HOW TO READ A METER
Fig. 655.--Recording dials of watt hour meter, illustrating method of reading electric meters. The unit of measurement of electrical energy is the watt hour. 1,000 watt hours make or equal 1 kilowatt hour. Some electric meters have 4 dials, the extreme right hand dial of which registers in kilowatt hours, while others have 5 dials, the extreme right hand dial of which registers in tenths of kilowatt hours. In making out bills to customers the extreme right hand dial of a 5 dial meter is ignored in order that the "state of meter" shown on bills uniformly requires the addition of 3 ciphers to correctly express the registration in watt hours. Each division on the right hand dial (ignoring the 5th dial mentioned) denotes 1,000 watt hours or 1 kilowatt hour; on the next dial 10 kilowatt hours, on the next dial 100 kilowatt hours and on the left hand dial 1,000 kilowatt hours. One complete revolution of any dial causes the hand on the dial immediately to its left to move forward one division. To take a statement from the meter begin at the left and set down for each dial the lower figure next to each hand, not necessarily the figure nearer the hand. In the above example the statement is 1,726 kilowatt hours or 1,726,000 watt hours. Subtract the previous statement to arrive at registration for a given period. Some meters are subject to a multiplying constant so stated on their face and the registration of such meters must be multiplied by the constant as shown, to determine the actual consumption of electrical energy. The constant is the measure of the mechanical adjustment in the register of the meter and is the ratio between the registration of the dial hands and the true consumption. This adjustment is made always by the manufacturer of the meter and is never changed in service.
Ques. What is the action of the motor in the Thompson watt hour meter?
Ans. It rotates at very slow speed, and since there is no iron in its fields and armature, it has very little reverse voltage. Its armature current, therefore, is independent of the speed of rotation, and is constant for any definite voltage applied at its terminals.
Fig. 656.--Interior of Thompson watt hour meter (type C-6) showing armature, small commutator and gravity brushes. A spherical armature moving within circular field coils is the construction adopted in this meter. The armature is wound on a very thin paper shell, stiff enough to withstand the strain due to winding and subsequent handling. The wire composing the armature is of the smallest gauge consistent with mechanical strength. The field coils, as before stated, are circular, and are placed as near each other as possible, one on either side of the armature, with the internal diameter just sufficient to give the necessary clearance for the rotating element. This construction prevents magnetic leakage. Ribbon wire is employed for the field coils, thus economizing space and further carrying out the idea of concentration.
The torque of this motor being proportioned to the product of its armature and field currents, must vary directly as the energy passing through its coils. In order then, that the motor shall record correctly it is necessary only to provide some means for making the speed proportional to the torque. This is accomplished by applying a load or drag, the strength of which varies directly as the speed.
Ques. Explain the operation of the Thompson recording wattmeter.
Ans. There being no iron in either field or armature of the motor element, no considerations of saturation are involved. The torque or pull of the armature is dependent upon the product of the field and armature strength. The strength of the field--there being no iron--varies directly with the current in the field. Thus the strength of the field with 10 amperes flowing to the load is exactly twice the strength of the field with 5 amperes flowing to the load. The strength of the armature is dependent on the voltage of the system to which it is connected, the armature element of the meter being practically a voltmeter. There is, therefore, a torque or pull varying directly with the strength of the armature multiplied by the strength of the field, or, in other words, varying directly with the watt load, and except in so far as influenced by friction, the speed of rotation varies directly with the torque or pull. The currents generated in the disc armature consist of eddy currents, which circulate within the mass of the disc.
