On Mon, 14 Oct 2002, Olin Lathrop wrote:

*>> I don't think that you need to interpolate at this level (256 steps on a
*>> 2kW heater is about 10 W per step - you'd have to have pretty good
*>> thermometers to see that amount of heat difference in a usual sized box or
*>> room). At most the output will jump between two adjacent values from time
*>> to time.
*>
*>Even better, why not screw the phase angle and switch in (or not) complete
*>power line cycles?  At 60Hz that gives you 13 different power choices and
*>still drive the heater with at least 5Hz to reduce stress.

That's the way the little project I posted to the piclist once works. But
most people who use a closed loop have concerns about the response time of
such a scheme. With some good reason, since even 10 power steps on an air
heating system implemented like this could cause a loop to oscillate (with
the integrating time of the heater proper you get time constants of the
order of magnitude of two seconds or more). You need a HUGE I component in
the regulator to get it steady and iron patience to tune it (it takes
ages to respond to user input).

However for large loads, like water heating, it is ok. Also synchronous
motor fans (the mass of the rotor does the integration here).

Peter

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