Electrical Engineering ⇒ Topic : Advantages and disadvantages of Permanent-Magnet Moving Coil (PMMC) Instruments

Advantages

(1) Uniform scale i.e., evenly divided scale.

(2) Very effective eddy current damping because the aluminium former moves in an intense  magnetic field of the permanent magnet.

(3) High efficiency as it requires very little power for its operation.

(4) No hysteresis loss as the magnetic flux is practically constant.

(5) External stray fields have little effect on the readings as the operating magnetic field is very strong.

(6)Very accurate and reliable.

Disadvantages

(1) Such instruments cannot be used for a.c. measurements.

(2) About 50% more expensive than moving-iron instruments because of their accurate design.

(3) Some errors are caused due to variations (with time or temperature) either in the strength of permanent magnet or in the control springs.