Better options for induction coils than water-cooled copper

Question

In an induction heater one of the main problems is that the coil carries a large current. This causes the coil to become hot. There is a limit to how large diameter the wire can be, because it needs to have multiple turns (I think). The standard solution to the problem is to use hollow tubing, then run water through the tubing, thereby cooling it.

Is there a better option for making the coil? The overall goal is to make a coil that can transmit large amounts of current without getting hot.

One obvious improvement I can think of is to make the coil out of silver. Silver is both more electrically conductive than copper and also more thermodynamically conductive, so it will be able to transmit electricity better and be less susceptible to heating up.

Is there an even better way, maybe superconductors?

Answer 1

Commercial Induction Heating Equipment is all about the economics of the equipment and the process of heating.

There are commercial units using super conductors Seen in this post and in This Post

Also, many uses of Induction Heaters are one of a kind for a specific product. Fabricating exotic induction coils for a one time sale might not make sense.

Induction Heating metals of low conductivity (aluminum and copper) would be candidates for using super conduction coils. Induction heating of copper components using copper coils results in efficiency less than 50%.

Heating ferrous objects (below the curie temperature) is probably not a good candidate for exotic induction coils since the efficiency could be quite high (85% to 90% efficient). However in multi-megawatt systems running continuously could save lots of power and $.

The efficiency gain of using silver rather than copper would likely still require water cooling.

Flux concentrators , see Fluxtrol also help with efficiency of the heating process. (not an endorsment for fluxtrol).

Flux Concentrator taken from Induction Presentation

The wall thickness of the copper tubing can be made thinner, so as to increase cooling. But, decreasing wall thickness begins to harm the efficiency (skin depth limitation).

Answer 2

One obvious improvement I can think of is to make the coil out of silver.

First of all conductivity tables: -

Silver conducts about 5% better than copper so not much of an improvement here. The better option is Litz wire because this counteracts something called skin effect.

Skin effect happens when AC current passes down a conductor - magnetic fields induced in the conductor tend to force current to flow at the periphery of the wire and so individually insulated multi-strand wire improves things. Skin effect: -

As you can see, most of the AC current flows at the periphery - this is why copper pipes work so reasonably well - the centre of the copper would hardly conduct anything so it can be filled with water for cooling purposes. Here's a table of skin depth versus material type versus operating frequency: -

This table is the backbone for designing Litz wire - depending on operating frequency, the individual strand diameters can be determined i.e. at 10 kHz there is little point in choosing a copper conductor that has a radius bigger than ~0.6mm.

Here's a good example of how increasing the number of strands is a benefit: -

So for the higher graph the construction is 16 strands of 0.2mm diameter. This is a cross section of copper of 16 x pi x 0.2 x 0.2 / 4 = 0.502 sq mm. For the next lower cable the cross sectional are of copper is the same at 25 x pi x 0.16 x 0.16 / 4 = 0.502 sq mm and the same all the way down to the wire made of 255 strands of 0.05 mm diameter wire (0.501 sq mm).

In conclusion, Litz wire is probably the best way to go but by how much depends on the operating frequency.

Answer 3

The heat in the coils comes from the radiant heat off the work object not from any resistance is the copper coils themselves. Water run thru the coil is the superior way for all but larger industrial units. Water via a pump that cycles thru a radiator with a electric fan attached. Balance the radiator size volume to the work size and power of the unit to get a 100% duty cycle. No need to get any fancier than that.