SOLUTION

The permanent magnet moving coil (PMMC)-type instrument is the basic dc measuring instrument. In the instruments, a permanent magnet, generally of horseshoe type, creates a magnetic field in which a coil of fine wire of the number of turns is placed.

Advantages of PMMC Type instruments

1. In moving-coil instruments, the power consumption is very small.
2. The torque to weight ratio is very high.
3. Moving-coil instruments have uniformly graduated scales.
4. The same instrument can be used both as an ammeter and a voltmeter by a suitable shunt or series-resistance (multiplier).
5. Moving-coil instruments are free from errors due to external magnetic fields since the working magnetic field strength itself is very high.
6. Very large scale deflection (300°) is possible using the “circular scale “type of instruments.
7. Damping employed in moving-coil instruments is perfect since an eddy current damping is used.
8. The torque-weight ratio is very high.
9. Due to the application of an intense polarized or unidirectional field, the stray magnetic field has no effect.
10. Since the PMMC instrument used a DC source there is no varying electromagnetic field on the core such and there will be no losses on this account.

Disadvantages of PMMC Type Instrument

• PMMC type of instrument can be operated in direct current only. In alternating current, the instrument does not operate because in the positive half the pointer experiences a force in one direction, and in the negative half, the pointer experiences the force in the opposite direction. Due to the inertia of the pointer, it remains in its zero position.
• Compared to moving iron instruments, these instruments are costlier,
• Friction error due to Jewel and pivot suspension, temperature, aging of control springs, and permanent magnets might introduce errors.

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