PID for heater control?

Hello.. I'm a little bit new to using PID and have a question about my particular application.

I will be turning on a heater using a solid state relay, so the element will either be on or off. We will measure the temperature using an RTD, attempting to reach some set point.

Is this a PID application? The reason I ask is because we have the digital output, and everything with PID I've seen has had an analog output. I've thought of using PWM on the heating element to make it more like an analog output, but the solid state relay would burn out fairly quickly.

From someone more experienced than I, what is the best method for controlling this?

Thanks in advance.

-nwt

You set up a time period for controlling the output to the SSR, for example 100 milliseconds. Then use the output of the PID to calculate the percentage of that time period that the output is on. When the PID function commands maximum output, the SSR is turned on all the time. When the PID commands 50% output, turn the SSR on for 50 mS, off for 50 mS.

If you're using a PLC output to turn the SSR on and off, be sure to use a solid state PLC output and not a relay output. A relay output won't last long at that kind of switching frequency. If you must use a relay output, lengthen your time base to the longest time period you can get away with that keeps the temperature within your tolerance limits.

but the solid state relay would burn out fairly quickly.

Click to expand...

what makes you say that? ... as long as they're correctly matched to the load, solid state relays usually last a long, long time ...

just follow the advice of my distinguished colleague, Steve Bailey and you should be on the right track ...

Steve Bailey said:

If you're using a PLC output to turn the SSR on and off, be sure to use a solid state PLC output and not a relay output.

Click to expand...

A related question: I have had trouble controlling SSR with other SSRs due to leakage current keeping the second SSR on. Can anyone shed some light on this? I have tried resistors to absorb the leakage current, and sometimes it works and sometimes they burn up (with that wonderful smell of burning electronics!)

What would the guidelines be to cascade the relays - all solid state?

I was trying to steer away from PWM, or fast switching of the SSR because it's rated for "millions of cycles of operation". So.. if I do it every 100ms then a million cycles would occur in 27.8 hours. It seems like this could easily burn out the device quickly.

However, I'm using an S7-300 and just found FB59 in the library to control a digital output with PID. It includes a deadband, which I think would be useful to prevent fast switching of the SSR.

The time base that you choose is a compromise between how tightly you can control the temperature and the life of the switching device. A longer time base means less frequent switching but a wider span between minimum and maximum temperature. A shorter time base can give you tighter temperature control, but the SSR needs to switch more often to accomplish it.

Read the ratings for the SSR carefully. The spec on the number of operations is conditional on many factors. If you are operating the SSR at 25% of its rated current you will likely get more operations than if you switch at 100% current. You will get more operations when the load is resistive (as is the case with heater elements) than if the the load is inductive.

Remember that the switching elements inside AC and DC drives are solid state switches and they last for many millions of cycles. With the proper selection of an SSR, you should be able to get an acceptable life for a heating application.

The timebase needed is dependent on the application and temp. tolerance as Steve said. Some applications can have time base of 10 minutes or even more.

billysmithde2 said:

A related question: I have had trouble controlling SSR with other SSRs due to leakage current keeping the second SSR on. Can anyone shed some light on this? I have tried resistors to absorb the leakage current, and sometimes it works and sometimes they burn up (with that wonderful smell of burning electronics!)

What would the guidelines be to cascade the relays - all solid state?

Click to expand...

Did you calculate the current draw, and the wattage of the resistors? If you are burning them up, you probably have too small of a resistor. Also, many solid state relays will have a leakage current spec, you need to size your resistor, to draw a current over this amount. But ussually 10 -20 mA is good.

northwest_trail said:

I will be turning on a heater using a solid state relay, so the element will either be on or off. We will measure the temperature using an RTD, attempting to reach some set point.

Is this a PID application? The reason I ask is because we have the digital output, and everything with PID I've seen has had an analog output. I've thought of using PWM on the heating element to make it more like an analog output, but the solid state relay would burn out fairly quickly.
-nwt

Click to expand...

It depends on what you're heating and how tight a control you need.

For instance, if this is a heater for fuel oil in a pipe on its way to a burner, a PID/PWM arrangement would probably work best. If you're heating a room, well, an on-off control with deadband seems to be adequate for most people.

Why not use a phase angle SSR and get away from duty cycle control?

http://www.hbcontrols.com/phase_angle.html

How SSR can be driven by analog?

Hello There, I am new with control system. I have a similar question. My application require to control an electric heater bank with a temperature controller. All the heater banks shall be modulated via new solid state relays in response to a 0 - 10 VDC analog control signal from temperature controller. How can this be achived /posible. Kind Regards,

If you just use relay control, no thyristor control, your best bet is to make a table grouping the element. FE, you have 4 elements, equal size, then group 1 is element 1, group 2 is element 1&2 etc. Then you use a 3 point step controller that steps up and down the groups.

sinbernal said:

Hello There, I am new with control system. I have a similar question. My application require to control an electric heater bank with a temperature controller. All the heater banks shall be modulated via new solid state relays in response to a 0 - 10 VDC analog control signal from temperature controller. How can this be achived /posible. Kind Regards,

Click to expand...

Here's one solution:

http://www.jumo.de/products/tempera...gbt-power-converter-709050.html?parentId=3690