A stupid question for this group!

I'm building a power coating oven 4'x4'x6' it will have three (3) heating elements with a total of about 9500 watts, I'll use a 50 amp circuit.

My question is what PID and functions should I be looking to use to control the heat at 350-400?

Thank You For Your Time
John

 

PID requires frequent 'on-off switching' of the electrical power to the heating elements.

An on-off control signal from the temperature controller to an electromechanical or solid state switch is used. The more frequently the heater can be turned on or off, the closer one can get to 'straight line' control.

Electromechanical contactors go 'clunk' every time the unit switches, which tends to wear them out after thousands or 10's of thousands of cycles.

The alternative, a solid state relay, has no moving parts to wear out. Solid state relays are rated for the circuit voltage (240Vac) and the amps (50) running through them. They need to be well "heat sinked" (heat sunk?) in order to dissipate the heat generated from IR drop across the semiconductor. Thermal conductive paste is used to ensure good conduction to a heat sink.

Zero cross solid state relays switch on or off when the AC input line crosses zero volts. Phase angle relays fire at any point in the AC cycle, and tend to generate lots of AC noise in the process.

There are commercial "SCR" products with the solid state relays packaged onto heat sinks which use a continuous analog control signal, like 4-20mA or 0-5Vdc, where 4mA/0V is zero percent output and 20mA/5V is 100% output.

Be very aware that any semiconductor in its off state will leak some voltage and just enough current that the leakage can be lethal. A technician in the Chicago area was killed some 30 years ago in a walk-in oven when working on a 480 heater element circuit thinking that when the controller was not calling for heat that its 'Off' state was an open circuit in the electrical 'hot' supply line to the solid state relay. It wasn't. He was electrocuted from the leadage current.

It is considered good practice to put a "high limit safety" on a thermal oven, a separate redundant circuit with its own temp sensor, high limit controller that drives an interposing electromechanical relay/contactor (shunt trip circuit breaker) upstream of whatever control switch is used, whether solid state or electromechanical, to create physically open circuit for an over-temp condition or, if necessary, open circuit the heater elements.

 

I applied a PID to On-Off control via SS relays for an 18 zone furnace used to heat automotive glass prior to bending. Each element was pulling 55 Amp and we obtained a very good "straight line" control. Heat sinking was crucial and we had no failures of the relays even with a 2 second on-off period. In fact, the element manufacturer was happy that we were not cycling the temperature of each element and causing additional physical stress to them.

One thing that came up, after about 2 weeks into the operations was that the first zone stopped heating and we found a fuse had blown in that circuit. We replaced it and all seemed well for a few minutes but it again blew.

After much head scratching we opened up every connection point and found that even after we had an electrician tighten every connection (1 week after first power had been applied) he missed a single terminal. That terminal was a charred mess and it also took out the 2 terminals above it on the strip. Copper wires and terminals do work loose after their initial heating up and this connection heated to the point of failure.

With proper application of devices, the On-Off control from a PID (actually Time Proportioning control) works great.

Russ

 

Controlling this oven can be very simple, but the accuracy of the solution to achieve this heating task depend on the ability to read the real-time temperature with sufficient accuracy and precision and your cleverness and ability to write a simple QBASIC program to control the heating system. It took me a little effort to experiment in each controller attempt to optimize the controlling software controller though the process of trial and error with a touch of careful reasoning.

A simple circuit for controlling 240V AC would require heatsinks with at least one 25-amp 800V triac mounted on it.

The solution, use parts from a discard PC power supply. The PC power supply already is the correct size for your controller, the mounting holes are already already correctly drilled on the heatsink for mounting the triacs, and the the fan to cool them and the optoisolators needed to control the triacs are also on the old PC supply printed circuit board, so your total cost is almost zero, except for the triacs and a few resistors and wire, all which cost less than a dollar apiece.

The heatsinks for your heater control would require a fan for cooling, and the salvaged PC power supply already has a competent fan mounted correctly for the job that runs off any 12V DC supply. The 12-volt fan can be powered from a discarded 12-volt wallwart power supply for other PC equipment, or you can use a LM7812 voltage regulator to power the fan from a -30V power supply supply salvaged from a PC printer. This 30V power supply will also provide and optimal triggering voltage for the triacs.

