3 phase PWM motor controller filtering

little

50%

electrical

oops sorry wrong group, moving to sci.electronics.design

Colin =^.^=

This is a real problem. I haven't come up with any way to build a good output filter for traditional PWM schemes like synchronous antiphase, in which either the high-side or low-side transistor are always on. I have a DC motor drive that uses sign-magnitude instead, and it works MUCH better. I haven't extended this to

3-phase yet, but am working on it. It seems that if the two poles that are to be driven according to the commutation signals are shorted except when the PWM says to apply power, then the circulating currents in the motor and filter will be much less. (This requires enough series inductance in the filter so the decelerating current never gets very large in a single cycle.)

Oh, my, that is a real problem! There is no way you can run such a motor without adding inductance externally. You either have to drastically limit the number of turns/core or use a modulation scheme that cuts down on the current ripple. A higher PWM frequency can help, up to a point.

Look at the IR 2113

Jon

opposed

little

50%

Thanks, Im not familiar with those terms, but all my 3 outputs are always switching, thus at idle each is at 50% pwm.

with synchronous 50% pwm the phases are also effectivly always shorted together, with one phase at 51% pwm the phases are shorted 99% of the time. with the pwm applying power between phases for 1%. although the CM signal is large this isnt a power consuming issue.

I tried making one output always low but this transfered most of the dissipation in the top FET to the lower one at low duty cycle.

yes it does seem a bit of a problem, ofc too much inductance will limit my top speed. Im wondering if I should feed the 3ph bridge with an adjustable voltage from a switching regulator to reduce the pk-pk HF content.

electrical

the low side level shifting only tolerates 5v, I was after 100-600v for both hi and low, so the logic input gnd is totaly isolated from the motor supply gnd.

Colin =^.^=

This is the same as what would be called synchronous-antiphase on an H-bridge, like for a DC brush motor. The continuous triangle-wave current is the problem. If the inductance is small, then the peak-peak current is large. without changing the PWM modulation scheme, the only fix is to add inductance, and then you move the heating problem from the motor and transistors to the inductor core. That is not a great improvement, in most cases.

Now, you are on the right track! Just add series inductance. The change in modulation from full voltage one way or the other across the inductance all the time, to full voltage only 1% of the time is an ENORMOUS improvement! You would need huge cores with the 50% scheme, but a MUCH smaller core will do for the 1% scheme. Your test didn't work out because you were shorting the motor - clearly a lot of loss there. With enough inductance in series, then the circulating currents both at the PWM frequency as well as the motor's synchronous frequency will be reduced.

They used to do this, but it is not necessary. And, you don't need much inductance, just enough to cut the P-P currents at the PWM frequency, not the motor's synch freq.

Opto-isolate then. I use the TLP2200 with the logic running at 12 V for noise immunity. The same 12 V is the FET gate bias supply, too.

Jon

most

but my 3ph pwm outputs are not antiphase. whatever scheme is used there must be enough inductance to limit the rise of the current during the on period. otherwise the FETs realy will get very hot indeed wich was my first problem.

The motor has much less than 10uh (limit of my measurer) initialy I tried winding 200uh with one of my cores from a big bag ive got, but this got way to hot so i had to use thicker wire with less turns.

I would agree that antiphase is as if its trying to deliberatly do the worst thing lol and anything would be better.

but with my in phase pwm outputs there is also definatly no reverse voltage across the inductors, as they are accross the outputs. there is only a forward voltage difference when the falling edges are skewed by different duty cycles wich wld be 1% at low power.

Ive also decided to switch to center aligned PWM in my dsPIC wich means power pulses between phases on both edges of the PWM output.

Incidently I used another method where I only used 2 PWM modules for 3ph. one PWM controlled the first phase completly. one of the other phases was either high or low as appropriate, and the other was pwm driven.

Its also neat to keep the motor center volts close to ground.

With

yes you need enough total series inductance to flywheel the current so that the motor sees just an average voltage. otherwise the current might also reverse wich will just multiply all losses.

another big reason why it was getting so hot was software, with such a incredibly low motor resistance not just inductance a huge current flows if the op voltage does not match the back emf from the motor, the motor speed is of course determined by the comutation frequency. took me a while to get this right, what with problems with hall sensors not working too well etc. and the smoke was kinda putting me off tbh.

yes I had considered that if I couldnt find what I wanted. also gate drive transformers driven from a HF chopped signal and rectified, as some mcu have PWM outputs wich can do this directly.

thanks for all the help and ideas, its clearer now.

Colin =^.^=

Please do not shoot the messenger, but i do not think that hobby 3-phase motor controllers do not do any PWM at all. Except at stop, to prevent burning the motor up. You can get one at any RC hobby shop, or on the Internet.

I think that running the waveforms look like this:

A. |-----_ |-----_ |-----_ |-----_ _____- _____- _____-

B. ---_ |-----_ |-----_ |-----_ _____- _____- _____- ___

C. -_ |-----_ |-----_ |-----_ _____- _____- _____- _____

This is intended to represent "square waves" with a little dead time.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller