3 PHASE PWM ON 18F452

[theredkid]

Hi Everybody,

I'm trying to achieve tri-phase angle control of a thyristor using PWM on 18F452. I know the code for single phase, but how can we implement in three phase? I'm restricted to use 18F452. Thank you:

[Ttelmah]

What you post, is not 'the code for single-phase', and has nothing to do with it....

Thyristor control does not want PWM.

With a thyristor, your signal needs to be synchronised to the mains, not 'free running' as a PIC PWM signal is.

Basically, you look for an input signal to signal the zero crossing point, and then a time after this signal, you send a pulse to the thyristor gate to turn it on. For full power, you turn it on immediately after the zero crossing, for 50% you turn it on half a cycle after the crossing etc..

All that is needed is a tiny pulse to trigger the gate. The power changes according to the time this pulse occurs, _not_ according to the pulse width as with conventional PWM.

The thyristor stays on from the moment it receives the pulse to the end of the half cycle.

To do three phase, ideally you have three zero crossing detectors, and for the 'power', you have a look up table of the time values required after this for each required level.
With three zero crossing signals, and a relatively simple code loop using a single timer, and working out the values required on this, it is quite do-able with your PIC. This is phase angle control.

Currently, if you are running single phase,, you have something possibly resembling 'maybe control', with semi-random beating of the switching point. It might vaguely work for something that wouldn't mind the beating (though as posted the range of control would be a few percent, then 'off'), like a heater, but for anything else it is not going to work at all.

You need to start by understanding how Thyristors work, and why PWM is not basically useable with them. Unless something special like GTO thyristors, there is no ability to turn them off once triggered.
Phase angle control can be considered as a very low frequency 'form' of PWM on the output, but it does not want 'PWM' from the PIC, or use the PWM hardware at all.....

[temtronic]

hmm...

Mr. T's "Phase angle control can be considered as a very low frequency 'form' of PWM "

I never thought of it like that but it really is!!
A type of fixed frequency(line or 'mains') analog (AC) PWM.

jay

[Ttelmah]

Yes, The 'trigger' is pulse position modulation, but the output from the actual thyristor is synchronous mains frequency PWM. However as I said it has nothing to do with PWM as generated at the PIC!...

[theredkid]

Thank you all for your detailed answers.