INDUCTION HEATING PROBLEM

[lectricman]

I recently installed an Induction Heater for heating carbon steel bars .The heating coil sits about 7" over the 3/4" steel plate that supports it . The problem is the 3/4" plate is heating up to well over 200 deg f after just a few minutes of operation .What can be put between the coil and the plate to to minimize the the affects of the eddy currents . The unit runs at about 8 KHZ .

 

[IRstuff]

Why do you have to use a steel plate?

TTFN

 

[biff44]

You can water cool it. Blow a fan on it. Move it further away. Not that familiar with induction heaters, but try aluminum plate replacing steel and see if it does not get hot.

 

[Higgler]

Based on these web notes below, I'd say just space the unit off your plate if possible,

"The amount of heating will depend on the field strength (and thus the amount of power running through the heating coil and how far it is from the material being heated), how quickly the field is changing (the heating coil power supply frequency), and the degree of coupling between the coil and what is being heated (how close the objects are to one another, what their geometry is).

kch
PS: or suspend it in the air.

 

[lectricman]

This is only part of a autoloading system for a press . The base plate ,which also supports other members of the system ,was thought to be ok if made from carbon steel to keep the cost down as it is approximately 18 sq. ft .Being that it runs at 140 KW / 9 KHZ , it generates a substantial field and the coupling is reasonibly good for this type of heater . I have been told that a 1/8 to 1/4 inch aluminum plate between the coil and the base might do the trick .Thanks for your input .

 

[biff44]

"Sometimes this condition can be corrected by applying a copper plate in the keyway during the heating operation, so that the high frequency current does not have to crowd through the thin area of the work only; but usually a modified design is to be preferred. Sometimes other forms of copper shield are used to cut off heat at limited areas, but these usually are applied on existing parts which were not designed for induction heating. Sharp corners, too, are liable to result in some overheating, sometimes referred to as "edge effect". Here again, by modification in design and a knowledge of induction heating requirements, such difficulties can be easily overcome."

http://www.electroheat-technologies.com/Designing for Induction Heating.htm

So, I would guess that you can cover the steel plate with a 1/4 copper plate, maybe 4 sq foot or so in diameter, right under where the coil is operating. The induced currents will flow in the copper (which due to its lower resistance, will not heat up as much) instead of the steel, and you will be all set.

If you wanted to get fancy, you could cut some thru-slots in the steel plate, which will limit the flow of eddy currents also, but you will have to experiment with placement and size of the slots.

 

[biff44]

typo, meant to say 1/4" copper plate. If you make it up of smaller plates, braze them together at the seams so the currents can flow easily. You might be able to get away with thinner plates--experimenting will tell.

 

[inductionexpert]

The carbon steel will always heat.

Use non magnetic stainless or aluminum 1" thick to knock down the field.

Fred Specht 847-606-9462

 

[electricuwe]

Working for an induction heating company in the past I remember one of the basic rules:

Do never forget that the materials we use for construction basicly are the same that we are going to heat or melt.

So you need to provide ad difference of several orders of magnitude in power density between laod and construction elements.

Sound design based on understanding the physical laws induction heating is based on is the best approach to avoid problems as you have expected.

 

[biff44]

I have to admit I never fully understood low frequency fields. However, wouldn't skin effect still apply, even at 60 Hz?

This link shows that the skin depth of 60 Hz in copper is 8 mm:

http://en.wikipedia.org/wiki/Skin_effect

So if you have a time varying magnetic field, and if you had a thick copper plate, say 40 mm thick, over the steel base, would not the field die off as it passed into the copper plate,and almost none of it would hit the steel base? And since the copper is much more electrically conductive than the steel, those currents would cause less ohmic heating, and thus remain cooler?

 

[inductionexpert]

Aluminum, copper or non magnetic stainless will all work, 1/4" tick is all that is needed to absorb the field.