RECTIFIERS
The purpose of a rectifier is to change alternating current into a
uni-directional or pulsating current. There are several classes of
apparatus to which the term rectifier may be applied, as
1. Mechanical rectifiers;
2. Electrolytic rectifiers;
3. Mercury vapor rectifiers, or, mercury arc rectifiers;
4. Electro-magnetic rectifiers.
~Mechanical Rectifiers.~--By definition, a mechanical rectifier is a
form of commutator operating in synchronism with the generator and
commutating or rectifying the negative waves of the alternating current
as shown graphically in figs. 2,076 and 2,078. The essential features
of construction are shown in fig. 2,079.
~Ques. Mention some application of a mechanical rectifier.~
Ans. It is used on a compositely excited alternator as illustrated on
page 1,192.
~Electrolytic Rectifiers.~--If two metals be placed in an electrolyte
and then subjected to a definite difference of pressure, _they will_
(under certain conditions) _offer greater resistance to the passage of
a current in one direction, than in the other direction._ On account
of this so called valve effect, electrolytic rectifiers are sometimes
called "valves."
~Ques. What metal is generally used for the cathode?~
Ans. Aluminum.
[Illustration: FIGS. 2,076 to 2,078.--Diagrams showing alternating
currents, and partial and complete rectification.]
~Ques. What is generally used for the other electrode?~
Ans. Lead or polished steel.
Metals of low atomic weight exhibit the valve effect at high
differences of pressure, and heavier metals at low differences
of pressure.
~Ques. Describe the "Nodon valve."~
Ans. The cathode is of aluminum or aluminum alloy, and the other
electrode, which has considerably more surface, is the containing
vessel. The electrolyte is a neutral solution of ammonia phosphate.
~Ques. Describe its action.~
Ans. It is due to the formation of a ~film~ of normal hydroxide of
aluminum, over the surface of the aluminum electrode. This film
presents a very high resistance to the current when flowing in one
direction but very little resistance, when flowing in the reverse
direction.
[Illustration: FIG. 2,079.--Mechanical rectifier. The rectifier
consists of two castings M and S with teeth which fit together as
shown, being insulated so they do not come in contact with each
other. Every alternate tooth, being of the same casting, is connected
together, the same as though joined by a conducting wire. There are as
many teeth as there are poles. The part M of the rectifier is connected
to one of the collector rings by F, and the part S to the other ring by
G.]
~Ques. What is the effect when a Nodon cell is supplied with
alternating current?~
Ans. Half of the wave will be suppressed and an intermittently
pulsating current will result as shown in fig. 2,077.
[Illustration: FIGS. 2,080 and 2,081.--Two views of Nodon valve.
This is an electrolytic rectifier in which the cathode is a rod
of aluminum alloy held centrally in a leaden vessel which forms
the anode and contains the electrolyte, a concentrated solution of
ammonium phosphate. Only a short portion at the lower end of the
cathode is utilized, the rest, which is rather smaller in diameter,
being protected from action by an enclosing glass sleeve. The current
density at the cathode ranges from 5 to 10 amp. per sq. dm. In the
larger sizes, the cells are made double, and a current of air is kept
circulating between the walls by means of a motor driven fan. In
order to utilize both halves of the supply wave, the Gratz method of
connection is adopted. The maximum efficiency is obtained at about
140 volts, and the efficiency lies between 65 and 75 per cent., and
is practically independent of the frequency between the limits of 25
~ and 200 ~. Above a pressure of 140 volts, the efficiency falls off
very rapidly, owing to breakdown of the film. The pressure difference
is high, being over 90 per cent. at full load. Temperature largely
influences the action of the valve, and should never exceed 122° Fahr.]
[Illustration: FIG. 2,082.--Oscillograph record from Nodon valve
showing original supply voltage and the corresponding pulsating current
at the terminals of such a valve.]
~Ques. How may both halves of the alternating waves be utilized?~
Ans. By coupling a series of cells in opposed pairs as in fig. 2,080.
[Illustration: FIG. 2,083.--Performance curves of five ampere Nodon
valve. Constant secondary voltage test. Loaded on non-inductive
resistances. Frequency 50. Maximum power factor on valve .7.]
~Ques. Upon what does the efficiency of the film depend?~
Ans. Upon the temperature.
It should not for maximum efficiency exceed 86 degrees Fahr.
There is also a certain critical voltage above which the film
breaks down locally, giving rise to a luminous and somewhat
disruptive discharge accompanied by a rapid rise of temperature
and fall in efficiency.
[Illustration: FIG. 2,084.--Mohawk electrolytic rectifier and
switchboard; diagram showing connections for charging storage battery.
~Operating instructions:~ After assembling battery as in fig. 2,085,
the ~film~ must be formed on the aluminum alloy electrodes so that
the rectifier will pass current only in the right direction. Open
switch B, close switch T to the right; discharge lever can be in any
position; charging regulator lever must be to the extreme left, the
zero position; now close main switch M. Moving regulator lever R from
the zero position to the first button or contact, let it remain there
for a time, ~not less than five minutes;~ _this is important_, as the
proper rectification of the current depends on the film formed on the
aluminum rods. The ammeter after the first rush of current may not
show any current as passing, or it may show a reverse current. In the
latter case, leave the contact finger on the first button until the
needle comes back to zero. This may take some time, but the needle
will eventually come back; it also indicates that the film is properly
formed when the needle returns to zero. Move regulator R to the extreme
right step by step and note that the ammeter continues to return to
zero, which indicates that the film on rectifier electrodes is formed
properly. Move regulator R to zero, close switch T to the left in
normal charging position. Close charging switch B. To regulate the
flow of current through the battery move charging lever R to the right
slowly until ammeter indicates the correct charging current. After the
batteries are charged and ready for use, discharge lever can be moved
to connect either set of storage batteries to the load terminal. The
voltage of the batteries can be read at any time, by pressing the strap
key. The discharge lever connects the batteries to the volt meter and
it is possible by moving it to measure the voltage of either set of
battery, charging or discharging. Trouble in the rectifier demonstrates
itself by the solution becoming heated. The condition of the rectifier
can be tested any time in a few seconds by opening switch B and closing
switch T to the right. If the rectifier be in proper condition the
ammeter will read zero. And if it be not rectifying and permitting
A. C. current to flow through the rectifier, the ammeter will read
negative or to the left of the zero. An old solution that is heating
and not rectifying properly will turn a reddish brown color.]
