Analogue Instruments PDF

ANALOGUE INSTRUMENTS LEC. (2)  OBJECT Analogue Instruments Classification of analogue instruments Classification according to principle of operation Constructional Details Moving System Controlling System Damping System   ANALOG INSTRUMENT...

ANALOGUE INSTRUMENTS LEC. (2)  OBJECT Analogue Instruments Classification of analogue instruments Classification according to principle of operation Constructional Details Moving System Controlling System Damping System   ANALOG INSTRUMENTS An analogue device is one in which the output or display is a continuous function of time and bears a constant relation to its input. Measuring instruments are classified according to both the quantity measured by the instrument and the principle of operation. Instruments depend for their operation on one of the many effects produced by current and voltage and thus can be classified according to which of the effects is used for their working. Classification Of Analogue Instruments Electromechanical Indicating electronic Analogue Instrument Recording Integrating  Classification According To Principle Of Operation Magnetic effect Hall Heating effect effect Electromagnetic Electrostatic effect effect  Magnetic effect Applied for Moving coil Moving iron Electrodynamic   MAGNETIC EFFECT When placed a current carrying conductor in a uniform magnetic field, the resultant field has a distortion and caused a force to act from left to right. The reversal of direction of the current will cause a force in the opposite direction from right to left while the direction of the existing field remains the same If the conductor is formed into a coil  1) Force Of Attraction Or Repulsion If we have a current carrying coil, it produces an imaginary bar magnet. When a piece of soft iron ( not magnetized before) is brought near the end of the coil, it will be attracted by the coil. We can use this effect in the attraction type of moving iron instrument. If there are two pieces of soft iron that magnetized with the same current carrying coil , they will repulse each other. This effect is utilized in repulsion type moving iron instruments. Force Between A Current Carrying Coil And A Permanent Magnet Consider the coil carrying current and a permanent magnet is brought near it, an attraction force or a repulsion force will be applied between them. This effect is utilized in Permanent Magnet Moving Coil instruments.  3) Force Between Two Current Carrying Coils If we have two current carrying coils each one can be considered as two magnets with the direction shown in figure. If one coil is fixed and the other is free, then an attraction force will be formed and the free magnet will move to the fixed one. This effect is utilized in dynamometer type of instruments. Heating effect Thermocouple Hotwire   Heating Effect ( Thermal Effect) The current to be measured is passed through a small element which heats it and so expands and this happened in type Hot Wire instruments. The thermocouple type operates on the fact that when the junction of two dissimilar electric conductors is heated by passing a current through it, an emf is developed. These instruments are free from errors due to frequency, waveform and external magnetic fields when used on AC and so can be used for measurement of current at extremely high frequencies.   Electrostatic Effect When two plates are charged, there is a force exerted between them. This force is used to move one of the plates. The instruments working on this principle are called electrostatic instruments and they are usually voltammeters. Instruments worked by this effect can be used to measure current and power with the help of external components. Induction Effect Induction Wattmeter Induction Energy meter  INDUCTION EFFECT When a non-magnetic conducting pivoted disc or drum is placed in a magnetic field by a system of electromagnets excited by alternating currents, an emf is induced in the disc or drum. If a closed path is provided, the emf forces a current to flow in the disc or drum. The force produced by the interaction of induced currents and the alternating magnetic fields makes the disc move. The Induction Effect is mainly utilized for AC energy meters.   HALL EFFECT If a strip of conducting material carries current in the presence of a transverse magnetic field, an emf is produced between two edges of conductor. The magnitude of the voltage depends upon the current, flux density and a property of conductor called ‘ HALL EFFECT CO- EFFICIENT’. The emf may be measured after amplification. It is mainly used in magnetic measurements. Instruments uses this effect is Poynting Vector Wattmeter that used for measuring the power loss density at the surface of a magnetic material.  CONSTRUCTIONAL DETAILS 1 Moving System 2 Controlling System 3 Damping System 4 Pointers and Scales  1. MOVING SYSTEM The moving system should have the following properties: The moving parts should be light. The frictional force should be minimum. These requirements should be fulfilled in order that power required by the instrument for its operation is small. The power is proportional to the weight of the moving parts and the frictional forces opposing the movement. The moving system can be made light by using aluminium as far as possible. The frictional forces are reduced by using spindle mounted jewel bearings and by carefully balancing the system. Supporting the moving system is important to make frictional forces minimum in order that the instrument reads correctly and is reliable. Supports Suspension used in vertical Pivot and Jewel position. bearings Taut suspension exact levelling is not required  PIVOT AND JEWEL BEARINGS The moving system is mounted on a spindle made of hardened steel. The two ends of the spindle are made conical and then polished to form pivots. These ends fit conical holes in jewels located in the fixed part of instruments. These jewels, which are preferably made of sapphire, form bearings. 2. CONTROLLING SYSTEM Operating Torques Deflecting torque Controlling torque Damping torque  I. DEFLECTING TORQUE/FORCE The deflecting torque is produced from the usage of one of the previous mentioned effects and it depends on the type of the instrument. The deflecting torque’s value is dependent upon the electrical signal to be measured; this torque/force helps in rotating the instrument movement from its zero position.  