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| Isolated zero cross detection w/ AC mains |
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| iroc86:
--- Quote from: Ian.M on October 17, 2019, 11:59:40 pm ---Re: second diagram in reply#55, Not quite. Remember the current lags the voltage in an inductor, so depending on the load resistance on the secondary side which 'reflects' to the primary scaled by the square of the turns ratio, the primary resistance and the primary inductance, it will reach zero current and commutate anywhere between the voltage zero crossing after the last firing pulse and the voltage peak of the next half cycle. If you add a trace for the primary current to your plot, you'll get a better idea of what's going on. --- End quote --- Okay, I think I got it... so the behavior would be something like this? The "region" being based on the variability of transformer design with respect to primary and secondary interaction. |
| Ian.M:
Again not quite, as the current waveform is phase shifted from the voltage by an amount that depends on the ratio of the load resistance to the load inductance. 0 deg for pure resistance, 90 deg lag for pure inductance. Therefore the red line is the LIMIT for the current phaseshift and the actual current waveform will have a phaseshift in-between the voltage and the red line. As you are building a spot welder the load is largely resistive when its working OK, so I would expect a large phase shift between making good welds and if there is poor contact with the work. Assuming you stop the gate firing pulses well before the end of the previous voltage half-cycle, the commutation time limits will be as you sketched, and the actual commutation point will be where the actual current waveform crosses zero, within that time range. |
| iroc86:
--- Quote from: Ian.M on October 23, 2019, 01:11:50 pm ---Again not quite, as the current waveform is phase shifted from the voltage by an amount that depends on the ratio of the load resistance to the load inductance. --- End quote --- Gotcha. I think that's how I was understanding it, but my image maybe wasn't the most clear way of depicting it :o. I probably should have blurred the red current waveform from 0-90 degree phase shift to show that it could fall anywhere in that range based on the specific characteristics of the circuit. So, back to the earlier discussion about turning on the transformer, it sounds like the ideal "on" point would not necessarily be at the voltage peak, but rather at the current zero crossing point according to the actual phase shift characteristics of the load and transformer? I think this is what beduino was describing in his circuit in Reply #44 with being able to adjust the offset time. |
| beduino:
--- Quote from: iroc86 on October 26, 2019, 01:39:54 pm ---So, back to the earlier discussion about turning on the transformer, it sounds like the ideal "on" point would not necessarily be at the voltage peak, but rather at the current zero crossing point according to the actual phase shift characteristics of the load and transformer? I think this is what beduino was describing in his circuit in Reply #44 with being able to adjust the offset time. --- End quote --- Nope, by offset time I mean between voltage peak maximum and next zero crossing, which is for 50Hz mains with T=20ms period T/4=5ms time span, but to ensure to do not triger thyristor in the case zero crossing missing, my idea is to limit this offset time to predicted 1ms before next ZC, so in reality for example by adjusting potentiometer between 0%-100% offset time will be in my case 0ms-4ms from voltage peak and additionally I'm able detect to fire pulse train from the same polarity since I can ditinguish which part of T/2 we are while I've sense positive/negative AC. Maybe, It is more visible in my post #47, when we do offset lets say half of T/4 lets say 2.5ms offset. In my software potentiometer will be used to choose spot weding program rather than set this offset directly, while sometimes we would like to do some kind of preheat or other things depending on what we spot weld, so MPU will be programmed to change those offsets in real time within e few seconds spot weld time in fully automated way. I'll heve some real data soon, since now I'm working on redesigning mechanics of spot welder to make it ready for testing new software. |
| iroc86:
--- Quote from: beduino on October 26, 2019, 05:27:10 pm ---Nope, by offset time I mean between voltage peak maximum and next zero crossing ... --- End quote --- So, kinda like the way a triac dimmer switch works with incandescent bulbs? You'd be outputting a clipped wave based on the starting offset time. This is a little different than the welder design I've been thinking about, where the user would select n number of full 60 Hz cycles to apply to the work piece. |
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