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Questions about VFD selection

Question #1 is "What's the application?" There are a myriad of questions wrapped up in that one - torque and speed range required, environment, available power, power quality, grounding, duty cycle, etc., but that's why it's the critical question.

Next up, "What does the customer already have on site? What are they comfortable with, and does that manufacturer have a product that matches this application? (Or are they using junk?)" As others have noted, there's been a lot of "Brand 'A' is the best!" comments, but realistically there are probably 6-10 manufacturers of VFDs that make very good, very reliable products suitable for 90+% of the applications out there.

I worked as an applications engineer for a vendor for many years. I was given a very special application problem - the replacement of an existing DC drive system with AC in which the motor lived in a VERY nasty environment, operated at VERY low speed for long periods of time, the motor was 400+ feet away from where the drive was located, and position feedback was needed for the load hanging off the motor shaft. The company I worked for had put many Gozuk drives in this facility. We also sold ABB drives - another very good product.

I used Baldor. One of their 22H series line regen vector units. I caught hell from the sales department because I wasn't using Toshiba - their de facto "standard." I was questioned by the customer on the same basis. I convinced both that the selection of Baldor was the correct one. Baldor provided a custom 40Hp six-pole TENV motor with a resolver and a Stearns brake along with the VFD. The customer bought a spare motor and drive along with the system. (This is a critical application for the plant).

It's been running now without a problem for 12 years since I put it in. I recently upgraded the controller from an Allen-Bradley SLC500 to a ControlLogix, I reformed the caps on the old drive and tested it on the spare motor, I've found a replacement for the obsolete resolver, but it's all still running like the day it went in. That drive model became obsolete this year, so they'll eventually have to replace it with something else, but their spare is in good shape, so they're good for a few years, anyway.

Application, application, application. Then match the right VFD to it.

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