  Alternating current ammeters or voltmeters indicate the virtual values of the current or pressure respectively, that is to say, they indicate, the square root of the mean square of a variable quantity. Fig. 2,491.—Line curve of alternating current, illustrating various current or pressure values. The virtual value, or .707×maximum value, is the value indicated by an ammeter or voltmeter. Thus, if the maximum value of the current be 100 volts, the virtual value as indicated by an ammeter is 100×.707=70.7 amperes. The virtual value of an alternating current or pressure is equivalent to that of a direct current or pressure which would produce the same effect. For instance an alternating current of 10 virtual amperes will produce the same heating effect as 10 amperes direct current. The relation of the virtual value of an alternating current to the other values is shown in fig. 2,491. When the current follows the sine law, the square root of the mean square, value of the sine functions is obtained by multiplying their maximum value by 1÷v2 or .707. Fig. 2,492.—Wagner tubular aluminum pointer. The word effective is commonly used erroneously for virtual, even among the best writers and the practice cannot be too strongly condemned In the operation of a steam engine, there are two pressures acting on the piston:
The forward pressure on one side of the piston is that due to the live steam from the boiler, and the back pressure, on the other side, that due to the resistance or opposition encountered by the steam as it exhausts from the cylinder. In order that the engine may run and do external work, it is evident that the forward pressure must be greater than the back pressure, and it follows that the pressure available to run the engine is the difference between these two pressures, this pressure difference being known as the effective pressure, that is to say effective pressure=forward pressure-back pressure Thus, electrically speaking, the effective voltage is that voltage which is available for driving electricity around the circuit, that is,
In the case of the steam engine, the forward pressure absolute, that is, measured from a perfect vacuum is the virtual pressure (not considering the source). The back pressure may vary widely for different conditions of operation as illustrated in figs. 2,493 and 2,494. Figs. 2,493 and 2,494.—Steam engine indicator cards, illustrating in mechanical analogy, the misuse of the term effective as applied to the pressure of an alternating current. The card fig. 2,493, represents the performance of a steam engine taking steam at 60 lbs. (gauge) pressure and exhausting into the atmosphere. The exhaust line being above the atmospheric line shows that the friction encountered by the steam in flowing through the exhaust pipe produces a back pressure of two lbs. Hence at the instant represented by the ordinate y, the effective pressure is 60 - 2=58 lbs., or using absolute pressures, 74.7 - 16.7=58 lbs., the virtual pressure being 60 lbs. gauge, or 74.7 lbs. absolute. Now, the back pressure may be considerably reduced by exhausting into a condenser as represented by the card, fig. 2,494. Here, most of the pressure of the atmosphere is removed from the exhaust, and at the instant y, the back pressure is only 6 lbs., and the effective pressure 74.7 - 6=68.7 lbs. Thus, in the two cases for the same virtual pressure of 60 lbs. gauge or 74.7 lbs. absolute, the effective pressures are 58 lbs. and 68.7 lbs. respectively. In the measurement of alternating current, it is not the average, or maximum value of the current wave that defines the current commercially, but the square root of the mean square value, because this gives the equivalent heating effect referred to direct current. There are several types of instrument for measuring alternating current, and they may be classified as
Electromagnetic or Moving Iron Instruments.—This type of instrument depends for its action upon the pull of flux in endeavoring to reduce the reluctance of its path. This pull is proportional to the product of the flux and the current, and so long as no part of the magnetic circuit becomes saturated, the flux is proportional to the current, hence the pull is proportional to the square of the current to be measured. Fig. 2,495.—A calibrated scale. This means that printed scales are not employed, but each instrument has its scale divisions plotted by actual comparison with standards, after which the division lines are inked in by a draughtsman. There are makes of direct current instruments employing printed scales in which the scale deflections are fairly accurate, even though the scales are printed, but printed scales should not be used on alternating current instruments. Ques. What are some objections to moving iron instruments? Ans. Instruments of this type are not independent of the frequency, wave form, or temperature and external magnetic fields may affect the readings temporarily. Fig. 2,497.—Plunger form of electromagnetic or moving iron type of ammeter. There are several forms of moving iron ammeters, which may be classified as
Ques. Describe the plunger type. Ans. This type of ammeter consists of a series coil and a soft iron plunger forming a solenoid, the plunger is so suspended that the magnetic pull due to the current flowing through the coil is balanced by gravity, as shown in fig. 2,497. Ques. How should the plunger be constructed to adapt it to alternating current, and why? Ans. It should be laminated to avoid eddy currents. Fig. 2,497.—One form of plunger instrument as made by Siemens. It has gravity control, is dead beat, and is shielded from external magnetic influence. The moving system consists of a thin soft iron pear shaped plate I pivoted on a horizontal spindle S running in jewelled centers. To this spindle S is also attached a light pointer P and a light wire W, bent as shown, and carrying a light piston D, which works in a curved air tube T. This tube T is closed at the end B but fully open at the other A, and constitutes the air damping device for making the instrument dead beat. Ques. What is the character of the scale and how should it be constructed? Ans. The scale is not uniform and should be hand made and calibrated under the conditions which it is to be used. Ques. What is the objection to moving iron ammeters? Ans. Since the coil carries the entire current they are large and expensive. Ques. What precaution should be taken in installing moving iron ammeters? Ans. Since gravity is the controlling force, the instrument should be carefully levelled. Ques. Describe an inclined coil instrument. Ans. It consists of a coil mounted at an angle to a shaft carrying the vane and pointer, as shown in fig. 2,498. A spring forms the controlling force and holds the pointer at zero when no current is flowing. Fig. 2,498.—Inclined coil form of electromagnetic or moving iron instrument. Ques. What is the principle of operation of the inclined coil instrument? Ans. When a current is passed through the coil, the iron tends to take up a position with its longest sides parallel to the lines of force, which results in the shaft being rotated and the pointer moved on the dial, the amount of movement depending upon the strength of the current in the coil. Ques. Describe a magnetic vane instrument. Ans. It consists of a small piece of soft iron or vane mounted Fig. 2,499.—Magnetic vane form of electromagnetic or moving iron instrument. Fig. 2,500.—Magnetic vane movement of a Wagner instrument; it is used both for voltmeters and ammeters. This type differs from the dynamometer movement in that a vane of very soft iron replaces the moving coil. The magnetic vane movement makes use of its controlling spring only for the purpose of resisting the pull on the vane and the returning of the needle to zero. The spring does not carry any current. Ques. How does it work? Ans. Its principle of operation is that a piece of soft iron placed in a magnetic field and free to move, will move into such position as to conduct the maximum number of lines of force. The current to be measured is passed around the coil, producing a magnetic field through the center of the coil. The magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible. Fig. 2,501.—Solenoid and plunger illustrating the operation of moving iron instruments. When a current flows through the coil, a field is set up as indicated by the dotted lines of force. The current flowing in the direction indicated by the arrow induces a north pole at N, which in turn induces a south pole in the plunger at S, thus attracting the plunger. The effect of the field upon the plunger may also be stated by saying that it tends to cause the plunger to move in a direction so as to conduct the maximum number of lines of force, that is, toward the solenoid. Thus if ABCD be the initial position of the plunger only five lines of force pass through it: should it move to the position A´B´C´D´, the number of lines passing through it will then be 9, assuming the field to remain unchanged. The operation of moving iron instruments of the plunger type may be explained by saying that the current flowing in the coil produces a pole at its end and induces an unlike pole at the end of the plunger nearest the coil, thus attracting the plunger, as illustrated in fig, 2,501 above. Figs, 2,502 and 2,503.—Wagner series transformers. Fig. 2,502, wound primary series transformer; fig. 2,503, open primary transformer. Wagner series transformers are made in three general types: One for switchboard mounting with wound primary; one for switchboard mounting with open primary, and one with open primary suitable for slipping over bus bars or switch stud. These transformers have 5 ampere secondary winding, and are intended for use in connection with instrument of scale capacity 0-5, although the scale should be calibrated to indicate the primary current. The capacities are from 2 watts to 50 watts, being suitable for operation on circuits of 750 to 66,000 volts. Hot Wire Instruments.—Instruments of this class depend for their operation on the expansion and contraction of a fine wire carrying either the current to be measured or a definite proportion of that current. The expansion or contraction of the wire is caused by temperature changes, which in turn are due to the heating effect of the current flowing through the wire. Since the variations in the length of the wire are extremely small, considerable magnification is necessary. Pulleys or levers are sometimes used to multiply the motion, and sometimes the double sag arrangement shown in fig. 2,504. As shown here, A is the active wire carrying the current to be measured and stretched between the terminals T and T´. It is pulled taut at its middle point by another wire C, which carries no current, and is, in its turn, kept tight by a thread passing round the pulley D attached to Hot wire instruments are equally accurate with alternating or direct current, but have cramped scales (since the deflection is proportional to the square of the current), and are liable to creep owing to unequal expansion of the parts. There is also the danger that they may be burnt out with even comparatively small overloads. They are not affected by magnetic fields but consume more current than the other types, these readings are inaccurate near either end of the scale. Fig. 2,504.—Diagram illustrating the principle of hot wire instruments. The essential parts are the active wire A, stretched between terminals T and T´, tension wire C, thread E, and pulley D to which is attached the pointer. Induction Instruments.—These were invented by Ferraris, and are sometimes called after him. They are for alternating current only, and there are two forms:
