JNTU B.Tech III ELECTRICAL MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (November/December - 2020 - R16)

Moving iron (MI) instruments operate on the principle of electromagnetic attraction or repulsion. When a current flows through a stationary coil, it creates a magnetic field that magnetizes a soft iron vane. This vane is then attracted towards or repelled from another fixed iron vane or the coil itself, causing a pointer attached to the moving system to deflect over a calibrated scale.

There are two primary types: Attraction type and Repulsion type. In the attraction type, a single iron vane is pulled into the coil. In the repulsion type, two vanes within the coil are magnetized with like polarity, causing them to repel each other.

To derive the torque expressions, consider the energy stored in the magnetic field of the coil, given by

From the principle of conservation of energy, the electrical energy supplied is equal to the change in stored magnetic energy plus the mechanical work done. For a small change in deflection dθ at constant current I, the change in stored energy is

The controlling torque is provided by control springs, which produce a torque proportional to the angle of deflection:

where k is the spring constant. At steady state, the deflecting torque equals the controlling torque (

Solving for the deflection angle:

This shows that the deflection is proportional to the square of the current, resulting in a non-linear (crowded at the beginning) scale.

A Permanent Magnet Moving Coil (PMMC) instrument operates on the principle of the Lorentz force, where a current-carrying coil placed in a uniform magnetic field experiences a mechanical force. When current flows through the coil, the interaction between the coil's magnetic field and the permanent magnet's field produces a torque, causing the coil to rotate against the restoring force of control springs.

The working mechanism involves:

• The coil is wound on a light aluminum frame pivoted between two jewel bearings.
• A soft iron core is placed inside the coil to concentrate the magnetic flux, ensuring a radial magnetic field.
• The deflection torque is proportional to the current flowing through the coil (T_d = N B L D I).
• The control springs provide a restoring torque (T_c = k θ), resulting in a linear scale.
• Damping is provided by eddy currents induced in the aluminum coil former.

Range extension is achieved as follows:

1. Ammeter Range Extension: A low-resistance shunt resistor (R_sh) is connected in parallel with the meter movement. The shunt carries the majority of the current, allowing the meter to measure currents higher than its full-scale deflection (FSD) limit. The required shunt resistance is calculated as R_sh = R_m / (m - 1), where R_m is the meter resistance and m is the multiplying factor (I_total / I_fsd).
2. Voltmeter Range Extension: A high-resistance multiplier resistor (R_se) is connected in series with the meter movement. This limits the current through the coil to its FSD value when measuring higher voltages. The required series resistance is calculated as R_se = R_m (m - 1), where m is the multiplying factor (V_total / V_fsd).