How accurate torque limit function of VFD? 1

[soulman]

Can someone tell me how accurate the torque limit functions of a voltage source VFD is? In other words, can one be sure that the motor output torque will never be exceeded (even for ms or so)? In a specific case, we are looking at a 4-Q MV GTO drive with an Active Frond End (GTO's) with a torque limit set to 140%.

Thanks for your feedback.

 

[Skogsgurra]

I am afraid that you have to ask the manufacturer if you want a value that you can use for design purposes. But, generally, a modern vector control drive shall be able to control/limit the torque to better than 5 percent in a sensorless application and down to 1 percent in closed loop.

The torque step response is generally better than two milliseconds if you have an IGBT inverter with a few kHz carrier frequency. Your GTO has a much lower carrier frequency - it might not even be vector controlled - so you really should ask the manufacturer.

 

[gcaudill]

Also, ask if the torque control loop is the primary loop or a secondary loop. In drives with less sophisticated torque control algorithms torque is really a secondary consideration to speed. Also, the quality of the modeling technique in the algorithm, the processor speed and bandwidth all come into play in the torqe control.

 

[soulman]

The question was more meant for some general feedback.

The essential question is: Can one rely on the electronic torque limit function of the VFDs or is it still necessary to size the mechanical equipment (couplings/reducers/shafts) according to the available torque of the motors which can be 220% or more.

 

[Skogsgurra]

I would say yes. But only if you have a vector control drive and if you have a true torque limit.

The torque loop is always closest to the motor (inside the speed loop) in modern vector drives. External torque limiters that act on the speed loop output (like those you could see in some DC servos te nor twenty years ago) do not exist today. And if they do, they shouldn't.

So, if you use AB, ABB, Danaher, Siemens, Schneider etc and there is a paramater called "max torque" or "torque limit" or similar wording, you can trust that the set limit will be kept under all circumstances. A hardware failure that produces a higher torque has yet to be "invented". In DC drives, almost all hardware failures seemed to be able to produce excessive torque. Not so in AC vector drives.

 

[jraef]

Although I agree with skogsgure, I want to add that ANY device such as a VFD that can be reprogrammed to ignore the safeguards you are trying to achieve, should have at least one back-up non-adjustable device as well. In your case, a Shear Pin perhaps. Use the VFD to keep from having nuisance Shear Pin failures. Use the Shear Pin as a backup for the day, and it WILL come, when someone can't figure out why you wanted the Torque Limit and turns it up.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Skogsgurra]

A VERY VALID point of view, jraef! Never forget the human factor.