Ques. Describe the shielded pole type of induction instrument. Ans. As shown in figs. 2,505, and 2,506 it consists, essentially of a disc A, or sometimes a drum and a laminated magnet B. Covering some two-thirds of the pole faces are two copper plates or shields C, and a permanent magnet D. Figs. 2,505 and 2,506.—Plan and elevation of shielded pole type of induction instrument. Ques. How does it work? Ans. Eddy currents are induced in the two copper plates or shields C, which attract those in the disc, producing in consequence a torque in the direction shown by the arrow, against Fig. 2,507.—Diagram showing construction and operation of Hoskins instrument. It is of the modified induction type in which the torque is produced from the direct repulsion between a primary and a secondary, or induced current. As shown in the diagram, the instrument embodies the principle of a short circuited transformer, consisting of a primary or exciting coil A, a secondary or closed coil B, linked in inductive relation to the primary by a laminated iron core C, constructed to give a completely closed magnetic circuit, that is, without air gap. The secondary is so mounted with respect to the primary as to have a movement under the influence of their mutual repulsion when the primary is traversed by an alternating current. This movement of the secondary B is opposed by a spiral spring, so that the extent of movement will be dependent upon and will indicate the strength of the primary current. To increase the sensitiveness of the instrument and also to adjust the contour of the scale, an adjustable secondary D, which has an attraction effect upon the coil B, is provided upon the core. The effect of this coil is inversely proportional to its distance from the end of the swing of the coil B. The vane, E, which is a part of the stamping B, is adjusted to swing freely and with a large amount of clearance, between the poles of a permanent magnet F, which acts as a damper on the oscillation of the moving element, but does not cause any friction or affect the accuracy of the calibration. The primary, like that of a transformer, is an independent electrical circuit and may be highly insulated. This meter will withstand several hundred per cent. overload for some time because of the very high value of the self-induction and the fact that the controlling spring is not in the circuit and therefore cannot burn off. Figs. 2,508 to 2,511.—Hoskins instruments. Fig. 2,508, voltmeter, small pattern; fig. 2,509, ammeter, large pattern; fig. 2,510, voltmeter, horizontal edgewise pattern; fig. 2,511, illuminated dial voltmeter. Ques. Describe the rotary field type of induction instrument. Ans. The parts are arranged similar to those of wattmeters, the necessary split phase being produced by dividing the current into two circuits, one inductive and the other non-inductive. Fig. 2,512.—Hoskins instrument with case removed. It has a very short magnetic circuit which is composed of silicon steel, permitting low magnetic densities to be used. Dynamometers.—This type of instrument is used to measure volts, amperes, or watts, and its operation depends on the reaction between two coils when the current to be measured is passed through them. One of the coils is fixed and the other movable. Fig. 2,513.—Diagram of Siemens' dynamometer. It consists of two coils on a common axis but set in planes at right angles to each other in such a way that a torque is produced between the two coils which measures the product of their currents. This torque is measured by twisting a spiral spring through a measured angle of such degree that the coils shall resume their original relative positions. When constructed as a voltmeter, both coils are wound with a large number of turns of fine wire, making the instrument sensitive to small currents. Then by connecting a high resistance in series with the instrument it can be connected across the terminals of a circuit whose voltage is to be measured. When constructed as a wattmeter, one coil is wound so as to carry the main current and the other made with many turns of fine wire of high resistance suitable for connecting across the circuit. Fig. 2,514.—Wagner dynamometer movement. In this type of instrument the deflection is proportional to the square of the current, producing a constantly decreasing sensitiveness as the pressure applied is decreased. The dynamometer movement is, for any indication, more accurate than the magnetic vane, but cannot readily be employed for the indication of current, as required in ammeters. Ques. Describe the construction of a dynamometer. Ans. It consists, as shown in fig. 2,513, of a fixed coil, composed of a number of turns of wire, and fastened to a vertical support. The fixed coil is surrounded by a movable coil composed of a few number of turns or often of only one turn of wire. The movable coil is suspended by a thread and a spiral spring attached to a tortive head which passes through the center of a dial. The ends of the movable coil dip into mercury cups, which act as pivots and electrical contacts, making connection with one end of the fixed coil and one terminal of the instrument as shown. The tortion head can be turned so as to place the planes of the coils at right angles to each other and to apply tortion to the spring to oppose the deflection of the movable coil for this position when a current is passed through the coils. A pointer attached to the movable coil indicates its position on the graduated dial between the two stops. Another pointer attached to the tortion head performs a similar function. Fig. 2,515.—Armature of Wagner dynanometer movement. Greater accuracy is claimed for this movement than the magnetic vane, but it cannot readily be employed for the indication of current flow, as required in ammeters. The magnetic vane movement is used on the A. C. ammeter, and can be used also in the A. C. voltmeters; it makes use of its controlling spring only for the purpose of resisting the pull on the vane and the returning of the pointer to zero. The dynanometer movement is recommended for voltmeters. Fig. 2,516.—Wagner 25 watt pressure transformer for use with various alternating current instruments, such as voltmeters, wattmeters, etc. They are made in capacities 25, 50, 100, and 200 watts, and are built for pressures of 750 to 60,000 volts. Ques. How does the dynamometer operate? Ans. When current is passed through both coils, the movable coil is deflected against one of the stop pins, then the tortion head is turned to oppose the movement until the deflection has been overcome and the coil brought back to its original position. Fig. 2,517.—Moving element of Keystone dynamometer instrument. The illustration shows the movable coil, pointer, aluminum air vane for damping the oscillations, controlling springs, and counter weights. Fig. 2,518.—Keystone dynamometer movement. Since the law governing this type of instrument is the law of current squares, it follows that in the case of voltmeters, equally divided scales cannot be obtained. In the case of wattmeters, the scale is approximately equally divided, due to the fact that the movement of the moving coil is proportional to the product of the current in the fixed and moving coils. The moving parts have been made as light in weight as is consistent with mechanical strength, and the entire moving system is supported on jeweled bearings. The motion of the pointer is rendered aperiodic by the use of an aluminum air vane moving in a partially enclosed air chamber. This method of damping the oscillations of the moving parts renders unnecessary the use of mechanical brakes or other frictional devices, which tend to impair the accuracy of the instrument. The illustration shows a voltmeter, which, however, differs but little from a wattmeter. In the case of a wattmeter the fixed coils are connected in series with the line, either directly or through a current transformer, while the moving coil is connected in shunt to the line. Ques. How is the dynamometer arranged to measure watts? Ans. When measuring watts, the instrument should be so arranged that one coil carries the main current, and the other a small current which is proportional to the pressure. Fig. 2,519.—Leeds and Northrup electro-dynamometer. It is a reliable instrument for the measurement of alternating currents of commercial frequencies. When wound with fine wire and used in connection with properly wound resistances, it is equally useful for measuring alternating pressures, and may thus be employed to calibrate alternating current voltmeters as well as ammeters. To give accurate results the instruments must be carefully constructed and designed with a view to avoiding the eddy currents always set up by alternating currents in masses of metal near, or in the circuits. The constant of a dynamometer may be obtained with a potentiometer, but this is usually done with precision by the manufacturer and a certificate giving the value of the constant is furnished with the instrument. The size and cost of dynamometers rapidly increase with the maximum currents which they are designed to carry, and when more than 500 amperes are to be measured, the use of other instruments and methods is recommended. Ques. In the construction of a dynamometer what material should not be used and why? Ans. No iron or other magnetic material should be employed because of the hysteresis losses occasioned thereby. The frame should be of non-conducting material so as to avoid eddy currents. Figs. 2,520 to 2,526.—Various types of Wagner instruments. Fig. 2,520, small round type; fig. 2,521, horizontal edgewise type; fig. 2,522, smallest switchboard type; fig. 2,523, portable type; fig. 2,524, combination voltmeter and ammeter in one case; fig. 2,525, vertical type; fig. 2,526, polyphase type. Watt Hour Meters.—A watt hour meter is a watt meter that will register the watt hours expended during an interval of time. Watt hour meters are often erroneously called recording or integrating watt meters. There are several types of the electromotor form of watt hour meter, which may be classified as
