Experiment o. 1 AME OF THE EXPERIMET To study the constructional features of ammeter, voltmeter, wattmeter and energymeter. OBJECTIVE 1. To be conversant with the constructional detail and working of common type of ammeters, voltmeters, wattmeters and energymeters. 2. To be able to select appropriate instrument for a given circuit and to take their readings. BRIEF THEORY 1. AMMETER AD VOLTMETER We mostly make use of indicating type of instruments in our electrical laboratories. Indicating instruments make use of a dial and a pointer for indicating the magnitude of the quantity to be measured. The action of all ammeters and voltmeters, with the exception of electrostatic type of instruments, depends upon a deflecting torque produced by an electric current. In an ammeter this torque is produced by the current to be measured or by a definite fraction of it. In a voltmeter this torque is produced by a current which is proportional to the voltage to be measured. This also explains the fact that ammeters are low resistance meters and connected in series with the circuit whereas voltmeters are high resistance meters connected across the voltage to be measured. Thus, all ammeters and voltmeters are essentially current measuring devices. Mostly we use moving iron type and moving coil type
Experiment o. 1 7 ammeters and voltmeters in the laboratory though thermal, electrostatic and induction type meters are also available. Operating Torques Following three types of torques are needed for the satisfactory operation of any indicating type instrument : (a) Deflecting torque (b) Controlling torque (c) Damping torque The deflecting torque is produced by utilising either magnetic, heating, electrostatic, electromagnetic or Hall effect. The deflecting system of an instrument converts the electric current or potential into a mechanical torque called the deflecting torque. The controlling system produces torque equal and opposite to the deflecting torque at the final steady position of the pointer so that the deflection of the pointer is definite for a particular magnitude of current being measured. In the absence of a controlling system, the pointer will always deflect to the end of the scale. The controlling torque which acts in opposition to the deflecting torque brings the moving system back to zero when the deflecting torque does not exist. The damping torque is necessary to bring down oscillations of the pointer, before coming to steady state position, under the influence of deflecting torque and controlling torque which act on the moving system in opposition to each other. Constructional Details (a) Moving system: The main requirements of a moving system are that it should be light in weight and its movement should be nearly frictionless. Aluminium is used to make the moving system lighter. Frictional forces are reduced by using a suitable support system and by balancing the moving parts. upports may be of following types: (i) uspension type (ii) Taut uspension type (iii) Pivot and jewel bearing type. A ribbon suspension, in addition to supporting the moving element, exerts a controlling torque when twisted. The ribbon suspension and taut suspension are generally used in galvanometers requiring high sensitivity and low friction. In case of pivot and jewel bearings, which are most common in portable measuring instruments, the moving system is
8 Experiments in Basic Electrical Engineering mounted on a spindle made of hardened steel. The two ends of the spindle are made conical and then polished to form pivots. In order that the wear on the bearing becomes uniform, the centre of gravity of moving part should coincide with the axis of the spindle for the all positions of the pointer. In mounting the moving part, a small amount of play should be allowed so that the pivots are not forced hard on to the jewels. A simple arrangement showing jewel bearing is given in Fig. 1.1. pindle pindle Jewel bearing Fig. 1.1 Jewel bearing arrangement for supporting the moving system of an indicating instrument (b) Controlling system: There are two types of Controlling system viz. (i) Gravity control (ii) pring control. In gravity control, as shown in Fig. 1.2, a small weight is placed on an arm attached to the moving system. This weight produces a controlling torque which is a function of sin φ where φ is the deflection of the moving system. Here, the instrument must be mounted in level position otherwise there will be a serious zero error. The scale will be non-uniform due to sin φ factor. For these reasons, gravity control is not in common use. Moving system Balance Weight Control Weight W W sin Fig. 1.2 Gravity control of the moving system
Experiment o. 1 9 In spring control, as shown in Fig. 1.3, a hair spring attached to the moving system exerts the controlling torque. For most applications, phosphor bronze has been the most suitable material for the springs. In order to eliminate the effect of temperature variation upon the length of the spring, two springs coiled in opposite directions are used. When moving system deflects, one spring gets wound while other gets unwound. pring control is most commonly used in portable instruments used in laboratories. 0 50 cale 10 Jewel Bearing Moving Iron piral pring Hair pring b Jewel Bearing (a) (b) Fig. 1.3 (a) pring control system (b) piral spring of a spring control system (c) Damping system: The damping torque should be of such a magnitude that the pointer quickly comes to its final steady position without much of over-shooting. To avoid error it is necessary that damping torque should act only when the moving system is in motion. The methods of producing damping torque are: (a) Air friction damping (b) Fluid friction damping (c) Eddy current damping In air friction damping, (Fig. 1.4 (a)) a light weight vane is attached to the moving system. The vane is allowed to move in an air tight chamber and thus produces damping force. In fluid friction damping, air is replaced by a fluid (generally oil) as shown in Fig. 1.4 (b). Though the fluid friction damping system gives a greater damping torque, it is not in common use due to the requirement of keeping the instrument vertical to avoid oil spilling.
10 Experiments in Basic Electrical Engineering Air Chamber Piston Air Chamber pindle (a) Instrument pindle Rotation Rotation Oil Damping Oil Disc (b) s Fig. 1.4 (a) Air friction damping (b) Fluid friction damping In eddy current damping system, Fig. 1.5 which is also called electromagnetic damping, eddy currents induced in a metallic disc or former constituting moving system experience a torque, T e in opposition to the direction of movement. T d T e Te is controlling torque Aluminium drum Td is deflection torque Radius, R Rotating metal disc Eddy Currents (a) (b) Fig. 1.5 Eddy current induced in rotating metal disc or aluminium drum produces damping torque. (d) and scale: The shape and size of the pointer used depends upon the type of instrument. For the sake of lightness, very thin aluminium strip or tube is used for the pointer. Usually full scale deflection varies between 90 degree and 120 degree.
