Electrical Engineering ⇒ Topic : Repulsion (or Double-iron) Type
A schematic diagram of the repulsion type of instrument is shown in Fig. It has a winding which carries a current that is proportional to the voltage or current to be measured. It also consists of two iron rods, one attached to the spindle and the other fixed to the former of the winding. The distance between the rods at all positions is usually small compared to their lengths. When the current flows through the coil, a magnetic field will be established along the axis of the coil, and both the rods will get magnetised alike thus establishing a repulsive force between them. This force is independent of the direction of the current in the winding. The spindle deflects due to this repulsive force. Since the deflecting torque in this type of instrument is produced due to repulsive forces, they are known as repulsion type
FIGURE Moving-iron repulsion type instrument
instruments. The controlling torque is provided either by a spring or by gravity control. But when gravity control is used, the spindle must be horizontal. In the type shown in the figure, air damping is employed. The condition for deflection of this type of instrument can be derived as follows. Let R be the radius of rotation of the moving rod, ¢ be the initial angular displacement between the rods, ¢ be the angle of deflection when a current of I amperes flows through the coil, d be the distance between the rods [see Fig(b), and let m1 and m2 be the pole strengths of the two rods [see Fig.(c)1. Then the magnetic force between the two rods,
From Figure (b)
Then the deflecting torque
We know that m1 α H, and m2 α H, where, H is the magnetic field intensity along the axis of the coil.
Also H α I, therefore, m1 α I, m2 α I and m1 m2 α 12
where k1 is a constant.
If gravity control is used, then the control torque is
under conditions of steady deflection, the two torques are equal, i.e.
It is clear from Equation (c) that the deflection is not proportional to O and therefore in the repulsion type of instrument, the scale is not uniform but is cramped. The meter reading is proportional to the rms value of the current or voltage measured.
Repulsion (or Double-iron) Type
In these instruments, there are two pieces of iron, one is fixed and the other is movable. This moving iron is mounted on a short arm fixed to the instrument spindle. There is one coil. The two iron pieces lie in the magnetic field due to this coil. If there is no current in the coil, no magnetic field will be produced. The two iron pieces - one moving and the other fixed-will touch each other and the pointer rests on the zero position. When the current to be measured, or a definite fraction of it proportional to the voltage to be measured, is passed through the coil, a magnetic field is set up. The two iron pieces are magnetized in the same direction and a repulsive force is set up between the two iron pieces. At this time, the moving iron piece is repelled by the fixed iron piece. This causes the deflection of the pointer.
The pointer comes to rest when the controlling force opposes the deflecting force. In these instruments, the spring control is provided and air friction damping is used. The iron piece is made of soft iron and shaped in such a way so that the uniform deflection is obtained. Figure (a) shows the schematic diagram of a moving iron instrument
figure (a) Moving iron instrument.
Let L be the self inductance corresponding to the angular deflection (total) θ. If dL be the change of self inductance corresponding to small change in deflection angle dθ, due to small changes in current, then the change in the energy of the magnetic field is given by
If TD is the deflecting torque, the work done dW due to the movement of iron by an angle dθ is given by
where I is in amperes and L is in henries.
From Eq. (3), it is clear that the deflecting torque is proportional to the square of current and the rate of change of self inductance L with θ. Therefore, the difficulty arises in designing these types of instruments
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