Eddy
current damping is the most efficient form of damping. The essential components
in this type of
damping
are a permanent magnet; and a light conducting disc usually of aluminium.
When
a sheet of conducting material moves in a magnetic field so as to cut through
lines of force, eddy currents are set up in it and a force exists between these
currents and the magnetic field, which is always in the direction opposing the
motion. This force is proportional to the magnitude of the current, and to the
strength of field. The former is proportional to the velocity of movement of
the conductor, and thus, if the magnetic field is constant, the damping force
is proportional to the velocity of the moving system and is zero when there is
no movement of the system.
In
Fig. (a) a thin disc of conducting, but non-magnetic material-usually copper of
aluminium is mounted on the spindle which carries the pointer of the
instrument. When the spindle rotates, the edge of the disc cuts through the
lines of force in the gap of a permanent magnet, and eddy currents, with
consequent damping, are produced.
Fig.
(b) shows the essential parts of a PMMC instrument. The coil is wound on a
light metal former in which eddy currents are induced when the coil moves in
the permanent-magnet field. The directions of the eddy-current which in turn
produce the damping torque due to the motion of the coil (clockwise) are as
shown in Fig. (b) and this will produce damping forces as indicated in the
figure.
Errors:
The
common errors are on account of friction, change in temperature, mechanical
unbalances and variation of the electrical and mechanical properties of the
materials employed with time.
Temperature Error: This type of error, apart from the change in room
temperature is mainly due to heating of the working coils and other resistance
coils connected internally in the instrument casing by the operating
current. As a result there is a change in resistance of the working coil
and other resistance coils. Such an error is not serious in case of an
ammeter but it is very important in case of a voltmeter which is of high
resistance and where constancy of resistance is very important. For
reducing such an error the power lost in a voltmeter should be kept very small
and arrangement for ventilation and cooling should be provided. Sometimes the
error is minimized by winding a series resistance coil of material of very
small temperature coefficient of resistance and having the working coil wound
of copper wire to have very low resistance in comparison with that of series
coil. This will reduce overall percentage change in resistance.
Friction
Error: The friction of the pivot in the
jewel produces a frictional torque which affects the instrument reading. This
error is more serious for sensitive instruments designed for low operating
torque. Such errors may be reduced by adopting a moving system of light
construction and large deflecting torque.
Observational
Error: Such errors are due to misreading
of the scale, parallax in readings and errors of estimation.
