CHAPTER XVI.
CURRENT CONTROL FOR ARC LAMPS.
_Voltages Required._--The commercial distribution of electrical energy is at voltages of 110 or 220, in most cases, and occasionally runs as high as 550. The direct-current arc requires for its best operation a voltage of from 45 to 50, while the alternating-current arc uses from 30 to 40. In order to secure satisfactory operation of arc lamps, it becomes necessary to provide some means of reducing the voltage at the arc to the proper amount.
_Resistance Control._--The simplest method and the one universally applicable is that of inserting resistance in series with the arc. The drop in voltage is equal to the current multiplied by the resistance; hence, if we wish to decrease our voltage, say 65 volts, as would be the case in connection with a 110-volt circuit and a 45-volt arc, using 25 amperes, we should need a resistance of 2.6 ohms. Twenty-five times 2.6 equals 65 volts lost, which leaves 45 volts to operate the arc with. In connection with arc lamps, however, it is not only necessary to lower the voltage but some provision must be made so that the current, when the electrodes are brought together, will not become excessive. At the time the arc is struck, i.e., at the time the electrodes are brought in contact with each other, the current is limited only by the extra resistance in the circuit, for the electrodes then form a short circuit. In the above case, 110 volts and a rheostat with 2.6 ohms resistance we should obtain, during the time the electrodes are together, a current equal to 110 divided by 2.6 which equals about 43 amperes. If it were not for this resistance the current would rise to several times this value and blow out any fuse we might provide. It is not necessary that this resistance be in any particular place; if we have a very long run of small wire from the service to the arc, there may be sufficient resistance in this so that very little extra resistance is required. Somewhere, however, there must be some provision inserted in the circuit to prevent the current from becoming too great at the time the electrodes are brought together. In passing it may be noted at this point that an arc can be started without any resistance in the circuit by bridging the space between the electrodes with a small fuse wire which will melt the instant the current is turned on and establish the arc.
The resistance method is very wasteful of energy, as the following tabulation will show; but with direct currents it is the only method available, unless we are willing to provide a motor generator to give us the proper voltage. For alternating currents, resistances are not much used, except with traveling shows where the portability of the control as well as its fitness for all possible conditions is an important consideration.
TABLE IV.
SHOWING WASTE OF ENERGY WITH USE OF RESISTANCES FOR VARIOUS VOLTAGES.
+-------+---------+------------+--------------+ | Volts | Current | Watts Lost | Useful Watts | +-------+---------+------------+--------------+ | 110 | 30 | 1950 | 1350 | | 220 | 30 | 5250 | 1350 | | 550 | 30 | 15150 | 1350 | +-------+---------+------------+--------------+
The tabulation in Table IV shows that the higher our voltage the greater the loss of energy caused by the use of resistances. The figures apply, as given, only to cases where only a single arc lamp is used. Where several can be used in series the loss due to high voltage need not be greater than with lower voltage.
In Figure 134 we have shown diagrammatically the usual representation of a resistance. The more wire there is in circuit, the higher will be the resistance and the greater the drop caused by a given current. If we lengthen the arc, the current will be somewhat decreased and the drop in voltage over the resistance will be less, thus allowing a rise in the voltage at the terminals of the lamp. The energy lost in resistances takes the form of heat and all resistances, used for the control of arc lamps, give off much heat and must be located in safe places. The heating also makes them objectionable in small operating rooms in summer, but somewhat welcome in winter. The heat generated in a wire is proportional to the square of the current; hence if we double the current through a certain resistance we shall have four times the heat.
If several resistances are connected in series the total resistance will be equal to the sum of the individual resistances, and the current will be correspondingly decreased. If we wish to get more current than can be obtained with the use of one resistance we may connect up two or more in parallel. Two equal resistances connected in parallel will give approximately double the current that can be obtained through one of them.
_Reactance Control._--A reactance, shown diagrammatically in Figure 135, may take the place of the resistance in alternating-current circuits and is preferable because it wastes comparatively little energy. It lowers the voltage over the arc but its operation depends upon a counter e.m.f. which opposes the impressed e.m.f. of the circuit and must be subtracted from the latter. The nature of reactances as well as of transformation, etc., has been fully treated in another work of the authors’, entitled, “Alternating Current Theory, Practice and Diagrams” and would carry us too far were it to be discussed in this work. Every reactive coil is made up of copper wire wound upon an iron core and contains both resistance and reactance. So far as the resistance in it is concerned, this causes a waste of energy, but it is always very small. There is also a waste of energy due to the hysteresis and eddy-current losses in the iron, but this is also small.
