Capacitor Usage Safety Guide | A Quick Guide to Induction Heating

This guide details briefly the electrical side of induction heating technology. Induction heating is a technology that is found primarily in mass-manufacturing which provides a superior alternative to other means of heating metals, such as flame, furnace or resistive heating, and is used for a wide variety of tasks including melting, brazing, hardening, annealing and others. The technology works on the same principle as a transformer, using the "losses" caused by eddy currents for producing useful heat.

In order to function efficiently, induction heating systems need to create the strongest possible magnetic field with maximum coupling to the part being heated. Consequently, a system operating at extremely high currents is desired. When induction heating is used at high frequencies for surface treatment, these currents exceed what can practically be generated or conducted over any distance. The solution to this is to use an oscillator "tank" circuit.

The oscillator circuit is perhaps best known in radio transceivers. A capacitor and a coil when connected together in series or parallel exhibit special characteristics at their resonant frequency. If excited, the circuit will begin to resonate, energy transferred between and stored alternatively in the electrical and magnetic fields of the capacitor and coil respectively. Induction heating takes advantage of this phenomenon to allow enormous coil currents while keeping the supply current at manageable levels.

A typical parallel-resonant induction heating tank circuit is shown here. It consists of a high frequency generator which supplies the power and initialises the oscillation and the resonator circuit. Due to the high currents flowing (often thousands of amps) the resistive losses in the components generate substantial amounts of heat, and consequently the coil and the capacitor both require cooling.

Coil design requires producing a coil which is physically suited to the heating system requirements, which has a high Q (minimal losses) and an appropriate inductance. Coils are normally manufactured from copper pipe, with cooling water flowing inside the pipe. The pipe should be wide enough to allow sufficient water flow and to minimise electrical resistance, and be rigid enough to not vibrate under the enormous magnetic fields.

The choice of capacitor is dependent on the required operating characteristics. The capacitor must operate safely at the desired operating voltage, and be capable of handling the desired currents at the operating frequency (determined by the depth of heating required), as well as the total reactive power. Capacitors may be conduction cooled^TM or water-cooled, and used singly or in banks. Conduction-cooled^TM capacitor banks allow variation of capacitance and therefore frequency, making the system more flexible, and are cheaper and easier to install than multiple water-cooled capacitors.

Capacitor requirements can be calculated as follows:

In order to choose an appropriate product, a number of decisions have to be made. It is often preferable to buy a solution where coil connections and water cooling are already integrated, which is the case with any respectable water-cooled capacitor. If more than one frequency of operation is required, then an assembly system of conduction-cooled^TM capacitors will allow hot-swapping of the capacitors in the bank in order to achieve this goal. Single capacitors are limited in current and power, and in some cases an assembly system is the only option, the more extreme cases requiring the assembly to be custom designed. Taking a high frequency example, Celem Power Capacitors manufacture the CSP1005 water-cooled capacitor which can handle up to 1000kVAr, but for power beyond this one must turn to an assembly system, such as the Celem AS150/5 which allows up to 2400kVAr.

If the designer is not confident in their own design knowledge, the capacitor manufacturers are normally happy to help find a product solution. They should be provided with an overview of the application and the frequency, voltage and reactive power requirements.