power resistor temperature? 1

[bjeservice]

i built 0.1 ohm power resistor in order to use it in induction furnace start cct.
the current that must pass throught is: 1000 amper for one second.
could any one help me finding an equation that connect the temperature with time for the resistor.
for more details the resistor width is 60 mm , length=2.85 meter of silicon steel and cooled naturally.

 

[Skogsgurra]

I understand that you are using a steel band to make the resistor. It would help if you give the thickness of that band. It could be "reverse engineered" from length, width and resistance. But better if you tell us.

Also, is the resistance given at room temperature?

Gunnar Englund

http://www.gke.org

 

[CarlPugh]

Resistor must be non magnetic. Non magnetic stainless steel is usually acceptable for use in this type application. Magnetic silicon steel should not be used.

Calculate joules dissipated (watts X time)
Multiply joules X specific heat. (Units should be the same, CGS or english or whatever)
This will give the temperature rise. (Assuming resistor cools down between on times)

Or you could do it the easy way and measure the temperature rise.

 

[bjeservice]

Dear skogsgurra
the thickness = 0.3 mm
and the resistor at room temp.
it is fitted horizontally for air convection heat transfer.

 

[bjeservice]

Dear CarlPugh
i built the resistor and test it and it is working well
but i need to know how long time it will stay working before failer
i would like to ask about an equation that take the case of heat transfer into account.
thank you alot for your replay

 

[Skogsgurra]

bjservice,

A very rough calculation says that the weight of your band is around 0.400 kg. The heat capacity of steel is around 0.5 kJ/kg and the energy developed is RxI2 = 0.1x1000000 = 100 kJ.

So the resultant temperature rise should be someting like 100 [kJ]/(0.4[kg]x0.5[kJ/kgxK] = 500 K. That means that the temperature rise will be around 500 kelvin. So, if your ban is at room temperature, say 25 centigrades, it will me slightly above 500 centigrades after 1 second with 1000 A. There will be some cooling, so the end temperature is probably lower than 500 centigrades. Iron melts at much higher temperatures - it will probably not even glow red at the end of the second.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

centigrades => Celsius.

 

[Skogsgurra]

Yes, Celsius. Also, I forgot to multiply power with time to get energy. But, since time is 1 second, the result will be the same. That's part of the beauty of the SI.

Gunnar Englund

http://www.gke.org

 

[CarlPugh]

Dear bjeservice
It's surprising that the resistor is working satisfactorily.

One time the test department used a cold rolled steel bar as a load.
The load was non linear, (doubling the voltage did not double the current) and the test results were not repeatable. (One time they would measure one value of current and when the identical test was repeated, they would get a different value of current)
It was decided that the inductance of the bar varied with the current and also with the temperature of the bar. Also that the resisistance of the bar varied with the temperature.
A non magnetic stainless steel bar was used and the test was satisfactory.

0.1 ohm at 1000 amp gives 100 volt. This seems high?

Why don't you just use the wire or bus used to connect to the induction furnance as the power resistor?

Good Luck
Carl

 

[bjeservice]

Dear CarlPugh
1. the linearity is not a problem for me and i think there was a type of power resistor called Ballast resistor( that has a high temperature coefficient ) which change its value rapidaly with temperature is more active to my application.

2. i tested the resistor by dc method,but if there is more reactance that i should has because of inductance it is more value for me to reduce the high starting current of furnace.

3. the resistor temperature rise quickly in my application ( as we calculate more than 500 deg.) i think the magnetic iron sheet lose its magnetic properties near that temp.

4. there was a contactor that cut the resistor from cct. after 1 sec. , so i could not use the cables that deliver the load current as a resistor as i need a more 0.1 ohm value in small area for a one second.

thank you very much for your discussion ideas.

 

[ScottyUK]

bjeservice,

Contact one of the commercial suppliers of high power resistors. Someone like Cressall Resistors

http://www.cressall.com

will probably have an off-the-shelf product which will meet you needs. Their products are not especially expensive and you'll save a lot of time which in the commercial world has a real monetary value. You may actually end up saving money by using the commercial product!

A ballast resistor is, in the power generation world at least, used to modify the L/R time constant of a generator's field circuit. I guess the term has similar meaning elsewhere - I've never heard it used to describe the characteristics of the resistor itself. High power resistors are usually stainless steel coils or perforated plates. As I understand it, stainless steel has non-magnetic properties and has good resistance to oxidation at high temperature.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[CarlPugh]

bjeservice

If the real and imaginary impedance of the resistor is changing, then it will be nearly impossible to calculate the temperature rise of the resistor.

The best method to obtain the resistor temperature rise would be to measure it.