DWH Technical Topic: Drive Surprise

By Clarence Klassen, P. Eng.

After decades in the business, I still get surprised by differences in drives. Here are a few:

Speed reference does not match the speed feedback. The speed setpoint was scaled by the maximum speed, and the feedback remained unchanged and had to be scaled in the PLC.

The drive will not run if the speed setpoint is greater than its maximum speed. I expected the drive would run at the maximum speed in this circumstance.

Autotune provides so many choices. Rotating, non-rotating, inertia. What do these choices all mean? In most cases, the manual does not help, and the vendor must be contacted.

I ran into one drive whose speed regulator could not be modified manually. We were stuck with the results of auto-tune. This provided a very satisfactory response, but I was looking to push it a bit harder for more bandwidth.

Another drive could not be left in a stall condition for more than 20 secs without faulting. This is very limiting for winder drives. When it comes to establishing tension, I like to "set and forget."

One drive does not tune the speed loop. Surprisingly the speed regulator response proved to be superior to most other drives.

You would think I had seen it all, but I am constantly amazed at how many parameters will limit the speed to 60 Hz. When the need to overspeed a motor crops up, there is one speed-limiting parameter after another until the last one is moved out of the way.

Have you ever seen a 500- or 700-page user manual that does not include a block diagram of the drive functions? I have. More than once. And more than once this year! Usually, the words are translated into our second language with varying degrees of comprehension. Please give us block diagrams. If the drive has 1,000 parameters and the manual has 700 pages, the block diagram should be at least 20 pages long. The secret is out. Someone in the product-design department has block diagrams.

Connect an encoder incorrectly. Anything may happen with an unfamiliar drive. We may get an understandable fault message. We may get a drive that runs at any incorrect speed it wants to. The drive may accelerate to infinity and beyond. In either direction. Please!

Drive status bits may be intermittent. We need to know a drive is ready to start, has started, is faulted, is stopping.

A motor can provide either torque or speed. It should be easy to switch between speed and torque. By easy, I mean a one-bit change or a one-reference change. This should be built into the basic or medium communication protocols. It should not require custom linking of parameters and hours spent with the user manual.

I recently ran into a drive that had trouble with forward and reverse. The speed reference was always a positive number with a direction bit. The speed and torque feedback were always positive numbers with a polarity bit. The polarity bit was not synchronized with the speed value.

Attention drive sales reps: Please provide solutions to these surprises, and you will have a technical advantage in winning your next order in the web-handling industry.