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# How does the power limit function of the Eurotherm 2404 work? Edit Subject

Can anyone explain to me how the power limit function of the Eurotherm 2404 works? Is it a current limiter, duty-cycle/PWM function, or something else?

Application: we use a number of 2404's to control 1000W 240V resistive band heaters in our pilot plant. Our oldest reactor is getting rebuilt and the wire mesh heating elements are being replaced with band heaters. We have a number of 2500W 240V band heaters in inventory while new 1000W 240V heaters have a lead time of at least a month.

If I use the 2500W units and set a power limit of 40% (1000W/2500W), will this system function roughly equivalent to the others using 1000W heaters at 100%?

Thanks!

Jason
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• Hello Jason,
at first the power limit depends on what kind of heat output you are using.
A real power limit is possible just with a DC-OP module where 0...100 % heat output
can be an analog signal, f.e. 0...10 V or 0...20 mA. And you need to have a thyristor stack with phase angle.
Otherwise you have any kind of On/ Off output ( Relay, Logic or Triac ) to switch a solid state relay On/ Off. That means 0 or 100 % output power, nothing in between.
The PID control and/ or the power limitation usually works with a so called time proportional functionality. That is the variation of the time the output switches On and Off.
2404. If you set the CYCH ( heat cycle time ) to 20 seconds
25 % OP = 5 sec On - 15 sec Off
50 % OP = 10 sec On - 10 sec Off
75 % OP = 15 sec On - 5 sec Off
The function is not realy linear, it is just approximately.
I hope that helps.
Rudi
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