3-Phase AC Motor Driver

I have a mystery motor that has 3 wires coming out. It is about the size of two 'C' size alkaline cells in series.

I was thinking of building 3, 6-bit R-2R networks and use 6 pins on each port BC&D to drive them to create a sine wave of various frequency.

Which device would you select for supplying the current to the motor? The motor came from some kind of laser set and has an octagon first surface cube, so it likely was not a high current application.

Is there a better idea or some tests I can do to help figure out how to proceed?


If you know it is 3-phase, there must be a name-plate. What voltage is it designed for? Are the 3 leads all connected to each other? If so, what is the resistance between the leads?


I'm pretty sure a motor that small is not a 3-phase motor. 3-phase motors are LARGE industrial motors... We don't have 3-phase power in our homes...

Maybe one of the wires is ground. Do you have a multimeter to measure the resistance? Or, maybe it's a stepper motor?* Or, sometimes the 3rd wire is a speed sensor...

Without knowing the voltage & current and without knowing if it's an AC or DC motor (or a stepper) there's no way of knowing how to drive it.

I was thinking of building 3, 6-bit R-2R networks and use 6 pins on each port BC&D to drive them to create a sine wave of various frequency

You can't drive a motor through a resistor network because there is too much current. As soon as you connect the (low resistance/impedance) motor, the output voltage will drop to (nearly) zero. And as you may already know, you can't get more than 40mA out of the Arduino so you can't driver a motor without an additional driver circuit.

  • Most stepper motors have 4 wires because there are 2 coils. Sometimes the coils are center-tapped and you have 6 wires.

Perhaps the key is in the comment the motor has an octagon shaped front surface mirror on the shaft. It cane from a laser scanner device. It is a motor with one lead for synchronizing the rotating mirror with the scanning laser and other electronics. Probably a DC motor with current regulation to stabilize the speed.


I was thinking it is a brushless DC motor. It would be sensorless. I had something like this in mind, but without the pic micro of course… I’ll try to get a picture later today. The motor just has a long model number inked on and I could not get any results back. It might be around 24V (as a wag). I think all the resistances between wires were about equal. I’ll check that too.


The motor says


Blue - Brown 1.6k ohm
White - Blue 800 ohm
White - Brown 800 ohm

I applied 15VDC Blue to Brown and the motor had a very slight tendency to pull one way or another from a natural resting location, meaning if I rotated the shaft slightly it would return, or move to the next sector. This tendancy was stronger with voltage applied between white and either other color wire.

I hooked the three wires to three scope probes, connected all probe grounds together, and gave the motor a spin. I did not pay attention, but maybe the yellow trace is the white wire since its voltage is about half the other two.

Any thoughts on how to drive this? Is my original idea in the right direction? I would feed the output of my R-2R resistor DAC to some power device, maybe an op-amp, transistor, or FET?

That looks like a classic 3phase output, but one wire is low, is it a brand new motor or dumpster variety.
The low phase output could be why its been discarded.

Tom… :slight_smile:

It came from a scrap collector who bought the lot mostly for the aluminum framing (about 4’x4’x4’ tubes and plates). I bought several laser tubes, HV power supplies, several motors like this and larger, with a bunch of mirrors. I think it came from some kind of research facility, or maybe industry. I have another identical motor that I’ll have to dig up and measure. Thanks.