JNTU B.Tech II ELECTRICAL AND ELECTRONICS ENGINEERING Previous Question Paper (March - 2021 - R13)

A Permanent Magnet Moving Coil (PMMC) instrument operates on the principle of the D'Arsonval galvanometer, where a current-carrying coil placed in a uniform magnetic field experiences a mechanical force. The construction consists of a lightweight rectangular coil wound on a metallic frame, pivoted between the poles of a permanent horse-shoe magnet. When current flows through the coil, the interaction between the magnetic field of the permanent magnet and the current in the coil produces a deflecting torque, given by T_d = N B I A, where N is the number of turns, B is the magnetic flux density, I is the current, and A is the area of the coil.

A control torque is provided by two phosphor-bronze hairsprings, which also serve as current leads to the coil. The controlling torque T_c is proportional to the deflection angle θ (T_c = kθ). Damping is provided by eddy currents induced in the metallic former on which the coil is wound, ensuring the pointer comes to rest quickly. At steady state, T_d = T_c, leading to θ = (NBA/k)I. Since N, B, A, and k are constant, the deflection is directly proportional to the current, resulting in a linear scale.

Given: R₁ = 50 Ω, R₂ = 10 Ω, V = 50 V.

Since the resistors are in series, the total resistance R_eq is:

The total current I in the circuit is:

Because the components are in series, the same current flows through both resistors:

The voltage across each resistor is calculated using Ohm's Law (V = IR):