Fig. 1,178.—The "Witham" charging board, for charging from any electric outlet on a direct current system. The instrument shows the direction of the current, and the candle power of the lamps used as resistance indicates approximately the strength of the current passing. Operation: From any convenient electric light fitting remove one of the lamps, replacing it by the plug attached to the flexible cord. Screw the lamp into one of the sockets on the charging board. Connect a wire to each binding post, and before joining up to the battery, hold the ends of the two wires together. The lamp will then light up and the indicator needle will point to that binding post which must be connected to the positive (+) terminal of the battery. The other binding post must, of course, be connected to the negative (-) of the battery. The charging current can be increased by inserting another lamp into the second socket on the charging board and by using lamps of higher candle power. If, when the lamp lights up, the indicator needle do not point to one of the binding posts, but retain its position midway, then the current is an alternating one and will not charge the battery.
Ques. What precaution should be taken with the jars?
Ans. They should be thoroughly cleaned with fresh water, no sediment being allowed to remain.
Putting Batteries into Commission.—When re-assembling a battery, it should be treated in the same manner as if it were new and the regular instructions for assembling and putting a new battery into commission followed.
Cleaning Jars.—The jars should be thoroughly cleaned with fresh water, no sediment being allowed to remain.
| 8 | hour | rate | .05 | volt |
| 6 | " | " | .065 | " |
| 4 | " | " | .09 | " |
| 3 | " | " | .11 | " |
| 2 | " | " | .14 | " |
| 1½ | " | " | .18 | " |
| 1 | " | " | .21 | " |
Condensed Rules for the Proper Care of Batteries.—The following general instructions should be followed in the care and maintenance of batteries:
1. A battery must always be charged with "direct" current and in the right direction.
2. Be careful to charge at the proper rates and to give the right amount of charge; do not undercharge or overcharge to an excessive degree.
3. Do not bring a naked flame near the battery while charging or immediately afterwards.
4. Do not overdischarge.
5. Do not allow the battery to stand completely discharged.
6. Voltage readings should be taken only when the battery is charging or discharging; if taken when the battery is standing idle they are of little or no value.
7. Do not allow the battery temperature to exceed 110° Fahr.
8. Keep the electrolyte at the proper height above the top of the plates and at the proper specific gravity. Use only pure water to replace loss by evaporation. In preparing the electrolyte never pour water into the acid.
9. Keep the cells free from dirt and all foreign substances, both solid and liquid.
10. Keep the battery and all connections clean; keep all bolted connections tight.
11. If there be lack of capacity in a battery, due to low cells, do not delay in locating and bringing them back to condition.
12. Do not allow sediment to get up to the plates.
Storage batteries are used for many purposes, such as to supply current for electric vehicles, gas engine ignition, lighting, and in connection with power stations and distribution work.
The latter is an important field, the storage battery being used in connection with the power station for the following purposes:
1. To carry the peak load, during hours of maximum demand;
2. To carry the entire load during hours of minimum demand, or for a short time in case of emergency;
3. To act as an equalizer;
4. For regulation of load and voltage;
5. As compensation for feeder drop;
6. As a preventive against shut downs.
In almost every electric lighting plant there are long periods during the day and late at night when the number of lamps lighted is so small that it may not pay to run the generating machinery. In such cases, storage batteries may usually be used to advantage to aid in carrying the maximum load and to supply the entire current at minimum load as illustrated in fig. 1,179. In other words, batteries are substituted for a certain portion of the machinery plant or are used in place of the latter.
Ques. What provision must be made in power plants when storage batteries are not used?
Ans. The capacity of the generating machinery must be sufficient for the heaviest overloads which may occur, and it must be operated continuously for 24 hours a day in the majority of central stations supplying current for lighting and power.
Ques. What results are obtained with this method of working?
Ans. The engines working under very variable loads, not only operate at low efficiency, but are continually subjected to severe mechanical strains.
Fig. 1,179.—Load curve showing use of storage battery as an aid to the generating machinery. In the diagram, it is seen that the battery discharges at minimum and maximum loads and is charged at other times, the battery furnishing current for the entire minimum load and part of the maximum load.
Ques. How may greater efficiency be secured with steam engines under variable loads?
Ans. Judicious selection of the number and sizes of the engines enable them to be worked in most cases at a considerable fraction of their full capacity nearly all the time.
Ques. What further improvement is secured in most cases with the storage battery?