The reactance is proportional to the square of the number of turns of wire, if the iron core is fixed, and may be controlled either by adjusting the position of an iron core in a helix, or by adjusting the number of turns of wire around a fixed iron core. The light obtainable through a reactance is not of the best quality and reactances are not much used.
_Transformation Control._--Another method of lowering the voltage is by means of the transformer. A diagram of an ordinary transformer winding is given in Figure 136. The fine-wire winding is the primary winding or coil, if the transformer is used to lower the voltage; and the other is known as the secondary winding or coil. The energy in both coils of the transformer, neglecting the iron and copper losses, is always exactly equal. The ratio of voltage between the primary and secondary terminals is in direct proportion to the number of turns of wire in each. If there are half as many turns in the secondary winding as in the primary, the voltage will be just one-half, but the current will be double. The transformer is self-regulating, within the limits of its capacity, and whatever energy is taken from the secondary, the primary will automatically supply.
A transformer must be specially built for the voltage and frequency, at which it is to be used; but many of them are provided with taps, such as shown in Figure 136, by which small adjustments of voltage or current can be made. A transformer must always be connected so that the switch, when open, will disconnect the primary wires. If these remain closed there will be a small current through the primary winding which will mean a considerable waste of energy.
_Auto-transformer Control._--The auto-transformer is a special type of transformer used to obtain reduced voltage and increased current. Its principle may be gathered from Figure 137. There is an iron core and two coils of wire as in ordinary transformers but the two coils are connected in series, as shown in the figure. It will also be noted that the arc is connected directly across one of the coils. The lower portion of the winding or coil is traversed by the alternating current from the mains at all times and this current also passes through the arc lamp when the circuit through it is closed. The current passing through the lower coil and the arc induces a current in the upper portion of the winding and these two currents then pass in parallel through the lamp.
When the arc circuit is open, both coils are in series and act as choke coils so that but very little current is used. The auto-transformer may be designed to reduce the voltage to any desirable amount and the current will be correspondingly increased, neglecting all losses.
If the reduced voltage were to be obtained from an ordinary transformer, the secondary coil would be called upon to carry the full current used by the lamp, while with this connection it carries much less. If the two coils are equal, the voltage will be reduced one-half, the current will be doubled, and only half of the current will pass through the secondary coil. The nearer equal the primary and secondary voltages are, the greater the saving in copper in the secondary coil. If it were intended to transform from 110 to 100 volts, the capacity of the secondary winding would need to be only one-eleventh of the total capacity. The auto-transformer is a very useful device but on account of the fact that the high voltage exists in all of its parts, it is not safe to use with the high commercial voltages outside.
If the auto-transformer is connected as shown in Figure 138, it can be used to raise the voltage; but in this case the current will be decreased. These auto-transformers, as well as the ordinary transformer, must always be connected to the source of energy by means of a switch so that they may be disconnected when not in use; otherwise there will be a small current in the primary circuit all the time which will show up quite strong on the watt hour meter. Transformers and auto-transformers are arranged to be portable. A general view is given in Figures 139 and 140; the former being the Edison and the latter the Fort Wayne.
_Motor-Generator Control._--The proper voltage for the operation of arc lamps can be obtained by the use of motor-generators. A motor-generator is a generator driven by a motor, the two armatures being placed upon one shaft or belted together. The motor may be driven by a current of any voltage desired. The diagram of such an outfit for direct current is shown in Figure 141. This type of machine is used, as a rule, only where the supply voltage is much higher than that used at the arc. Resistance must be used at each arc lamp.
Figure 142 shows the connections of an alternating-current to a direct-current motor-generator of the Fort Wayne Electric Company. The switch _A_ is used to start it and is shown connected to a three-phase line. Aside from the field winding there are three wires leading to the generator. The wire _B_ carries a compound winding inside of the generator which opposes the magnetization of the shunt winding. The wire _C_ carries another compound winding which is arranged to strengthen the shunt field. _D_ is a box containing two resistances, one for each arc lamp shown.
If only one lamp is to burn, the switch _E_ is closed and the arc started in the usual way. When ready to change to the other arc lamp, switch _E_ must be opened, the switch on the second arc lamp closed, and the arc struck. Then extinguish the first arc and close the switch _E_ again. If both lamps are to be used continually, switch _E_ must be left open.
As long as current is used through wire _B_, there is no loss of energy in any resistance and should the current in the arc rise, as when the electrodes are brought together, the increased current in the series winding, cut into this wire, would weaken the field and thus keep the current down. When current is used through the wire _C_, the series field winding strengthens the field and builds up the voltage sufficiently so that the lamps may be operated through the resistances. The field strength may be further regulated by the rheostat _R_.
