0
\$\begingroup\$

I am planning on using a WATLOW DC2T-24C0-0000 power controller for a three phase electric kiln.

It is a zero-cross 3 phase, 2 leg controlled contactor capable of driving 46 amps with its huge heat sink. Its principle of operation is basically the same as a contactor (as far as I understand), except of course that it can switch much faster since it's zero cross and on a 60 Hz system there will be 120 zeros per second for each AC wave.

Mine is just activated with a 4.5-32 VDC signal, though WATLOW offers other DIN-A-MITE power controllers which utilize analog input signals (4-20mA or 0-10VDC) to automatically adjust power as shown in the manual:

enter image description here

enter image description here

I quote

"With variable time base control, the power controller automatically adjusts the time base and output power with respect to process input. Accelerated life testing verified that variable time base control significantly reduces expansion and contraction of the heater element. This extends heater and SCR life while improving process temperature control. You save money on heaters, downtime and maintenance."

Since the model I bought is basically just a really fast contactor that lasts a lot, I was planning on using a 1 second switching period and have an algorithm that determines how much % of time it needs to be ON and the rest is OFF. Clearly this isn't as precise as the variable time base models, so what I want to know is:

  • For a big kiln with >300 kg of ceramics inside, will the thermal response justify getting a fancy power controller or is my plan more than adequate?
  • Is variable time base possible to program for the contactor models? I am already using a microcontroller (ESP32) to control the kiln with a PID algorithm.

On the last note, today I tried using a Arduino NANO to test the switching capabilities of the SSR with different codes. I used a filament lamp and an oscilloscope in order to see how the controller switched. I can conclude that a) given that the maximum switching frequency is 120 Hz, I couldn't see much of an effect on the oscilloscope and b) the switching is nowhere near as smooth as that of a modern dimmer.

\$\endgroup\$
4
  • \$\begingroup\$ I've used an MSP430, adjusting its DCO to phase lock to the zero crossing events and to guarantee an integer count divisor for each half cycle and then directly controlled a pair of inverse coupled SCRs to replace a TRIAC, which I dislike for this. I used 40 A SCRs in my case, as that was more than enough. You can select the number of half cycles needed for whatever PID output resolution you feel balances well with your need, I suppose. \$\endgroup\$
    – jonk
    Apr 26, 2022 at 11:29
  • \$\begingroup\$ How did you phase lock the zero crossing events? I am going to be using the ESP32 and already know how to output a square wave with variable frequency / duty cycle on a GPIO. The issue will be on how to synchronize the timing of this output with the mains zero crossing. \$\endgroup\$
    – PabloL
    Apr 26, 2022 at 17:50
  • \$\begingroup\$ I dithered the MSP430 DCO. I could haul out the old code, I suppose, and show you. But you'd probably prefer to see all the careful notebook work that went into the design. It wasn't done in a day. That's for sure. \$\endgroup\$
    – jonk
    Apr 26, 2022 at 19:08
  • \$\begingroup\$ Yes, if it's possible I would like to see the design. I'm not sure I could implement it but I can try \$\endgroup\$
    – PabloL
    Apr 29, 2022 at 21:08

1 Answer 1

Reset to default
1
\$\begingroup\$

Thermally there is little to no advantage in going much less than 10-20 seconds in a typical hobby ceramic kiln. The advantages are marginal, like reducing the thermal cycling so the thyristor die bonding lasts longer, as the quoted paragraph suggests. There are also possible disadvantages such as flickering lights at an annoying frequency or possibly audible sounds. Of course if you're programming it you can always change that.

If you sense the zero crossing and generate the timing synchronously with the mains then you can program a variable timebase, provided you know the algorithm. If not, it won't work very well and you'd probably be better off sticking with a couple seconds or so minimum timebase.

\$\endgroup\$
2
  • \$\begingroup\$ The kiln is quite bigger than the typical hobby ceramic kiln and will fire to temperatures of up to 1300 C, however it will have a large thermal mass so as you say maybe there won't be much advantage of doing a variable time base. I was also going to do 1-2.5 seconds for the SSR not 10-20 seconds, that shouldn't be a problem right? \$\endgroup\$
    – PabloL
    Apr 26, 2022 at 17:46
  • \$\begingroup\$ @PabloL It should not be a problem unless the rapid switching causes annoyances. A very short non-synchronized timebase might cause some beat frequency disturbances in the controlled temperature but I doubt the load will see much variation. \$\endgroup\$
    – Spehro Pefhany
    Apr 26, 2022 at 17:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.