Ans. The plant is made more flexible, and the economy of the engines is increased by making their loads nearer uniform, and nearer to full capacity while they are running.
Ques. What is the effect of a battery connected in parallel with a dynamo, as in fig. 1,180?
Ans. It is not necessary for the dynamo to have a capacity exceeding that which is sufficient for the average daily load, at which it may be worked practically all the time.
Fig. 1,180.—Storage battery connected in parallel with a dynamo. This arrangement enables the dynamo to be stopped for a considerable portion of the time, and thus save labor and attention. It also acts to prevent fluctuations as in a dynamo driven by a gas engine whose speed varies periodically because of the nature of its cycle of operation.
When the load is below the average, the dynamo charges the battery, and when the load rises above the average, during the hours of maximum demand, the battery discharges into the line in parallel with the dynamo. During the hours of minimum demand the engines may be shut down and the necessary current supplied from the battery alone, thus not only increasing the efficiency of the plant, but serving to maintain a steadier pressure under fluctuating loads.
Ques. What is understood by the expression "floating the battery on the line"?
Ans. A storage battery is said to float on a line when connected across the circuit at some distance from the power station, so that a heavy load on the line, within the range of the battery influence, causes sufficient line drop to allow the battery to discharge, while with a light load on the line, the drop is small and the impressed voltage at the battery high enough to charge the battery. This usage is confined chiefly to electric railway service, where large voltage changes are permissible.
Fig. 1,181.—Diagram showing effect of storage battery in regulating the dynamo load in a combined railway and lighting plant. In this case the average and line loads are about equal and the battery covers the instantaneous fluctuations. It will be noted that while the line load fluctuations vary between 780 and 1,420 amperes, those of the dynamo load are kept at an average between 1,030 and 1,160 amperes.
Ques. When the battery is floated on the line, how may the amount of charge be made to approximately equal the amount of discharge?
Ans. By properly proportioning the number of cells in series.
Connections and Circuit Control Apparatus.—When a storage battery is used in an electric lighting plant, provision must be made for feeding the lamps, etc., from either the dynamo or battery separately, or from the two working in parallel, and it should be possible to charge the battery at the same time the lamps are being supplied. To accomplish these results requires three switches, for the following connections:
1. To connect the lamps to the dynamo;
2. To connect the lamps to the battery;
3. To connect the battery to the dynamo.
Fig. 1,182.—Diagram showing action of storage battery as a reservoir of reserve power. The figure shows an actual load curve from an Edison station for 24 hours. A sudden storm caused the load to be thrown on very quickly, the peak of the load being higher than usual.
In some plants, the first switch is omitted, because the lamps are always fed by the battery alone, the latter being charged during the day, when no lamps are in use.
It is desirable, however, to have all three switches in every plant in order to be able to supply lamps and charge the battery at any time.
In the battery circuit there should be an ammeter having a scale on both sides of zero, to show whether the battery is being charged or discharged, as well as the value of the current. Another similar ammeter is required in the circuit between the dynamo and the battery, to show the direction and amount of current. A third ammeter is desirable in the lamp circuit, to show the total current supplied to the lamps, but it need only indicate on one side of zero, since the current there always flows in the same direction.
Fig. 1,183.—Diagram showing three wire system with one dynamo and storage battery. A 220 volt dynamo charges a storage battery of corresponding pressure, which in turn subdivides the pressure and supplies a three wire system, the neutral wire of which is connected to the middle point of the battery as shown.
A voltmeter is required with a three-way switch to connect it to the dynamo, battery or lamps, and a circuit breaker must be inserted in the battery circuit in order that it may be opened when the current becomes excessive.
A discriminating cut out or reverse current circuit breaker is required between the dynamo and the battery to open the circuit when the charging current falls below a certain value, and thus avoid any danger of the battery discharging through the dynamo, if from any cause the voltage of the latter drop below that of the battery. This completes the ordinary measuring and circuit controlling apparatus employed with storage batteries.
