14-02-2012, 07:13 PM
Moving Coil Instruments
There are two types of moving coil instruments namely, permanent magnet moving coil type which can only be used for direct current, voltage measurements and the dynamometer type which can be used on either direct or alternating current, voltage measurements.
Permanent Magnet Moving Coil Mechanism (PMMC)
In PMMC meter or (D’Arsonval) meter or galvanometer all are the same instrument, a coil of fine wire is suspended in a magnetic field produced by permanent magnet. According to the fundamental law of electromagnetic force, the coil will rotate in the magnetic field when it carries an electric current by electromagnetic (EM) torque effect. A pointer which attached the movable coil will deflect according to the amount of current to be measured which applied to the coil. The (EM) torque is counterbalance by the mechanical torque of control springs attached to the movable coil also. When the torques are balanced the moving coil will stopped and its angular deflection represent the amount of electrical current to be measured against a fixed reference, called a scale. If the permanent magnet field is uniform and the spring linear, then the pointer deflection is also linear.
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