Fig. 2,527—Interior Weston single phase wattmeter. The general appearance of the dynamometer movement and the relative positions of the various parts are clearly shown. The parts are assembled on one base, the whole movement being removable by unfastening two bolts. The fixed winding is made up of two coils, which together produce the field of the wattmeter. The movable coil is wound to gauge with silk covered wire and treated with cement. While winding, the coil is spread at diametrical points to allow the insertion of the staff, which is centered by means of two curved plates cemented to the inside surface of the coil and forming a part thereof. The coil is held in a definite position by two tiny pins which pass through the staff and engage with ears on the curved plates. Fig. 2,528.—Westinghouse single phase induction type watt hour meter removed from case. The friction compensation, or light load adjustment, is accomplished by slightly unbalancing the two legs of the shunt magnetic circuit. To do this a short circuited loop is placed in each air gap, and means are provided for adjusting the position of the loops so that one loop will enclose and choke back more of the flux than the other loop, and thus produce a slight torque. It will be noted, that this torque depends on voltage alone, which is practically constant, and is entirely independent of the load. Adjustment is accomplished by means of either of two screws which makes micrometer adjustment possible. It is clamped when adjusted by means of a set screw, which prevents change. This method makes possible an accuracy of adjustment which effectively prevents creeping. The power factor adjustment consists of an adjustable compensating coil placed around the shunt pole tip. This is adjusted at the factory by twisting together the leads of the compensating coil, thus altering its resistance until the desired lagging effect is had. Frequency adjustment. 133 cycle meters are first calibrated on 60 cycles and the leads then untwisted to make them correct on 133 cycles. To change such a meter for use on 60 cycles it is necessary only to retwist these leads to the point shown by the condition of the wire. Ques. What are the essential parts of a watt hour meter? Ans. A motor, generator, and counting mechanism. Fig. 2,529.—Pointer and movable system of Weston wattmeter. The coil is described in fig. 2,527. The pointer consists of a triangular truss with tubular members, an index tip of very thin metal being mounted at its extremity. The index tip is reinforced by a rib stamped into the metal. The pointer is permanently joined to a balance cross, consisting of a flat center web, provided with two short arms and one long arm, each arm carrying a nut by means of which the balance of the system may be adjusted. The longest arm, which is opposite the pointer, carries a balance nut, consisting of a thin walled sleeve provided with a relatively large flange at its outer end. The sleeve is tapped with 272 threads to the inch, the internal diameter of the sleeve being made slightly smaller than the outside diameter of the screw, and the sleeve is split lengthwise; therefore when sprung into place and properly adjusted it will remain permanently in position. A sleeve which is forced over the end of the staff carries the pointer firmly clamped between a flanged shoulder and a nut. By perforating the web plate of the balance cross with a hole having two flat sides that fit snugly over a similarly shaped portion of the sleeve, the pointer is given a definite and permanently fixed angular position. The air damperconsists of two very light symmetrically disposed vanes, which are enclosed in chambers made as nearly air tight as possible. These vanes are formed of very thin metal stiffened by ribs, stamped into them and by the edges, which are bent over to conform to the surface of the side walls of the chambers. They are attached by metal eyelets to a cross bar carried on a sleeve similar in construction to the one at the upper end of the staff. This cross bar is held in place by a nut, and is provided at the center with a hole having two flat sides, being similar in shape to the one in the balance cross. This hole likewise fits over a sleeve and definitely locates the vanes with reference to the other parts of the system. The damper box is cast in one piece to form the base that carries the field coils and the movable system. Ques. What is the function of the motor? Ans. Since the motor runs at a speed proportional to the Ques. What is the object of the generator? Ans. It furnishes a suitable counter torque or load for the motor. Fig. 2,530.—Westinghouse polyphase induction type watt hour meter, covers removed. This type is made for two phase three wire and four wire, and three phase three wire and four wire circuits. Meters for circuits of more than 300 amperes or 500 volts require transformers, but, like the self-contained meters, are calibrated to read directly in kilowatt hours on the dial, without a multiplying constant. Ques. Is there any other resistance to be overcome by the motor? Ans. It must overcome the friction of all the moving parts. Ques. Is the friction constant? Ans. No. Figs. 2,531 to 2,533.—Diagram of electromagnetic circuit of Westinghouse induction type watt hour meter, and diagram showing rotation of field. The dotted lines show the main paths of the magnetic flux produced by the two windings, the directions, however, are constantly reversing owing to the alternations of the current in the coils. Denoting the shunt and series pole tips by the letters as shown, a clear statement of the relation of the fields for each quarter period may be given. The signs + and - represent the instantaneous values of the poles indicated. Thus, at one instant the shunt pole tips A, C, and A1 are maximum +, -, and +, respectively because the instantaneous value of the current is maximum, while the value of the series flux is zero. At ¼ period later the shunt current is zero, giving zero magnetic pressure at the pole tips, while the series current has reached a maximum value, giving maximum-and + at the pole tips B and D. At the next ¼ period the shunt current is again maximum, but in a direction opposite to what it was at the beginning, making the pole tips A, C, and A1 +, -, and +, respectively, while the series current again is zero, etc., the values for the complete cycle being given in fig. 2,533. It will be observed from the table that both the + and - signs move constantly in the direction from A1 to A, indicating a shifting of the field in this direction, the process being repeated during each cycle. Ques. What provision is made to correct the error due to friction? Ans. The meter is compensated by exciting an adjustable auxiliary field from the shunt or pressure circuit. Ques. What is the construction of the generator? Ans. In nearly all meters it consists of a copper or aluminum Ques. How is the counter torque produced? Ans. When the disc is rotated in the magnetic field, eddy currents are induced in the disc in a direction to oppose the motion which produces them. Ques. For what services is the commutator type meter used? Ans. It is used on both direct and alternating current circuits. Figs. 2,534 and 2,535.—Cross section of bearings of Westinghouse induction type watt hour meter. The lower bearing consists of a very highly polished and hardened steel ball resting between two sapphire cup jewels, one fixed in the end of the bearing screw and the other mounted in a removable sleeve on the end of the shaft. Owing to the minute gyrations of the shaft the ball has a rolling action, which not only makes a lower friction coefficient than the usual rubbing action, but presents constantly new bearing surfaces and thus produces long life. The upper bearing is only a guide bearing to keep the shaft in a vertical position, and is subject to virtually no pressure, and consequently little friction. It consists of a steel pin fastened to a removable screw and projecting down into a bushing in a recess drilled in the shaft. The bottom of this recess is filled with billiard cloth saturated with watch oil. A film of oil is maintained around the pin by capillary action. Ques. What is the objection to the commutator meter? Ans. The complication of commutator and brushes, and the fact that the friction of the brushes is likely to affect the accuracy of the meter. Fig. 2,536.—Diagram of Fort Wayne, induction watt hour meter. It is designed to register the energy of alternating current circuits regardless of the power factor, and embodies the usual induction motor, eddy current generator and registering mechanism. The electrical arrangement of the meter consists of a current circuit composed of two coils connected in series with each other and in series with the line to be measured, and a pressure circuit consisting of a reactance coil and a pressure coil connected in series with each other and across the line to be measured. In addition, the pressure circuit contains a light load coil wound over a laminated sheet steel member, adjustably arranged in the core of the pressure coil and connected across a small number of turns of the reactance coil so as to give a field substantially in phase with the impressed pressure. The light load winding is further provided with a series adjustable resistance furnished for the purpose of regulating the current flowing in the light load winding, thereby providing a means of lagging the meter on high frequencies, such as 125 or 140 cycle circuits. The pressure circuit also comprises a lag coil wound over the upper limb of the core of the pressure circuit and provided with an adjustable resistance for obtaining a held component in quadrature with the shunt field. Ques. What are its characteristics? Ans. It is independent of power factor, wave form, and frequency when no iron is used in the motor. Ques. What meter is chiefly used on A. C. circuits? Ans. The induction meter. Fig. 2,537.