Question

A Permanent Magnet Moving Coil (PMMC) instrument can be used to measure:

• A. DC quantities only
• B. AC quantities only
• C. Both AC and DC quantities
• D. Only high-frequency AC

Hint:

PMMC measures the average value of any waveform.
Since the average value of a pure AC wave is zero, a PMMC meter connected to an AC line reads zero.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks about the operating range and applicability of a Permanent Magnet Moving Coil (PMMC) instrument regarding AC and DC quantities.
PMMC instruments are standard analog meters based on electromagnetic torque.

Step 2: Key Formula or Approach:
The deflecting torque in a PMMC instrument is given by:
\[ T_d = BIAN \] Because the permanent magnet establishes a constant magnetic field (\( B \)), the deflecting torque is directly proportional to the magnitude and sign of the current (\( T_d \propto I \)).
If the direction of the current alternates, the direction of the deflecting torque will also alternate.

Step 3: Detailed Explanation:
Let us analyze the behavior of the instrument under both DC and AC excitations:

• Under DC Excitation: The current is constant in both magnitude and direction. This results in a stable, unidirectional deflecting torque, leading to an accurate pointer reading.

• Under AC Excitation: When an alternating current of frequency \( f \) is passed through the coil, the torque alternates direction at the same frequency.
Because the moving coil assembly has significant physical mass and mechanical inertia, the pointer cannot follow these rapid reversals (e.g., at 50 Hz).
Instead, the pointer settles at its mean position, which corresponds to the average value of the applied AC torque over a complete cycle:
\[ T_{d,\text{avg}} = \frac{1}{T}\int_0^T T_d(t) dt \propto I_{\text{avg}} \] For a symmetric AC signal, the average value is zero. Hence, the PMMC pointer will remain at zero or merely vibrate around the zero mark.

• Conclusion: Therefore, PMMC instruments cannot measure AC directly and are restricted purely to DC measurements.

Step 4: Final Answer:
A PMMC instrument can be used to measure DC quantities only.