Eddy current damping is not used on repulsion type instrument because
Right Answer is:
The presence of a permanent magnet required for damping would affect the deflection and hence the reading of the instrument
SOLUTION
The operating magnetic field in moving iron instruments is very weak. Hence eddy current damping is not used since it requires a permanent magnet which would affect or distort the operating field.
Eddy Current Damping:
This is the most effective way of providing damping. It is based on Faraday’s law and Lenz’s law. When a conductor moves in a magnetic field cutting the flux, e,m.f. got induced in it. And the direction of this e.m.f. is such as to oppose the cause producing it.
Eddy current damping is not used on repulsion-type instruments because the presence of a permanent magnet required for damping would affect the deflection and hence the reading of the instrument.
This force always acts in opposite direction to that of the cause producing it, that is, motion according to Lenz s law. This provides necessary damping torque. The magnitude of damping torque is directly proportional to the speed of the moving system.
In this method, a thin disc of non-magnetic conducting material like aluminum or copper is mounted on the spindle which carries the moving system and the pointer of the instrument. The disc is allowed to move along with the spindle inside the magnetic field provided by a permanent magnet. The disc is placed such that, when it rotates it cuts the magnetic flux between the poles of a permanent magnet Hence, eddy currents (eddy currents are currents produced on the surface of conducting materials are produced in the disc. According to Lenz’s law, the eddy currents produce a damping force in such a direction as to oppose the very cause of producing them. Since the cause here is the rotation of the disc, the eddy current forces, damp the movement of the disc and hence the pointer attached to the spindle.