—Fort Wayne multiphase induction watt hour meter. The construction of the mechanism is essentially two single phase motor elements, one at the bottom of the meter in a suitable position, the other inverted and placed at the top of the meter. Each element acts on a separate cup, but both cups are mounted on a single shaft so that the registration is due to the resultant torque of the two elements. The meter is provided with three supporting lugs, the one at the top being keyholed and one of the bottom two, slotted to facilitate leveling. The registering mechanism is mounted on a cast iron bracket at the middle of the meter between the two motor elements. The supporting bracket is attached to the meter base by two screws and aligned by two dowel pins. The register is of the four dial type, reading in kilowatt hours. Each division of the right hand circle, or that passed over by the most rapidly moving pointer, equals one kilowatt hour in meters without a dial constant. In meters of larger capacities, dial constants of 10, 100 and 1,000 are used, in which case it is only necessary to add one, two or three ciphers to the observed reading. Principles of Induction Watt Hour Meters.—Every commercial meter of this type is made up of a number of elements, 1. The field producing element; 2. The moving element; 3. The retarding element; 4. The registering element; 5. The mounting frame and bearings; 6. The friction compensator; 7. The power factor adjustment; 8. Frequency adjustment; 9. The case and cover. Figs. 2,533 to 2,541.—Connections of Fort Wayne multiphase watt hour meters (type k3—forms MAB and MAK), for 100-625 volt circuits, 5-150 amperes. Fig. 2,538 two and three phase, three wire circuit, 25-36 cycles; fig. 2,539 two and three phase, 3 wire circuit, 36 cycles and above; fig. 2,540, two phase 4 wire circuit, 25-36 cycles; fig. 2,541 two phase, 4 wire circuit 36 cycles and above. Fig. 2,542.—Fort Wayne single phase induction watthour meter with cover removed. The rotating parts consist of an aluminum disc mounted on a short shaft of small diameter. The lower end has inserted in it a hardened steel pivot which rests in a cup shaped jewel bearing. The top of the meter shaft is drilled and provided with a small washer having the central hole of very small diameter. Into this hole there extends a steel pin around which the shaft turns. Two micrometer screws are provided for load adjustment—one for the full load and the other for the light load adjustment. The adjustment for accuracy on full load is secured by varying the position of the permanent magnets, sliding them either in or out from the center of the rotating disc of the meter depending on whether it is desired to increase or decrease the speed of the disc. The micrometer screw shown in the figure serves to vary the position of the permanent magnets, causing the shoe in which the two magnets are firmly clamped to slide on the milled magnet support which is cast as an integral part of the meter frame. When the proper position of the magnets has been accurately determined by adjustment and test, the shoe which holds the two magnets is clamped firmly to the milled magnet support by two screws, one of which is shown in the figure. The adjustment for accuracy on light load is secured by varying the position of a metal punching, known as the starting plate, laterally under the pressure pole in the path of the pressure flux. This lateral movement is accomplished by means of the micrometer screw. When the proper position of this punching has been accurately determined by adjustment and test, it is secured in place by tightening the two brass screws which serve to clamp it to the meter frame. 1. The Field Producing Element.—This consists of the electromagnetic circuit and the measuring coils. One of these coils, connected in series with the circuit to be metered, is wound of few turns and is therefore of low inductance. The current through it is in phase with the current in the metered circuit. The other coil, connected across the circuit, is highly inductive, and therefore the current in it is nearly 90 degrees out of phase with, and proportional to the voltage of the metered circuit Ques. How is this angle made exactly 90 degrees? Ans. By means of the power factor adjustment. Fig. 2,543.—Rear view of Fort Wayne single phase induction watthour meter with back cover plate removed. The pressure and current coils and their respective cores lie behind the main frame of the meter. This complete electromagnetic unit can be removed as a whole from its mounting in the case. The pressure coil is wound from enameled wire, the number of turns being very high. The current coils have but few turns each and are wound from cotton covered wire. All coils are treated with insulating compound before assembling in the meters. The laminated iron cores placed within these coils are built up from magnetic steel. The magnetic circuits formed by the cores of the pressure and current coils are so arranged that they exert a high torque upon the disc of the rotating element in order that minute variations in the friction of the moving parts, which are likely to occur will not cause any appreciable error in the registration of the meter. The iron case surrounding the electrical elements protect that part of the meter from the effects of external stray fields, while the astatic arrangement of the permanent magnets tends to prevent any influence on the damping system. The fact that the iron frame of the meter lies between the permanent magnets and the current coils protects the magnets from the effects of short circuits which create a strong magnetic field within the meter itself. Ques. How are the coils mounted? Ans. They are so mounted on the core that the currents in Fig. 2,544.—Fort Wayne single phase induction watthour meter with cover register and permanent magnets removed to show solid meter frame. A heavy steel back plate held in place by two screws inserted from the front of the central casting encloses the back part of the completely assembled meter. A felt gasket lying on a suitable ledge seals the joint against the entrance of dust or moisture when the back plate is drawn down firmly by the screws. The cover which encloses the back part of the meter is a non-magnetic metallic stamping. It is held in place by wing nuts on the two light brass studs extending forward from the meter frame. This joint between the main frame and the cover is also sealed against the entrance of dust and moisture by the use of a suitable felt gasket. Two glass windows are provided in this cover, one to permit the reading of the register dials, the other to permit observation of the disc's rotation. The cover is sealed in place in the usual way by passing a sealing wire through a hole drilled in the cover sealing stud and thence through a hole provided in the wing of the seal nut. The terminal chamber is an extension of the casting which supports all the inner parts of the meter. The heavy brass terminals used for connecting the meter in circuit are held permanently by a non-combustible insulating compound which is moulded in place around them. This construction gives excellent insulation and is a safeguard against accidental short circuits across terminals. A punched terminal cover which fits over the terminal chamber is hinged at the upper left hand corner so that it will of its own accord swing out of the way when the terminal cover sealing screw is removed. This hinged style of cover will be found convenient when installing and connecting the meter in circuit. When this cover is swung back into closed position it is fastened in place by passing a seal right through the seal screw and through a lug provided on the cover. Ques. What is the strength of the rotating field with 90 degrees phase difference between the currents? Ans. It is proportional to the product of the currents in the At any other power factor the field is proportional to this product multiplied by the sine of the angle of phase difference between the two meter currents. If the current in the voltage coil be in quadrature with the voltage of the metered circuit, at any power factor the sine of the angle of phase difference between the currents in the meter circuits will be equal to the cosine of the angular displacement between the current and voltage in the metered circuit. Under these conditions therefore the strength of the shifting field is proportional also to the power factor of the circuit. In other words, the strength of the rotating field is proportional to the product of the volts, amperes and power factor and is therefore a measure of the actual power. Fig. 2,545.—Sangamo single phase induction watt hour meter; view with cover removed showing mechanism. Ques. In what part of the meter is energy consumed? Ans. In the field producing element. Fig. 2,546.