2 phase (90deg) PWM variable frequency drive for a 2 phase induction motor  - Page 1
Forum section: electronics / projects

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: November 08, 2020, 10:43:52 pm
content: So here is my deal. I have to figure out how to controll thish quite oddball induction motor. The windings are 90 degrees apart which is a headache in itself. This induction motor is a 4W one and operates off 48V AC. Seems irrational right? The motor is attached to quite the reduction also. The idea is to use this for a overkill grill rotator (and since there is a insane torque available maybe for some other things too..).So I figured the PWM part is easy, make a triangle oscillator no big deal and then have two comparators for two outputs and have that running into a H bridge driver and to the fets. Thats the easy part. But how am I going to generate the controll signal ? A RC shift network even with a op amp wont produce perfect 90 degrees across 20-400hz range (and yes the motor is totally not built for that range). How do I make a variable oscillator that has two outputs 90 degrees apart with a frequency range of 20-400hz? Thats the problem I have. I see a lot of three phase designs all working as I would want to, but absolutely nothing for a two phase 90 degree induction motors. Anyone has an idea?Arduino would be good here but I have absolutely 0 knowledge of any programming whatsoever. So it has to be analog. I kind of ran out of ideas. I cant drive this thing off of a bench frequency generator forever.I wonder if there are any oscillators that can run off of a pulse. I can create a puls 90 degrees apart precisely at any frequency, But I dont know of any contraption that would make a sine wave on demand based on pulse count.Many thanks in advance for any help.

author: richard.cs
author_rank: Super Contributor
timestamp: November 08, 2020, 10:56:44 pm
content: A useful English search term for this is "quadrature oscillator" which gives you lots of two-opamp results.The other way to get two signals at 90 degrees is to do it digitally, generate twice the frequency you want and feed it into two flip-flops, one with an inverted clock. You then have two square waves at 90 degrees. The angle actually ends up dependent on the duty cycle, so it may be better to generate 4*f and divide once up-front to get a perfect 50/50 duty cycle in your 2*f signal. This gives you square waves of course, which is probably OK for motor driving, but if desired they could be shaped or filtered to sinusoidal.

author: langwadt
author_rank: Super Contributor
timestamp: November 08, 2020, 11:13:53 pm
content: that's what a stepper motor driver does

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: November 08, 2020, 11:40:10 pm
content: Quote from: richard.cs on November 08, 2020, 10:56:44 pmI did try the digital way an then I realised well filtering a square wave into a sinusoidal wont be fun. It sure is possible assuming you have a constant frequency and it would have been much easier in the 400Hz range but its impractical to say the least at 20Hz and it wont work at 400Hz aswell. Basically what I need is a VFD for a 2 phase 90 degree shift INDUCTION (not stepper) motor.I think a arduino would excell here in this task since it could directly create modulated PWM of two variable sinewave outputs at a 90degree shift.the good news is I did come up with a circuit that can convert a series of pulses into a triangle waveform with a updown counter a nor latch and a DA to get a voltage range from 0-5V. SO to get another triangle waveform I would need to just delay the other contraption by 4 clock cycles (because its a 4bit setup)So. In essence I would need one variable timebase which is now a square wave, Square wave oscillators are easy to make variable in very wide ranges in my case it would go from 1kHz up to 10kHz, that would go into a up down counter contraption tha would count up from 0 to 16 and back down by itself, and then that would go into a DA that would convert those 16 steps into a voltage ranging from 0 to 5VThe copy this counter DA contraption and somehow offset it by 4 clock cycles compared to the first counter contraption. Now no matter what clock I would have I would ALWAYS get a triangle 90 degrees apart from the first one. And I think driving a induction motor with a triangle is much more acceptable than a square wave that has been filtered partly because the amplitude stays the same and the shape remains the same over a wide frequency rangeEDIT: I was wrong I meant to say delay by 8 cycles

author: David Hess
author_rank: Super Contributor
timestamp: November 09, 2020, 01:44:28 am
content: One way in hardware is to generate a variable frequency 100 times greater, or maybe 64 times for convenience, divide it down, create the quadrature low frequency digital outputs, and then run each one through a switched capacitor filter which is clocked by the higher variable frequency. Now as the high variable frequency is changed, the switched capacitor filters track the low frequency outputs to produce a clean output.However up to 400 Hz is well within the range of software only direct digital synthesis on a microcontroller.If maximum torque is not required which seems to be the case, then driving the motor with square waves should be acceptable which also considerably simplifies the power amplifier. Then no filtering is required and variable frequency quadrature square waves are almost trivial to implement with some flip-flops and an oscillator.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 09, 2020, 02:57:16 am
content: That's basically a split phase motor (but with symmetrical windings?), generating a pair of PWMed sine waves 90 degrees apart is trivial with any "digital power" microcontroller. Send those signals into a power stage and you basically have a motor controller. Beware that when decreasing the frequency, you also have to decrease the voltage to keep a constant V/Hz ratio.It's similar to the "Shannon Liu Quadrature Drive" intended to run a small refrigeration compressor from DC, except rather than a constant V/Hz ratio, it does some fancy downconversion tricks with the torque pulsation of the compressor piston to detect the actual rotor speed and automatically vary the voltage to get the best efficiency.