Where I worked we wound low voltage tantalum heaters for use in a process done in a vacuum. When I converted some systems from phase angle firing into a variac to DC, heater lifetime improved significantly.
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This
https://www.elprocus.com/power-control-using-scr/ isn't a bad explanation, except for the last few paragraphs.
They mention zero cross and controlling the SCR with pulses which is good. They used dual SCR's rather than a Triac. It's important to use pulses with motors.
The first thing you need a good zero-crossing detector.
With two SCR's the firing angle varies from 0-180 degrees, HOWEVER, your 0-100% signal would not be a linear relationship of firing angle.
It should be more of a table of.
0-100%("power"), Firing angle, delay time@Line freq, RMS(V)^2(normalized)
0
1%
2%
The table is a little messy to create. Since V^2 is proportional to power, use V^2 and not the RMS value of voltage directly.
Firing angle works for all frequencies. The delay time is line frequency specific and it;s when you start applying pulses after the zero cross detection.
Your also getting contributions from both the upper and lower half cycles.
This
RMS(V)^2(normalized) column is a bit more difficult.
P=V^2/R, so P is k*(V^2
) so just make k=1 and normalize to the line voltage. e.g. at line voltage (240^2) is the max value, which is 1. it doesn;t matter what the line voltage, is because time and phase angle won;t change.
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Typically it's a resistive heating load so R is constant. If your using a lamp to heat, you should convolute the effects of resistance temperature change.
Finally, you should implement slow start.
What I don't know is if both SCR's are fired simultaneousy. I suspect they are. Only one will conduct.
Current limiting needs to be implemented if operating into an inductive load. Not sure how that is done. It might just stop the gate pulses for the rest of the cycle.
So, 50% power output does occur at 90 degrees, but 75% power does not occur at 90+(90/2) for a resistive load.
Caveot: If you pulse and accidently include the next zero crossing, you end up applying 100% voltage.
Aside:
Some, usually stand-alone controllers can control V, I, R or P and do line voltage compensation for a 0-100% input.
Initially try to control at say 0, 45, 90, (90+45) and 178 degrees just by a single pulse and look at the output on a scope. then add a stream of pulses at a fixed frequency.
M`easure the frequency and change modify the timing.
Now add 0-100% in V and use an RMS meter to see how your doing. Now add 0-100% in RMS(V)^2 and finally try to add the effects of a tungsten load which has about a 10-15x lower resistance when it's cold.
Maybe I've explained things better this time.
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A high quality zero cross detector:
https://cdn.hackaday.io/files/1597066832861504/SimpleIsolatedZeroCrossDetector.pdf