—Main grid or supporting frame of Sangamo single phase induction watt hour meter. The grid is of cast iron and its design is such that the weight of the permanent magnets, series laminated element and return plate are carried on the main portion, the smaller projecting brackets carrying no weight except that of the moving system. The supporting grid is removed by taking out the three screws locating and holding it in position, to the iron base, also removing at the same time the screws connecting the leads of the series coils to the binding posts at the bottom. The meters are all built with four binding posts so that they may be connected either with two series leads and a tap for the pressure connection or with both sides of the circuit carried through the meter. The wire meters employ a 220 volt shunt coil, connected across the binding posts within the meter, one series coil being in each of the outer lines of the three wire system. This renders unnecessary the use of a pressure tap. It is upon the design of this element that the losses in the meter depend. Current is flowing through the shunt coil continuously, even when no energy is being taken, and the higher the inductance of this coil, the smaller will be the energy component of the constant flow. The series coil causes a loss of energy proportional to the square of the current flowing. It also causes a drop in voltage, both inductive and resistive, hence, the resistance and inductance of the series coil of the meter should be as low as possible. Ques. How should the magnetic circuit be designed? Ans. The design should be such that the increase of magnetic flux with high voltage or high current will not have a retarding action but will act only to increase the torque. If the retarding effect be not prevented, the meter will, of course, run slow at overloads. A comparative test of meters at varying load and at varying voltage will reveal the characteristics of the magnetic circuit. 2. The Moving Element.—This usually consists of a light metal disc revolving through the air gap in which the rotating field is produced. Fig. 2,547.—Moving element of Sangamo single phase induction watt hour meter. It consists of a light aluminum disc mounted on a hard brass shaft, the entire system weighing 15.6 grams. The disc is swaged under heavy pressure, to render it stiff. The arrangement of the disc, shaft, and bearings is shown in fig. 2,548. By unscrewing the upper and lower bearings the disc and shaft can be removed without disturbing the magnets or adjustments. Ques. What is the action of the disc? Ans. It acts like the squirrel cage armature of an induction motor, developing the motive torque for the meter. Ques. How is this torque counter balanced? Ans. By the retarding element so that the speed is proportional to the torque. Ques. How should the disc be made and why? Ans. As light as possible to reduce wear on the bearings to a minimum. Fig. 2,548.—Bearing system of Sangamo single phase induction watthour meter. The upper pivot, or bearing is made of tempered steel wire and of sufficiently small diameter to be quite flexible in the length between the top of the brass shaft and the guide ring in which it rotates. The guide ring, made of phosphor bronze, has the heavy hole lined and burnished. The upper bearing screw, in which the bronze bushing is carried, is so constructed that a long brass sleeve closely surrounds the upper pivot of the spindle. Any blow against the moving system, caused by accident or short circuit, will slightly deflect the shaft until the steel pivot touches against the side of the shell, thus preventing danger of breaking off or bending the upper pivot. At the same time a cushioning or flexible action between the shaft and the bearing shell is secured, thus eliminating the effect of vibration in the moving system, which would tend to produce rattling. The lower bearing consists of a cup sapphire jewel, supported in a threaded pillar, the upper end of which is provided with a sleeve so located that it prevents the moving element dropping out during shipment. This protecting sleeve is held friction tight on the shaft and can be removed if it be desired to inspect the jewel. 3. The Retarding Element.—This part acts as a load on the induction motor and enables the adjustment of its speed to normal limits. In order that the speed shall be proportional to the driving torque, which varies with the watts in the circuit, it is necessary that the torque of the retarding device be proportional to the speed. For this reason a short circuited constant field generator, consisting of a metal disc rotating between permanent magnet poles, has been generally adopted. Ques. How is the retarding torque produced? Ans. Eddy currents are induced in the disc in rotating through the magnetic field which, according to Lenz's law, oppose the force that produces them, thus developing a retarding torque. Ques. How is the constant field for the retarding disc produced? Ans. By permanent magnets. The retarding disc may be the same disc used for the moving element, in which case the meter field acts on one edge while the permanent magnet field acts on the edge diametrically opposite. This arrangement simplifies the number of parts and saves space and weight of moving element. Ques. What error is likely to be introduced by the retarding element? Ans. If the strength of the permanent magnets change from any cause, the retarding torque will be changed and the calibration of the meter rendered inaccurate. Ques. How may the strength of the permanent magnets be changed? Ans. They may become weak with age, or affected by the proximity of other magnetic fields. The series coil of the meter may, under short circuit so affect the strength of the permanent magnets as to render the meter inaccurate. Ques. What precautions are taken to keep the strength of the permanent magnets constant? Ans. Weakening with age is prevented by the process of "Aging." The effect of neighboring fields is overcome by iron shields; this prevents the electromagnets affecting, through overloads, the strength of the permanent magnets. 4. The Registering Element.—This mechanism comprises the dials, pointers, and gear train necessary to secure the required reduction in speed. This gear train is driven directly by the rotor and therefore its friction should be low and constant. The dials should be easily read and should register directly in kilowatt hours. If a constant be used to reduce the reading to kilowatt hours, it should be some multiple of 10, to avoid errors in multiplication. By means of suitable gears in the meters this is easily accomplished. Fig. 2,549.—Register dial of Sangamo single phase induction watt hour meter (full size). The dial circles read 10, 100, 1,000, and 10,000 kilowatt hours from right to left. 5. The Mounting Frame and Bearings.—These parts have an important influence on the accuracy of the meter, as it Initial friction is unavoidable in any meter construction and can be easily compensated for. A change in the initial friction, however, due to wear of bearings, makes readjustment necessary. In selecting a meter the special attention should therefore be given, to the construction of the bearings, particularly the lower, or "step" bearing which supports the weight of the moving element. Fig. 2,550.—Canadian dial of Sangamo single phase induction watt hour meter. It has a small test circle indicating one kilowatt hour per revolution in all sizes where the first regular circle indicates 10. This is provided to conform with the requirements of the Canadian government and it is intended that the hand on the test circle shall make not less than ½ revolution in one hour with full load on a meter. In the case of a 10 ampere meter, it will make one complete revolution in one hour and for a 20 ampere, two revolutions, and so on. The train or indicating mechanism is carried on a rigidly formed and swaged brass bracket accurately located by two dowel pins set in the top face of the main grid, and is held to the grid with two screws easily accessible when it is desired to remove the train for any purpose. All indicating trains used on type "H" meters are marked with symbols on the back of the train and on the compound attachment to indicate the gear ratio of each combination; this ratio being different for meters of different capacities in order to obtain a direct reading in kilowatt hours on the dial. Ques. Describe a good construction for the step bearing. Ans. A desirable construction would consist of a very highly polished and hardened ball with jewel seats. 6. The Friction Compensator.—The object of this device is to overcome the initial friction of the moving parts. It is evident that if this initial friction were not compensated some of the driving torque of the meter would be used in overcoming it, and the meter would therefore not rotate at very light load, and not fast enough at other loads, thus rendering the registration inaccurate, especially at light loads. Fig. 2,551.—Base and shunt coil of Sangamo single phase induction watt hour meter. Since the shunt or pressure coil sometimes breaks down or burns out, due to abnormal line conditions or accident, provision is made for easy replacement. The shunt magnet with its coil is held to the base by two dowel pins and four screws, enabling it to be removed as a unit as shown. A new core and coil may then be substituted without the necessity of removing and replacing laminations. The shunt coil in 25 cycle meters is wider and contains more steel than the 60 and 133 cycle coils, the winding also being correspondingly increased. The return plate and series coil laminations are also changed in proportion to correspond to the increased width of the shunt magnet. The laminations forming the core are laced into the shunt coil, and subjected to enormous hydraulic pressure, the rivets being set at the same time, to form a compact unit and eliminate humming. The laminated core of the shunt element has but a single air gap in which these discs rotate. Since meters are usually run at light loads it is important that an efficient light load adjustment or friction compensator should be provided. Ques. What important point should be considered in the design of the friction compensator? Ans. The compensating torque must not cause the moving element to rotate or "creep" without current in the series coil. The rotation of a meter is caused by two distinct torques, the varying meter torque, dependent on the power in the circuit, and the constant torque adjusted to compensate the initial friction. The friction at all speeds is not exactly the same as the initial friction, and therefore the friction compensating torque may be in error a few per cent. at high speeds. Figs. 2,552 and 2,553.