Installation of Wattmeters.--The various types of wattmeter differ so widely either in mechanical details, or operating principles, that it is customary for manufacturers to furnish detailed instructions for the installation of their meters. Such instructions should be carefully followed in all cases, but the following will be found generally applicable to all types of motor meter:
[B] NOTE.--The most practical and accurate method of plumbing a meter is to level it by means of a small brass weight placed upon the retarding disc. Place the weight upon the front or back upper surface of the disc, close to the edge. If the disc and weight rotate toward the right, move the bottom of the meter in the same direction so as to raise the disc on the right. When the disc is level, the weight and disc will remain stationary when the weight is placed on either the front or the back of the disc. Next, place the weight on the disc close to the edge on either side. If the disc rotate towards the front, swing the bottom of the meter away from the wall or board until the disc remains stationary when the weight is placed upon it on either side. If the disc rotate toward the back, raise it up on that side by bringing the top of the meter away from the wall or board. It is possible that the second levelling operation will alter the position of the disc obtained by the first operation, therefore, the first should be repeated, and after that the second also, until the disc remains stationary when the weight is placed at any point upon its surface. This method of levelling is more reliable than any method in which a spirit level is employed.
Fig. 657.--Interior view of Thompson watt hour meter (type CQ). The capacities of this type range from 50 to 400 amperes inclusive, two wire, and 50 to 200 amperes inclusive, three wire, and for voltages of from 100 to 600 volts. These meters are made with either front or back connections. In front connected meters the positions of the leading-in wires and cables are the same as in the type C-6, fig. 654, so that either type of meter may be installed in the same location.
Fig. 658.--Specimen record from General Electric recording ammeter. The record is made on a band of specially prepared paper four inches wide and sixty feet in length. On this paper are ruled lines corresponding to time, and the instrument calibration. The lines ruled across the paper represent time; those ruled lengthwise represent volts, amperes, or watts, depending upon the instrument construction. This form of paper has the advantage of permitting the use of time divisions of equal length throughout the entire range of the recording pen. The recording pen is attached to the moving element in such a manner that its motion is transmitted in a straight line parallel to the time division on the chart. As the paper is unwound and passed under the recording pen, it is paid into a space at the bottom of the instrument case. To assist in removing paper, the instrument is provided with a stripper, which enables the paper to be torn off evenly and without damage. The paper feeding mechanism is simple. By means of suitable gearing, the clock drives a drum having peg teeth which engage the holes located near the edge of the paper. These teeth not only feed the paper under the recording pen, but also give it a definite and accurate position along the axis of the drum. The feeding drum is driven by a friction clutch.
Fig. 659.--Westinghouse type CW-6 watt hour meter with cover off. This meter is of the commutator type without iron in the magnetic circuit. The spherical armature is closely surrounded by circular field coils which provide the shortest magnetic path and smallest magnetic leakage, thus securing high torque with small consumption of energy. The armature winding is wound on a hollow sphere of prepared paper which is moulded in corrugated form to secure strength. Uniform brush tension is maintained by gravity. Each brush consists of two small round wires placed side by side and held against the commutator by a small counterweight whose distance from the fulcrum is adjustable. The current winding consists of two flat coils of strap copper, one clamped rigidly on either side of the central mounting frame which supports the armature bearings. These coils are connected either in series or parallel, depending on the capacity. In three wire meters one of the coils is connected in series with each side of the line. The retarding element consists of a light aluminum disc rotating between two pairs of permanent magnets. The magnets are prepared by a special aging process to insure permanence. Full load adjustment is made by shifting the position of the permanent magnets. Ample light load adjustment or friction compensation is provided by means of the movable coil, which can be shifted horizontally or radially on loosening one screw. The meter registers directly in kilowatt hours.
