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.
[Illustration: FIGURE 134.]
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.
[Illustration: FIGURE 135.]
_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.
[Illustration: FIGURE 136.]
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.
[Illustration: FIGURE 137.]
[Illustration: FIGURE 138.]
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.
[Illustration: FIGURE 139.]
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.
[Illustration: FIGURE 140.]
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.
[Illustration: FIGURE 141.]
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.
[Illustration: FIGURE 142.]
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.
[Illustration: FIGURE 143.]
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.
[Illustration: FIGURE 144.]
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.
[Illustration: FIGURE 145.]
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.
[Illustration: FIGURE 146.]
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.
[Illustration: FIGURE 147.]
[Illustration: 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.
[Illustration: FIGURE 149.]
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.
[Illustration: FIGURE 150.]
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.