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Help needed to reduce Harmonics from a firing angle control circuit

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ArunPS

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Hi, I have pure resistive heater load which consumes 1000W to deliver power to heat a liquid. To constantly generate 1000W at multiple voltages, I have implemented a Solid State Relay (SSR) based firing angle control mechanism to deliver AC supply to the heater. This circuit and logic is working fine. However, this method is leading to lot of current harmonics in the multiples of 50Hz and 60Hz which are its operating frequencies. How can I reduce the harmonics? I am performing a cycle by cycle phase angle control of the AC to deliver power to heater. I have used an SSR with built in snubber circuit. Also I have a line filter on the AC input line to reduce overall emissions. Operating voltage in my case is 100VAC to 240VAC and 16A current. Need ideas that can be implemented like soft start (with cycle by cycle control) or suppress harmonics.
 

The problem is not the control circuit, it's the hard chop
at turnon (turnoff ought to be zero current / voltage -ish).

Since heater is probably not very fussy about how the
power comes, maybe you could get to a "pulse skip"
mode where you turn on "right after" zero crossing,
ride the half-cycle up and back down until quench,
then either do it the next time or don't (hysteretic
control loop, maybe with some smart PID massaging
of the temperature feedback so you don't overshoot
and undershoot on temperature, to more than the
hysteresis band).
 
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    ArunPS

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Power quality regulations (e.g. IEC 61000-3-2) don't allow phase angle control for heating applications, only for lighting control. Full wave switching is an appropriate method, the switch rate must consider the thermal time constant of the heater and also flicker regulations.
 
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    ArunPS

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Power quality regulations (e.g. IEC 61000-3-2) don't allow phase angle control for heating applications, only for lighting control. Full wave switching is an appropriate method, the switch rate must consider the thermal time constant of the heater and also flicker regulations.

Thank you dick_freebird and FvM. I was able to pass the power harmonics IEC 61000-3-2 by using pulse skip method. We have tested for passing few burst cycles and also distributed half cycles over a period of 1 second or so. However, I have two questions on this:

- We are planning to connect our devices to house hold connections (100V to 240V, 15A) sockets. Passing full cycles (230V) to heater (9 ohms load) for two cycles out of 30 cycles would still make the cycle current to be 25.6A. However, such current is passed only for about 2 cyles (40mSeconds) out of 30 cycles (600mSeconds). Having house hold rating of about 1500VA (raw or UPS lines), will it not lead to tripping of power considering over current? Do we have any standard/ test data available on behavior of sockets in house hold circuits?
- Even though we can pass power harmonics test (IEC61000-3-2) with this method, we fail in flicker test (IEC61000-3-3) where test standard counts the number ON/OFF of loads and if it is more than 25 cycles per minute then we fail. current considering one ON/OFF per minute, we are looking at 60 cycles which is more than prescribed 25 times/min. Redesigning the algorithm for switching only ones every 3 seconds will pose challenge related to over current tripping of socket. Any thoughts?
 

The low maximum duty cycle demands for redesigning the heater.
 

intelligent full cycle control needed, 5 out of 6, 2 out of three, 1 out of three, 9 out of 10 ( full cycles ) depending on temp target, a max input current limit needed ( CT or calculated based on Vac in and % of full cycles out ) however unless your switching is very precise about the zero crossing - you may well get a small net average DC component in the neutral ( & phase ) many countries limit thjis to 5mA DC max for continuous operation ( for domestic earth stake corrosion reasons ) - so there is this to be considered.

Also there are flicker implications as FvM mentions

An SSR with a suitable inline choke would at least soften the current waveshape, turn on at zero voltage, off at zero current ...
 

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