480V heating element control

shooter said:

For every element use a SCR, and 6 outputs on a simple PLC (or arduino)No need for fancy stuff, switch on a SCR when temperature is deviating for 1 degrees, the next one on 2 etc.

Click to expand...

+1

Going to be doing this myself soon, we have a 12kW water heater running off 240V 3P with a broken controller. The SCRs are already there, just going to bypass the existing controller board.

On/off control may be good enough at maintaining set point for this application or it may not - and you'll need to PWM the SCR to achieve power levels in between the full output levels. If you already have SCRs on there then that would lead me to think that they were needed (why would they have paid to put them on otherwise)

Rather than switching every degree - use a PID loop using proportional only control, at least this way if you get to the point that the process needs integral (to correct error over time) then you don't need to reprogram anything

Ignoring deadband - if you want the full output to be on at 15 deg from set point this is a proportional band of 15, or a gain of 6.667 (100/15) etc.

Assume you have 6 * 2kW stages, 100%/6 = 16.667%. So stage the elements on the output from the PID loop so that every 16.67% the next stage kicks in

I would recommend against that, that is how we used to do it and that never resulted in good performance.

In stead of a continuous controller, we use a 3-point step controller now. The controller steps up and down the number of active groups.

You want 3 phase SCR and you probably want phase angle control.
Yes you can get SCRs that control either 2 phases (called "wild leg" SCRs) or all 3 phases.

Zero-fired is commonly known as ON/OFF, like a contactor but you can change frequency of on/off with SCR. Called Zero-fired because they fire when phase frequency is at "0". Cannot use on primary of transformers.

Phase Angle is proportional...so if you have 50% heat, you have 240VAC....etc. This give far more uniform control of power. My favorite... a little more cost but way more capabilities. You can use these on Transformer primary too. More even control of power.

For best safety, use contacotor above SCR so you can completely shut down power to tanks.

I use Control Concepts SCRs...excellent company and very well made device.
Their Fusion line is really nice...
http://ccipower.com/product-category/three-phase-ac

What are your current loads? Anything about 25amp?

Woehner makes a product that called the OMUS. Its a Solid State Relay that can be used in single phase & three phase, PLUS each phase has a built in current transmitter with a 4-20ma output for phase monitoring. PLUS, it has built in short circuit protection (fusing). Can be DIN mounted, or busbar mounted to safe space.

http://www.woehner.de/en/news/omus/

Definitely not a Woehner salesman, just love a lot of their products.

boneless said:

I would recommend against that, that is how we used to do it and that never resulted in good performance.

In stead of a continuous controller, we use a 3-point step controller now. The controller steps up and down the number of active groups.

Click to expand...

The gain I used was an example of how you can stage elements based on a degree value like the previous suggestion. Staging the outputs in steps as I described is based on them all being equal size. Tuning is always required for good performance. Model the process.

By 3 point controller I think you are referring to floating valve control (or split range time proportional in controllogix)? If so you still need an output from a control loop into it for it to calculate how long to pulse open or closed based on actuator stroke time - I'm not sure how this would help to stage binary equipment? Have an accumulator that increments up time on and decrement down time on? Sounds easier to me to just switch the stages based of the output directly. Certainly simpler to follow

Unless you're implying that using a modulating valve rather than binary electric elements is better? There is a reason it is electric now though, maybe they don't have suitable heat source etc.