High Frequency Eddy Current Heating - Working, Advantages, Disadvantages

The eddy current heating is the process of heating the charge by inducing eddy currents in it. In this method, the charge or material to be heated is placed inside a current-carrying coil that induces an eddy current in the charge, due to which heat is produced and the temperature of the charge is increased. For eddy current heating the input current supplied is of high frequency hence the method is also called high-frequency eddy current heating.

Eddy Current Heating :

The below diagram shows the arrangement of high-frequency eddy current heating. The charge or material to be heated is surrounded by a coil through which alternating current at high frequency is supplied. This current-carrying coil is known as a heater coil or work coil and is magnetically coupled with the charge to be heated.

Working of Eddy Current Heating :

When the coil is connected across a high-frequency ac supply, it setups an alternating magnetic flux due to which eddy currents are induced in the charge. This eddy current loss in the charge will generate heat in the charge thereby increasing the temperature of the charge.

The heat produced in the charge is mostly due to eddy current loss, but to some extent, hysteresis loss also contributes to the production of heat in charge in the case of magnetic material. The eddy current loss caused by eddy currents in a magnetic material is given by,

P_e = K_e f² B_max² t² V

From the above equation, it can be seen that eddy current loss is proportional to the square of the supply frequency and magnetic flux density i.e., P_e ∝ f² B². Therefore by controlling the frequency and flux density, the heat produced in the charge can be controlled.

But the magnitude of eddy current induced in the charge will be large at the surface of the charge and goes on decreasing as we go inside the charge. Thus the eddy current loss i.e., the heat produced, will be high at the surface of the charge and low at the center. The depth up to which the eddy currents are induced in the charge can be known by,

Where,
ρ = Resistivity of charge.
f = supply frequency.
μ_r = Relative permeability of charge.

Since the depth of penetration of eddy currents into the charge is inversely proportional to the square root of the supply frequency. Therefore using eddy current heating, the depth of heat produced in the charge can also be controlled by judicious selection of frequency of the supplied current. A frequency in the range of 10 kHz to 40 kHz is used for eddy current heating.

Advantages of Eddy Current Heating :

The advantages of high-frequency eddy current heating are,
The temperature control is easy in eddy current heating.
The penetration of heat into the charge can be controlled to any desired depth.
The coil surrounding the charge is not required to fit closely around the charge.
By using this method, materials of different sizes and shapes can be heated.
This type of heating method can be operated in any atmosphere conditions (such as vacuum or any other special atmosphere).
The amount of heat produced can be accurately controlled by using suitable timing devices.
This method of heating is quick, clean, and convenient.
Since heat produced is inside the charge, the loss or wastage of heat is less.
The eddy current heating of a material can be carried out even by unskilled labor.

Disadvantages of Eddy Current Heating :

The disadvantages of high-frequency eddy current heating are,
The cost of the equipment required for dielectric heating is high.
The efficiency of the equipment is quite low.
The cost of heat produced is high.

Applications of Eddy Current Heating :

Some of the applications of eddy current heating are,
The high-frequency eddy current heating is used for surface hardening in which the desired depth of penetration of heat can be obtained by judicious selection of frequency which reduces the cost, labor, and time considerably.
This method is employed in the annealing of metals which saves a lot of time along with the prevention of scales on metals obtained by conventional methods.
The soldering can be done precisely and economically using eddy current heating.
This method of heating is also used for welding, drying of paints, melting of precious metals, sterilization of surgical instruments, and forging bolt and rivet heads.