I would like to use an audio sine wave generator, plus four amplifiers, to drive a custom-made four phase motor at low speed.
To enable adjustment of the rate of rotation, it needs to operate over a range of audio and subaudio frequencies, i.e. 1 to 100Hz. In other words, not just at a single frequency.
What is the simplest way in circuitry to obtain four quadrature phased signals for this purpose?
Robert Miller
I wouldn't try to do it from a bench sine generator, for starters. It'll be much easier to make an oscillator that'll do the job than to try to take a single sine wave and generate a 90-degree phase shifted one over a
100:1 frequency ratio.If you're going to be doing this a lot you might want to just buy an arbitrary waveform generator with two channels output, that can be programmed to do what you want.
How much distortion can you stand? If I thought I could do it with a bit of distortion allowed, I'd make a two-integrator oscillator with a stereo pot to adjust frequency. I'd probably have it work in two ranges, so that setting the frequency could be done by human beings. You'll need honkin' big caps and high impedances to get it to go that slow reliably. Using the light-bulb method of controlling the amplitude is right out at
1Hz, so unless you can get really fancy you'll either want to use diode limiting or just let the op-amps bang into the rails (which is why I suggested to only do it if you could stand some distortion).Personally, I'd do it with one of the processor eval boards that I have lying around, with PWM channels running at about 10kHz into analog low- pass filters -- but if you're a pure analog guy, you'll want a pure analog solution.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
My Rigol function generator has two outputs that can be set 90 degrees apart. I've made a quadrature oscillator out of some opamps in a all pass filter configuration.. maybe one or so degree of error. Or try the infamous phase sequence filter.
George H.
Be careful
When driving a PM motor in open loop you will only be able to run at very low speeds before the magnets demagnetize
(If it is a PM, for induction motor it's easier)
Cheers
Klaus
A little analog mixing makes any phase angles from that.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
A phase-shift oscillator, with transconductance amplifiers feeding matched capacitors, generates as many different phases as you have stages. Just feed identical program currents to all the amplifier sections.
See Fig. 35 here for a three-phase example (you might want some gentle diode-clipping instead of using the Schmitt comparator for amplitude setting)
Quadrature oscillator?
It isn't cheap - at $A55 in small quantities - and telling it what outputs to generate would be tedious, but it should be able to do exactly what you ask for, if you set it up right.
You will need to buffer each of the four outputs to get enough current to d rive a motor, but that shouldn't be difficult, though it might not be all t hat cheap.
A purely digital solution, producing outputs that produced three level appr oximations to the desired sine waves, could be cheaper and simpler.
A waveform that sits at ground from 0 to 30 degrees, at high (positive rail )from 30 degrees to 150 degrees, at ground from 150 degrees to 210 degrees, at low (negative rail) from 210 degrees 330 degrees and goes back to groun d from 330 degree to 360 degrees, has no third harmonic content and not a l ot of fifth harmonic content.
The digital part could be done in a fairly small programmable logic part, a nd a microcontroller could do the digital part of the job if it had enough output pins.
-- Bill Sloman, Sydney
Thanks to everyone for their replies.
I was hoping for something self-contained and portable, but have decided the simplest approach is to generate two identical signals (tones), in audio editing software such as Audacity, with a 90 degree phase difference.
This also allows for experimentation with different frequencies and wave shapes.
If I use a cheap USB soundcard and short the filter cap, I can even output sub-audio frequencies.
Then obtain the 180 and 270 degree phases using external inverting op amps. These, combined with two non-inverting op amps for the 0 and 90 degree signals, can drive the motor via emitter followers.
Robert Miller
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