Temperature Control Advice Sought

[brianafischer 2]

Hello, First of all, thanks for reading my post, this is a great community. I have been given the task of creating a heating machine. The specs are the following: All components are powered from the plant 3-phase 480VAC drop 3-phase 480VAC VFD controlling a 3-phase 480V blower, which sends air into an 3-phase 480VAC 16kW heater (20 Amps), which sends air into a chamber with temperature monitored via thermocouples The machine will "ramp and soak" the chamber temperature with cycle times between 1-5 minutes. The blower flow rate will be constant during this. A computer will be connected for data acquisition, setting parameters, and possibly PID control for the heater. I am looking for advice on the machine in general and any of the following points: How can I keep power to 120V outlets and the PC when the main disconnect is turned off (while passing NEC code and local inspection? How can I determine what type of control to use on the heater? I am debating between ON-OFF control via a contactor OR an SCR Controller like the Omega SCR19Z-48-040 I want to use a limit controller and a contactor to prevent over-heating (in case of a failure). Is there a cheaper limit controller than the Omega CN3101 ($410)? Is an isolation transformer worth the money for the 480VAC circuit (isolated from the plant 480VAC system) or just a rfi/line filter on the VFD? Any advice/examples on 480VAC in a panel? Would it be worthwhile to have a 120VAC/24VDC control panel and a seperate 480VAC control panel? Thanks!

[Sleepy Wombat 27]

Look at an Omron temperature controller with the RAMP SOAK function inbuilt also add comm's capabilty for connection to a PC as you have stated Secondly, it is basic ON/OFF or PID ON/OFF, control the heater with a 3 pahse solid state relay, this will enable you to have better control of the temperature without over burdening a contactor with short switching times.

[DanW 41]

Cal Controls has a great little 1/32 DIN high limit temperature controller, but try to find it on their web site. Find your local distributor on the CALs web site and call them. You should absolutely, definitely, positively use a shunt trip circuit breaker to interlock the power to the SCR or solid state relays. I have a personal horror story I will bore you with. A manufacturer of high temp ceramic kilns refused to interlock his newly designed SCR control (the one I supplied) with any sort of switch, against my strong protests to the contrary. He called me on a Monday morning asking about device I had so strongly advised to use to interlock the SCR. I asked why he wanted to know, since it had been a dead issue. He told me he'd built his first kiln with SCR control the previous week and left it running over the weekend. He came back that morning to find the outside steel skin GLOWING RED HOT. The SCRs had failed, as SCRs do, in the 'on' state, pouring unmodulated heat into the kiln, on an uninterrupted basis all day Saturday and all day Sunday. He was fortunate that the kiln was left in the spot where it had been built, in a fairly open area of the production floor. If it had been located closer to a wall, the wall could have caught fire from radiated hea and burned his shop down. To save the price of a $400 shunt trip breaker and a $200 high limit, it cost him $30k in material and labor, a missed shipping date, and the possibility of burning down his shop. Consider the liability if he'd sold it and it burned someone else's shop down. So be warned about solid state output devices. They fail 'on'. As to the choice of an SCR or solid state relays: SCRs cost more than solid state relays. What justifies the cost? More precise control. Does your process warrant more precise control? Modern controllers (whether standalone or PLC) can be purchased with solid state outputs to drive solid state relays, without the wear and tear of yesteryear's electromechanical relays. So short, time-proportional cycle times (on the order of a second or two) are easy to do without wearing out the relay outputs. (this presupposes that the loop warrants short cycle times). It really depends on whether your ability to supply energy (KW) is on the low or high capacity side with relation to whatever's being accomplished in the chamber. If the KWs you supply is on the low capacity side, the tuning constants will approximate on-off control, ie. high gain (narrow proportional band) and a high rate term. Why pay for SCR control, if you have a loop that's essentially on-off? If, on the other hand, the ability to supply heat is high capacity (compared to the consumption of heat), then an SCR might pay for itself in slightly more precise control by doing what it's designed to so - modulate at fractional percentage outputs.. In other words, if an SCR runs in almost on-off mode, switching between 100% and 0, why bother? Just use solid state relays. CAL also has very nice ramp soak controllers. The folks on PLC lists always want to re-invent the wheel and do ramp soak in the PLC, to my utter amazement. But it's their time, not mine. CAL even has Windows PC monitor software and the ability to write tuning constants to the controller. They were on the OPC bandwagon early-on. I'm told that they even have a DIN rail mount process controller with PLC objects like timers and counters, but with thermal PID functions like ramp and soak. Again, call the distributor, unless you can wait through that silly intro screen on CALs web site. Dan