Methods of Control for Storage Batteries.—As the external voltage of a storage battery varies with the amount of charge it contains and with the direction of the current, it is necessary to employ some means for compensating this variation in order to maintain a constant voltage on the line supplied by the battery. The various devices used for this purpose are as follows:
1. Variable resistances;
2. End cell switches;
3. Reverse pressure cells;
4. Boosters.
Fig. 1,184.—Diagram showing connections for ignition outfit. The charging switch has four indications—"Off," "Battery," "Dynamo" and "Charge." When engine is at rest switch is turned to "Off." The first turn brings it to "Battery," enabling the engine to be started. Next turn cuts battery off and puts "Dynamo" direct on engine. The next turn brings the switch to "Charge." Dynamo then charges the battery and surplus current is stored up. Next turn is "Off," which stops engine and disconnects battery from dynamo. Test the dynamo wires with test paper (negative makes mark). Put positive of dynamo to positive of battery. Dynamo should be regulated to charge at about four amperes.
The particular method selected will depend upon the size of the battery, the purpose for which it is used, the allowable limits pf current and voltage variations, the cost of the system, etc.
Variable Resistance.—Regulation by variable resistance may be used advantageously only with batteries of small capacity, and in small lighting plants such as those of yachts, where the space available for battery auxiliaries is limited, and where the cost of energy is so low that the loss of power in the resistance is not objectionable.
Fig. 1,185.—Variable resistance method of regulation for storage battery; diagram showing connections for charging two halves of a battery in parallel.
The connections for one of the simplest methods is shown in fig. 1,185. The battery is divided into two halves, which are connected in series for discharging and in parallel for charging. Since the voltage of each cell at the end of a discharge should not be lower than 1.8 volts, a battery intended for use on a 110 volt lighting circuit will require 110 ÷ 1.8 = 62 cells. The voltage necessary, however, for each cell at the end of a charge is about 2.6 volts, or a total of 2.6 × 62 = 161 volts for the battery, a value which is far above the line voltage. By dividing the battery into two halves and connecting them in parallel only 80.5 volts are necessary for charging. The excess voltage of the line, 29.5 volts is taken up by the resistance, which also controls the output of the battery on discharge.
End Cell Switches.—These may be used to advantage in small installations where there is not demand for current during the day, or where the charging is done by means of boosters.
Fig. 1,186.—Diagram of connections of a battery equipment for a residential lighting plant. In the diagram the voltmeter and voltmeter connections have been omitted. The bus bars on the battery panel are connected directly to the bus bars on the dynamo panel. In this installation the dynamos are run during the afternoon on discharge, being regulated by means of an end cell switch. On charge, the pressure above that of the bus bars, required to bring all cells up to full charge, is supplied by means of a motor driven charging booster, the voltage at the armature being suitably varied by changing the field excitation.
Ques. What is an end cell switch?
Ans. A form of switch employed in connection with a storage battery in order to control the end cells for regulating the voltage.
Ques. Describe the construction of an end cell switch.
Ans. This is shown in fig. 1,187. The switch contact arm is made in two parts, A and B, which are insulated from each other as shown, and connected with each other through the protective resistance R. The end cell contacts are so spaced that when the main current carrying part A of the switch arm is squarely on one end cell contact such as X, the part B, does not touch any other contact such as Y, but when the switch arm is advanced for cutting into circuit another end cell, the part B, reaches the contact Y before the part A, leaves the contact X, thus keeping the battery circuit closed, while the resistance R, limits the current in the short circuited cell at the instant the switch arm passes from one end cell contact to the next.
Fig. 1,187.—Diagram of end cell switch. This form of switch controls several cells at one end of a storage battery and is used for regulating the voltage. The requirement of an end cell switch is that in switching from one end cell contact to another, the discharging circuit must not be opened, neither must the moving arm touch one contact before leaving the one adjacent, since the joining of two contacts will short circuit the cells connected thereto. To accomplish this, the spacings of the two arms and contacts are such that when the main arm A is squarely on an end cell contact, the advance or auxiliary arm B touches no other contact, but in passing from one point to the next, the advance arm reaches the contact toward which it is moving before the main arm leaves its contact. The resistance X, between the two points prevents short circuiting, and the current to the main circuit is never broken.
Ques. How should the conductors joining the end cells to the end cell switch contacts be proportioned?
Ans. They must have the same sectional area as the conductors of the main circuit.
The reason for this is that when any end cell is in use, the conductor connecting it to the switch becomes a part of the main circuit. An allowance of 1,000 amperes per sq. in., when the battery is discharging at the two-hour rate, is considered good practice.
Ques. Describe some of the features of end cell switch construction.
Ans. Those of small capacity are made circular; the larger sizes are made horizontal in form, and both types may be either operated by hand or motor driven.
