PWM of 2.5 kW Heating Element

Hi all!


Helping a co-worker out with a home brewing setup.


He wants an accurate temperature control, so we have divided up the element in two groups (1ph 230V), and I am wanting to use PWM.


I got that part to work, but was wondering about what frequency to use? I read conflicting things online, everywhere from 10Hz to 0.something Hz.


Anyone has any experience using PWM for smaller heating elements?

You don't need to use PWM, the heating element doesn't respond fast enough to need a PWM signal. The two simplest ways to do temperature controls is either with an SCR to vary the voltage to the heater with an analog output or to use a solid state contractor on a digital output with time proportional control.

In general, you will use a PID loop to control the temperature. If you where doing a SCR, you would just scale the PID output range to from 0 to 10v or 4-20mA. For the time proportional output, you will scale it to a timer. If your PID loop had a 50% output and you had a 3 second cycle time, your contactor would come on for 1.5 seconds and then be off for 1.5 seconds. Its very simpler to PWM, but switching is not near as fast as traditional PWM applications. A solid state contactor is highly recommended, as a mechanical one will wear out quickly being cycled every few seconds.

What kind of controller are you using? A lot of manufactures provide special instructions or code that makes doing this task very simple.

Hi Helliana,


Thanks! He got his hands on a used Siemens 313C, the coding is not a problem, I got that covered.



If the ideal frequency is indeed sub-hertz, I agree that PWM wouldn't be necessary. We already have a SSR in place, I think the SCR would make it a little to costly right now.

PWM is time proportional. ??


Regardless, if you are not using a dedicated device for switching (power controller SCR), then you will want to be sure to use SSR relays. For base period times, for heating applications, I would use 10hz-1hz. The faster base period will give you finer resolution and control. If you are using electromechanical relay/contactor (not advisable) I would use 10sec or more.

It is a good idea/safety to put a mechanical contactor ahead of the SSR or SCR as an enable/overtemp safety. The SSR/SCRs will likely fail "ON" when they fail, and they make have leakage current when you don't want them to be on. An enable contactor will allow you to completely cut electric to the heating element.

Thanks!

Well, for the PWM I can set the period time and the minimum on time.


And yes, that's what I already have in place:

For my plastic extruders, I use a 3 second cycle time. For your application, it can be longer. A tank full of water has a pretty large thermal mass, it shouldn't react very quickly to the heating.

In the AB world, there is a Split-Range Time Proportional (SRTP) function in the Function Block Diagram language of Studio5000 that does what you need as well as has the capability of adding a cooling output as well. Siemens probably has something similar.

Zero crossing switch solid state relays, a low noise clean solution.

Carlo Gavazzi has a nice range of this kind of relays.

I've used one model in the past that converted a 4-20mA onto a voltage to be "eaten" by the heating resistor, but can't for the life of me find it now.

Here's a link to exactly what you need

http://www.crydom.com/en/products/

Set up a PID loop in you PLC output it to the SSR and your done

I use a SSR driven from a DC output card for my home brew set up. I use a PID to calaculate a duty cycle and move that to two timer presets that act as on/off timers. For example if PID is calling for 50%, and my resolution is 1000ms. The on timer would be 500ms and off for 500ms. I have found the cycle rate does not have to be fast as all.

General rules(as always there are exceptions)

If this is a convective or conductive element:
Use a zero cross solid state relay and some variant of pid control

If this is a radiant element:
1. If fast response (like quartz tube)
Purpose built phase Angle power controller from watlow or control concepts

2. If slow response element (like resistive embedded in radiant material)
Pwm/zero cross power controller like you can get from watlow or control concepts

I'm a big fan of running a PID (or just P if you don't need super accurate control) on your controller, and using a PWM output to a standard zero-crossing SSR. 5Hz works very well for resolution and quick response.

Thanks guys, I appreciate all the responses!