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PWM of AC current for resistive load

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We need to modulate the power to a resistive water heating element. Rectifying to DC does not make much sense and just looking to modulate AC current with solid state switching. The requirement will be 240V/30A. What is the smartest route for this? Dirty output won't matter as we are just feeding the heating element.

Would a pair of transistors work? Triacs? I know things like ceiling fan dimmers do this but not sure how we might want to approach this. Simple is best but don't want any problematic harmonics being bad fed and causing other equipment problems.
The time constant of a water heating system will be quite long so a very low PWM frequency can be used. This could have a period of a few seconds so just switching a solid state relay on and off will work. You can buy ready made PID (Proportional Integral Derivative.) temperature controllers that us this slow PWM method for less than you can make one. **broken link removed** but there are many more on ebay.

resistive water heating element
If you had DC you can turn on/off in a millionth of a second. But why?
I have used "zero crossing" TRIACs so the resistor can be turned on/off in 1/2 of a power cycle. Even this is faster than needed. Most resistive heaters for water can have a large delay with no problem.

I have used one of these because I have several:

It was designed for 24V ac but works with dc and almost any voltage. I use it to turn on a power relay that turn on the "240V/30A".
The lumber yard also has these. 120/220V ac/dc 22A. Used in base board heaters.

I can't find a picture now, but electric hot water heaters have a thermostat much like the above.
I have some bigger versions of these in the green houses. The ones I have a switch for a heater and one for a cooling fan. (normal open verses normal closed)
The application is making a cleanable, tankless water heat that needs to produce water at a very specific and consistent temp even with variable flow rates.

I agree that the actual switching speed could be rather slow in comparison to say a VFD, but what we are trying to avoid is something that would switch on and produce over temp water, then switch off and its too cold. There needs to be consistent heat. We might have to test but I was thinking something like 60-90% duty cycle and around 5-20hz. Thoughts?
I found some water heaters that when they need 50% they turn one for 1/2 cycle and then off for 1/2 cycle. This causes some power and current meters to go nuts. (they see dc) [positive 1/2 cycles draw power but negative 1/2 cycles do not draw power] Some power companies could not accurately measure the power.

Just saying don't use 1/2 cycle to get 50%.
Switching at 5 to 20hz will be fine but switching at 100 or 120hz will get you into this trouble.
If you are running a 'tankless" heater, there is no way a 180-on and 184-off will work. That will just be a hot cold sandwich on the outlet. The actual value is maybe not as important as being consistent. I did not say 1/2*F, I said +/-2*F. That should be reasonable with decent thermistors and calibrations.
The usual method is 'burstfire' control, as heating is a very slow responding system there's no need for any great speed.

You simply use zero-voltage switching to turn a triac ON and OFF, which generates no interference as all switching is done at zero voltage.

So assuming you only want ten different power settings?, half power would be ON for 5 mains cycles, and OFF for 5 mains cycles, 90% would be ON for 9 and OFF for 1.

If you want more precision, then use more cycles - 100 cycles would give 1% precision, if you need that much?.
It sounds like calculating the required heat input from the flow rate and difference between the water input temperature and required output temperature might be a better method. This would get quite complicated as you would need to take into account varying supply voltage and the change in resistance of the heating element at different temperatures to calculate the required duty cycle of the PWM.

The real goal is to set the temperature with in +/-2F. If you measure the temp and turn on at 180 and below and off at 182 and above the power will be on and off much like PWM. My heater (filled with cold water) will be on 100% until it reaches 182 then it will be off for a while and it will come back on for a while. A couple of cycles like that and the tank stabilizes in range. After hours the temperature drops and it needs another blast of power.

The goal is "in temp range" so it must be measured. How you get there is not as important.

If I was going to use PWM at 5hz:
If temp is at 181 (center of range) keep the duty cycle at what it is.
If the temp is 181.5 decrease the duty cycle by 10%/minute. (180.5=increase 10%/minute)
If temp is at 182 decrease the duty cycle by 30%/minute. (180=increase by 30%.minute)
If temp is at or above 182.5 go directly to 0%. (179,5 and below =100%)
This is some thing like an analog regulator. (using op-amps and comparitors)

Maybe one minute is too short or long. The point is, measure the error and use it to increase/decrease the duty cycle.
Before even messing with it, can a guy buy what I need somewhere? I am not even sure what I am looking for though! lol
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