Introduction to Permanent Magnet Moving Coil instrument

Permanent Magnet Moving Coil: 

Principle :  
as per Fleming’s left hand rule, When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction .

Moving-coil instruments with a permanent-magnet field respond only to direct current. for Measurement of AC voltage requires a rectifier in the circuit so that the coil deflects in only one direction.

Construction:

A coil of thin wire is mounted on frame of an aluminum  (spindle) positioned between the U shaped permanent magnet poles of a  which is made up of magnetic alloys like alnico.

The coil is pivoted on the jeweled bearing , the coil is free to rotate. The current is fed to the coil through spiral springs which are two in numbers. The coil which carries a current, which is to be measured, moves in a strong magnetic field produced by a permanent magnet and the measured value shows by a pointer is attached to the spindle which shows .

Working:

When a will current flow through in the coil, it generates a magnetic field which is proportional to the current in case of an ammeter. In the meter The deflecting torque is produced by the electromagnetic action of the current in the coil and the magnetic field.

Advantages:

-      The PMMC consumes less power and has great accuracy.

-   It has uniformly divided scale and can cover arc of 270 degree.

-A high torque to weight ratio in PMMC.

-    With suitable resistance,  It can be modified as ammeter or voltmeter .

-       efficient damping characteristics and is not affected by stray magnetic field.

- No losses produces  due to hysteresis.

Disadvantage:

·        As the reversal of current produces reversal of torque on the coil ,so The moving coil instrument can only be used on D.C supply

·        It’s very delicate and sometimes uses ac circuit with a rectifier.

·        Itis costly as compared to moving coil iron instruments.

·        due to loss of magnetism of permanent magnet,it can show error .