PMMC :- A permanent magnet moving coil instrument can measure DC voltage & current. The working principle of these instrument is the same as that of the D' Arsonval type of galvanometer, the difference being that a direct reading provided with a pointer & a scale.
Construction :- It has a permanent shoe magnet & a rectangular coil having large number of turns wound on a light aluminium or copper former. A iron core is provided inside the former to provide an easy path for the magnetic lines of force.
The coil is mounted on a spindle. The springs are used to supply current to the moving coil. The pointer is provided on the spindle to indicate the magnitude of quantity being measured.
- Deflecting torque is provided by the force acting on the coil when current flow through it.
- Controlling torque is provided by the springs provided on the spindle.
- Damping torque is provided by eddy current induced in the iron core of the moving coil.
- Scale of these instrument is uniform.
- These are accurate & reliable.
- Eddy current damping is quit effective.
- There is no effect of stray magnetic field as it has a strong permanent magnet.
- These instrument require low driving power, hence their power consumption is low.
- These instrument can operate DC supply only.
- These are costlier than moving iron instrument.
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