A few months ago I was asked what it would take to drive a 3-phase AC induction motor from a lighting console. A specific variable frequency drive by Delta Electronics (VFD-L series) was provided. Coincidentally, miceuz was working on a variable frequency drive of his own, so a motor that fit the purpose just happened to be lying around.

So everything that had to be done was a translation from DMX512 (the console) to Modbus (the VFD).

I took my trusty Metaboard and meditated. Quite conveniently, both DMX512 and Modbus are RS485 protocols, so a 75176 interface chip for each is all the hardware that is needed. Here’s the schematic I drew then and fished out from under the desk now. The table on the left is for bookbinding signatures and absolutely unrelated.

75176 and ATMega interconnection

Writing the code was pretty simple, since I took most of it from the DMX dimmer project of mine. Which is going slow and steady, thank you.

The rest, dealing with Modbus, required a skim through the VFD’s manual, and is pretty much hard-coded. I let myself do this, since the goal was to see if it can be done at all, and the finished thing needed only to have speed control, nothing else. A lot more is possible, though, with the mentioned VFD. If I had to do that, I’d probably go with FreeMODBUS, a library for a number of embedded architectures.

To simulate a lighting console, I used QLC (software) and ENTTEC’s DMX USB Pro (hardware). Keep in mind that if you want to use this setup on GNU/Linux, the former uses a not-invented-here approach to communicate with the latter, and you might need to blacklist the ftdi_sio kernel module.

Keeping the story short, after 6 hours of copying code around, I came up with this slackjob proof-of-concept to control an AC induction motor using DMX, taking a few givens for granted. Mind you, a VFD for industrial automation is quite more common (and, therefore, cheaper) than professional theatre special effects equipment that does the same thing.

The last move I made was film it all, once, on the first camera that happened about. Here it is, in all its pixelated, shaky, moory glory.

And yes, I do call that a workshop.