Fig. 1,188.—End cell switch control for storage battery; connections showing main line open when the battery is being charged.
Ques. Where are end cell switches of large capacity located?
Ans. Generally they are placed as near the battery room as possible to avoid the cost of running the heavy conductors, and when such switches are motor driven, the usual practice is to control their operation from the main switchboard.
In fig. 1,188 is shown the method of regulation with an end switch. The diagram shows the battery being charged with the main switch open, and the voltage of the dynamo raised to the charging pressure. During discharge the cells are connected in series, and as the voltage of each cell at the beginning of discharge is at least 2.1 volts, only 52 or 53 cells are required to give the desired pressure of 110 volts, but as the discharge continues, and the voltage of each cell decreases, the end cells, 1, 2, 3, 4, etc., are cut into circuit successively by means of the end cell switch, thereby adding to and compensating the drop in the total voltage until, at the end of discharge when the voltage of each cell has fallen to 1.8 volts, the entire 62 cells are in series to supply the required line pressure.
Fig. 1,189.—Diagram of connections arranged for charging battery in two parallel groups and discharging in series, the charge and discharge being controlled by variable resistances. In yacht lighting the limited space generally prohibits the use of a charging booster, and in such instances this method of charge and discharge control is the usual practice. In case the dynamo from which the battery is charged has sufficient range in voltage to charge all cells in series, a charging booster is not required, nor is it necessary to connect groups of cells in parallel, as the dynamo voltage may be varied as charge proceeds.
For a 110 volt circuit, the number of cells required is 110 ÷ 1.8 = 61, and the number in series when the battery begins to discharge is 110 ÷ 2.1 = 52. Hence, in a 110 volt circuit an arrangement must be provided whereby 61 - 52 = 9 cells may be cut out or switched in, one by one.
The number of end cells for any voltage may be obtained by the following formula:
Number of end cells = (E/1.8) - (E/2.1)
E = voltage of supply circuit;
1.8 = minimum voltage of cell during discharge;
2.1 = voltage of fully charged cell.
Reverse Pressure Cells.—These consist of unformed lead plates immersed in the ordinary electrolyte of dilute sulphuric acid. As they have no active material, they possess no capacity, but are capable of setting up an opposing pressure of about 2 volts each to the discharging current flowing through them, thereby cutting down the total voltage of the battery, so that the net voltage across the line depends on the number of reverse current cells in series in the battery circuit. As the voltage of the battery falls during discharge, the reverse pressure cells are cut out, successively, thus keeping the external or line voltage constant.
Fig. 1,190.—Regulation with reverse pressure cells. These cells are merely lead plates placed in an electrolyte of dilute sulphuric acid. They have no capacity but set up an opposing or reverse voltage of approximately 2 volts per cell if current be passed through them. In using these cells for controlling discharge, the total number of active cells in the battery will be the same as if the method of end cell control had been used. Reverse pressure cells represent an increase in equipment of about 8 per cent. or more. These cells, as shown, are connected in the circuit in opposition to the main battery, and conductors are run from each of them to points on a switch similar to an end cell switch. At the beginning of discharge, all the reverse cells are in circuit, acting in opposition to the main battery. As discharge proceeds and the battery voltage falls, the reverse cells are gradually cut out of circuit. The only advantage in this method of regulation is that the discharge throughout the battery is uniform, but this fact alone does not warrant such means of regulation on account of the additional expense involved, and the energy loss when discharging against reverse cells is the same as if resistance had been placed in the circuit.
It is obvious, that as these cells do not possess any capacity, the number of active cells required in the battery will be the same as when end cell control is employed. Therefore, the reverse pressure cells represent an increase in equipment, which entails an additional expense of at least 8 per cent. For this reason, and also on account of the fact that the amount of energy lost in discharging against reverse pressure cells, is the same as when the resistance methods of controlling the discharge are employed, the use of cells for this purpose is now practically obsolete.
Fig. 1,191.—Holzer-Cabot dynamotor (type K). A dynamotor is a combination of dynamo and motor on the same shaft, one receiving current, usually of different voltage, the motor being employed to drive the dynamo with a pressure either higher or lower than that received at the motor terminals. A machine of the dynamotor form, with its windings exactly alike, is often used in three wire systems to balance or equalize the two halves of the circuit as in fig. 798.
