18-01-2013, 03:07 PM
Electro-Dynamic Instruments
Electro-Dynamic.pptx (Size: 593.98 KB / Downloads: 34)
Introduction
Measuring instruments are classified according to both the quantity measured by the instrument and the principle of operation.
Three general principles of operation are available:
electromagnetic, which utilizes the magnetic effects of electric currents.
electrostatic, which utilizes the forces between electrically-charged conductors.
electro-thermic, which utilizes the heating effect.
Electric measuring instruments and meters are used to indicate directly the value of current, voltage, power or energy.
Various forces/torques required in measuring instruments
Deflecting torque/force: The defection of any instrument is determined by the combined effect of the deflecting torque/force, control torque/force and damping torque/force. The value of deflecting torque must depend on the electrical signal to be measured; this torque/force causes the instrument movement to rotate from its zero position.
Controlling torque/force: The defection of any instrument is determined by the combined effect of the deflecting torque/force, control torque/force and damping torque/force. The value of deflecting torque must depend on the electrical signal to be measured; this torque/force causes the instrument movement to rotate from its zero position.
Damping torque/force: A damping force is required to act in a direction opposite to the movement of the moving system. This brings the moving system to rest at the deflected position reasonably quickly without any oscillation or very small oscillation. This is provided by i) air friction ii) fluid friction iii) eddy current. It should be pointed out that any damping force shall not influence the steady state deflection produced by a given deflecting force or torque. Damping force increases with the angular velocity of the moving system, so that its effect is greatest when the rotation is rapid and zero when the system rotation is zero. Details of mathematical expressions for the above torques are considered in the description of various types of instruments.
Construction and Basic principle operation of Moving-iron Instruments
It is a electromagnet type instrument in which magnetism is not permanent.
Electromagnetic instruments are classified as (i) moving-iron instruments (ii) electro-dynamic or dynamometer instruments, (iii) induction instruments.
In moving –iron instruments the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil.
There are two general types of moving-iron instruments namely:-
Repulsion (or double iron) type.
Attraction (or single-iron) type
Dynamometer type instruments
Electrodynamic type instruments are similar to the PMMC-type elements except that the magnet is replaced by two serially connected fixed coils that produce the magnetic field when energized.
The fixed coils are spaced far enough apart to allow passage of the shaft of the movable coil.
The movable coil carries a pointer, which is balanced by counter weights. Its rotation is controlled by springs.
The motor torque is proportional to the product of the currents in the moving and fixed coils. If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same time, and the turning force continues in the original direction. Since the reversing the current direction does not reverse the turning force, this type of instruments can be used to measure AC or DC current, voltage, or its major application as a wattmeter for power measurement.