A Circuit Cellar reader sent me a lengthy note describing his approach to slow-motion AC motor drives, designed for an already ancient truck mounted radar antenna back in 1972-ish, that prompted me to try it his way.
The general idea is to pulse the motor at full current for half a power line cycle with an SCR (rather than a triac) at a variable pulse repetition rate: the high current pulse ensures that the motor will start turning and the variable repetition frequency determines the average speed. As he puts it, the motor will give off a distinct tick at very low speeds and the maximum speed will depend on how the motor reacts to half-wave drive.
Note that this is not the chopped-current approach to speed control: the SCR always begins conducting at the first positive-going 0 V crossing after the command and continues until the motor current drops to zero. There are no sharp edges generating high-pitched acoustic noise and EMI: silence is golden.
The existing speed control circuitry limits the peak current and assumes that the motor trundles along more-or-less steadily. That won’t be the case when it’s coasting between discontinuous current pulses.
When I first looked at running the motor on DC, these measurements showed the expected relationship:
Later on, plotting RPM against current (50 mA/step starting at 550 mA):
Eyeballometrically, the slowest useful speed will be 2 stitch/s = 120 shaft RPM = 1300 motor RPM. At that speed, under minimal load, the motor runs on about 20 V and draws 550 mA. At that current, the 40 Ω winding drops 22 V, which we’ll define as “about 20 V” for this discussion, so the back EMF amounts to pretty nearly zilch.
That’s what you’d expect for the fraction of a second while the motor comes up to full speed, but in this case it never reaches full speed, so the motor current during the pulses will be limited only by the winding resistance. At the 200 V peak I’ve been using for the high-line condition, that’s about 5 A peak, although I’d expect 4 A to be more typical.
So, in order to make this work:
- the optocoupler driving the base needs more current
- the differential amp from the Hall effect sensor needs less gain
Given the ease with which I’ve pushed the hulking ET227 transistor out of its SOA, the motor definitely needs a flyback diode to direct the winding current away from the collector as the transistor shut off at the end of the pulse. Because it’s running from full-wave rectified AC, the winding current never drops to zero: there will definitely be enough current to wreck the transistor.
The firmware needs reworking to produce discrete pulses at a regular pace, rather than slowly adjusting the current over time, but that’s a simple matter of software…
The Smell of Molten Projects in the Morning 
What keeps the SCR from conducting until the next zero crossing? Is there a zero crossing detector in the circuit?
Yeah, something like that. He gave an overview of the idea, not the detailed circuit layout, with the key feature of hitting the motor with a single half sine wave: full power as quickly as possible.
Seeing as how it happened four decades ago, the rig probably used a vacuum tube or two; I’m not going that hardcore!