Choice of Frequency for Electric Heating: Induction Heating vs Dielectric Heating

The choice of operating frequency for electric heating is a significant factor for heating because it greatly affects the work to be heated and the method of its heating, whether dielectric heating or induction heating.

In practice, the furnaces operating on power frequency of 50 Hz can be of 1 MW capacity, those operating on medium frequencies (about 500 Hz to 1000 Hz) have a capacity of 50 kW and those operating on high frequencies (about 1 MHz to 5 MHz) have capacities ranging from 200 kW to 500 kW.

Therefore, the factors which are considered while selecting the frequency for electric heating, either induction heating or dielectric heating, are described below.

Induction Heating

The following factors are considered for the selection of frequency for induction heating −

• Thickness of the body or surface to be heated, i.e., higher the frequency, thinner the surface that will get heated.

• The temperature to be obtained, i.e., if higher the temperature, higher the capacity of the generator required.

• The time of continuous heating, i.e., if longer the duration, the deeper is the penetration of the heat in the work to be heated due to conduction.

Dielectric Heating

During the dielectric heating, the power consumed is given by,

$$\mathrm{\mathit{P}\:=\:\mathrm{2\pi}\mathit{fCV^{\mathrm{2}}}\epsilon _{\mathit{r}}\:\mathrm{cos\phi }}$$

Hence, from the above equation is can be seen that the rate of heat production or power consumed can be increased either by increasing the voltage or by increasing the frequency.

In practice, the voltage across the body to be heated is limited by its thickness. Normally, voltages ranging from 500 V to 3000 V are employed for dielectric heating. However, sometimes voltages of the order of 20 kV are also used.

Also, the rate of heat production can be increased by applying high frequencies but it is also limited due to following considerations −

• At high frequencies, it is essential to use special matching circuit due to the fact that maximum power transfer takes place when the oscillator impedance equals to the load impedance.

• At high frequencies, it is difficult for tuning the inductance resonate with the charged capacitance.

• At high frequencies, it is almost impossible to get uniform voltage distribution.

• With the use of high frequencies, there is a possibility of formation of standing waves between the surface of two electrodes having wavelength nearly equal to or more than one quarter of the wavelength of the particular frequency used.

• The high frequencies disturb nearby communication services, therefore special arrangement required to be made to stop radiation from the high frequency generator employed for the heating purpose.

The following table shows the frequencies required for various heating purposes and the type of equipment used.

Types of HeatingFrequencySource or Equipment
Induction Heating
Low temperature heating of metal, annealing50 Hz to 500 HzRotating generator or diode frequency converter.
Deep heat penetration, melting, through heating500 Hz to 10 kHzRotating generator or diode frequency converter.
Surface heating of metals10 kHz to 200 kHzSpark gap generator
Hardening100 kHz to 500 kHzSpark gap generator, Vacuum tube oscillator.
Heating metal pieces, wires and metal strips400 kHz to 1000 kHzVacuum tube oscillator
Dielectric Heating

1 MHz to 50 MHzVacuum tube oscillator