Dear list,

A client want to use temperature controller for their heaters. I prposed the use of SSR with a PLC. However, the client insists on using thyristors (SCR) for linear control of the heaters.

Could you please guide me whether this solutions is better than using SSR (Pro + Con)? Any ready-made solutions?

With best regards
Shahid Waqas Chaudhry

 

A lot would depend on what you are heating. Phase control would be preferred for fast processes, those with little thermal mass or relatively large heaters. That is, where temperature changes quickly relative to the rate you can cycle the SSRs. Either would work for loads that change temperature slowly. Phase control is also better for heater life due to the elimination of temp cycling and fatigue. I'm certain some of the usual suspects produce controllers that can operate in this mode. I know Avatar does. And I've seen units that take a 0-10 volt input for 0-100% power. I don't recall whose they were but I suspect they were Watlow.

Regards

cww

 

Given the time constant of most heaters, duty-cycle control through a SSR (typical industrial solid state relay) would be just as effective as a more elaborate truly linear heater drive circuit. If such a circuit uses SCRs, then it is also using duty-cycling to approximate linear control, because SCRs are not linear devices. Depending on how much power these heaters draw, you may run into power limitation problems with the SSR approach. SCRs can handle more power. Also, the SSR approach requires that you synthesize the duty-cycle control in the PLC, which is extra programming overhead. However an analog output from a PLC driving a linear heater controller, while it costs more, involves less programming overhead.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

Sorry, I haven't been following this thread very closely... but a couple of additional things to consider if they haven't been mentioned already... If you said how large the heaters are, I didn't catch it. That said, I have controlled heaters of up to several megawatts with SCRs, and hundreds of small heaters with SSRs. One often ignored aspect of SCRs is that they can introduce a small DC component into your power lines. If the heaters are an insignificant or minor percentage of the load on your service, the effect of the DC component is negligible. However, in the case of a service dedicated to the heaters, and if the heaters are the majority load, the DC component can cause saturation of the transformers feeding the service, causing them to overheat.

Also, there are several methods of firing SCRs. Phase-firing was mentioned, but this can introduce noise into the line. Zero-cross firing is preferable as it eliminates switching noise. Phase-firing is used more with inductive loads. Often, zero-cross SCRs are burst-fired or time-proportioned; e.g., on for 3 AC cycles, off for 7 AC cycles = 30% output. Certainly not a linear control, but the end result is the same for most applications. I'm assuming that your heaters are resistive heaters and not saturable reactors or something. I am also assuming the heaters are small (under 100 amps - maybe even a throw-away item, like tubular heaters) as you were considering SSRs. What reason did your client give for using SCRs? What is the application? Is it heating air, or liquids, or is there significant mass, or extreme tolerance required (e.g. growing crystals)? More info would help understand what is driving the stance on SCRs. You should also have an independent limit controller which opens a contactor feeding the heater. In the event of a control failure, the limit control can positively de-energize the heater. It's a big topic, but hopefully the LIST will give you avenues to research.

There's lots of ways to skin the cat... they all have pros and cons...it depends on the application. It's also easy to go overboard. It's like a tone-deaf man buying a top-of-the-line audio system with .00001% THD. Is the client willing to pay for the Cadillac if he only needs a Cavalier? Certainly, the SSR route is less expensive than the SCR route. But in the end, the customer is always right. Unless it's unsafe, give him what he wants. I suspect you underbid the job.

Best of luck.

Bruce Axtell

 

For the vast majority of resistive heaters that are heating a thermal mass, a zero crossing power controller (SSR solid state relay, solid state contactor or a zero-crossing SCR) will turn on and off several times per second and will provide excellent control at a low to medium cost. Also, there are several companies who now make solid state contactors that accept a 4-20mA signal, thus making it easier for PLC applications to use zero crossing with little programming overhead.

For transformer coupled loads or for heaters that have variable resistance (silicon carbide heaters or moly disilicide heaters), then you need a “phase angle SCR” that might need features like current limiting. Some information is at: http://www.power-io.com/library/appnotes/solid-state-relay-terminology.htm

Regards;

Mike Johnson