# Tic 36V4 used for VFD with Permanent Magnet Synchronous Motor

Hello!

I have a custom-built single phase permanent magnet synchronous motor that I would like to drive with a sinusoidal current wave as a variable frequency drive.

The current wave needs to have an amplitude between 3-8 A which I could adjust for a range of voltages by varying a series resistor in-line with my coil windings. It must be of adjustable frequency with an accuracy of 0.01 Hz or better.

From my understanding, the questions I need to ask are:

• Can the refresh rate of the microstepping be fast enough so that I’ll be able to differentiate between a sine wave of 100.00 Hz and of 100.01 Hz? I’m using the STM32F411 MCU at a clock speed of 72 MHz if that helps.

• I’ve seen forum posts elsewhere state that you can increase the current output for a single phase by wiring the two phases from the bipolar driver in parallel - can this be done with this driver?

I’m trying out sinusoidal pulse width modulation on the 24v13 (Pololu 2992) but I figured it wouldn’t hurt to look at this board as a backup in case the accuracy isn’t what I want it to be

Thank you,
Nick

Hello, Nick.

We haven’t heard of anyone using a stepper controller as a variable frequency drive for an AC motor like you’re describing, so I don’t have a good idea of whether this is a practical thing to attempt in general, but I can try to answer your questions without that context.

1. It will depend on some factors like the microstep resolution, but it’s unlikely that you will be able to get the timing resolution you’re looking for. For example, if you use 1/256 microstepping (for the best approximation of a sine wave), then it will probably jump from about 99.3 Hz to 100.2 Hz, with proportionally better resolution for lower microstepping ratios.

2. Because of the way the stepper controller works, the two phases are always off by 90 degrees, so I do not see how wiring them in parallel would give you something useful.

Brandon