Experiment o. 1 11 Moving Iron Instruments The commonest ammeters and voltmeters for laboratory or switchboard applications at power frequencies are moving iron instruments. They are cheaper than any other type of ac measuring instruments of equal accuracy and ruggedness. Moving iron instruments can be classified as under: (i) Attraction type; (ii) Repulsion type; (iii) Combination of the above two. (i) Attraction type : The earliest and simplest instruments were of the attraction type in which a vane or a plate of soft iron of high permeability steel attached to the moving system was drawn into the field of a solenoid. Fig. 1.6. Ampere-turn for ammeter and voltmeter action remains the same, therefore, identical moving iron can be used for both the instruments. When the electromagnet is excited the iron vane moves in such a way as to increase the flux through it, i.e., to increase the inductance of the combination. Construction of a moving iron instrument is shown in Fig. 1.6. Moving iron is a flat disc or a sector eccentrically mounted. Air Damping Chamber winding carrying current to be measured Balance Weight Magnetic field Moving Iron ( or a plate) Control Weight Fig. 1.6 Attraction type moving iron instrument The controlling torque is provided by spring. Gravity control can be used for panel type instruments. Usually air damping is used. In another arrangement, called Thompson inclined coil instrument (Fig. 1.7), tendency of a metal strip or disc to align itself parallel to a magnetic field is utilised. uch an arrangement will give 180 degree angular displacement. Where torque requirement is high, two vanes may be used.
12 Experiments in Basic Electrical Engineering pindle (a) Jewel bearing (b) pindle Jewel bearing Fig. 1.7 Thompson inclined coil moving iron instrument (ii) Repulsion type: In the repulsion type instruments, there are two vanes inside the coil, one fixed and other movable. These are magnetised identically and a force of repulsion moves the spindle and the pointer even the graduated scale. Two different designs are in use: Radial vane type and co-axial vane type. In radial vane type two radial vanes in the form of an iron strip are placed within the coil. (Fig. 1.8 (a)). In co-axial vane type, the fixed and moving vanes are sections of co-axial cylinders, as shown in Fig. 1.8 (b). The shapes of moving vane and stationary vane have been shown in Fig. 1.8 (c). Movable pindle Movable Fixed Movable Fixed pring (a) (b) Moving tationary (c) Fig. 1.8 Repulsion type moving iron instrument of two different designs
Experiment o. 1 13 (iii) Combination Type : The construction of such instruments is similar to that of the co-axial vane type except that there are three vanes as shown in Fig. 1.9. By such combination it is possible to achieve a displacement over 240 degree and a more uniform scale. As the deflection increases, the outer stationary vanes exert attractive force on the moving vane and add to the torque. An attraction type moving iron instrument will usually have a lower inductance than the corresponding repulsion type instrument; a voltmeter will, therefore, be more accurate over a wide range of frequencies and there is a greater possibility of using shunts with ammeters. On the other hand, repulsion instruments are more suitable for economical production and nearly a uniform scale is more easily obtained, these are, therefore, more common than the attraction type. It may be noted here that both the types of moving iron instruments can be used for ac as well as dc measurement. Top pindle Moving Iron Middle Moving Iron Bottom Top Middle Bottom Fig. 1.9 Long scale instruments using combination of attraction and repulsion principle Permanent Magnet Moving-coil Type Instruments The permanent magnet-moving coil instruments are suitable for accurate dc measurements. Working principle of these instruments is the same as that of the d Arsonval type galvanometer, except that a direct reading instrument is provided with a pointer and a scale. The moving-coil is wound with many turns of fine copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jeweled bearings. The coil moves freely in the field of a permanent magnet.
14 Experiments in Basic Electrical Engineering Old style magnet systems as shown in Fig. 1.10 consisted of a relatively long u-shaped permanent magnet having soft iron polepieces. Movement of coil is restricted in this design. This is because no actual part of the coil is allowed to reach the extreme positions near the pole piece where there is fringing. Thus scale is limited to almost 90 degree. Counter Weight pring Mirror &Former Pivot &Bearing pring Permanent Magnet Radical Field tationary Core Moving Fig. 1.10 Permanent magnet moving coil instruments In order to obtain longer movement and a longer angular swing of the coil a concentric magnet construction is used as shown in Fig. 1.11. oft teel Ring Upper Control pring Moving Permanent Magnet Core Lower pring Fig. 1.11 Concentric type magnet assembly ince the magnet is concentric type, it produces a radial flux pattern which extends over 240. In another arrangement, as shown in Fig. 1.12, an air cored coil offset from the axis of rotation is used which gives displacement between 120 and 240, thereby giving better resolution. In recent magnet construction, an alnico permanent magnet itself serves as the core as shown in Fig. 1.13.
Experiment o. 1 15 and uspension Ribbon uspension Field Magnet Arrangement Fig. 1.12 Long scale moving coil instruments cale Pole hoe of the magnet Moving Yoke Fixed Magnet Fig. 1.13 Core magnet construction for moving coil instruments The moving coil moves over the magnet. This arrangement has the obvious advantage of being relatively unaffected by the external magnetic fields. The controlling torque is provided by two phosphor bronze hair springs. These springs also lead current in and out of the coil. Invariably damping torque is produced by eddy currents induced in the moving aluminium former. Due to presence of a permanent magnet, such instruments can only be used for the measurement of dc current. As the deflection is directly proportional to the current passing through the meter, we get almost a uniform (linear) scale.