Boosters.—In general, a booster may be defined as a dynamo inserted in series in a circuit, to change its voltage. It may be driven by an electric motor, in which case it is sometimes called a motor-booster. The function of a booster is to add to an electric pressure derived from another source.
For instance, if a storage battery be used in conjunction with one or more dynamos to supply current to an electric light installation, the battery cannot be charged from the machines which are feeding the lamps, because it requires a pressure higher than that required for the lamps to complete the charge. A small dynamo is therefore connected in series, with the main machines and the battery, acting in conjunction with the former to provide the necessary pressure.
Fig. 1,192.—Dayton launch lighting outfit. It consists of an "Apple" dynamo, switchboard and storage battery. The dynamo is fitted with a bevel friction drive governor. The dynamo gives a three ampere charging rate on a six volt battery at its normal speed of 1,050 R. P. M. The switchboard is provided with a combination volt-ammeter which shows the voltage of the battery, the ampere charging rate of the dynamo and the ampere discharging rate of the battery. The automatic cut out in the back of the switchboard automatically severs the connections between the dynamo and the storage battery when the engine stops and so prevents the storage battery current running back through the dynamo when the dynamo is not generating current. A 6 volt, 60 ampere hour battery, consisting of 3 five plate units connected in series, is used with the size dynamo shown in the illustration.
The power for running such a dynamo is obtained in various ways. The dynamo or charging booster may be belt driven or arranged on an extension of the armature shaft of the main dynamo; again, it may consist of a single armature with a double winding (fig. 1,191), or a motor and dynamo coupled together on one bed plate as in figs. 800 and 805. Boosters may be divided into several classes as follows:
1. Series boosters;
2. Shunt boosters;
3. Compound boosters;
4. Differential boosters;
5. Constant current boosters;
6. Separately excited boosters.
Series Boosters.—The series booster acts so as to compound the battery, and tends to maintain a constant voltage on the line, whatever the load may be. Its operation depends on the fact that the dynamo voltage must rise and fall with the load. It can, therefore, be used only with a shunt dynamo or its equivalent as the source of supply.
Ques. What use is made of the series booster system?
Ans. It is suited to power, but not to incandescent lighting purposes, being similar in operation to a floating battery. It is not extensively used as the other types give better service, under the same conditions.
Fig. 1,193.—Diagram of Joseph Bijur's storage battery system (General Storage Battery Co.). The booster field winding has one terminal connected to the middle point of the battery and the other terminal, to the wire joining the resistances A and B. A lever, pivoted at L, carries at either end a number of contact points which dip into troughs of mercury when one end of the lever moves upward or downward. These points are connected to corresponding points on their respective resistances, and therefore all of the resistances connected to contact points which are immersed in the mercury are short circuited. The points are of various lengths, so that when the lever operates, they contact progressively with the mercury. If more of the A points than the B points be immersed in the mercury, the resistance of B is less than that of A, more sections of it being short circuited. Current will therefore flow from the middle point of the battery, through the booster field, and through B to the negative side of the system, exciting the booster field and producing a booster voltage to charge the battery. Again, if more of the A points be immersed, the A resistance becomes the smaller, and current then flows from the positive side of the system through resistance A, through the booster field to the middle point of the battery, the field excitation and the booster pressure produced being in a direction opposite to the first described, and tending to discharge the battery. When the resistances A and B are equal, there is no pressure to send current in either direction through the booster field coil. When the load on the external circuit is normal, the lever is in a horizontal position, A and B being equal, no current flows through the booster field hence, no current passes into or out of the battery. With increase of external load, the pull of the solenoid is strengthened by a small increase in dynamo current passing through the winding. This draws down the left end of the lever producing a current in the booster field such as to discharge the battery and assist the dynamo to supply the load demand. A decrease in external load is attended by a slight diminution in dynamo current, the solenoid is weakened and the pull of the spring predominates. This results in a downward movement of the right side of the lever causing excitation of the booster field to produce a pressure to send charge into the battery.
Ques. Describe some characteristics of the series booster.
Ans. It is automatic and adjusts its voltage to produce the proper ratio of charge or discharge with varying external load, and it also tends to maintain a constant voltage across the line, under all conditions of change in circuit.
Fig. 1,194.—Load diagram, showing kind of service to which the shunt booster is adapted.
