Basic Measuring Instruments

Branch : Electrical and Electronics Engineering Subject : Measuring Instruments and Batteries Topic : Basic Measuring Instruments 1 Recap In the previous class we have learnt about Gravity control : Controlling torque is produced due to the gravitational force of the weight. Damping torque : Due to the inertia and the two opposite torques, the pointer will oscillate. Types of damping: • Over damped • Critical damped • Under damped 2 Objective On completion of this period, you would be able to know • Air friction damping • Fluid friction damping • Eddy current damping 3 Classification Of Damping Torque • Air friction damping • Fluid friction damping • Eddy current damping 4 Air Friction Damping • Air is used as the medium to suppress the oscillations in an instrument as shown in the figure. 5 Air Friction Damping Cylinder Scale Piston Head Piston Compressed Air Pointer Spindle Air chamber 6 • A light aluminium piston is moving in the air chamber. • The clearance between the piston and sides of chamber should be less and uniform. • When the piston moves inside the chamber pressure is developed in the chamber. • The pressure inside the chamber opposes the motion of the piston. 7 Air Friction Damping Cylinder Piston Head Scale Piston free Air Pointer Spindle Air chamber 8 • When the piston moves away from the piston head vacuum is developed. • The vacuum inside the chamber opposes the motion of the piston, which in-turn arrests the oscillations of the pointer. 9 Precautions • The piston should not touch the walls of the chamber. • Applications • Hot wire instruments • Moving iron instruments • Dynamometer type instruments 10 Fluid Friction Damping • Fluid (high viscosity damping oil) is used as the medium to suppress the oscillations in an instrument as shown in the figure. 11 Fluid Friction Damping Spindle Rotation Vanes Disc Oil Oil 12 • The vanes or discs are attached to the spindle of the moving system. • It dips into the pot of damping oil . • The friction of the damping oil opposes the motion of the moving system. • The damping force increases with the increase in velocity of vane and viscosity of the oil. Example : Kelvin electro-static voltmeter 13 The properties of damping oil, • A good insulator . • Non evaporating. • • Non corrosive. Viscosity should not change with temperature. Disadvantages: • Oil creeps out if the sealing is not perfect. • The instrument should be in vertical position. 14 Eddy Current Damping • Eddy current (current induced in a conducting metal disc) is used as the medium to suppress the oscillations in an instrument as shown in the figure. 15 Eddy Current Damping Scale Pointer Magnetic field Disc Spindle Damping magnet 16 • A disc of conducting material moved in the magnetic field. • The eddy currents produced in it as per Faraday’s laws of Electromagnetic Induction. • According to Lenz’s law, the induced currents opposes the cause of it’s production i.e. motion of the disc. Hence damping torque is produced. 17 When the disc rotates in the magnetic field: • The disc cuts the magnetic lines. • The eddy currents are produced in the disc. • So that the damping force is produced. • The force opposes the very cause of producing it (Lenz’s law) • The damping force opposes the rotation of the moving system. 18 The damping force •Proportional to the magnitude of the current. • Strength of the magnetic field. • Damping force is proportional to the velocity of the disc. If velocity is zero then the damping force is zero. 19 Applications : • Hot wire instruments • Moving coil instruments • Induction type instruments 20 Summary • Air friction damping : The pressure inside the air chamber opposes the motion of the piston (deflecting torque). • Fluid friction damping: The friction of the damping oil opposes the motion of the moving system. • Eddy current damping : The eddy currents produced in the disc causes damping torque 21 Quiz 1. The most effective damping is a) Air friction b) Fluid friction c) Eddy current d) Hysteresis 22 Quiz 2. Fluid friction damping can be employed in a) Vertically mounted instruments b) Horizontally mounted instruments c) Both a&b d) None of the above 23 Quiz 3. In eddy current damping, the disc employed should be of a) Conducting &magnetic material b) Conducting but non magnetic c) Magnetic but non conductive d) Non magnetic & non conductive 24 Frequently Asked Questions • What are the different damping systems adopted in an instrument ? • Differentiate between air friction damping and fluid friction damping. • Explain the damping systems with neat sketches. 25