Fig. 660.--Interior of Thompson prepayment watt hour meter. The actuating force is a large flat coil spring enclosed in a barrel or drum to which its outside end is attached. The operating knob winds this main spring by turning the drum. The spring has many turns and as the operation of the device never equals one whole turn, the spring always exerts a practically constant force. The rate device consists of a small train of gears secured to the front of the frame directly back of the register. Each device is marked with the price per kw-hr. for which it should be used. The switch is of the double pole double break type with leaf contacts. The coin receptacles are placed at the back of the meter. To make an advance payment, the winding knob is turned so that the arrow points upward. A quarter dollar is then inserted in the slot and the knob turned to the right, the coin serving as a key which operates the mechanism within the device, turning the registering wheel and placing the coin to the credit of the customer. If the circuit be open when the coin is deposited the same motion of the knob which moves the registering mechanism closes the circuit switch contained within the case. The dial contains a scale marked in plain figures over which a pointer passes indicating the number of coins remaining to the credit of the depositor. When the first coin is deposited and the knob turned closing the main switch, the pointer rests opposite the first division on the scale. If a second coin be deposited before the current purchased with the first coin has been consumed, a second motion of the knob will bring the pointer opposite the second division on the scale. Twelve coins can thus be deposited consecutively, after which the slot is automatically closed and further prepayment cannot be made until the value of one or more coins has been consumed. Whenever energy to the value of one coin has been delivered through the meter, the escapement is mechanically released turning the pointer back one division. This process continues until all the energy has been delivered for which payment has been made. Thus the depositor can ascertain at any time how much energy can be obtained without further payment. When all energy has been delivered, the circuit switch is opened so that no more current can be obtained until one or more coins have been deposited. The indicating mechanism shows only the number of coins which stand to the credit of the customer, but, by consulting the meter dial, one can determine what fractional part of the prepayment next to be cancelled remains to the credit of the customer. A coin or washer larger than the coin for which the device is designed cannot be introduced into the receiving slot and a smaller one will not operate the device.
How to test a meter.--A simple test for ascertaining whether a customer's meter is fast or slowC, may be applied as follows:
The difference between the first and second readings of the dial will be the indicated consumption of two hours, and if this be greater than the amount of power that ought to be consumed by the number of lamps turned on, the meter is fast, but if it be less, the meter is slow.
The best results obtained by this method are only approximations, however, on account of the variations in the watts consumed by the different makes of lamp, the uncertainty as to the actual voltage on the line at the time of the test, and the lack of knowledge as to the age of the lamps. Therefore, if the meter test within five per cent., or do not record more nor less than one-twentieth of the assumed lamp consumption it is safe to assume that the meter is correct as the result of the test is not likely to be any closer to the truth.
[C] NOTE.--A meter operates under more varied and exacting conditions than almost any other piece of apparatus. It is frequently subjected to vibration, moisture and extremes of temperature; it must register accurately on varying voltages and various wave forms; it must operate for many months without any supervision or attention whatever; and, in spite of all these conditions, it is expected to register with accuracy from a few per cent. of its rated capacity to a 50 per cent. overload. As a meter is a type of machine, its natural tendency is to run slow; but occasionally, through accident, a meter may run fast. When a meter runs fast the consumer is paying a higher rate per kilowatt hour than his contract calls for. He is being discriminated against. The periodic testing of meters is therefore a necessity and is an indication of the honesty of intention of the manager toward the customers of his company. Meters controlling a very large amount of revenue may be tested as often as once a month, while the ordinary run of meters should be tested at least once a year, once in eighteen months, or once in two years, the period varying with different companies, different types and different civic requirements. Commutator type meters, having comparatively heavy moving elements with consequent rapid increase in friction due to wear on the jewel and bearings, and a commutator also increasing in friction with age, must have frequent and expert attention to insure their accuracy under all conditions.
Ques. How should a roughened commutator be cleaned and smoothed?
Ans. By means of tape.
