Variable reluctance stepper

DigitalBob

Has anyone used a Prop to drive a variable reluctance stepper with three phases. I just finished my own prop version and was wondering if anyone else has fiddle with these. The one I encountered had windings like a unipolar stepper but with only three phases. So one main power wire and three coil wires A, B, and C. It had a feedback pot so I used that for my position holding. Seem to draw a lot of cuurent if one or two phase were left on. I would only send a step command if the shaft was nudged off null.

ManAtWork

I have tried to drive such a motor with my P1 based servo drive. It didn't run very well. The problem was that the digital PID current control loop couldn't cope with the largely varying winding inductance. Depending on the air gap (rotor poles matching stator poles vs. rotor/stator poles interleaved) the inductanced changed by an order of magnitude. That either caused instabilities or very slow reaction.

A standard stepper driver with fast/slow decay chopper switching possibly runs better with this kind of motors.

DigitalBob

The standard step driver didn't have the correct step sequence. I tried the standard driver and it worked, but it had funky steps.

ManAtWork

What is the correct sequence? How many poles, windings and wires does the motor have?

DigitalBob

ABC is the sequence, reference my first post for winding config.

ManAtWork

So it's three phases and four wires? (star point and three windings)

Then it should be possible to drive it with an unipolar stepper driver for a two phase motor (5 wires, star point + 4 windings) by leaving out one output. Of course, the counter must be reset after 3 instead of 4 steps.

Phil Pilgrim (PhiPi)

Typically, in a 5-wire unipolar drive, two motor windings are activated at once. (One-at-once is called "wave drive.") In the case of three windings, you could activate one or two at a time. Two-at-a-time might give you more torque. But you're already complaining about current draw, so maybe that's not what you want.

There are a couple ways to deal with current draw in a stepper:

1. R-nR drive, wherein you put n times the winding resistance in series with the coils and use a few times the usual drive voltage. What this accomplishes is to provide a high voltage into the motor's inductive loads until the coils saturate, whereupon they see less drive voltage, hence less current. The downside is that you waste a lot of energy from I²R losses in the resistors.

2. PWM drive. With this you can control the average current in each winding by the PWM duty cycle driving each coil. Moreover, by providing each coil with more than the rated drive voltage, you get more torque off of the starting block, but can back off the duty cycle when holding a fixed position for less average current draw.

The thing to realize about inductive motors -- steppers included -- is that they're current-driven, not voltage-driven. Whatever you can do to regulate the current they draw will affect their performance more than anything else.

-Phil

DigitalBob

Resetting the counter after three steps would probably work