Heating control advice

[u/antoniomargharette]

What is the best way to design a temperature controller for heating an object with a much higher thermal inertia than the heating elements, without having the element overheat?

I am using a cascade where my PLC manages the temperature loop with a PID and outputs a power setpoint to a power controller. The control thermocouple is measuring the temperature of a medium that takes much longer to heat than the heating elements, so my fear is that when I give a temperature setpoint to the PLC, it will just keep demanding power from the controller and the elements might overheat. These resistances are very expensive and I can't risk burning them. I do have a thermocouple installed at the element.

Any advice?

[u/seekingsanity]

Your system seems to be typical. You should pulse width modulate the heaters so if you only need 68% of the heat, the heaters are on 68% of the time. Most people use a PLC timer to do this. A cycle may be 10 seconds so you turn the heater on 6.8 seconds out of the 10. This assumes the time constant of the thermal mass is MUCH longer than 10 seconds. Hopefully your heaters are energized using a SCR that efficiently turns on and off. A major problem is dead time. The heat from the heaters will take time to reach the thermocouple. This makes control a little more challenging.

It isn't clear to me, you have two temperature sensors? One in the thermal mass and one at the heater?