JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year ELECTRICAL AND ELECTRONIC MEASUREMENTS Previous Question Paper (August/September - 2022 - R16)

Measuring instruments are broadly classified into two categories: Absolute instruments, which give the value of the quantity in terms of physical constants (e.g., Tangent Galvanometer), and Secondary instruments, which must be calibrated by comparison with an absolute instrument. Secondary instruments are further subdivided into Indicating (display instantaneous value), Recording (keep a continuous record over time), and Integrating (totalize the quantity over a period, e.g., Energy meters).

The basic requirements of any measuring instrument include:

• Accuracy: The ability of the instrument to provide a reading close to the true value.
• Precision: The consistency or reproducibility of successive measurements.
• Sensitivity: The ratio of the output response to the input change.
• Linearity: The ability of the instrument to maintain a linear relationship between input and output.
• Stability: The ability to maintain calibration over time.
• Low Power Consumption: Minimal disturbance to the circuit being measured.
• Robustness: Ability to withstand mechanical stress and environmental conditions.

A Permanent Magnet Moving Coil (PMMC) instrument, also known as a D'Arsonval galvanometer, consists of a lightweight coil wound on a rectangular aluminum frame, suspended in the uniform radial magnetic field created by a permanent horseshoe magnet with soft iron pole pieces. A soft iron core is placed inside the coil to concentrate the magnetic flux.

The working principle is based on the electromagnetic effect. When current flows through the coil, a mechanical force is exerted on the sides of the coil due to the interaction between the magnetic field and the current, given by the formula:

where N is the number of turns, B is the flux density, I is the current, and l is the length of the coil side. This force creates a deflecting torque (T_d = BINA), causing the coil to rotate. A spiral spring provides the restoring torque (T_c = kθ), and eddy current damping is provided by the aluminum former to bring the pointer to a steady state quickly. The final deflection is proportional to the current, resulting in a linear scale.