Shunt Boosters.—This type of machine is simply a shunt dynamo, having its armature circuit in series with the line from the main dynamo to the battery. A rheostat controls the field excitation. Its function is to send charge into the battery. It is used in plants where the battery is not designed to take up load fluctuations, but is in service only to carry the peak of the load, being charged during periods of light loads and discharged in parallel with the dynamo.
The shunt booster acts to increase the voltage applied to the battery so that the charging current will flow into the latter.
Ques. How is a battery used with a shunt booster proportioned?
Ans. Usually sufficient battery is provided to carry the entire load during the light load period.
Ques. Explain the use of the rheostat controlling the field excitation.
Ans. It is used to vary the booster voltage so as to hasten the charging of the battery if desired.
Fig. 1,195.—Entz' carbon pile booster system (Electric Storage Battery Co.). The booster field winding is connected at one end to the middle point of the battery. The other end is connected to the upper contact points of two carbon pile resistances, A and B. The lower end of A is connected to the negative side of the battery, and the corresponding end of B, to the positive side. This arrangement constitutes in effect a potentiometer. If the resistance of A be equal to that of B, there is no pressure in the booster field to establish current through it. The drop through A + B is equal to the total battery voltage, and if A = B, the drop from either side of the battery through A or B is one-half the total drop, hence the end of the booster field winding, connected to the upper ends of A and B is also at the pressure of the middle point of the battery which is likewise the pressure of the other side of the booster field coil. Accordingly when A = B, there can be no current through the coil. When the two resistances are unequal, there will be current through the booster field, its direction depending on which of the resistances is the less, and its magnitude will be proportional to the difference between the two resistances. Variations in the pressure on a carbon pile causes variations in its resistance and the solenoid, M, opposed by spring S, both pulling on lever L which rests on the two piles A and B, controls the relative resistances of the two piles to cause charge and discharge of the battery. The solenoid winding is in series with the dynamo circuit and when the load is normal, the spring pull is just equal to the magnet pull, and the resistance of A and B are equal. When external load varies, a small but proportional variation in the pull of P charges the relative resistances of the piles and the booster field is energized to produce a voltage to cause battery charge or discharge.
Ques. For what service is the shunt booster not suited?
Ans. It is not adapted to circuits where there are sudden fluctuations that are great compared with the capacity of the dynamo.
Ques. What is its action in changing from charge to discharge?
Ans. It is not automatic, the switching must be done by hand.
Fig. 1,196.—Diagram showing usual connections of a non-reversible shunt booster and battery system. In charging, the switches A and B are closed, and C put on contact m; the end cell switch D is put on the last contact. Part of the dynamo current will go into the line and part through the booster into the battery. The charging current is adjusted by the field rheostat E. To discharge, throw the end cell switch D to first contact; next turn switch C to contact s. The battery is then in parallel with the dynamo with all end cells cut out. As the voltage of the battery falls, end cells are cut in by the end cell switch D.
Ques. How may it be used reversibly?
Ans. It will give a pressure to assist the battery to discharge when excited from the bus bars and provided with a reversing rheostat.
In this case it will assist the battery to discharge when the direction of the field magnetization is changed. When so used, no end cells are necessary, but the booster must be run continuously during the entire period of discharge.
Ques. What should be the battery capacity on a 110 volt circuit with a reversible booster?
Ans. 56 cells will be sufficient.
The voltage to fully charge is 56 × 2.6 = 146, or 36 volts above dynamo voltage. Minimum voltage of discharge = 1.8 × 56 = 100 volts, or 10 volts less than that of the line. Hence, the booster need give only 36 volts maximum, and is required to add 10 volts to the battery voltage toward the end of battery discharge. In this case, the booster voltage is only 36/49 or about ¾ of that required in the preceding case; five cells less of battery are necessary and the end cell switches and leads are eliminated.
Fig. 1,197.—Diagram of compound booster connections.
The machine will be larger, however, than it would be if used only for charging, because the discharge current is unusually greater than that of charge, and the current carrying of the armature must be great enough to take care of the heaviest currents.
Compound Boosters.—These machines are used on railway and power circuits where there are great fluctuations in load, the battery acting to prevent excessive drop and to assist the generating machinery in carrying the load, relieving it from the strain of sudden rushes of current.
The connections are shown in the diagram fig. 1,197. Under ordinary working conditions, the shunt field of the booster creates an electric pressure in the same direction as that of the battery, tending to discharge it.
