INVENTOR OF EDDY CURRENT AND SPEED OF LIGHT
JEAN BERNARD LÉON FOUCAULT – French physicist who determined the speed of light and showed that it travels slower in water than in air; invented the Foucault pendulum and the gyroscope.
He first directed his attention to the improvement of Louis Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy.
In 1850, he did an experiment using the Fizeau–Foucault apparatus to measure the speed of light.
In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis (diurnal motion).
Foucault achieved the demonstration by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon, Paris. The experiment caused a sensation in both the learned and popular worlds, and "Foucault pendulums" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the gyroscope as a conceptually simpler experimental proof.
In September 1855 he discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current or "Foucault currents" induced in the metal.
He died on 11th February 1868
He lived for 48 years in this planet but his name is one of the 72 names inscribed on the EIFFEL TOWER.
CHOICE OF FREQUENCY
Low temperature heating of metal, annealing – 50 – 500 HZ.
Melting, deep heat penetration – 500 HZ – 10 KHz
Surface heating of metals – 10 KHz – 200 KHZ
Surface hardening – 100 KHZ – 500 KHZ
Heating metal pieces, wire and metal strips – 400 – 1000 KHZ
Dielectric heating – 1MHZ – 50 MHZ.
Low temperature heating of metal, annealing – 50 – 500 HZ.
Melting, deep heat penetration – 500 HZ – 10 KHz
Surface heating of metals – 10 KHz – 200 KHZ
Surface hardening – 100 KHZ – 500 KHZ
Heating metal pieces, wire and metal strips – 400 – 1000 KHZ
Dielectric heating – 1MHZ – 50 MHZ.
EDDY CURRENT – An electric current in a conducting material that results from induction by a moving or varying magnetic field. “Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. The heat can be harnessed for induction heating.”
THE EDDY CURRENT LOSS - Eddy current are those which are produced or induced in the masses of metals, whenever these metals are moved in magnetic field, or the magnetic field moves through the metals. The direction of the eddy currents is always in opposite direction to the cause (motion) to produce them. It is the loss of power i.e. I2R due to eddy currents, and it is expressed in watts, which causes the output of the machine to decrease.
Pe = Kef2Bm2 watts
Pe = Kef2Bm2 watts
HYSTERESIS LOSS - The term "hysteresis" is derived from an ancient Greek word meaning "deficiency" or "lagging behind". It was coined around 1890 by Sir James Alfred Ewing to describe the behaviour of magnetic materials. It is the loss of power due to hysteresis, and it is expressed in watts or kilowatts. This actually raises the temperature of the part where the magnetic reversal occurs. Hysteresis cannot be avoided but can be minimized by selecting proper materials which has lesser hysteresis constant. Wh = KhfBm2 watts
At high frequency the heating due to hysteresis becomes very small as compared to eddy currents.
At high frequency the heating due to hysteresis becomes very small as compared to eddy currents.
PRINCIPLE OF EDDY CURRENT HEATING
1. It works on the principle of electromagnetic induction.
2. A finite value of diametre and thickness of a metal disc surrounded by a copper coil in which an alternating current is flowing.
3. We find that a secondary current I is caused to circulate around the outer surface of the disc.
4.In transformer the electrical energy is available in secondary while in induction heating it is used to heat the charge itself which acts as a short circuited secondary.
5. The high frequency current carrying coil is known as heater coil or work coil. The material which is to be heated is known as the charge or load.
6.The current flows on the outer surface of the metal disc and in so doing, heats this surface. The heat energy is transferred to the metal at an extremely rapid rate, much faster than conventional methods of heating metal.
7.The heat is generated within the metal without any physical contact between the source of electrical energy and the metal being heated.
8. The medium of energy transmission, the magnetic field, can penetrate any non-metallic substance placed between the heating coil and the material being heated.
9.The process employed is referred to as high frequency eddy current heating.
10. The heat in the disc can be increased by increasing the coil current, increasing the number of turns, frequency of supply, close spacing between the coil and work, using higher permeability magnetic material and higher electrical resistivity of the disc.
1. It works on the principle of electromagnetic induction.
2. A finite value of diametre and thickness of a metal disc surrounded by a copper coil in which an alternating current is flowing.
3. We find that a secondary current I is caused to circulate around the outer surface of the disc.
4.In transformer the electrical energy is available in secondary while in induction heating it is used to heat the charge itself which acts as a short circuited secondary.
5. The high frequency current carrying coil is known as heater coil or work coil. The material which is to be heated is known as the charge or load.
6.The current flows on the outer surface of the metal disc and in so doing, heats this surface. The heat energy is transferred to the metal at an extremely rapid rate, much faster than conventional methods of heating metal.
7.The heat is generated within the metal without any physical contact between the source of electrical energy and the metal being heated.
8. The medium of energy transmission, the magnetic field, can penetrate any non-metallic substance placed between the heating coil and the material being heated.
9.The process employed is referred to as high frequency eddy current heating.
10. The heat in the disc can be increased by increasing the coil current, increasing the number of turns, frequency of supply, close spacing between the coil and work, using higher permeability magnetic material and higher electrical resistivity of the disc.
Eddy current loss is responsible for the production of heat although hysteresis loss also contributes to some extent in the case of magnetic material. As the eddy current loss is proportional to Bm^2 F^2, this loss can be controlled by controlling Bm and the supply frequency f.
The depth of penetration ‘d’ of eddy currents into the charge is given by:
D = [1/2∏] √ρx 10^9/μf cm, ρ – specific resistance of molten charge in ohm-cm
f – frequency in Hz , μ – permeability of the charge
This shows higher the frequency of supply the lower the depth of penetration.
The depth is directly proportional to 1/√f. The supply frequency is usually employed between 10kHZ to 40kHZ
The depth of penetration ‘d’ of eddy currents into the charge is given by:
D = [1/2∏] √ρx 10^9/μf cm, ρ – specific resistance of molten charge in ohm-cm
f – frequency in Hz , μ – permeability of the charge
This shows higher the frequency of supply the lower the depth of penetration.
The depth is directly proportional to 1/√f. The supply frequency is usually employed between 10kHZ to 40kHZ
ADVANTAGES
1. It is quick, clean and convenient method.
2. Heat produced in the body to be heated up directly, hence wastage is less.
3. Eddy current heating can easily take in vacuum or special atmosphere.
4. Temperature control is easy. 5. Heat can be made to penetrate into the metal surface to any desired depth.
1. It is quick, clean and convenient method.
2. Heat produced in the body to be heated up directly, hence wastage is less.
3. Eddy current heating can easily take in vacuum or special atmosphere.
4. Temperature control is easy. 5. Heat can be made to penetrate into the metal surface to any desired depth.
APPLICATIONS
It is used for surface hardening, annealing, soldering, welding, drying paints, melting of precious metals, sterilization of surgical instruments and gorging of bolts and rivet heads.
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