no bits stepper drive

neiltechspec

Senior Member
Very neat.

Just tried it myself & sure enough it does work (with what was handy),
18VAC Wall Wart, NEMA17 Motor, 100uF Bi-Polar Cap.

Neil.
 

rq3

Senior Member
Yes. The inductance of coil A lowpass filters the square wave to something approximating a sinusoid. The capacitor adds a 90 degree phase shift, which in conjunction with the inductance of coil B, makes the whole thing work. This will work on any stepper motor, but you have to get the phasing right. 5,6, or 8 lead motors need a bit of deciphering, and the LED trick is a good one, especially since it lets you figure out the coil phase by observing whether the LED lights with a clockwise or anticlockwise rotation of the motor.

The motor speed varies only with the frequency of the applied AC, and two electrolytic capacitors connected anode to anode (or cathode to cathode) in series are equal to half the value (in Farads) of one cap of the same individual cap value, but aren't subject to the "DC cap on an AC circuit" issue, if the individual caps are rated for twice the applied peak to peak voltage.

EDIT: non-polarized to polarized, and various clarifications regarding cap voltage. The reason electrolytic capacitors are used is that you need a LARGE microfarad value, which isn't easily or cheaply available in non-polarized (not electrolytic) capacitor styles.
 
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stan74

Senior Member
What would the maximum frequency be for a unipolar 5V motor?..with a smaller cap. What advantage over a comparable dc motor?
Does it have to be real AC or would a dc square wave work? I got a stepper motor but it's late.
 

AllyCat

Senior Member
Hi,

Basically, it's using the stepper motor as a mains (line) synchronous motor, which "throws away" almost all of the usual benefits of a stepper motor, i.e. (instant) stop/start, variable speed and reversing. If you happen to have a convenient mains supply, a suitable step-down transformer, two capacitors (to put in reverse-series as above) and want a motor that rotates continuously in one direction at the one available speed with modest torque, then you have a solution. ;)

Most of those questions are unanswerable except by "It depends...." (on the motor, capacitor, transformer voltage, etc, etc.). Synchronous and stepper motors are not really "comparable" with dc motors; when you increase the load on a dc motor it slows down and draws more current, a synchronous/stepper will just stop or judder.

It might "work" (or at least rotate) with a square wave, but the capacitor can't give anything close to phase-shifting the square waveform by 90 degrees, as it can with a sine wave, so the performnance will be (very) poor. But why bother when you may be able to drive it from a "real" H-bridge with a 20 pence module and two PICaxe pins (or perhaps a 556). Wow, already 8 pages back, but I will give an update in due course. ;)

Cheers, Alan.
 
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