Question

A Moving iron instrument measures:

• A. DC only
• B. AC only
• C. AC and DC
• D. Pulsating DC

Hint:

MI instruments read the RMS value on AC.
They are inexpensive, robust, and highly suited for commercial low-frequency power measurements.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the types of electrical quantities (AC, DC, or both) that can be measured using a Moving Iron (MI) instrument.
Moving Iron instruments are widely used in power-frequency applications as ammeters and voltmeters due to their robust construction.

Step 2: Key Formula or Approach:
The deflecting torque (\( T_d \)) in a moving iron instrument is derived from the change in self-inductance (\( L \)) with respect to angular deflection (\( \theta \)):
\[ T_d = \frac{1}{2} I^2 \frac{dL}{d\theta} \] Where:
\( I \) is the current flowing through the coil.
\( \frac{dL}{d\theta} \) is the rate of change of inductance with position.

Step 3: Detailed Explanation:
Let us analyze the operational physics from the torque equation:

• Unidirectional Deflection: The deflecting torque is proportional to the square of the operating current (\( T_d \propto I^2 \)).
Since the square of any real value (positive or negative) is always positive, the deflecting torque is unidirectional.
Even when the current alternates direction (AC), the physical force on the moving iron vane remains in the same direction.

• AC Measurement (RMS Value): When an alternating current \( I(t) = I_m \sin(\omega t) \) is applied, the moving system experiences an average torque:
\[ T_{d,\text{avg}} \propto \text{Average of } \big(I^2(t)\big) \propto I_{\text{rms}}^2 \] Therefore, the instrument naturally registers the Root Mean Square (RMS) value of the AC quantity.

• DC Measurement: When direct current is passed, it simply deflects proportionally to the constant value \( I_{\text{dc}}^2 \).
Thus, MI instruments are highly versatile and work on both AC and DC systems.

Step 4: Final Answer:
A Moving Iron instrument is capable of measuring both AC and DC quantities.