JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRICAL AND ELECTRONICS ENGINEERING Previous Question Paper (September - 2021 - R15)

A repulsion-type moving iron instrument operates on the principle of magnetic repulsion between two soft iron vanes placed inside a current-carrying coil. When current flows through the coil, both vanes become magnetized with the same polarity, causing them to repel each other, which produces a deflecting torque.

The construction consists of a stationary coil that carries the current to be measured. Inside this coil, there are two soft iron vanes: one is fixed to the coil frame, and the other is attached to a spindle that carries the pointer. When current passes through the coil, it creates a magnetic field that magnetizes both vanes. Because the vanes are placed in the same magnetic field, the adjacent edges of the vanes develop like magnetic poles. The resulting force of repulsion between these like poles causes the moving vane to rotate, moving the pointer across the calibrated scale.

The deflecting torque is proportional to the square of the current (T_d ∝ I²), which allows the instrument to be used for both AC and DC measurements. Damping is typically provided by an air-friction mechanism using a piston moving inside a closed cylinder. Control torque is provided by a spiral spring or gravity control, which also brings the pointer back to zero when the current is removed.

Key features include:

• The scale is non-linear, being cramped at the beginning and open at the higher end.
• It is robust, inexpensive, and simple in construction.
• It measures the RMS value of alternating currents.

A diagram would show the stationary coil, the fixed and moving iron vanes positioned such that their edges are parallel, the spindle supporting the moving vane and pointer, and the air-damping chamber connected to the spindle.

To calculate the equivalent resistance between terminals A and B for a network, one must identify the series and parallel combinations of resistors. If the network contains a bridge circuit, check for balanced conditions where the ratio of resistances in opposite arms is equal, allowing the removal of the central resistor. For general circuits, apply series and parallel reduction rules:

1. Identify resistors in series: Add their values directly,
   R_eq = R₁ + R₂ + ... + R_n
2. Identify resistors in parallel: Use the reciprocal sum formula,
   1 / R_eq = 1 / R₁ + 1 / R₂ + ... + 1 / R_n
   or for two resistors,
   R_eq = (R₁ * R₂) / (R₁ + R₂)
3. If the network is complex (e.g., Delta or Star configuration), use the Star-Delta transformation formulas to convert the network into a simpler series-parallel structure.
4. Continue reducing the circuit step-by-step until a single equivalent resistance value is obtained across terminals A and B.

Note: As the specific circuit diagram was not provided in the prompt, these systematic reduction steps are the required methodology to determine the equivalent resistance for any resistive network.