Summary: I want to build a phase controlled heater controller, thats huge 240 Volt 200 Amps. SCRs that size cost $150 each (would need two, and Id really hate to blow one). Is there any way to parallel several cheap, say 25-40 Amp, triacs or inverse pairs of SCRs?
Details: Helping my son-in-law, glass artist, with a glass melting furnace.
There are 6 heating elements all rated at 6KW, but designed to run on approximately 12 to 18 volts. They are darn near 0 ohms when cold but the R goes up as they get white hot. So the problem is starting up the furnace. (These things are lined with several inches of refractory, so the heat slowly builds up inside.)
He picked up a huge old welder for cheap and I think we can use it alright. The transformer has a 240 Volt, 198 Amp primary, and the secondary puts out about 48 volts (at darn near 1000 Amps), so we can parallel 2 sets of 3 elements in series. I want to phase control the primary of the transformer. To fire up the furnace wed start out with the thing turned way down and then slowly over an hour or two turn it up. (I would tie in the pyrometer/thermostat too which would cycle the whole thing off and on as it reaches the desired temp, but cools down each time he opens the door.)
Ive looked at the ST_AN308 Fig 11 diagram referenced on a recent post here. I think thats a good one to use. What Id like to know is since there is one small triac triggering the larger one, why could it not trigger several paralleled triacs or inverse pairs of SCRs (say, 5 40 Amp jobs)? Wouldnt that make all of them turn on at exactly the same moment (phase angle) - thus not causing an imbalance (which is the reason Ive heard you cant parallel triacs and SCRs)?
Another question about this: Even though the measured DC resistance of the bank of cold heater coils is close to zero, do you think the 120 cycle on-off switching of the primary, along with whatever time it takes for the transformer core to saturate, will sort of act as a buffer and therefore not look like a dead short to the transformer or the line?
Thanks all for your help!
Details: Helping my son-in-law, glass artist, with a glass melting furnace.
There are 6 heating elements all rated at 6KW, but designed to run on approximately 12 to 18 volts. They are darn near 0 ohms when cold but the R goes up as they get white hot. So the problem is starting up the furnace. (These things are lined with several inches of refractory, so the heat slowly builds up inside.)
He picked up a huge old welder for cheap and I think we can use it alright. The transformer has a 240 Volt, 198 Amp primary, and the secondary puts out about 48 volts (at darn near 1000 Amps), so we can parallel 2 sets of 3 elements in series. I want to phase control the primary of the transformer. To fire up the furnace wed start out with the thing turned way down and then slowly over an hour or two turn it up. (I would tie in the pyrometer/thermostat too which would cycle the whole thing off and on as it reaches the desired temp, but cools down each time he opens the door.)
Ive looked at the ST_AN308 Fig 11 diagram referenced on a recent post here. I think thats a good one to use. What Id like to know is since there is one small triac triggering the larger one, why could it not trigger several paralleled triacs or inverse pairs of SCRs (say, 5 40 Amp jobs)? Wouldnt that make all of them turn on at exactly the same moment (phase angle) - thus not causing an imbalance (which is the reason Ive heard you cant parallel triacs and SCRs)?
Another question about this: Even though the measured DC resistance of the bank of cold heater coils is close to zero, do you think the 120 cycle on-off switching of the primary, along with whatever time it takes for the transformer core to saturate, will sort of act as a buffer and therefore not look like a dead short to the transformer or the line?
Thanks all for your help!