Construction of Moving Iron Repulsion Type Instruments

Construction of Moving Iron Repulsion Type Instruments Points : Construction of Moving Iron Repulsion Type Instruments In repulsion type instrument there are two irons. A curved iron of soft iron is fixed to the inside of the bobbin former and another curved iron is mounted on a spindle, which passes axially through the solenoid. These two irons lie in the magnetic field produced by a solenoid. When there is no current in the solenoid, the two irons (moving & fixed) are almost touching each other and pointer rests on zero position. The solenoid is wound with insulated copper wire on a cylindrical non-magnetic timer. it has few turns of thick wire in case of an ammeter and a large number of turns of fine wire in case of voltmeter. When the current to be measured or proportional to the voltage to be measured is passed through the coil, a magnetic field is set up inside the solenoid. The two irons are magnetized in the same direction. These sots up a repulsive force, so moving iron is repelled by fixed iron, thereby results in the motion of the moving iron carrying the pointer. The pointer comes to rest when equilibrium is attained between repulsion force and the controlling force. Controlling Torque The controlling torque is produced either by hair-springs, when the instrument is to be employed in any position or by gravity, when the instrument is to be operated vertically. Damping Torque The damping in these instruments is air-friction one Eddy current damping is not possible because of presence of permanent magnet needed for such purpose would affect the deflection i.e. reading of the instrument. Shape of Scale As the angular deflection is proportional to the square of the operating current, and the instrument has a square law response. The deflection is in terms of r.m.s. value of current or voltages so the scale of such an instrument is non- uniform. If there is no saturation, the change of inductance with angle of deflection is uniform; the scale of such instrument can be easily laid as the measured quantity is proportional to the square root of the deflection.