EDDY CURRENTS AND IT’S APPLICATIONS
DEFINITION:
EDDY CURRENT: When magnetic flux linked with a coil changes, induced e.m.f. is produced in it and the induced current flows through the wire forming the coil.
Foucault found that when magnetic flux linked with a metallic conductor charges, induced currents are set up in the conductor in the form of closed loops . These currents look like eddies or whirl pools and likewise are known as Eddy currents. They are also known as Foucault’s currents .
Thus , eddy currents are the currents induced in a conductor, when placed in a changing magnetic field.
APPLICATIONS OF EDDY CURRENTS.
Here are the some important applications of eddy currents
1. Dead beat galvanometer. The oscillations of a moving coil galvanometer generally take a long time to die out . But by winding it’s coil on a magnetic frame made of copper or aluminum, the galvanometer can be made dead beat. It is because due to the production of eddy currents in the metallic frame , the coil of the galvanometer comes to rest very soon.
It may be pointed out that the working of ballistic galvanometer requires that the damping should be minimum. It is achieved by winding the coil on a frame made of non -conducting material, such as bamboo or paper.
EDDY CURRENTS AND IT’S APPLICATIONS
2. ELECTROMAGNETIC DAMPING:
The oscillations of the coil of a galvanometer can also be stopped by short circuiting the two ends of the coil through a tapping key . When the key is pressed, the coil circuit is closed and the eddy currents are produced due to the motion of the coil inside the magnetic field produced by the magnet of the galvanometer . Due to Lenz’s law , the direction of eddy currents is such that it oppose the oscillations of the coil.
3. INDUCTION MOTORS.
A rotating magnetic field is produced by means of two single phase currents A metallic rotor placed inside the rotating magnetic field starts rotating due to large eddy currents produced in it . These motors are commonly used in fans.
4. INDUCTION FURNACE
In an induction furnace very high temperature can be produced by producing large eddy currents . The high frequency eddy currents are preferred because of the reason that the heating effect of current is proportional to the square of the frequency of the current.
5. DIATHERMY.
Eddy currents have been used for deep heat treatment called DIATHERMY.
6. SPEEDOMETER.
In the speedometer of an automobile, a small magnet is geared to the main shaft of the vehicle and it rotates according to the speed of the vehicle. The magnet is mounted in an aluminum cylinder with the help of hair springs. When the magnet rotates , it produces eddy currents in the drum. Due to the effect of eddy currents, the drum to oppose the motion of rotating magnet. In turn , the drum experiences a torque and gets deflected through certain angles. A pointer attached to the drum moves over a calibrated scale , which directly indicates the speed of the vehicle.
7. ENERGY METERS.
In energy meters , the armature coil carries a metallic aluminum disc, which rotates between the poles of a pair of permanent horse shoe magnets. As the armature rotates the current induced in the disc tends to oppose the motion of the armature coil. Due to this braking effects , the deflection is proportional to the energy consumed.
8.ELECTRIC BREAK.
A metallic drum is coupled to the wheels of the train, so that when train runs , the drum also rotates . In order to stop the train, a strong magnetic field is applied to the rotating drum. The large eddy currents produced oppose the motion of the drum. Since the drum is connected to the wheels of the train, the latter comes to a halt.
These days electric brakes are provided in the trains in addition to usual vacuum brakes.
EDDY CURRENTS AND IT’S APPLICATIONS
UNDESIRABLE EFFECTS OF EDDY CURRENTS.
Since the resistance of a metallic conductor is quite low, the magnitude of eddy currents produced is quite large. AS such , considerable amount of heat is produced in the conductor due to Joule ‘s heating effect. If the large eddy currents are allowed to produce in the core of a choke coil , transformer dynamo, etc , it may produce undesirable effects.
The eddy currents produced in the core of a transformer are reduced by making use of a laminated core . The thick core of a transformer is formed by joining identical iron strips together, after insulating them from each other by coating them with varnish. Since the resistance of an iron strip will be much larger than the resistance of the thick core, only very feeble eddy currents are produced in the iron strips and the transformer is saved from getting heated.
EDDY CURRENTS AND IT’S APPLICATIONS
