3 Phase Angle Control

[sougata]

Hi,

By measuring the period between one zero cross to other it is possible to to get the frequency of the input frequency. If you are turning off the TRIACS/SCR while a zero cross occurs then there should be no problem. All you need is a timer locked to the first AC input (R) and have some pseudo timers running in software which gets reset at zero cross signals from the other sources (You need 3 interrupt sources). The only problem is that with a 8 bit timer you get a low resolution. With one 16 bit and 2 virtual 16 bit counters the process is further complicated. I posted a thread (I do not remember now) for dimming various channel with a PIC16F84. It was not tested by me and basically an adaption of my big brother program. It is heavily commented and might help out in your algo design.

My suggestions are :

1. Strictly asm interrupts
2. High Priority 18F interrupt vector
3. High Oscillator speed (I use x4 PLL @ 10 MHz with 18F452 )

My design is a commercial one and cannot share the code thus. However always willing to help out (within my scope of little knowledge).

I have the habbit of looking into the PRO forum only so overlooked this thread.

BTW Pramod I lost your telephone number. One of my partner is right now in Delhi for some component purchase and needs your assistance. Please drop me a private mail or to my regular email with your Phone No.

Thanks and Regards

Sougata

[oldtoddler]

Why don't u simply use optoisolator like MOC3063? it's zero crossing triac driver and I have used few thousands of those without any problems... and they cost about 1eur@farnell.. If u are after power adjusting, then thyristors are not a proper componets due RF-noice.. IMHO..
/MRa

[dukea84]

Hi! I have a question for the same problem..
I'm using 3 zero crossing detector with 3 optoisolators..
The problem is that the load of the motor is an inductive type and the triac stops at the zero current...that is not the zero voltage.
I think that the solution could be starting the firing angle at the shift current phase..

Any suggest?
Thanks.

[mark_s]

Hi! I have a question for the same problem..
I'm using 3 zero crossing detector with 3 optoisolators..
The problem is that the load of the motor is an inductive type and the triac stops at the zero current...that is not the zero voltage.
I think that the solution could be starting the firing angle at the shift current phase..

Any suggest?
Thanks.

Take a look at the data sheet for the PIC18F2331, specifically "motion feedback module". It has three capture inputs and the chip also has power control PWM outputs. The Microchip site as many motor control app notes. Keep us posted on your results

Mark

[sougata]

Hi,

You can use two reverse connected SCRs with two gate drivers (common feed) for switching inductive loads.

[dukea84]

Thanks for yours answers..but I am confused anyway..

@sougata: do u think that use 2 scr and not 1 triac for any phase improve the load controller? why? In that way I need 6 pwm output (instead of only 3).

@mark_s: I'm reading "motion feedback module" section and it's really interesting. The problem is that my pic input are three zero crossing phase and I would like control inductive load (like motors) with different phase displacement.

[sougata]

Hi,

Triacs commute at the zero cross. However due to current lag the voltage zero cross and the current zero cross do not coincide. So the other half part of the triac can come into conduction thus phase angle control is not achieved. If you use a 1:1:1 pulse transformer then the firing signal is fed to the pulse transformer. You can AND a logic drive with a 10 KHz PWM of 10%-25% duty to derive the final driving signal for the PulseTr. The two isolated windings then serve the purpose of firing to Anti-Parallel connected SCRs. So you need 3 drive signals only and not six.