Jawaharlal Nehru Technological University Hyderabad B.Tech III MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (August - 2022 - R18)

In a Permanent Magnet Moving Coil (PMMC) instrument, the deflecting torque (T_d) is produced by the interaction of the magnetic field and the current-carrying coil. The force on a conductor is given by F = BIlN, where B is the flux density, I is the current, l is the length of the conductor, and N is the number of turns. The deflecting torque is:

where A is the area of the coil. Thus, T_d = G I, where G = BNA is the displacement constant.

For a spring-controlled instrument, the controlling torque (T_c) is proportional to the angular deflection (θ):

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

Therefore, the deflection equation is:

Since G and k are constants, the deflection is directly proportional to the current (θ ∝ I), resulting in a linear scale.

Moving Iron (MI) instruments operate on the principle of electromagnetic attraction or repulsion between a fixed coil and a soft iron vane. The instrument consists of a stationary coil carrying the current to be measured, which creates a magnetic field proportional to the current.

There are two main types:

• Attraction type: A single soft iron vane is mounted on a spindle. When current flows through the coil, the vane is attracted into the coil, causing the pointer to deflect.
• Repulsion type: Two soft iron vanes are placed inside the coil, one fixed and one moving. When the coil is energized, both vanes become magnetized with the same polarity, causing them to repel each other. The moving vane rotates the spindle and pointer.

The deflecting torque in an MI instrument is proportional to the square of the current (T_d ∝ I²), which allows these instruments to measure both AC and DC quantities. The controlling torque is provided by springs, and damping is typically provided by air friction using a vane moving in a closed chamber.