Fig. 661.--Internal connections of Sangamo watt hour meter (type D). A, copper disc armature, submerged in mercury; B, bridge wire between binding posts, for main load current, when both sides of the line are carried through the meter; CT, compounding series turns around pressure circuit magnet, building up field as load increases, to compensate for falling off in speed otherwise found; D, aluminum damping, or brake disc, controlling speed of meter; E, copper contact ears, imbedded in insulating wall of mercury chamber, leading current into and out from armature; F, hardwood float on armature proportioned to give slight "lift" to entire moving system, when armature and float are immersed in mercury; H, soft steel disc above permanent magnets, riveted to fine pitch screw working in bracket above, so that adjustment of the disc up or down gives variation in damping effect of permanent magnets, and therefore of main speed. K, clamp slider with thumb screw, for obtaining light load adjustment by moving K to right or left, as may be necessary. K spans and connects parallel wires of light load adjustment, BR and RR'. MM, powerful permanent magnets, acting on disc D, giving main speed control for meter. N, high resistance heavy wire, forming part of series adjustment between armature and any shunt with which meter may be used, to set drop through meter correct for drop of the shunt. P, spirally laminated soft steel ring, moulded in mercury chamber above the armature space, to act as a return for magnetic lines of force from and to energizing magnet below. R, resistance card unit, in series with pressure circuit coils; in 110 volt meters one card is used, in 220 volt meters two cards, or one card and a thermocouple. BR, small brass wire, connected to ingoing end of pressure circuit coils and forming RR' and the slides K the light load adjustment. RR', high resistance wire having opposite ends connected to ears EE by low resistance wires. Current energizing the pressure circuit coils SC passes from RR' through K to BR and thence to the coils, and if K be near the end of RR' and BR, the least compensation is obtained; if near right end, maximum light load compensation is obtained. S, shaft or spindle. In actual meter S is divided, the lower shaft carrying armature A, and the upper shaft damping disc D. SA, series resistance adjustment, for setting meter to correct drop for shunt. SC and SC', pressure coils connected in series. TT, binding posts at bottom of meter. Y, laminated soft steel yoke, carrying coils SC and SC', and giving a powerful and concentrated magnetic field on the armature. W, worm, driving recording dial train. WW, worm wheel.
Fig. 662.--Interior view of Columbia watt hour meter (type D), showing construction and principal parts and connections. The armature winding consists of three coils approximately circular in shape. The coils are form wound, interlocked with one another and with the light impregnated fibre disc which serves as a spacer for them. The aluminum damper disc has the conventional anti-creep provision in the shape of the three small soft iron plugs, mounted close to the central staff, which the illustration shows. These in their revolution come successively within the influence of an adjustable iron screw which is magnetized by an extension from one of the damper magnets. The angular relationship of the armature windings and of the three iron plugs is such that at the time that the armature is exerting a maximum torque the magnetized screw is exerting the maximum pull to hold back a given plug and conversely when the armature pull is a minimum the magnetic screw is attracting a plug with the maximum effort to cause ahead rotation. The irregularities of torque are in this way smoothed out. The commutator has three segments and is made of chemically pure silver. Each brush is formed of a length of phosphor bronze wire bent like a hair pin and secured at its "U" end to a brass sleeve, which in turn is secured to an insulated stud by a set screw. An extension on the sleeve carries a micrometer screw brush adjustment. The main speed adjustment is secured by providing a soft iron bridge plate, bridging over the extremities of each magnet end and adjustable, by means of a set screw and lock nut, to any desired distance therefrom. This gives a regular micrometer means of varying the effective magnet strength. Interposed between the series coil and the permanent magnets is a heavy soft iron shield to guard the magnets against disturbance by short circuiting. Light load adjustment is obtained by providing in the coil circuit a series of small resistance spools, equipped with pin terminals, to which connection can be selectively made by means of a split bushing terminal on a flexible cord. This series of spools is strung on a metal arbor located within the case.
Fig. 663.--Diagram showing internal connections of the Duncan watt hour meter. Its operation depends upon the principle of the well known electro-dynamometer, in which the electromagnetic action between the currents in the field coils and an armature produces motion in the latter. It also embodies the other two necessary watt hour meter elements required for the speed control and registration of the revolutions of the armature, these being embodied in the drag magnet and disc, and the meter register respectively. The motion of the armature is converted into continuous rotation by the aid of a commutator and brushes, the commutator being connected to the armature coils and carried on the same spindle therewith.
Waste of Electricity in Lighting.--In large residences where a good many servants are employed or in any place where the power consumed is not directly under the supervision of the person who must pay the bills, a great deal of waste usually occurs.
If the meter be read before retiring, the reading in the morning will show how much energy was consumed during the night, which will show in turn how many lamps were burning all night.
