Variable Transformer & Home Made Resistor

[dpayne1022]

We currently have a control box at work that is composed of the following:
Payne Engineering 18D-1-30i Variable Transformer (120v, 30a, 1p)
A variable resistor (not sure of its rating)
30 Amp Breaker (used for power on/off switch)
Home made resistor ( a strip of metal in a zig zag configuration )

Basically what we are trying to do is replace the home made resistor to something more sensible. It's not professional looking and creates a hazard since it gets really hot. What else can we do here?

 

[KMoffett]

Can you post a photo?

Ken

 

[duke37]

Any resistor passing current will get hot. If you put it into a cabinet, there will be no health hazard and it could be made to look pretty. To keep the temperature to a reasonable level, use a fan.

 

[dpayne1022]

Attached are the images.

 

[KJ6EAD]

Put something like this:

http://www.postglover.com/ribbon.html

in a screened metal enclosure with a fan or purchase a portable load bank.

 

[dpayne1022]

Portable load bank? Is that small? I want something compact. The transformer is in a tool box and all this is on a cart we push around to where we need it.

 

[KJ6EAD]

It can be large or small, depending on your wattage and runtime requirements. A 4kW unit could fit into approximately 150 cu in.

I'm guessing based on the appearance of your homemade unit that it's less than 1kW.

 

[dpayne1022]

Yeah I think we are only running 3.6 kw. And only run this unit at 20 minutes at max.

 

[dpayne1022]

That explains why it gets soooo hot. We don't call it the "hot dog cooker" for nothing. Do you have any cheap recommendations as far as a ribbon or grid resistor?

 

[dpayne1022]

Or should we just order some of the ribbon and make a longer resistor and just make a housing all ourselves?

 

[KJ6EAD]

Do you have any cheap recommendations as far as a ribbon or grid resistor?

No. I've used some for testing power supplies but never purchased them. Google or check Digi-key, Mouser, etc. Precision costs more and higher wattage costs more so you need to get your specifications nailed down before looking for prices.

If no precision is needed, you could do something kludgy like rewiring a couple of cheap portable heaters.

 

[CDRIVE]

Yeah I think we are only running 3.6 kw. And only run this unit at 20 minutes at max.

Or should we just order some of the ribbon and make a longer resistor and just make a housing all ourselves?

No matter how you cut it you have 3.6KW to dissipate either as heat or as mechanical energy. It would be nice if you could do something useful with it instead of merely wasting it. I say wasting it because, unless I'm reading this wrong, this is a dummy load? On the up side it helps heat the shop in the winter and in the summer you can make a lot of toast.

Chris

 

[Lenp]

Years ago I made some high power load resistors for audio testing. To be non-inductive I used 2 square sheets of copper and drilled them with a hole pattern. High wattage axial lead resistors were mounted through the holes and soldered to each plate. Think Oreo cookie..the cookie is the copper and the cream is the resistor!

Just use ohm's law to figure the value since all resistors will be in parallel. Cooling should be easy with a small fan. If space permits make the copper plates larger than needed and add vent holes to enhance cooling.
Hope this helps!
Len