster than your speed reference is dropping. But the motor continues to speed up as it crosses your speed reference.

I agree that this should not be happening, so something else is in play

If you are sending a velocity reference, it should not matter that you are in flux vector control, (which gives you more torque at low speed, lower voltage and current at high speed, etc but should not overspeed your motor)

The acceleration and deceleration times can cause your speed to lag your setpoint by a significant amount. That can also happen when you reach current limit on the drive output ... but I don't think that's what is happening.

I've never used 'jog (MAJ) command with a velocity reference' ... how does that work?
  Reply With Quote
Old September 16th, 2020, 06:34 PM   #12
norunningintheplant
Member
United States

norunningintheplant is offline
 
Join Date: Aug 2020
Location: Washington
Posts: 6
Quote:
Originally Posted by thingstodo View Post
I've never used 'jog (MAJ) command with a velocity reference' ... how does that work?
Like this apparently: https://imgur.com/a/Q3CMQvU

I've been digging deeper. I thought the velocity commands were being sent over Ethernet but it appears that is only used for feedback to the PLC. In reality the velocity reference is passed to an analog motion module (1756-M02AE) where it compared to the motor feedback to compute a torque reference, which is then sent to an analog input on the VFD. I still don't know why torque control is being used but this equipment has been running like this long before I got here.
  Reply With Quote
Old September 16th, 2020, 08:55 PM   #13
OkiePC
Lifetime Supporting Member
United States

OkiePC is offline
 
OkiePC's Avatar
 
Join Date: Mar 2005
Location: ENE of Nowhere Oklahoma
Posts: 10,686
I didn't realize this was a motion application. Again, my experiences are antique now, but many (perhaps all?) of the motion control applications I worked on had the servo drives set up for torque mode and the motion controller closed the position loop using a position feedback and drove it with a command reference to the servo amplifier. Your VFD being set up in torque mode sounds like the same situation.

In other words, my previous comment can be ignored.

Is this an application that worked well and now doesn't? Or is it new (or relatively recent) and now exhibits this problem?
__________________
You can choose a ready guide in some celestial voice. If you choose not to decide you still have made a choice.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Connect to RSEmulate from another PC running RSLinx ASF LIVE PLC Questions And Answers 7 March 5th, 2019 09:17 PM
Simovert Master Drive -Profibus Enabling Running Problem Engrtalha LIVE PLC Questions And Answers 5 June 9th, 2017 07:39 PM
Directly buried FO cables vs Cables running in Underground ducts Charbel LIVE PLC Questions And Answers 5 March 27th, 2017 02:55 PM
Kinetix 7000 Is Running Without Drive Enable mjp123gp LIVE PLC Questions And Answers 10 December 10th, 2016 09:52 PM
1746-hsce2 counter running flat JNS LIVE PLC Questions And Answers 1 July 11th, 2010 02:03 PM


All times are GMT -4. The time now is 11:10 AM.


.