A great deal of light can be saved by placing the lamps so that they will throw the light where it is needed and by placing small lamps such as 8 candle power and 4 candle power in places where not much light is needed, such as bathrooms, halls, cellars, etc.
When the lamps get old and dim they should be replaced with new ones, as it costs about the same to burn an old lamp as a new one. An old 16 candle power lamp which is very dim will give only about 8 candle power and use about as much current as is required for a new 16 candle power. If the dim light be light enough, it should be replaced by an 8 candle power lamp, which will not consume as much power as the old 16 candle power.
Before Starting a Dynamo or Motor.--When the machine has been securely fixed, it should be carefully examined to see that all parts are in good order. The examination should be made as follows:
In the subsequent working of the dynamo it will of course be unnecessary to follow the whole of these proceedings every time the machine is started, as it is extremely unlikely that the machine will be damaged from external causes while working without the attendant being aware of the fact.
Adjusting the Brushes.--The adjustment of the brushes upon the commutator requires careful attention if sparking is to be avoided. There are two adjustments to be made:
Ques. At what point on the commutator should the brushes bear?
Ans. The points upon the commutator at which the tips of the brushes (carried by opposite arms of the rocker) bear, should be, in bipolar dynamos, at opposite extremities of a diameter. In multipolar dynamos the positions vary with the number of poles and the nature of the armature winding.
Ques. What provision is made to facilitate the correct setting of the brushes?
Ans. Setting marks are usually cut in the collar of the commutator next to the bearing.
Figs. 664 and 665.--Diagrams illustrating how to set brushes. Some brush holders require brushes set with the direction of rotation of the commutator, and others, set against the direction of rotation. In fig. 664 is shown a brush holder of the first class, which must always be set as indicated by the arrow. If set in the opposite direction, trouble will ensue, as an inspection of the figure will show, because the surface of the commutator and the brush would form a toggle joint, and the brush would tend to dig into the commutator and either break itself or bend the brush rigging. In fig. 665 is shown a brush holder of the second type. This brush is set against the direction of rotation, but an inspection of the cut will show that there is, in this case, no tendency for the brush to dig into the commutator surface. Each type of brush holder, of which there are several, should be adjusted as recommended by the manufacturer to secure proper working.
Ques. How are the brushes set by these marks?
Ans. The tips of all the brushes carried by one arm of the rocker are set in correct line with the commutator segments marked out by one setting mark, and the tips of the brushes carried by the other arm or arms are set in correct line with the segments marked out by the other mark or marks.
If one or more of the brushes in a set be out of line with their setting mark, it will be necessary to adjust the brushes up to this mark by pushing them out or drawing them back, as may be required, afterwards clamping them in position. When adjusting the brushes, the armature should always be rotated, so that the setting marks are horizontal. The rocker can then be rotated into position, and the tips of both sets of brushes conveniently adjusted to their marks. In those brush holders provided with an index or pointer for adjusting the brushes, the setting marks upon the commutator are absent, length of the pointer being so proportioned that when the tips of the brushes are in line with the extreme tips of the pointers, the brushes bear upon the correct positions on the commutator.
Fig. 666.--Method of soldering cable to carbon brush. Drill a hole in the end, also in the side of the brush, as shown in the sketch, and after thoroughly tinning the "pig-tail," place it in the end hole and fill the holes up with solder through the side hole. Another method is to drill a hole through the carbon so that the cable will just slip through, countersink the edge of the hole a little, clean the cable thoroughly and pass it through the hole. Then with any good flux and solder, fill the countersunk part on both sides.
Ques. What should be done after adjusting the brushes to their correct positions upon the commutator?
Ans. Their tips or rubbing ends should be examined while in position to see that they bed accurately on the surface of the commutator.
In many instances it will be found that this is not the case, the brushes sometimes bearing upon the point or toe, and sometimes upon the heel, so that they do not make contact with the commutator throughout their entire thickness and width. The angle of the rubbing ends will therefore need to be altered by filing to make them lie flat.
