Induction Heater Components

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We will talk about each component making up the induction heater. First, there is the workcoil. This is what heats the workpiece. The workcoil will get very hot from the high current going through it and the radiation of heat from the workpiece.

The workcoil is attached to the LC tank. This can either be a series or parallel resonant tank. The tank and coil need to be cool, so I implemented a plumbing-type design that allows me to pump water through the coil using a fountain pump.

The resonant tank is coupled to the power source with a coupling transformer. The transformer is connected to the inverter.

The inverter chops the DC power source at a particular frequency. This is the resonant frequency of the tank. Now, as the workpiece heats and goes through its curie point - the temperature when the metal is no longer ferromagnetic - the resonant frequency changes. The inverter needs to stay locked on as closely as possible to the current resonant frequency to achieve the fullest power. Some will do this manually, using an oscilloscope to monitor the waveforms, or using a voltmeter on the tank and tuning the frequency to the highest tank voltage. Another method is using a phase locked loop (PLL) to monitor the phase relationship of the inverter voltage and tank voltage. This is the method I use and I will discuss this in detail later on.

Let's start with how to easily make a workcoil. We will be using frequencies in the 10s to 100s of kilohertz (kHz), so metals will conduct the current only slightly below the surface. This is the skin effect. The current depth in mm is

Depth (mm) = 76/√(F)

So, the wider the tubing, the lower the resistance. We also want to use tubing so we can water-cool the coil. I purchased some refrigerator 3/8" copper tubing from Home Depot. You will also need some 1/2" copper pipe and the necessary fittings so you can feed water through one end, have it circulate through the coil, and come out the other end. I have brass fittings with nipples so I can attach some tubing to my fountain pump, and a return tube to my ice water bath.

This is the tubing I got from Home Depot.

I want to mention a few points about the workcoil:

More turns allows you to heat a bigger piece of metal. The coil should allow you to easily heat your workpiece, or to do so with small movements in and out of the field. The more turns, the less induced voltage, and less induced current in the workpiece. If the induced current is too low you may never achieve a high enough temperature to get beyond the Curie point, where you will then get a significant boost in heating. I believe this occurs, because of the change in the workpiece molecular arrangement, reducing the quenching effect on the coil.

You will also have a lower Fres for the same tank capacitance. This results in deeper current penetration into the workpiece, which may or may not be desired depending on your application. All this means it will take longer to heat the metal for the same input power. To compensate you will need a higher voltage going to the workcoil if you want to maintain the same rate of heating. You can compensate for more turns on your workcoil with fewer turns on your coupling transformer. However, you will still be faced with the issue of needing more input power to achieve the higher excitation voltage on the workpiece. You can get more input power by having a higher input voltage or drawing more current.