—Arrangement of magnetic circuit of Sangamo single phase induction meter. Fig. 2,552, front view; fig. 2,553, rear and side view. As shown, the gap of the shunt held in which the disc rotates, projects in between the poles of the series magnet, the return plate bending around so as to clear the upper leg of the shunt magnet. This gives the desired proximity of shunt and series fields with a maximum radius of action for both sets of field. In all capacities up to and including 60 amperes, 2 wire and 3 wire, round wire and taped series coils are used, and in capacities of 80 and 100 amperes, strap windings. Meters exceeding 100 ampere capacity have five ampere coils and are operated from external current transformers having 5 ampere secondaries. The series windings or coils are mounted on a laminated iron U shaped magnet having a laminated return path above the disc of the meter, thus forming air gaps in which the disc rotates. The series coils in all capacities not having strap windings are held firmly in position on the yoke so that they cannot slip up from the lowest position. This is accomplished by means of a pair of spring brass clips slipped through the coils on the rear face of the yoke, the clips being held by the two screws which fasten the series magnet to the main grid. As an additional precaution, spring steel lock washers are put beneath the heads of the holding screws, thus eliminating any chance of the series magnet loosening and changing position. If the compensating torque be small compared with the driving torque, this small error percentage is negligible in its effect on accuracy. The smaller it is, the greater will be the accuracy at all loads, and therefore, as the compensating torque is adjusted to balance the initial friction, the initial friction should be small compared with the driving torque. A high driving torque and low initial friction are therefore desirable, but any increase in the driving torque which necessitates an increase in friction, is obviously useless. The desirable feature of a meter is high ratio of torque to friction. As the friction is practically proportional, to the weight of the moving element, in meters having the same form of bearing, the ratio of torque to weight of rotor gives an approximation to the ratio of torque to friction, but the design of bearing should not be overlooked. A meter having a high torque obtained by using a thick and consequently heavy disc, often has a lower ratio of torque to weight than another with lower torque, and is consequently likely to be less accurate over a given range. Furthermore, the heavy disc is a distinct disadvantage because it produces more wear on the bearings and thus reduces the life. Figs. 2,554 and 2,555. Connections of Sangamo single phase induction meter. Fig. 2,554, 2 wire meter, 5-100 ampere capacity; fig. 2,555, 3 wire meter, 5-100 ampere capacity. 7. The Power Factor Adjustment. This adjustment is necessary to make the phase angle between the shunt and series field components 90° with unity power factor in the metered circuit. Owing to the resistance and iron loss in the shunt field circuit, that field is not shifted quite 90° with respect to the voltage. Yet exact quadrature is necessary to make the strength of the resultant field, and consequently the rotor speed, proportional to the power factor, as explained in the discussion of the field producing element. Ques. What is the usual construction of the power factor adjustment? Ans. It usually consists of a short circuited loop enclosing part or all of the shunt field flux. Ques. How does this loop act? Ans. It acts like the secondary of a transformer. The flux induces a current in it which, acting with the current in the shunt coil, produces a slightly lagging field. By shifting the position of the resistance of the short circuited loop, the lag may be so adjusted that the shunt field flux is in exact quadrature with the voltage. It should be noted, however, that this adjustment makes the meter correct at or near one frequency only. This feature is not objectionable if reasonable accuracy be maintained within the limits of normal variation of frequency. Figs. 2,556 and 2,557. Connections of Sangamo single phase induction meter. Fig. 2,556, 2 wire meter exceeding 100 amperes; fig. 2,557, 3 wire meter exceeding 100 amperes. 8. Frequency Adjustment.—This is often desirable, particularly for systems operating at 133 cycles. Most makes of meter are provided with means for changing the adjustment from 133 to 60 cycles in case of change in the system. 9. The Case and Cover.—These parts should be dust and bug proof, to avoid damage to the bearings, insulation and moving parts, and should of course be provided with means for sealing. Terminal chambers so arranged that the cover of the meter element need not be removed in connecting up, are an important A window through which the rotation of the disc can be observed in checking, should be provided for the same reason. Fig. 2,558.—Faraday disc, or mercury motor ampere hour meter; view showing electric and magnetic circuits. The Faraday Disc, or Mercury Motor Ampere Hour Meter.—On this type of meter the mercury motor consists essentially of a copper disc floated in mercury between the poles of a magnet and provided with leads to and from the mercury at diametrically opposite points. The theoretical relations of the various parts are shown in fig. 2,558. Fig. 2,559.—Diagram showing relative direction of current, magnetic flux, and motion of disc in Faraday disc, or mercury motor ampere hour meter. Ques. Explain its operation. Ans. The electric current, as shown in fig. 2,558, enters the contact C, passes through the comparatively high resistance mercury H to the edge of the low resistance copper disc D across the disc to the mercury H and out of contact C'. The magnetic flux cuts across the disc on each side from N to S, making a complete circuit through M and M'. The relative directions of the magnetic flux and the current of electricity as well as the resulting motion are shown in fig. 2,559. According to the laws of electromagnetic induction, if a current carrying conductor cut a magnetic field of flux at right angles, a force is exerted upon the conductor, tending to push it at right angles to both the current and the flux. When connected to an eddy current damper or generator which requires a driving force directly proportional to the speed of rotation, the mercury Fig. 2,560.—Sectional view of Faraday disc or mercury motor ampere hour meter as made by Sangamo Electric Co. The illustration does not show the magnets and indicating mechanism. Ques. How is the flux produced in the alternating current form of Faraday disc meter? Ans. By the secondary current of a series transformer. Frequency Indicators.—A frequency indicator or meter is an instrument used for determining the frequency, or number of cycles per second of an alternating current. There are several forms of frequency indicator, whose principle of operation differs, and according to which, they may be classed as
Fig. 2,561.—Circuit diagram of simple shunt Sangamo ampere hour meter. It is rated at 10 amperes, larger currents being measured by using shunts. In operation, the main or line current to be measured passes through the shunt, while a part proportional to the drop across the shunt, is shunted through the meter and measured. The only effect of reversing the current will be to reverse the direction of rotation of the meter. In battery installations it is never possible to take the same number of ampere hours from a battery as are put into it, hence, if the simple shunt ampere hour meter be used for repeated and successive charges and discharges, it will be necessary to reset the pointer to zero each time the battery is fully charged. When the shunt meter is equipped with a charge stopping device, the pointer is reset while charging, to allow for a predetermined overcharge. Fig. 2,562.—Circuit diagram of Sangamo differential shunt type ampere hour meter for use in battery charging. Since a battery absorbs more energy on charge than it will give out on discharge, at its working voltage, it is usually given a certain amount of overcharge. This makes desirable a meter that automatically allows for the proper amount of overcharge. Such a meter indicates at all times the amount of electricity available for useful work without resetting the pointer every time the battery is charged. In other words, the battery and the meter would keep in step for considerable periods of time without readjustment. The Sangamo differential shunt meter is designed to meet these requirements, and it consists essentially of a Sangamo meter with two shunts connected as shown. The relative value of shunt resistance is adjustable by means of slider G, so that the meter can be made to run slow on charge or fast on discharge, whichever may be desired. The usual method is to allow the meter to register less than the true amount on charge and the exact amount on discharge, the difference representing the loss in the battery, or the overcharge. If the meter be provided with a charge stopping device, the battery can be given an amount of overcharge predetermined by the setting of the slider G. Therefore the amount of overcharge can be fixed in advance by a skilled man and the actual charging done by any unskilled person, since all there is to do is to make the connection. Ques. How is a synchronous motor employed as a frequency indicator? Ans. A small synchronous motor is connected in the circuit of the current whose frequency is to be measured. After frequency=(revolutions per second×number of poles)÷2. Figs. 2,563 and 2,564.