Another connection of the Fort Wayne motor-generator is shown in Figure 143. In this case the lamps may be operated either from the compensarc _C_ or the generator. By throwing either one of the switches connected to the arc lamps up, the corresponding arc lamp is connected to the compensarc. By throwing the switch down it is fed from the generator. The lamp, by which the picture is being projected, should be fed from the generator and when nearly ready to change, the other may be started on the compensarc. This lamp will burn with a short arc and when it is connected in parallel with the one on the generator, it will immediately extinguish the latter. No attempt must be made to burn both arcs from either the compensarc or the generator. This generator is also wound to protect itself against an overload.
Where these connections are to be installed, it will be best to consult the local inspection departments concerning the necessary fusing for the compensarc and the generator. In some localities the possibility of throwing both arcs on either compensarc or generator, where these have capacity for but one arc at a time, will be considered very objectionable.
Another combination of motor and generator sometimes used is shown in Figure 144. By tracing out the circuits it will be seen that the armatures of both are in series and that the electrodes, when they come together, form a shunt about _B_. With the electrodes separated, if current is turned on, it must pass through both armatures in series. Thus the counter e.m.f. of both armatures opposes that of the line and they operate at a certain speed. Each motor has a natural tendency to send current in opposition to that impressed upon it by the line. If then the electrodes are brought together, they at once form a short circuit around the armature of _B_. The current in _B_ reverses and it then begins to act as a generator and sends current through the arc lamp. The current which passes through the armature of _A_ also passes through the arc lamp. _A_ is then a motor and operates _B_ as a generator.
The voltage at the arc is less than the line voltage by as much as the counter e.m.f. of motor _A_ amounts to, neglecting the drop in voltage due to resistance. No resistance is needed if the winding is properly arranged and there is not the less in heat which goes with the use of resistances. This arrangement can be used with direct-current circuits only. It is not suitable where the supply voltage is very much higher than the voltage used at the arc. A field rheostat is provided to adjust the field strength of _B_. _A_ is equipped with the ordinary motor-starting rheostat only.
_Rotary Converter Control._--This is a machine used only where the supply is alternating current. The voltage delivered to the converter must be the same as that desired at the direct-current terminals. This machine has an armature essentially similar to that of a direct-current dynamo. Alternating current is supplied to it at one set of terminals and direct current is taken from the others. This armature acts as motor and generator at the same time. Whatever voltage regulating is necessary with this machine must be done on the alternating-current side. Changing the field strength does not materially affect the voltage so that no means for regulating the field strength is provided.
The polarity of the direct-current terminals depends upon the position the armature happens to be in when the alternating current is applied to it and is very apt to come in wrong when the machine is started. It is therefore necessary to have a polarity indicating voltmeter in the circuit and to watch it when starting the machine. If the polarity is wrong, the switch must be opened and in a moment thrown in again; and if still wrong, this process must be repeated until the polarity comes right. Each arc lamp fed from a converter must be equipped with resistance.
The Martin rotary converter is especially designed for motion-picture work and may be provided with the proper connections for either single-phase, two-phase, or three-phase work. There is a stator ring which entirely surrounds the armature. This ring is made up of laminated disks with squirrel-cage bars and slots alternating. The squirrel-cage bars are joined at the end to a copper bar and it is by the aid of this squirrel-cage that the motor may be started and brought into step. The squirrel-cage also prevents “hunting” which is one of the common troubles experienced with synchronous motors or converters. Into the slots are wound special compensating coils to balance the armature reaction and keep the neutral point in constant position from no load to full load. This prevents sparking at the brushes. On the outside of this damper ring or squirrel-cage winding is the regular shunt-field winding used with direct-current motors or generators.
Figure 145 is a diagram showing the connections of the Martin Rotary Converter as installed by the Northwestern Electric Company of Chicago. This switchboard is equipped to operate two moving-picture arcs, two dissolving stereopticon lamps, and one spot light. Each lamp is provided with a throw-over switch so that current may be used, either from the alternating-current mains direct or from the direct-current side of the converter.
Figure 146 is another panel board for moving-picture work made up by the same company. In this case resistances are provided for use when the arc lamps are operated from the converter. In case it is desired to run from the alternating-current mains, transformers or compensarcs are used. The emergency feature of these panel boards is highly to be recommended. It must be borne in mind that one may suddenly be forced to deal with an operator who has never seen a converter and knows nothing of its operation; and there is also always the possibility of some trouble with the machine.
A Martin rotary converter to be operated from a single-phase line is shown in Figure 147. This machine is started through the commutator side. In order to start this machine it is necessary first to close the main switch. Next throw the switch 2 to the right and leave it there for about five seconds. It may then be thrown over to the running position at the left and allowed to remain in this position. If the polarity is not correct, the switch must be opened again for an instant and closed again; and this process must be repeated until the polarity comes in right. To stop the converter, first open the main switch and then the throw-over switch. The manner in which the above machines are preferably set up is shown in Figure 148.
