Information on VFD torque control and torque limiting

[ctolbert]

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.

 

[jbartos]

Suggestion:
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.

 

[ctolbert]

I'm looking at the standard drive rated 3-phase motor. No frills, the same kind if a maintenance guy called Marathon, GE, or US motors and got the cheapest 10hp 1800rpm unit they could buy.

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

 

[redtrumpet]

Torque control aka vector control, field-oriented control, flux control, slip control, etc. is a VFD control system that uses mathematical models and current vectors to closely control the speed and torque of an AC induction motor. The total current is separated into its magnetizing current vector and its torque producing current vector. By knowing the vector angle required, and having control of when to fire the output transistors on the inverter, the motor torque and speed can be more precisely controlled.

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.

 

[ellcobrad]

Torque mode controls the output using integral & proportional sums. It is used to maintain a certain amount of load such as on a pump or fan motor.

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.

 

[jbartos]

Suggestion: Reference:
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