st7mc sinusoid pwm - turning off one pwm

bigsmile · ‎2004-10-28

Posted on October 28, 2004 at 14:58

bigsmile · ‎2011-05-17

Posted on May 17, 2011 at 10:17

hello

i want to drive an ac motor with the st7mc. for controll I need to turn off one PWM during a short time. but I haven't found a way to turn off only one PWM an to leave the other ones running. the only way I've found is to change to BLDC mode during this time and to turn off the Phase. but meanwhile the other two phases drive a PWM signal with a 50% duty cycle an not the needed duty cycle for the sinusoid voltage.

can anyone help me please

thanks for your help

bigsmile

gaetano · ‎2011-05-17

Posted on May 17, 2011 at 10:17

Hi,

with STMC configured in BLDC mode you can provide only a PWM signal with a duty cycle value common for each leg of the inverter (both in voltage and in current mode).

Due to that, it is impossible to generate 2 independent sinusoidal signals as you need when you switch in BLDC mode.

Suggestion: what about to put the third leg at ground by providing 0% of duty cycle: is it compliant with your apply?

Regards,

Tanio

bigsmile · ‎2011-05-17

Posted on May 17, 2011 at 10:17

hi

first I want to thank you for your help.

i've had the same idea too. but I need to turn off the third phase completely. this means neither to switch it to high or to low pontential. I have to do this, because I wand to measure the back emf voltage on that phase.

thanks and greetings

bigsmile

gaetano · ‎2011-05-17

Posted on May 17, 2011 at 10:17

I suppose you're talking about an AC motor with permanent magnets and not an asinchronous one: is it right? That's the reason as you want to measure the BEMF.

Moreover, the reason related to a BEMF measurement, I guess, is related to a sensorless stategy...

If all my hipothesis are right, with ST7MC there isn't any algorithm to drive a PMAC motor without any sensor: so your request cannot be satisfied.

I confirm again that, in AC mode, it is not possible to open the third leg as you would want to do.

Best regards,

Tanio

bigsmile · ‎2011-05-17

Posted on May 17, 2011 at 10:17

hi

thanks for your help.

I've found a hardware solution to turn off one half bridge.

but now I have the next problem. Is it possible to use the C, D and Z interrupt in AC mode? now I'm able to see the BEMF but not to detect the zero crossing with the mcu.

can you help me please

thanks a lot

bigsmile

gaetano · ‎2011-05-17

Posted on May 17, 2011 at 10:17

Hi,

once again I have to give you a bad news: when you are in AC mode it is impossible to speak about C, D and Z event that are related only to the BLDC mode or six-steps mode: that's because the MTC peripheral registers used to capture those events are used, in AC mode, to perform different operations (like the speed feedback using an encoder or a tachogenerator).

You can consult the datasheets for more details.

On the other hand, what I feel to suggest you is to use the comparator used for current monitoring as comparator to sense the BEMF zero crossing. When the ouput of this comparator toggles an interrupt can be generated and this information could be useful to your purpose.

If you intend to use this comparator for current monitoring purposes, then you could try to use the embedded OPAMP in conjuction with the current comparator by acting on CFAV bit opportunely (see page 225 of ST7MC datasheet for reference).

Anyhow, these are just ideas on how to perform what you asked. Personally, I never tested them in lab.

With my best regards,

Tanio