Interfacing AC power lines

[sougata]

Hi,

If you are doing a phase angle control then make sure you have a EMI/RFI filter between your zero cross sensing path. Using an opto-coupler is a good choice and the LED currents are not that much sensitive to the line noise (third harmonic injection by your thyristor control module). On the photo-transistor side use a constant current source followed by a schmitt trigger buffer. You can also use a low pass filter there. This gives you a rock-solid sync. If you are not doing a phase angle control then make sure you are firing the thyristors right at the zero cross. I use a sustained fence-pulse circuit (using the HPWM module of PIC). And for low current applications I use opto-triacs (both zero cross and non zero cross depending on the application). As far as the field experience goes , phase reversal are common problems that drive the synchronized trigger algo crazy. Post your schematic if possible.

[BobEdge]

Hi,

There is the current feedback, and phase angle thyristor driver drawings.

When the machine switches on U14 figures out the phase rotation, and whether a star star or star delta transformer is being used. As you can see the only connection to the pic's is the primary and secondary windings. There is no connection to neutral or earth. This works fine in most cases, but with certain mig welders nearby the current loses control, & during the weld some thyristors seem to stay open for a cycle, at random intervals. This got me thinking how does the pic detect the zero crossing point when the phase inputs are not refferenced to the pic power supply, I am wondering how it ever worked at all.

Bob...

[Luciano]

Hi,

See page 2 of this PDF.
http://www.atmel.com/dyn/resources/p...ts/doc2508.pdf

Best regards,

Luciano

[sougata]

Hi,

You mentioned that your circuit works for most of the time. Give me some idea how you have used your zero-cross routine. Have you taken all precautions to prevent a rollover triggering ?? Best practice is to limit the maximum firing angle (minimum output) to 150 degrees. Anyway down to the last 30 degrees you would be getting average power that is probably unsusable for your design. This prevents that you are not triggering the thyristors close to a zero cross. Due to the phase lag and current lag your thyristors may experience the current zero cross a little late after the voltage zero cross (your pic). I use an asm interrupt to turn-off the outputs and do not fire (in case of full firing) before a certain delay to make sure that the thyristors turned off at the zero cross. For firing I use a timer interrupt.

Also from your schematic I understood that you are using a PIC for Phase health check and measuring the shunt-current. Why not use a single PIC that has three external interrupt sources? You can then do away with the DAC part. I would suggest that at each zero-cross you turn off the outputs and do the AD to find out the next firing angle.

Seems it is time to look at your code.