The triacs can be turned on and off at zero crossing with zero-crossing triac optical couplers than you can purchase at Radio Shack or you can use ordinary optoisolators salvaged from the same PC power supply. (Some PC power supplies have 2 or 3 optoisolators, some use only one.) Almost any optoisolator will work. (Approx 15 to 25mA triac trigger current trough a >30V output transistor). These parts are also readily available from Radio Shack for less than a dollar.

You can easily control the whole system with a PC parallel port interface, which can give you direct and simple control of 8-13 TTL level (0V-5V logic level) input/output pins. You can then use input and output statements in QBASIC code to control the PC parallel port. Using QBASIC, you can write a simple program running directly from bootup from MSDOS 6 and not even bother with Windows. QBASIC uses single OUT &H378,x and x=INP(&H378)...where x is an 8-bit word and thus so very simply communicate and control anything in the external world.

If you want a Windows interface, Windows XP SP3 using a DOS shell program is best, or write a more complicated program in Visual BASIC 6 or Microsoft C+ 6. You only need DOS for the operating system for QBASIC, and it can be freely downloaded from the internet, so an old laptop could make a competent, intelligent, and reliable controller and display for this task.

The whole heating power control circuitry could fit comfortably in the salvaged PC power supply chassis on a breadboard you build to fit.

The actual circuit is very simple. Wire the individual heating elements in series with each 25-amp 800V 4-quadrant triggering triac. You could get by with just one 25-Amp triac or you can use several to individually control the each heating element. Use an ordinary NPN transitor output optoisolator that can be easily controlled by your PC Parallel Port output port with a serial resistor to the LED input. A -30 volt supply is connected in series with each optoisolator output transistor emitter through a in-series 330-ohm 2W resistor and the collector of the optoisolator connects to the gate of each triac. The 0V return of the -30V printer power supply connects to the 240V line and the Main Terminal gate return of the triac. The other case (MT2) of the triac connects to the one side of the heating element. The other side of the heating element connects to the hot side of the 240 Volt line. The triacs are reliably triggered with a negative current and -25 mA is sufficient to achieve reliable triggering. Since you have at least 13 I/O pins available on the PC's parallel port, you have plenty of control lines to turn on the triacs individually or in tandem. You could also use the parallel port to input the temperature from your temperature sensing circuit by interfacing to a V to F output or from a serial output A2D convertor.
In the design of controllers I have built I communicate directly to an 8-bit port on a MCU. I am usually use a PICC 16F887 MCU that has several A2D input pins to interface with temperature sensor circuitry and deliver the result to the PC parallel port using one of MCU 8-bit ports and other control lines for the interface.

PID control for this system would only require a simple scheme:

(1) Apply full power until the temperature is somewhat close to the required temperature.

(2) Modulate (lower and raise) the duty cycle of power to the heating elements by skipping several full 60-Cycle ON cycles to achieve a slower heating to get to the final precise operating temperature.

(3) Dither around the correct operating temperature by using a slow (fractional seconds to minutes) on/off of the heating elements. This means if the temperature is too hot, lower the duty cycle, too low, increase it with some significant time delay to allow the heating elements to achieve the required temperature.

(4) Use some form of failsafe over temperature electromechanical switch or alarm to prevent mishaps in case of some PC program crash.

The success of the whole controller will depend on the accuracy of your temperature feedback device and your cleverness and ability to write a very simple controlling program. You must be sure of the precision, accuracy and and placement of one or more thermocouples or infrared feedback temperature sensors to give your PC controller an accurate reading of the actual temperature.

You must remember in controlling the heater that any effect will be manifest later in time and you need only to slow down the control of the turning on/off of the heating elements to not overshoot the correct heat set point.

You really do not need a PID controller to accomplish this task.

You can control the power to the heating elements in proportion to the difference between the current temperature and the exact temperature goal, or just use a full on/full off approach until you are near the correct temperature. You can then dither the on/off duty cycle of the heating elements slow enough, so slowly enough as to achieve and maintain the exact temperature required. No complicated integral or derivative control is actually necessary to successfully accomplish this task. Remember that it might be more quick achieving heating than cooling of your heating elements.