Ques. How is the proper brush contact secured?
Ans. When the brushes do not bed properly they should be refitted to secure proper contact.
Ques. How is the pressure adjustment made?
Ans. This is effected by regulating the tension of the springs provided for the purpose upon the brush holders.
Ques. With what pressure should the brushes bear against the commutator?
Ans. The tension of the springs should be just sufficient to cause the brushes to make a light yet reliable contact with the commutator.
The contact must not be too light, otherwise the brushes will vibrate, and thus cause sparking; nor must it be too heavy, or they will press too hard upon the commutator, grinding, scoring and wearing away the latter and themselves to an undesirable extent, and moreover, giving rise to heating and sparking.
The correct pressure is attained when the brushes collect the full current without sparking, while their pressure upon the commutator is just sufficient to overcome ordinary vibration due to the rotation of the commutator.
Figs. 667 to 669.--Method of winding cables with marlin. When connecting the feeders and dynamo and service leads to a switchboard, the wires are often served with marlin. By serving is meant to tightly wrap the wires of each set together with marlin. A tool for serving may be made as in fig. 667, using a piece of oak 2 ins. wide, 7/8 in. thick and 14 ins. long, having four holes drilled through it, as shown. The marlin is passed through the holes, commencing at the hole nearest the handle, the object being to cause a strain on the marlin at the point where it passes around the wire, so that the marlin may be wrapped tightly. It is necessary to serve the first four or five inches by hand, pushing the winding into the conduit as far as possible. This acts as an additional protection to the wires where they leave the conduit. The serving is continued, as in fig. 668, to within four or five inches of the first lug by means of the serving tool, passing the ball of marlin around the wires with the serving tool. The wires are then bent in shape, as in fig. 669. To serve the wires properly it is necessary to tie the ends of the wires taut. The wires should be straightened and run together so as to be parallel, being bound with tape at different points to keep them so. When the serving is complete the marlin should be thoroughly painted with a moisture resisting compound. The marlin serving will stiffen the wires and they can be bent very neatly to avoid touching the bus bars of the board. When painted the marlin hardens so that it is difficult to bend the wires after the paint has dried. It then requires a strong pressure to bend them. The marlin acts as an additional insulation and mechanical protection to the wires, and while no harm would result from the wires coming in contact with the bars while thus protected, it looks better to bend them so as to avoid touching the bars.
Direction of Rotation.--This is sometimes a matter of doubt and often results in considerable trouble. As a general rule, a dynamo is intended to run in a certain direction; either right handed or left handed according to whether the armature, when looked at from the pulley end, revolves with or against the direction of the hands of a clock. Dynamos are usually designed to run right handed, but the manufacturers will make them left handed if so desired.
It may be necessary to reverse the direction of rotation of a dynamo, if the driving pulley to which it has to be connected happen to revolve left handed, or if it be necessary to bring the loose side of the belt on top of the pulley, or to place the machine in a certain position on account of limited space. The direction of rotation of ordinary series, shunt, or compound bipolar dynamos may be reversed by simply reversing the brushes without changing any of the connections, then changing the point of contact of the brush tips 180°.
In multipolar dynamos, a similar change, amounting to 90° for a four pole machine, and 45° for an eight pole machine, will reverse their direction of rotation. It will be understood that under these conditions, the original direction of the current and the polarity of the field magnets will remain unchanged.
This rule does not apply to arc dynamos and other machines, which have to be run in a certain direction only, in order to suit their regulating devices.
If the direction of current generated by a dynamo be opposite to that desired, the two leads should be reversed in the terminals, or the residual magnetism should be reversed by a current from an outside source.
Fig. 670.--Method of assembling core discs. For this operation two wooden "horses" should be provided to support the core at a convenient height, as shown in the illustration.
Starting a Dynamo.--Having followed the foregoing instructions, all keys, spanners, bolts, etc., should be removed from the immediate neighborhood of the machine, and the dynamo started.
