Current control

[adrianh]

I have a small electrical furnace. The machine is basically a temperature controller connected to a contactor which connects to an element. The problem is that the when the element turns on it draws 15A continuously until the thermocouple shuts it down.

What would be the best way to limit this to say 5A or 10A withou wasting energy or creating additional heat?

Is there an industrial current limit unit available?

[ians]

Remove the big element, fit 2 or 3 seperate elements with a selector switch, 1 and 2 and 3, which would give you 5 - 10 - 15 amps.

I dont know on any current limiting device for big elements. I have used a light dimmer for a cutter device (special wire) for polystyrene, to control the temperature.

I can find out from people in the know if you dont come right.

[adrianh]

The element is a simple wire twised together in the correct shape.

[Justloadit]

The easiest would be to use a lamp dimmer in series with the element. Ensure the dimmer is rated for the current. Turn the knob and set at the desired setting.

[ians]

Andrian's element is around 4000 watts give or take a few.

Where is Andy ?

[Yuri]

You can instal a thyristor drive and install a hockey puck on the thermocouple to have a 4-20mA output to control the drive

[adrianh]

@justloadit - I'll see if I can find one that can cope with the current.

[Justloadit]

Hi Adrian,

Stick a big heatsink on the triac, being a resistive load, there should be no problem to do this. Don't forget to isolate the triac tab.

[adrianh]

I contacted R & D Electronics in Cape Town who told me that I have to order from their suppler Bellco. Bellco told me that the don't stock the item but that R & D would have to build it specially. Bellco said that they would come back with a price and lead time.... 3 hours ago....

I'm not holding my breath....

[adrianh]

The 5KW dimmer is going to set me back R980. I'm going te get it tomorrow and see how it goes.

[AndyD]

Why do you need to limit the element heat output in the first place?

Just less than a grand for a 5 kW 'dimmer' sounds about right. The guy you need to speak to at R&D is Owen. I've dealt with him on several technical matters for projects in the past and I can tell you he's forgotten more about dimmers and PWM stuff than I'll ever know.

[bergie]

a variac (adjustable transformer) would be ideal. probably too expensive though. why not just replace the element . its very cheap too make up.