_The Mercury-Arc Rectifier Control._--The mercury-arc rectifier has three essential parts: the rectifier tube, the main reactance, and the panel. The rectifier tube, Figure 149, is a glass vessel from which the air has been exhausted and in which there are two graphite electrodes, _A_ and _A´_, and one mercury electrode _B_. From the two upper electrodes current can pass in the direction of the mercury only. They are always positive and the term _anode_ is usually applied to them. _B_ is always negative and the term _cathode_ applies to it. Each anode is connected to a separate side of an alternating-current circuit and is thus subject alternately to positive or negative potential.
When current has once been started, the tube is filled with ionized mercury vapor through which the electricity can flow, from whichever of the two anodes is positive, toward the cathode _B_. Under no conditions, however, can electricity flow from the mercury in the tube toward the anodes. The action of the tube is started by tilting it sufficiently, so that the mercury in the bottom of it connects the starting anode _C_ to _A_. This starts the current and when the tube is returned to its upright position, the mercury bridge from _C_ to _A_ is interrupted; but the current then continues from one or the other of the anodes.
Should the current be interrupted, even for an instant, the tube would cease to work until it had been tilted again. In order to provide that the current, which is alternating and comes to zero twice in every cycle, may never cease in the tube, it is necessary to provide some reactance. Such a reactance causes the current to lag behind the e.m.f. and in consequence lap over the time when it would otherwise fall to zero. While the current from the rectifier is always in the same direction, positive from _B_ to the lamp, it is also a pulsating current changing in value to some extent.
In Figure 149 a complete diagram of the connections of the General Electric Company Mercury Arc Rectifier for moving-picture arcs is given. This type of rectifier is entirely automatic and is much used. The front and back connections are shown in Figure 150. The following instructions are taken from a publication of the General Electric Company:
The leads marked _A C_ should be connected to the lower side of a double-pole switch located near the moving-picture machine. The upper studs of the switch should be connected to the _A C_ source of supply.
The leads marked + and - should be connected, respectively, to the positive (upper) and negative (lower) electrodes of the moving-picture lamp.
If the _A C_ supply voltage is 110 volts; then connect the flexible lead marked _Z_ to stud marked _12_; and flexible lead marked _Y_ to stud marked _6_.
If the _A C_ supply voltage is 220 volts; then connect lead _Z_ to stud _7_, and lead _Y_ to stud _1_.
NOTE:--Do not disturb the other connections that are made on studs _1_, _6_, _7_, and _12_, but only place leads _Y_ and _Z_ as directed.
The tube holder should be reversed so that the clip and support will be turned away from the panel instead of towards the panel, as it is when shipped.
Remove the tube from its box, being very careful not to handle it roughly and not to strain the seals in any way whatever. Care must also be taken to prevent the mercury from suddenly flowing into any of the arms; otherwise the resultant pounding might damage them.
Examine the tube for vacuum by noting the sound the mercury makes when allowed to roll gently about in the large chamber. If it makes a clear, metallic click, the vacuum is good; but, if the sound is dull and the mercury sluggish in moving, the vacuum is either partially or wholly destroyed. If the vacuum is poor, the life of the tube may be short or it may not start at all. To insure careful handling and safe delivery, Mercury-Arc Rectifier tubes are always shipped by express in the special box as they come from the factory.
Place the tube in the holder by inserting the small part of the tube just above the anode arms in the upper clip; then gently lower it until it rests firmly on the lower support. Connect the tube and beaded leads according to the above diagram.
Adjustment of current (number of amperes) at the arc is obtained by connecting leads marked _X_ to studs marked _11_, _9_, _7_, _5_, _3_, or _1_ of the regulating reactance. Stud _1_ gives the maximum and stud _11_ the minimum number of amperes. In starting up the first time it is best to start with lead _X_ on stud _11_ and move toward the maximum position by steps until the desired current is obtained, as indicated by the ammeter. For this adjustment it is advisable to connect an ammeter in series with the arc in the moving-picture machine.
With the above instructions carried out, all that is necessary to start is to close the switch in the _A C_ line; then bring the electrodes of the arc together. The automatic shaking device should then rock the tube until the arc in the tube starts; as soon as the arc in the tube starts separate the electrodes.
The best and whitest light can be obtained when a 5/8-inch cored-carbon electrode is used above and a 1/2-inch solid-carbon electrode below, care being taken not to get solid carbons too hard. The average current in the arc should not exceed 30 amperes and it will be found that excellent pictures can be obtained with 25 amperes or even less and the cost of energy, carbons, and condensers will be less.