Powerflex 755 Torque Limiting

[ASF]

Hi all,

I'm looking at an application with a Powerflex 755 with encoder feedback, being run as a standard VSD (as opposed to a motion axis), with a simple frequency reference, but the requirement to limit the torque to avoid damage to the mechanical system in the event of a jam. This is replacing an existing dual-winding two-speed DOL motor with some kind of electronic torque limiting arrangement for which I have no details yet.

In reading through the programming manual I can see that I can set torque limits in % (parameters 670/671), and that I must have the drive in flux vector mode (parameter 35 = 3). My questions are:

1. Are there any other drive parameters I need to be aware of in order to set this up correctly?

2. What will happen if there is a mechanical obstruction and the drive hits the torque limit? Will it trip on some kind of fault or just sit there not moving (or moving slower than the desired speed reference) and drawing the specified amount of current until someone notices?

3. How can I calculate the equivalent % torque limit to match the old motor, given that the only information I have is that it's set to "6.7kW at 1430rpm"?

Thanks for any insights!

[TomekPLC]

Maybe you can set it as SLAT min/max?

[ASF]

I haven't come across SLAT before. I'll have a read up on it. Have you used it in this sort of application before?

[chelton]

Originally Posted by ASF

....
2. What will happen if there is a mechanical obstruction and the drive hits the torque limit? Will it trip on some kind of fault or just sit there not moving (or moving slower than the desired speed reference) and drawing the specified amount of current until someone notices?
....

Assuming it's similar functionality to a 525 if the drive is running on the torque limit it will run for a short time then trip on a speed reference no reached or similar fault.

[TomekPLC]

Originally Posted by ASF

I haven't come across SLAT before. I'll have a read up on it. Have you used it in this sort of application before?

We use SLAT Max to limit speed when we are in torque control. One of the analog inputs is the speed target, while the other analog input is torque target.
The way it works on our application is the motor attempts to reach the torque target but it will not rotate any faster that the speed target. There aren't any faults generated, and all the motor is doing is torque control (if the torque target is reached before reaching speed target) or it runs at the speed target when the torque is not reached.


Maybe by manipulating torque and speed targets you can accomplish what you need in your application...

[ASF]

Your application sounds a little different to mine - I might be able to make it work by setting a low torque setpoint and monitoring the status word to see if it's running in torque control instead of speed control, and generate a fault if so. But it does seem a little hacky, I'd prefer to use the torque limit feature in its intended manner if I can