JNTUH B.Tech II ELECTRICAL AND ELECTRONICS ENGINEERING Previous Question Paper (March 2022 - R13)

In a moving coil instrument, three distinct torques are required for proper operation:

• Deflecting Torque (T_d): This torque is produced by the interaction between the current-carrying coil and the magnetic field. It causes the moving system to rotate from its zero position. For moving coil instruments, T_d is proportional to the current (I).
• Control Torque (T_c): Also known as restoring torque, it is provided by springs. It acts in the opposite direction to the deflecting torque and brings the pointer to rest at a position where T_c = T_d. It ensures that the deflection is proportional to the current.
• Damping Torque (T_damp): This torque is necessary to bring the pointer to its final steady position quickly without oscillation. In moving coil instruments, eddy current damping is typically used, where a metallic former moves through the magnetic field to produce a braking effect proportional to the velocity of the coil.

To calculate the current through the 6 Ω resistor, one must apply circuit analysis techniques such as Mesh Analysis or Nodal Analysis based on the provided network. Assuming a standard circuit configuration where the 6 Ω resistor is in a branch with a voltage source or part of a bridge/parallel network:

1. Identify all independent loops in the network.
2. Apply Kirchhoff's Voltage Law (KVL) to each loop to establish a system of linear equations.
3. Solve the system of equations for the mesh currents.
4. If the 6 Ω resistor is shared between two loops with currents I₁ and I₂, the current through the resistor is the algebraic difference (I₁ - I₂) or sum depending on the assumed directions.
5. If the circuit is complex, use Thevenin's Theorem by finding the Thevenin voltage (V_th) and Thevenin resistance (R_th) across the 6 Ω resistor terminals, then calculate current using I = V_th / (R_th + 6).