120 degrees phase shift for three-phase pwm? Any ideas?

[declan]

Hi, everyone! newbie's here.

How to write a three-phase pwm train pulses with 120 degrees phase offset from each ole ther? The sample code here is only for one pulse? So I wonder if anyone could help?

[sayzer]

There are examples at Microhip's web site but all in ASM.

If you find some examples in PBP, pls post here. I am also on the same thing.

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[declan]

Sorry Sayzer, I have been searching around but came out nothing, that's why I come here to ask.
So if anyone has something same as this, please post.
it is much appreciated!!

[mister_e]

Download the application note 900 of microchip. It use a PIC18F4431.
http://www.microchip.com/wwwappnotes....aspx?lang=en#

Not in PBP but maybe a good kick in the ... to start

[sougata]

Hi,

Please let me know your application. It is highly doable in PBP-with mixed asm. In fact you can use darretl's routine of multipwm. You need another interrupt that is 3 times your base interrupt frequency and offset the main pwm value into the other. Still you will have offset errors (I suppose), since all the three PWMs are not updated at once. May be okay for driving a motor or UPS in general purpose application. Choice of the PIC is important as if you are playing with soft PWM then you might not have enough computing time left for other jobs.

[sayzer]

Hi Sougata,

For me, what I need is not sophisticated.

At first I would like to control the speed of the motor via a pot or couple of buttons.
Then I can get it modified as I needed.

So far, mister_e advised me to consider 18F2331 or 18F2431 or 4431. Thanks to him very much.

I need to use PBP so that I can modify and understand the concept easily.

Not a very sensitive control I need. Errors are very flexible in my case also.

But I have no idea where to start. I have good examples in ASM which I am no good at.

I would appreciate some information.



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[declan]

hi, everyone!
the pwms that i need is very simple: just need to be same in amplitude and frequency as well as duty cycle.
but have to be 120 degrees offset from each other.
the pwms will pass through a 3-phase bridge driver and appear as control signals at the gate terminal of the 3-phase inverter.
the examples on microchip website shows that they are using a lookup table to update the duty cycle. what is a lookup table? how to do that?

by the way what is soft pwm?

have a nice day!!