—Frahm resonance type frequency meter. Fig, 2,563, portable meter; fig. 2,564, switchboard meter. The readings are correct in either the vertical or horizontal position. The energy consumption at 100 volts is about 1 to 2 volt amperes, and is approximately proportional for other pressures. The regular portable meters are arranged for pressures of from 50 to 300 volts, and for this purpose they are fitted with terminals for 65, 100, 130, 180, and 250 volts. In order to obtain full amplitude at intermediate pressures, a milled headed screw is provided for adjusting the base piece mechanically, and thereby permitting of regulating the pressure range within ±20 per cent; this insures indications of maximum clearness. Should it be desired to extend the standard pressure range of 50 to 300 volts, up to 600 volts, two further terminals for 350 and 500 volts are necessary, so that these instruments are provided with eight fixed terminals in addition to the mechanical regulating device. Instruments which are intended for connecting to one specific supply or to the secondary of a pressure transformer, require only a single pressure range, say 100 volts, with the aforementioned regulating device. The frequency range is from 7.5 to 600 cycles per second. In order to obtain easily readable indications, one reed is provided for every quarter period for frequencies below 30, for every half period for frequencies between 30 and 80, and for every whole period for frequencies between 80 and 140. The use of a smaller number of reed, that is to say, of larger intervals between the periods of vibration of adjoining reeds, is only recommended for circuits having very variable frequencies, as otherwise no reed might respond to the vibrations caused by intermediate frequencies. The arrangement of the separate reeds on a common base piece, permits supplying any combination of interval that may be required. It is often desirable to secure two ranges with one set of reed. To do this a second electromagnet is supplied. It is polarized, and operates on the same base plate. In the case of alternating current when the unpolarized magnet is used the reeds receive two impulses during each cycle, while with the polarized magnet they receive but one impulse per cycle. A commutator is provided to easily make the change from one range to the other. If there be two sets of reed, the one commutator may be connected to change both. This device is only applicable when alternating current is measured. Instruments with unpolarized magnets are made with frequencies of 15 to 300 cycles per second. Ques. Describe the resonance method of obtaining the frequency. Ans. In construction, the apparatus consists of a pendulum, or reed, of given length, which responds to periodic forces having the same natural period as itself. The instrument comprises a number of reeds of different lengths, mounted in a row, and all simultaneously subjected to the oscillatory attraction of an electromagnet excited by the supply current that is being measured. The reed, which has the same natural time period as the current will vibrate, while the others will remain practically at rest. Figs. 2,565 and 2,566.—Side and end views of Frahm resonance type frequency meter reeds. Owing to the principle employed in the meter it is evident that the indications are independent of the voltage, change of wave form, and external magnetic fields. The construction and operation of the instrument may be better understood from figs. 2,565 and 2,566, which illustrates the indicating part of the Frahm meter. This consists of one or more rows of tuned reeds rigidly mounted side by side on a common and slightly flexible base. The reeds are made of spring steel, 3 or 7 mm. wide, with a small portion of their free ends bent over at right angles as shown in fig. 2,566 and enameled white so that when viewed end on they will be easily visible. The reeds are of adjustable length, and are weighted at the end. A piece of soft iron, rigidly fastened on the base plate which supports the reeds, forms the armature of a magnet. When the magnet is excited by alternating current, or interrupted direct current, the armature is set in vibration, and that gives a slight movement to the base plate at right angles to its axis, thereby affecting all the reeds, especially those which are almost in tune with its vibrations. The reed which is in tune will vibrate through an arc of considerable amplitude, and so indicate the frequency of the exciting current. Ques. For what use is the resonance type of frequency meter most desirable? Ans. For laboratory use. Fig. 2,567.—Westinghouse induction type frequency meter. The normal frequency is usually at the top of the scale to facilitate reading. The damping disc moves in a magnetic field, thus damping by the method of eddy currents. The standard meters are designed for circuits of 100 volts nominal and can be used for voltages up to 125 volts. For higher voltages, transformers with nominal 100 volt secondary should be used. Ques. Describe the induction type of frequency meter. Ans. It consists of two voltmeter electromagnets acting in opposition on a disc attached to the pointer shaft. One of the magnets is in series with an inductance, and the other with a resistance, so that any change in the frequency will unbalance the forces acting on the shaft and cause the pointer to assume a new position, when the forces are again balanced. The aluminum disc is so arranged that when the shaft turns in one direction Fig. 2,568.—Langsdorf and Begole frequency meter. The operation of this meter is based on the fact that if an alternating pressure of E Volts be impressed on a condenser of capacity C, in farads, the current in amperes will be equal to 2p ~ EC, provided the pressure be constant. In construction, the scale is mounted on the same axis as the pressure coil, across the mains so as to render the instrument independent of variation of voltage. For a discussion of this meter, see Electrical Review, vol. LVIII, page 114. Fig. 2,569.—General Electric horizontal edgewise, induction type frequency indicator. It is provided with an external inductance and resistance placed in a ventilated cage for mounting on the back of the switchboard. Means are provided for adjusting the instrument for the characteristics of the circuit on which it is installed. Standard instruments are wound for 100 to 125 volt circuits only, but can be wound for circuits up to and including 650 volts. Instruments for use on circuits in excess of 650 volts are always provided with pressure transformers. The normal operating point is marked at approximately the center of the scale, thus giving the advantage of very open divisions. The standard frequencies are 25, 40, 60,125 and 133. Ques. What is the object of the aluminum disc? Ans. Its function is to damp the oscillations of the pointer. Fig. 2,570.—Westinghouse rotary type of synchroscope or synchronism indicator. The indication is by means of a pointer which assumes at every instant a position corresponding to the phase angle between the pressures of the busbars and the incoming machine, and therefore rotates when the incoming machine is not in synchronism. The direction of rotation indicates whether the machine be fast or slow, and the speed of rotation depends on the difference in frequency. The pointer is continuously visible, during both the dark and light periods of the synchronizing lamps. Synchronism Indicators.—These devices, sometimes called synchroscopes, or synchronizers indicate the exact difference in phase angle at every instant, and the difference in frequency, between an incoming machine and the system to which it is to be connected, so that the coupling switch can be closed at the proper instant. There are several types of synchronizer, such as
The simplest arrangement consists of a lamp or preferably a voltmeter connected across one pole of a two pole switch connecting the incoming machine to the busbars, the other pole of the switch being already closed. If the machines be out of step, the lamps will fluctuate in brightness, or the voltmeter pointer will oscillate, the pulsation becoming less and less as the incoming machine approaches synchronous speed. Synchronism is shown by the lamp remaining out, or the voltmeter at zero. Fig. 2,571.—General Electric synchronism indicator. The synchronism indicator is a motor whose field is supplied with single phase current from one of the machines to be synchronized, and its armature from the other. The armature carries two inductance coils placed at a large angle, one supplied through a resistance, the other through an inductance. This arrangement generates a rotating field in the armature, while the stationary field is alternating. The armature tends to assume a position where the two fields coincide when the alternating field passes through its maximum; hence, the armature and pointer move forward or backward at a rate corresponding to the difference of frequency, and the position when stationary depends on the phase relation. When the machines are running at the same frequency and in phase the pointer is stationary at the marked point. In construction, it is like a small, two phase, bipolar synchronous motor, the field being supplied with alternating instead of direct current. The armature is mounted in ball bearings in order to make it sufficiently sensitive and smooth in operating. The armature coils are not exactly 90 degrees apart, since it is not possible to get the current in the two coils exactly in quadrature without introducing condensers on other complicated construction. Standard ratings are for 110 and 220 volt circuits. Synchronism indicators should be ordered for the frequency of the circuit on which they are to be operated, although the instruments may be used on circuits varying 10 per cent to 15 per cent from the normal. The words "Fast" and "Slow" on the dial indicate that the frequency on binding posts E and F is respectively higher or lower than that on A and B; or, in other words, clockwise rotation of the pointer means that the incoming machine is running at too high speed, counter clockwise rotation indicating too low speed. Ques. How does the resonance type of synchronism indicator operate? Ans. On the same principle as the resonance type of frequency indicator, already described. Ques. What is the principle of the rotating field type of synchronism indicator? Ans. Its operation depends on the production of a rotating field by the currents of the metered circuits in angularly placed coils, one for each phase in the case of a polyphase indicator. In this field is provided a movable iron vane or armature, magnetized by a stationary coil whose current is in phase with the voltage of one phase of the circuit. As the iron vane is attracted or repelled by the rotating field, it takes up a position where the zero of the rotating field occurs at the same instant as the zero of its own field. In the single phase meter the positions of voltage and current coils are interchanged and the rotating field is produced by means of a split phase winding connected to the voltage circuit. Fig. 2,572.—General Electric external resistance and inductance for 110 volt synchronism indicator. Both the resistance and inductance are intended to be placed behind the switchboard. Figs. 2,573 to 2,576.