The advantages of moving coil permanent magnet type instrument are

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Examoogle · Accepted Answer

All of the above

Explanation: Permanent Magnet Moving Coil (PMMC) instruments are highly efficient DC measuring devices that operate on the principle of the Lorentz force acting on a current-carrying coil in a static magnetic field. They are characterized by linear scales, high sensitivity, and excellent damping properties, making them the standard choice for precision DC measurements.

Working Principle: When a direct current passes through the moving coil, it experiences a torque $T_d = N B I l d$ due to the interaction between the coil's magnetic field and the static field produced by the permanent magnet. The coil is wound on a light aluminum former, which acts as a short-circuited loop; as the coil moves, eddy currents are induced in this former, creating a braking torque that provides highly effective electromagnetic damping.

Key Formulas: $T_d = N B I l d$ — The deflecting torque developed in the PMMC instrument; $T_c = K 	heta$ — The restoring torque provided by the control springs

Key Points: PMMC instruments only work on DC supplies because the torque direction reverses with polarity.; The scale is linear because the deflecting torque is directly proportional to the current ($T_d \propto I$).; They provide the highest torque-to-weight ratio among analog instruments.; Hysteresis loss is negligible as there is no iron core within the moving coil itself.

Advantages: Very low power consumption due to high sensitivity.; Absence of hysteresis loss ensures high accuracy.; Effective eddy current damping provides quick, dead-beat settling.; Uniform scale distribution allows for precise readings.

Disadvantages: Limited to DC measurements only.; Control springs are subject to fatigue and temperature aging.; Higher cost compared to Moving Iron (MI) instruments.

Applications: Used as DC Ammeters and Voltmeters.; Used in galvanometers for detecting minute DC currents.; Basis for many electronic test equipment measurements.

Additional Info: Low power consumption: The high torque-to-weight ratio allows for a low number of turns and minimal current required for full-scale deflection.. No hysteresis loss: Since the magnetic circuit involves only an air gap and no soft iron core inside the moving coil, magnetic reversal losses are absent.. Eddy current damping: The aluminum former moving within the permanent magnetic field induces an EMF, resulting in circulating currents that oppose motion.

Answer: D is correct — PMMC instruments offer low power consumption, zero hysteresis loss, and superior eddy current damping, making them ideal for high-precision DC measurements.

Exam Tip: Always remember that PMMC = DC only; if you see AC measurement, think Moving Iron (MI) or Electrodynamometer types instead.

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