Figs. 671 and 672.--Tinning block for electric soldering tool. It is made with two soft bricks. One brick is used to support the soldering tool, and the other to contain the tinning material and to furnish a material which will keep the copper bit bright enough to receive its coating of "tin." Fig. 671 represents a section of the tinning brick, which is scooped out on top as shown by the lower line. Into one end of the hollow in the brick, some sal-ammoniac is placed to help tin the copper bit. Sal-ammoniac is a natural flux for copper and aids greatly in keeping the tool well tinned. Next, some melted solder is run into the hollow of the brick, and lastly enough resin to fill the cavity nearly to the top. When the tool is not in use, the electricity is switched off and the tool permitted to lie in the resin. If it be desired to repair the tin coating a little when the tool is in use, the latter is rubbed on the brick below the layer of solder, and the layer of resin. If the tool be in very bad condition, it may be pushed into the sal-ammoniac once or twice and then rubbed in the solder again. It requires but little heat to keep the brick and its contents ready for use. In fact, the brick is a fair non-conductor of heat and prevents the escape of heat from one side of the tool. When momentarily not in use, the tool remains in the solder which becomes melted underneath the layer of resin. When the copper bit becomes too hot, it will begin to volatilize the resin, thus calling attention to this fact, whereupon, the electricity should be turned off from the tool.
Ques. How should a dynamo be started?
Ans. A dynamo is usually brought up to speed either by starting the driving engine, or by connecting the dynamo to a source of power already in motion. In the first case, it should be done by a competent engineer, and in the second case by a person experienced in putting on friction clutches to revolving shafts, or in slipping on belting to moving pulleys.
Fig. 673.--Connections for two shunt wound dynamos to run in parallel. The positive lead of each machine is connected to the same bus bar. In starting, if but one machine is to be used, the dynamo is first brought up to speed and the voltage regulated by means of the rheostat R and the voltmeter V. The main switch is then thrown in. The connections for the field are taken off the dynamo leads so that the opening of the main switch will not open the field circuit and for this reason the field will begin to build up as soon as the machine is started. When but one of the machines is running, the idle machine is brought up to speed with the main switch open, and the voltage regulated by means of the rheostat and voltmeter until the voltages of the machines are the same. Then the main switch is thrown in and the load on the machines (which is ascertained by the ammeters) is equalized by means of the rheostats. Should there be any great difference in voltages, the higher one will run the other as a motor without changing the direction of rotation. The field current will remain unchanged, and the armature current of the low dynamo will be reversed, which will cause it to run as a motor in the same direction as it ran as a dynamo. When dynamos feeding current to motors are to be shut down, the switches on the motors should first be opened. Otherwise some of the motor fuses will blow. As the voltage goes down the motors will draw more current to do the work. If a plant be shut down with the motor switches "in" it will generally be found impossible to start a shunt dynamo, the low resistance in the mains not allowing enough current to flow around the shunt fields to energize them.
Ques. Should the brushes be raised out of contact in starting?
Ans. The brushes should not be in contact in starting if there be any danger of reverse rotation, as might happen when the dynamo is driven by a gas engine. Aside from this, it is desirable that the brushes be in contact, because they are more easily and better adjusted, and the voltage will come up slowly, so that any fault or difficulty will develop gradually and can be corrected, or the machine stopped before any injury is done.
Fig. 674.--Connections for two shunt dynamos to run on the three wire system. The two machines are connected in series, three wires being carried from them, one from the outside pole of each machine and one from the junction of the two machines. The voltage between the outside wires is equal to the combined voltage of the two machines and the voltage between the outside and the central or neutral wire is equal to the voltage of the corresponding machine. If the load on each side of the system be equal, there will be no current in the neutral wire, while if the loads be unequal, the neutral wire will have to carry the difference in current between the two outside wires.
Ques. How should a series machine be started?
Ans. The external circuit should be closed, otherwise a closed circuit will not be formed through the field magnet winding and the machine will not build up.
Ques. What is understood by the term "build up"?
Ans. In starting, the gradual voltage increase to maximum.