II. CONTROLLING TORQUE/FORCE The act of this torque/force is opposite to the deflecting torque/force. The functions of the controlling system are To produce a torque equal and opposite to the deflecting torque at the final steady position of the pointer in order to make the deflection of the pointer definite for a particular magnitude of current To bring the moving system back to its zero position when the force causing the instrument moving system to deflect is removed  Controlling Force Spring Control Can be obtained by Gravity Control  A. SPRING CONTROL  In this type of control, there are two phosphor bronze spiral hair spring that coiled in opposite directions and acting one against the other.  The spring materials should be nonmagnetic, proof against mechanical fatigue, and in particular of low resistivity and low temperature coefficient.  Silicon-bronze, hard-rolled silver or copper, platinum- silver, platinum-iridium or German silver have been used as spring material but for most applications phosphor-bronze except in low resistance instruments (ex. millivoltmeters) › Suppose that a spiral spring is made up of a total length L m of strip whose cross section is rectangular, the radial thickness being t m and the depth b m. Let E be Young’s modulus (N/m2) for the material of the spring. Then, if θ radians be the deflection of the moving system to which one end of the spring is being attached, the expression for the controlling torque is › The springs should be stressed well below their elastic limit at maximum deflection of the instrument in order that there is no permanent set or that no change in deflection ( or zero shift) will occur from inelastic yield.  B. GRAVITY CONTROL  Due to the displacement in the zero position of the moving system because of the effect of temperature on the stiffness of the spring, Gravity control can used in electrical measuring instrument.  A small weight is attached to the moving system in such a way that it produces a controlling torque, when the moving system is in deflected position.  Gravity control is cheap, unaffected by change in temperature and is free from fatigue or deterioration with time but it gives a cramped scale and the instrument has to be kept in a vertical position. › When the pointer at zero position, the control torque is zero. › Suppose the system deflects through an angle θ. The weight acts at a distance l from the centre, the component of weight trying to restore the pointer back to zero position is W sin θ. Therefore, controlling torque is :  EXAMPLE (1) › A weight of 5 g is used as the controlling weight in a gravity controlled instrument. Find its distance from the spindle if the deflecting torque corresponding to a deflection of 60 ◦ is 1.13 * 10-3 Nm. Consider an instrument in which the deflecting torque TD is directly proportional to the current (say) to be measured. Thus, if I is the current, If the instrument is spring-controlled, the controlling torque being TC, when the deflection is θ, Thus, the deflection is proportional to the current throughout the scale. Now if the same instrument is gravity controlled, TC= kg sin θ (kg is a constant that depends upon the control weight and its distance from the axis of rotation of the moving system).  EXAMPLE (2) › The deflecting torque corresponding to a deflection of 60◦ is 100 * 10-2 Nm in an instrument. The control is exerted through two phosphorus bronze springs. Allowing a maximum stress of 65 MN/ m2 and taking the value of modulus of elasticity as 112.8 GN /m2, calculate suitable dimensions for control springs. The width of spring strip is 1 mm.  3- DAMPING SYSTEM  A damping force generally works in an opposite direction to the movement of the moving system. This opposite movement of the damping force, without any oscillation or very small oscillation brings the moving system to rest at the final deflected position quickly.  When the deflecting torque is much greater than the controlling torque, the system is called underdamped. If the deflecting torque is equal to the controlling torque, it is called critically damped. When deflecting torque is much less than the controlling torque, the system is under overdamped condition. In practice to obtain best results the damping is adjusted to the value slightly less than the critical value. Methods of Damping Torque Damping Torque Air Friction Fluid Friction Eddy Current Damping Damping Damping With Metal Disc With Metal Former  a) AIR FRICTION DAMPING › Air friction damping provides a very simple, cheap method and does not need the use of permanent magnet which lead to distortion in the operating field. It can be used in moving iron and dynamometer instruments.  b) FLUID FRICTION DAMPING › Fluid friction damping consists of a van or disc that impressed in a damping oil. Oil employed should be a good insulator, non evaporating, non corrosive upon the disc or vane and of viscosity not subject to change with temperature changes. › It has advantages that, the oil can be used for the insulation purposes in some instruments, and also reduces the friction errors. It has several drawbacks such as creeping of oil and the necessity of using the instruments in vertical position. It is used in electrostatic laboratory instruments. c) EDDY CURRENT DAMPING  Eddy Current damping is the most efficient. It based on the principle of whenever a sheet of conducting but non magnetic material moves in the way of lines of force of a magnetic field, eddy currents are set up in the sheet which yields a force opposes the motion of the sheet.  It can be used in instruments where a metallic disc or former and permanent magnet already form part of the operating system such as moving coil, hot wire and induction instruments.  It can not be used in instruments that require a permanent magnet for causing eddy current, and this will distort the existing magnetic field as in moving iron and dynamometer instruments. Eddy-Current Damping Torque of Metal Former Eddy-current Damping Torque Of Metal Disc  4- POINTERS AND SCALES › Instrument scales and pointers may be considered together in two classes: › Those intended for reading quickly › Those intended for close accurate reading. › Pointers should have small weight and inertia to reduce the load on the bearing and to avoid necessity of excessive damping torque.  The pointer motion is limited by buffers stops to a little more than the scale.  These stops are constructed as very light springs so that the pointer is not bent when it strikes them sharply on a sudden overload or reversal of the operating current.