—Connections of General Electric synchronism indicator. Fig. 2,573, connections with grounded secondaries on pressure transformers; fig. 2,574, connections with ungrounded secondaries on pressure transformers; fig. 2,575, connections for 200 to 240 volt circuits, with six point receptacles; fig. 2,576, connections for checking location of needle. The various letters referred to in the diagrams will be found marked on the ends of the instrument studs and back of reactance coil box. It is important that the instrument be connected in circuit in the proper manner so that the needle will come to the mark on the upper part of the scale when synchronism is obtained. In case the pointer become moved or a change in its position be necessary, it is advisable to make a check on the indication before relocating the needle. This test can be made as follows: Connect together (fig. 2,573) studs marked B and E and connect stud A to terminal F on the external reactance box. When these connections are made, the instrument can be connected to a single phase circuit of normal voltage and if the instrument be correct, the pointer will stand vertically at the point of synchronism. If it do not, the needle can be moved and should be fastened in the correct position. The synchronizing lamps when connected as illustrated in the diagrams, show dark when synchronism is reached. This is the only connection possible when grounded secondaries are used, as in fig. 2,573, and for the high voltage indicators when used as in fig. 2,575, but with ungrounded secondaries (fig. 2,574) the lamps may be connected as indicated, when they will show bright at the moment of synchronism. The connections to the synchronism indicator remain the same as before. Power Factor Indicators.—Meters of this class indicate the phase relationship between pressure and current, and are therefore sometimes called phase indicators. There are two types:
Fig. 2577.—General Electric synchronizing receptacle and plug for use with synchronism indicator. Fig. 2,578.—Westinghouse rotating field type power factor meter. The rotating field is produced by the currents of the metered circuits in angularly placed coils, one for each phase of the system, in the case of polyphase meters. In the three phase meter the rotating field is produced by three coils spaced 60° apart; in the two phase meter by two coils spaced 90°; in the single phase meter the positions of voltage and current coils are interchanged and the rotating field is produced by means of a split phase winding, connected to the voltage circuit. There are no movable coils or flexible connections. Single phase meters indicate the power factor of a single phase circuit, or of one branch of any polyphase circuit. Special calibration is necessary in order to use a single phase instrument on a three phase circuit unless the voltage coil be connected from one line to the neutral. Polyphase meters indicate the average angle between the currents and voltages and are superior for polyphase service to meters having only one current coil. In the wattmeter type, the phase relation between the pressure and the current fluxes is such that on a non-inductive load the torque is zero. For instance, in a dynamometer wattmeter, the pressure circuit is made highly inductive and the instrument then indicates volts×amperes×sinf instead of volts×amperes×cosf, that is to say, it will indicate the wattless component of the power. A dynamometer of this type is sometimes called an idle current wattmeter. Fig. 2,579.—Single phase power factor meter of the rotating field or disc type. Ques. Describe a single phase power factor meter of the disc or rotating field type. Ans. It consists of two pressure coils, as shown in fig. 2,579, placed at right angles to each other, one being connected through a resistance, and the other through an inductance so as to "split" the phase and get the equivalent of a rotating magnetic field. The coils are placed about a common axis, along which is pivoted an iron disc or vane. The magnetizing coils FG are in series with the load. If the load be very inductive, the coil M experiences very little torque and the system will set itself as shown in the figure. As the load becomes less inductive, the torque on S decreases and on M increases so that the system takes up a particular position for every angle of lag or lead. Ground Detectors.—Instruments of this name are used for detecting (and sometimes measuring) the leakage to earth or For systems not permanently earthed anywhere, these instruments are nearly all based on a measurement of the pressure difference between each pole and earth, two measurements being required for two wire systems, and three for three wire, whether direct current single phase, or polyphase alternating current. In the case of direct current systems, the insulation, both of the network and of the individual lines, can be calculated from the readings, but with alternating current, the disturbance due to capacity effects is usually too great. In any case, however, the main showing the smallest pressure difference to earth must be taken as being the worst insulated. For low tension systems moving coil (for alternating current) or moving iron instruments (for direct current) are the most used, while for high tension systems electrostatic voltmeters are to be preferred. On systems having some point permanently earthed at the station, as for instance the neutral wire of direct current system, or the neutral point of a three phase system, an ammeter connected in the earth wire will serve as a rough guide. It should indicate no current so long as the insulation is in a satisfactory state, but on the occurrence of an earth it will at once show a deflection. The indications are, however, often misleading, and serve more as a warning than anything else. Fig. 2,580.—Westinghouse single phase electrostatic ground detector. Fig. 2,581.—Westinghouse three phase electrostatic ground detector. Fig. 2,582.—Wallis-Jones automatic earth leakage cut out. It is an instrument which so protects a direct current circuit that the circuit is broken whenever a leak occurs from either main to earth, and so that the circuit cannot be permanently re-established until the leak has been removed. The instrument and its connections may be explained by the aid of the accompanying diagram, in which T1 and T2 represent the points of connection from the mains, and T3 and T4, the points of connection to the circuit to be protected. So S2, and S3 will preferably be ordinary tumbler switches, but they are diagrammatically represented as plain bar switches, their fixed contacts being diagrammatically represented by dotted circles. When the three switches S1, S2, and S3 are closed, the current passes from T1 to T3 through the small resistance R1, through circuit L to T4, and back through the resistance R2 to T2. In shunt with R1 and R2, are the two moving coils C and C2, working in the magnetic field of the magnets NS, NS, and rigidly fixed on one spindle, which is broken electrically by an ebonite block E. The points of connection to the shunts are adjusted so that when the same current passes out through one and back through the other, the effect on the two coils is equal and opposite, and there is thus no movement. Should, however, any minute portion of the current through R1 leak to earth instead of returning via R2, the balance is disturbed, C becomes stronger than C2, the system is deflected, and a contact is made by the arm A at B, no matter in which direction the coils deflect. The system is similarly deflected for a leak on the other lead. In the diagram these contacts are shown at right angles to their normal plane. As soon as the contact is made, the electromagnet M is energized, the arm of S1 is released and the spring at once pulls it off its contact, at the same time breaking S2. The positions of the blades when the switches are open are shown dotted. The only means the user has of closing the circuit is by putting on S3 by the handle H, which is outside the locked box. The first effect of putting on S3 is to break its circuit; it then by means of the slotted bar P begins to pull on S2 and S1, which can thus be closed again, and held closed by the trigger as before. The circuit, is, however, still broken till S2 is pushed back. Then if the leak be still on, the slot in P allows S1 and S2 to open at once as before. It is therefore impossible to keep the circuit closed while the leak exists. The working condition of the instrument can be tested at any time by switching a lamp on in the circuit and depressing one of the keys K1, K2. This short circuits R1 or R2, throws the coils out of balance, and the switch opens. The contact arm is closed in an inner dust tight case, and it will be noted that it makes contact only; the break occurs at the switches, thus avoiding any sparking. Since the two coils work in the two gaps of one and the same field, changes in the strength of the magnets have no effect, the apparatus is enclosed in a locked metallic box, and the only part to which the user has access is the handle H, and, if desired, the testing keys K1 K2.] HAWKINS PRACTICAL LIBRARY OF They are not only the best, but the cheapest work published on Electricity. Each number being complete in itself. Separate numbers sent postpaid to any address on receipt of price. They are guaranteed in every way or your money will be returned. Complete catalog of series will be mailed free on request. ELECTRICAL GUIDE, NO. 1 Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery. ELECTRICAL GUIDE, NO. 2 The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos. ELECTRICAL GUIDE, NO. 3 Electrical Instruments, Testing, Practical Management of Dynamos and Motors. ELECTRICAL GUIDE, NO. 4 Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries. ELECTRICAL GUIDE, NO. 5 Principles of Alternating Currents and Alternators. ELECTRICAL GUIDE, NO. 6 Alternating Current Motors, Transformers, Converters, Rectifiers. ELECTRICAL GUIDE, NO. 7 Alternating Current Systems, Circuit Breakers, Measuring Instruments. ELECTRICAL GUIDE, NO. 8 Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation. ELECTRICAL GUIDE, NO. 9 Telephone, Telegraph, Wireless, Bells, Lighting, Railways. ELECTRICAL GUIDE, NO. 10 Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers. Theo. Audel & Co., Publishers. 72 FIFTH AVENUE,
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