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I want to control the temperature of mica band heaters mounted on extruder barrel very precisely with a power rating of 5kW and 220VAC. What should be the best for the switching purpose of these heaters?

Null
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Zeeshan Khan
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2 Answers2

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Contactors are usually acceptable for this application. The current is 22A so you can use a 40A or 50A contactor. Check the life curves for when preventative maintenance should take place.

Temperature sensor should be a type K thermocouple (maybe type J if you're in the U.S.) and autotune PID control with overshoot inhibition would be normal. Mica heaters are somewhat loosely coupled so they tend to overshoot more than better types, and that can degrade the plastic in the barrel.

Normally there are several zones on an extruder barrel with each controlled separately and you may need air or water cooling to maintain the temperature accurately when the plasticizing energy is added, which can add some complexity as the dynamics of heating vs. cooling tend to be quite different.

Spehro Pefhany
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  • Is "autotune" a marketing statement (like the "plug-and-play), or it really auto-optimized PID? If yes, why the manual (hard) optimization process still there? – GR Tech Dec 16 '14 at 21:00
  • @GRTech Autotune often works pretty well with thermal processes. It is an off-line process-- you run it to set the values- it drives the process around the setpoint enough to get some information on the system dynamics. If there is something unusual in the process (like multiple similar lags) it may result in the controller having less phase margin than desirable, similarly if it's a very simple process it might be a bit on the conservative side, so it can be optimized by diddling the autotuned values (or you can do it all manually if you want). – Spehro Pefhany Dec 16 '14 at 21:31
  • @GRTech Disclaimer: My experience is with top-tier manufacturers and the firmware implemented in my own companies and clients, the knock-offs may not work quite as well (I know other things they do such as accuracy of cold-junction compensation are often not as good). – Spehro Pefhany Dec 16 '14 at 21:33
  • Beware of using relays (or contactors) in PID applications. The relay will cycle a lot and have a short life. Eventually the contacts may fail by sticking closed. – Robert Endl Oct 04 '18 at 09:46
  • @RobertEndl Cycle time is unrelated to the control algorithm unless it's badly tuned and oscillating. Typical fixed time-proportioning cycle of about 20-30 seconds will give many years life with a properly rated contactor, and contactors should never fail sticking closed, much much more unlikely than solid-state relays which almost invariably fail 'on'. They are also much more robust against heater failures without expensive I2t semiconductor fuses. – Spehro Pefhany Oct 05 '18 at 02:34
  • This has not been my experience, but your point about poor tuning may well be true. I've seen heating applications using commercial controllers that gave cycle times much shorter than 20 seconds. The result was relay failures (short) in maybe 6 months or so. – Robert Endl Oct 05 '18 at 05:12
  • @RobertEndl I was heavily involved in the plastics industry back in the 80s/90s and mechanical contactors were standard. Other options were solid state (expensive and unreliable and required removing a great deal of waste heat) and mercury contactors (moderately expensive and occasionally exploded, releasing a fair amount of Hg, unacceptable today in most countries). Nothing is really a wonderful solution, and not much has improved since then except costs have shifted – Spehro Pefhany Oct 05 '18 at 16:11
  • One mitigating factor is that extruders are often run close to adiabatic so the barrel heaters are mostly used for startup with slight heating or water cooling used during steady-state operation. Most of the heat is supplied by the screw motor through shear heating of the resin in steady state. – Spehro Pefhany Oct 05 '18 at 16:13
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    I tend to lean toward contactors if well derated. I've heard that mercury wetted relays are going away, but I'm retired now and don't keep up. – Robert Endl Oct 05 '18 at 18:16
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Solid state relays (SSRs) with a heat sink will be your best bet. You can get a 40A SSR with heatsink for about $14, on Amazon of all places. You can add an auto-tuning (non-exclusive-or manual tuning) PID controller with thermocouple for another $25. I usually recommend against Amazon for component purchases, but that's a pretty good deal.

I did this on a smaller scale recently to heat an aluminum torus to 220°C ±1°C. It was only ~300W, but a small torus, so it only took about 2 minutes. I wrote a PID controller application for an MSP430 microcontroller. It worked splendidly. But there are off the shelf versions you can buy.

Samuel
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