[ssommers 20]

Other people have addressed the control questions so I'll address the code issues... If you have the PC and outlets installed inside the main control cabinet, they should be turned off when the main disconnect is turned off. If you are only using the PC for data collection & parameter changing, NOT for I/O or control, you could install it outside of the main cabinet and have a separate 120VAC power source for it. If the PC is for control, like a PID loop, the control could be trying to turn on something that isn't actually available since it's powered down by the main disconnect. Then when you turn the main disconnect on, unpredictable things could happen. Not a good situation. As for mixed voltages in a panel, that's up to you. I like to make my I/O be 24VDC whenever possible simply because it draws less power. Currently I'm working in a plant where the I/O points could be 500 ft away from the controller so I use 120VAC. Most of the panels here are small so separating 480 VAC to another panel would be inconvenient. However, there are several resistance welders in the plant where we do have most of the 480VAC in one panel & the 120VAC in another. It's part noise immunity, part safety and another part maintainability that drives me to layout my control panels the way I do. Go read NFPA 79 Electrical Standard for Industrial Machinery and consult with your plant electrician. This way you and they will be safe once the machine is built. HTH! Susan

[DanW 41]

I was thinking single phase when I wrote that last reply, even though you were explicit in stating 3 phase. That makes some difference in the decision between solid state relays (SSR) versus an SCR package. SSRs have long lives, if they are kept cool. Heat kills 'em. (So do SCRs if mounted where they can cool themsolves) So most people do some form of heat sinking, which can range from screwing them to the sub panel, or mounting them on commercial extruded aluminum heat sinks with heat transfer compound (white paste). Remember too that the output of the controller has to be able to drive 2 SSRs for 3 phase (the third leg is straight through). Some inexpensive controllers just can't handle the load of 2 SSRs in parallel. Both factors combine to close the gap on SCRs, because an SCR is engineered as a package, includes the heat sinking, and has paid attention to little things like bleeder resistors. For 3 phase, it's a smaller gap between SSRs and SCRs than it is for single phase. Dan

[brianafischer 2]

For my application (20A), it looks like a SCR39Z-48-040 is $503 (SCR power controller) vs around $180 for a SSRL660DC50 for ($45 + $15) * 3. This is from omega.com. Do you have another source you are looking at. Another question to bring up: why do I only need 2 SSR's?

[DanW 41]

The 3 phase SCR controllers only have 2 SCRs, one for each of the 2 of 3 phases. The 3rd phase is wired directly through and is not switched. So, whether you get a 3 phase controller or do it yourself with 2 SSRs, only 2 phases of 3 are switched. It sounds like the price differential between SSRs and SCR is such that you're considering SSRs, which is fine. Just be sure - to heat sink 'em (add in cost of heat sinks) - whatevr controller you use can drive 2 SSRs simultaneously - the input signal to the SSR is the 3-32Vdc (not AC) input. As to sources, the pricing doesn't seem unreasonable. but I don't have time to look up prices. [soapbox] I use local distributors for vendors, because they - are there when something goes wrong - don't steer me wrong and sometimes keep me out of trouble (I've had one rep tell me not to buy some items because they were dogs. That kind of advice is invaluable. Guess who I keep going back to) I have no idea how Omega stacks up. other than the fact that they're known as a mail order outfit. Because of that I don't deal with them. Not because they're bad, but because there are times in this biz when someone's gotta show up and see what's happening, and I've found local distributors that will stand behind their stuff and show up if necessary. Dan So

[TimWilborne 108]

Why couldn't you go with something like a AB 156-A30BA2 of 2 phase or a 156-A20BA3 for 3 phase. They seem to be less expensive and also have built in heatsinks. I have had very good luck with them