Treat the variable frequency drive with kindness

Variable frequency AC drive controllers are all sorts of amazing. They rectify AC power to DC then give you 3 phase AC at the desired frequency to let you run the motor at the desired speed, anywhere from just a few RPM up to full tilt. They can also, if misconfigured, drive you up the wall at 60 cycles per second.

I’ll save you the pain of having to watch me scream about Johnson Controls Metasys, here’s the data showing that this one air handler is not happy. This graph is showing its air output temperature. This morning I came in to find half the facility nearing meltdown and decided to see if I could do anything about it.

There’s the drive for that unit’s blower. I found it shut off on a DC bus overvolt fault. The automatic fault restart was not enabled, so it just sat there.

I restarted it and watched it ramp up to full speed…. fearsomely. After it’d let the place cool down a while I revisited the settings. This Yaskawa controller actually has pretty good documentation and a setup routine designed to aid in quick deployment (I CANNOT say that for all the controllers I’ve come across in the wild). Right away I noticed the amperage and wattage limits set in the controller did not match the motor, which did not appear to be original to the unit.

Yeah uh I’m gonna have to recommend you not do that. I filled out the proper values in there, turned on fault restart, and ran the auto tune, which sounds like angry crickets on this unit.

Aside from the drain pan looking suitably foul, I’d say it’s happy again.

I dunno, I realize that variable frequency drives are probably a bit of crazy black magic to a lot of HVAC people, but pleeeeeeeeeaasseeeee make sure you have the thing configured right for whatever motor you’ve wired to it! It does make a lot of difference!

Oh, and the Johnson Controls corporation is a bellended bagbiting cockwomble. There i said it ok

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