Brief
Details:- In this lecture we will cover Moving Coil, Moving Iron and
dynamometer type of instruments.
Moving Iron
instrument consists of a coil of copper having many turns. There is an iron
spindle in front of the coil axis. Iron spindle is connected to weight to
balance the spindle in equilibrium position. Iron spindle is also connected to
a pointer which moves on a scale. Spindle is also connected to an air chamber
provided with a piston. Purpose of air chamber is to provide air frictiondamping to the needle to avoid unnecessary oscillations of pointer.
When current
is allowed to pass through the coil there exist a strong magnetic field along
the axis of coil due to which iron spindle experiences magnetic attractive
force towards the coil axis. When spindle moves towards the axis of coil,
weight attached to the spindle provides gravitational controlling torque.
Amount of
deflection produced is directly proportional to the square of electric current.
Thus scale of the moving iron instruments is non uniform. Moving iron
instruments are suitable for both ac as well as dc currents. Moving iron
instruments are bulky in size and have high cost.
