What is the drawback of a Permanent Magnet Moving Coil (PMMC) instrument?

Question

Examoogle · Accepted Answer

The correct answer is Option D: Relatively high-cost as compared to moving iron instruments. PMMC instruments are essentially expensive to manufacture due to the requirement of high-quality permanent magnets, precision coil winding, and delicate spring-jewel bearing assemblies. Compared to Moving Iron (MI) instruments, which use simple iron vanes and a stationary coil, the complex construction of a PMMC meter significantly increases its unit production cost.

Step-by-Step Solution & Concepts:
PMMC instruments are essentially expensive to manufacture due to the requirement of high-quality permanent magnets, precision coil winding, and delicate spring-jewel bearing assemblies. Compared to Moving Iron (MI) instruments, which use simple iron vanes and a stationary coil, the complex construction of a PMMC meter significantly increases its unit production cost.

Working Principle: The principle of PMMC is based on the Lorentz force, where a current-carrying coil placed in a uniform magnetic field experiences a deflecting torque $T_d$. The torque is given by $T_d = N B A I$, where $N$ is the number of turns, $B$ is the flux density, $A$ is the coil area, and $I$ is the current. This force interacts with control springs to provide a deflection proportional to the average value of the current.

Key Formulas: $T_d = N B A I$ — where $T_d$ is the deflecting torque in Newton-meters (N-m); $T_c = K 	heta$ — where $T_c$ is the restoring torque provided by springs and $	heta$ is the angular deflection

Key Points: PMMC instruments respond only to DC and measure the average value of current.; They provide a linear scale because the deflection is directly proportional to the current.; High sensitivity is a major feature, but the instruments are delicate and sensitive to mechanical shock.; Eddy current damping is used to bring the pointer to rest quickly.

Advantages: Uniform scale; High torque-to-weight ratio; High sensitivity; Excellent damping

Disadvantages: High cost; Cannot be used for AC measurements; Sensitive to temperature variations due to change in magnet strength; Susceptible to mechanical wear of pivot-jewel bearings

Applications: DC Ammeters; DC Voltmeters; Galvanometers; Battery charging monitoring systems

Additional Info: Option A is incorrect because PMMC instruments actually have very low power consumption.. Option B is incorrect because PMMC instruments are known for having a very high torque/weight ratio.. Option C is incorrect because PMMC instruments utilize highly effective eddy current damping.. The primary construction constraint of PMMC is the high cost of permanent magnets and precise assembly.

Answer: D is correct — PMMC instruments are relatively more expensive than moving iron instruments due to their complex, high-precision assembly and permanent magnet requirements.

Exam Tip: Always remember that PMMC is for DC only; if an AC input is applied to a PMMC, it will oscillate around zero and show no reading due to the inertia of the moving system.

Examoogle · Answer

Option A: High power consumption

Examoogle · Answer

Option B: Low torque / weight ratio

Examoogle · Answer

Option C: Absence of effective and efficient current damping

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