Fig. 1,198.—Fairbanks-Morse lighting outfit. The above cut illustrates a 2 horse power vertical special gasoline or kerosene oil engine belted to a .9 kw. compound wound 32 volt dynamo. It will supply a maximum of 42-20 watt, or 50-15 watt 32 volt Tungsten lamps and is built and balanced, so that current can be taken direct from the dynamo without flicker in the lights. The storage battery has 16 cells and a capacity of 4½ amperes for 7½ hours at 32 volts. This will supply seven 20 watt Tungsten lamps for 7½ hours, or nine 15 watt lamps for 7½ hours. The switchboard is arranged so as to give 24 hours service. It is customary to run the engine during most of the lighting period and to use the battery for lights late at night. If the whole number of lights be not used when the engine and dynamo are in operation, the surplus is used to charge the battery.
When no current is flowing into or out of the battery, the following relation exists:
Dynamo voltage = booster voltage + battery voltage
In this case the dynamo carries the whole external load. If the load increase, the dynamo voltage decreases, so that the booster voltage + battery voltage is greater than the dynamo voltage, and the battery begins to discharge.
In discharging, the current passes through the series field of the booster and produces a proportional pressure acting with the shunt field to raise the voltage of the booster, thus increasing the battery discharge and shifting more of the load from the dynamo, until the system becomes balanced.
Fig. 1,199.—Diagram showing method of charging a storage battery at one voltage and supplying lights at a different voltage. As may be seen, two end cell switches are required. The voltage of the supply current is adjusted by the number of cells in series on switch S', while switch S is moved to cut out cells as they become fully charged. In this instance the end cells included between the contact arms of the two end cell switches must be of sufficient size to receive the charging current, plus the current to the supply circuit. If the battery can be charged at times when the dynamo is supplying no other load, only one end cell switch is required.
If the load on the external circuit be small, the dynamo voltage rises and current flows into the battery. In this case the series field acts against the shunt field and decreases the booster voltage so that the pressure at the dynamo is greater than booster and battery voltage combined, thus increasing the rate of charge of the battery until the load causes the dynamo voltage to drop to normal and the system is again balanced.
The battery and booster can be placed at the power house or where the greatest drop is likely to occur. As this system, like the series booster, depends for its action upon the drop of voltage with increase of load, it is only adapted to shunt wound dynamos.
From the foregoing description it will be seen that the compound booster is automatic within certain limits of battery charge. Any marked change of battery voltage will be followed by a corresponding change in dynamo current, unless the rheostat be manipulated to bring battery voltage + booster voltage back to normal.
While the theoretical dynamo current variation is small for a given change of load, there is always a sudden, momentary, current rush from the dynamo on increase of load, the duration of which is equal to the time lag of magnetization of the booster field.
Lights on a circuit with variable load will "wink" on sudden changes of load. In this respect the compound booster is not so satisfactory as the constant current booster, as in the latter all dynamo current passes through the series fields, which, by reason of their self-induction, oppose and check any sudden current rush, giving the booster field time to change its magnetization to the proper degree.
Fig. 1,200.—Diagram of connection of one form of differential booster. In operation, the dynamo current passes through the series winding of the booster, and the current in this winding is to remain practically constant. The shunt coil produces a field which opposes the field produced by the series coil, the resulting magnetization being, in direction and amount, the resultant of the two field strengths. The adjustments are so made that when the normal dynamo current is passing through the series coil, the shunt field just neutralizes its effect, and the resultant magnetization is zero. Since the open current voltage of the battery is equal to that of the system, neither charge nor discharge takes place. With increased demand on the line, the slight increase in dynamo current in the series coil overpowers the shunt field, and causes a pressure in the booster armature in such direction as to assist discharge. If the external load fall below the average demand, the current in the series coil decreases slightly so that the shunt field predominates, producing a booster armature pressure in a direction to assist charge. Although the voltage of the battery falls while discharging by an amount proportional to the outflowing current the increased excitation due to this current through the series coil is also proportional to it, and the booster voltage rises as that of the battery falls, their sum being always equal to that of the system. In other words, the booster serves to compound the battery for constant pressure.
Differential Boosters.—In this type of booster, a series coil energized from the main current, tends to discharge the battery, and a shunt coil, excited from the battery, tends to charge the cells. These two coils are opposed to one another, and the difference in their respective strengths represents the net strength available for boosting. In order to produce quicker reversal, additional compound coils are sometimes added.