Information on VFD torque control and torque limiting
Information on VFD torque control and torque limiting
(OP)
This is an elusive topic....I always thought torque control and torque limiting were the same thing. One of my contemporaries expressed that this was incorrect and then couldn't give me enough detail to support it. Can someone enlighten me to the difference.
Websites, articles, personal exp, or just your opinion.
All would be appreciated.
Websites, articles, personal exp, or just your opinion.
All would be appreciated.





RE: Information on VFD torque control and torque limiting
1. Torque Control
1a. Torque control is often imbedded in AC Motor Drives to produce the motor shaft torque characteristics suitable for certain motor shaft load category.
1b. The amortisseur or damper winding at a synchronous motor rotor adds a starting torque to the synchronous machine, which has no starting torque, thus making the synchronous motor self-starting.
2. Torque Limiting
2a. It can be accomplished by a soft starter at a squirrel cage induction machine to produce smooth start of the shaft load and reduce the motor starting and inrush currents.
2b. It can also be accomplished by a proper selection of the squirrel cage induction motor NEMA design, e.g. A, B, C, D, E.
3. DC Machines have wider torque characteristics diversity; therefore, the torque control and torque limiting can take more forms. Please, post which category of motors is to be addressed.
RE: Information on VFD torque control and torque limiting
I guess the question should have been geared more towards AC VFD's. I understood that torque control in the sense of modern VFD's comes from vector drives only. There is a lot of marketing going on with sensorless vector and torque limiting. Other than the obvious difference of external feedback...there are not many dissimilarities (in basic theory.)
It does, however, vary from manufacturer to manufacturer. This is the perspective that I seek.
Thanks,
Carl
RE: Information on VFD torque control and torque limiting
Torque limit I think is what it sounds like, a limit beyond which the controller will not increase the torque to the motor, even if the load calls for increased torque.
RE: Information on VFD torque control and torque limiting
Torque limiting, limits the max torque output. Mechanical/electrical breakdown precaution (shaft, coupling etc.) Controlling torque with the limiting parameter usually results in large control oscillations.
AC VFD's without speed reference are historically not very good at controlling torque. Usually leaving the user to control the torque output with the Torque limiting parameter.
RE: Information on VFD torque control and torque limiting
Muhammad H. Rashid, "Power Electronics Handbook," Academic Press, 2001
Chapter 28 "Sensorless Vector and Direct-Torque-Controlled Drives" outlines various aspects of motor torque controls. It indicates that the torque controls of AC machine is similar to DC machines. Also, it addresses stator-flux-oriented control, rotor-flux-oriented control and magnetizing-flux-oriented control for the induction machine vector control. The vector control may have the torque controller, flux controller, and speed controller.
Also, visit
http://www.baldor.com
http://www.baldor.com.au/source/entry.htm
http://www.robicon.com/library/index.html
http://www.drivesys.com/asdis.html
http://www.actechdrives.com/Library.htm
http://www.abb.com
http://www.siemens.de
etc. for more info