~Ques. When an electrolytic rectifier is not in use for some time what
happens?~
Ans. The electrodes will loose the film.
[Illustration: FIGS. 2,085 and 2,086.--Mohawk electrolytic rectifier.
To put in commission, clean out the jar. Fill with distilled or rain
water. Add six pounds of electro salts, stir and after all salts are
dissolved place the cover in position. The specific gravity of the
solution should be 1.125. The middle iron electrode must hang straight
down in the solution and not touch either of the other aluminum alloy
electrodes. The aluminum alloy electrodes are mounted on an insulated
bracket that slides up and down on a ¼" rod. This rod screws in the
hole taped in the middle of the cover. The electrodes give the best
results only when perfectly smooth. Should they get rough, covered
with a deposit or a white coating remove from the solution, and clean
with fine sand paper. Finish with fine sand paper. Form the film
again and the electrodes will be as good as new. Clean iron electrode
occasionally.]
~Ques. What must be done in such case?~
Ans. The electrodes must be reformed.
~Ques. How is the loss of film prevented?~
Ans. By Removing the electrodes from the electrolyte and drying them.
[Illustration: FIG. 2,087.--The Fleming oscillation valve. It depends
for its action on the well-known Edison effect in glow lamps. The valve
consists of a carbon filament glow lamp with a simple central horseshoe
filament. Around this filament inside the exhausted bulb is fixed a
small cylinder of nickel, which is connected by means of a platinum
wire sealed through the bulb to a third terminal. ~The valve is used as
follows~: _The carbon loop is made incandescent by a suitable battery.
The circuits in which the oscillations are to be detected is joined
in series with a sensitive mirror galvanometer, the nickel cylinder
terminal and the negative terminal of the filament of the valve being
used._ The galvanometer will then be traversed by a series of rapid
discharges all in the same direction, those in the opposite direction
being entirely suppressed.]
~Ques. What attention must be given to the electrolyte?~
Ans. Water must be added from time to time to make up for evaporation.
This is necessary to keep the solution at the proper density.
~Ques. What is the indication that the rectifier needs recharging?~
Ans. Excessive heating of the solution with normal load.
~Ques. What is the indication that a rectifier is passing alternating
current?~
Ans. It will heat, and if the solution be very weak, it will cause a
buzzing sound.
[Illustration: FIG. 2,088.--The Churcher valve. This is of the modified
Nodon type. It differs from the latter in that it has two cathodes of
aluminum and an anode of lead or platinum, suspended in the one cell.
This permits the complete utilization of both halves of the supply wave
with one cell instead of the four required in the Gratz method. The
connections of such a cell are shown in the figure. The secondary of
the transformer carries a central tapping, and is connected through the
direct current load to the central anode, while each of the cathodes
is connected to the ordinary terminals of the transformer itself. The
practical limits of the cell are 50 volts direct current, or 130 volts
at the transformer terminals AB. F, is the anode; C, cathode I; D,
cathode II.]
~Ques. What harm is caused by operating a rectifier with a weak
electrolyte?~
Ans. The electrodes will eat away.
A few of the so called electrolytic valves are here briefly
described:
~The Audion Valve.~--This valve was invented by De Forest in
1900 and is practically identical with the Fleming oscillation
valve, the latter being illustrated in fig. 2,086.
~Grisson Valve.~--In this valve the cathode is a sheet of
aluminum, and the anode, a sheet of lead, supported, in
the original form, horizontally in a vessel containing the
electrolyte, consisting of a solution of sodium carbonate.
Cooling is effected by circulating water through metal tubes in
the electrolyte itself.
[Illustration: FIGS. 2,089 and 2,090.--The De Faria valve. This is an
aluminum lead rectifier. The cathode is a hollow cylinder of aluminum
placed concentrically in a larger cylinder of lead, and the whole
immersed in electrolyte of sodium phosphate in an ebonite containing
vessel. Cooling is effected by promoting automatic circulation of
the electrolyte by providing the lead cylinder with holes near its
extremities; the heated electrolyte then rises in the lead cylinder,
passes out at the upper holes, is cooled by contact with the walls of
the containing vessel, and descends outside the lead cylinder. It is
claimed that this cooling action is sufficient to allow of a current
density of 8 amp. per sq. dm. of aluminum.]
~Pawlowski Valve.~--This is an electrolytic valve employing
a solid electrolyte. It consists of a copper plate which has
been coated with a crystalline layer of carefully prepared
copper hemisulphide, prepared by melting sulphur and copper
together out of contact with air. The prepared plate is placed
in contact with an aluminum sheet and the combination is then
_formed_ by submitting it to an alternating pressure until
sparking, which at first occurs, ceases.
~Giles Electric Valve.~--This consists of a combination of
spark gaps and capacity used to protect electrical apparatus
against damage due to atmospheric discharges and resonance
surges. The spark gaps are formed between the edges of sharp
rimmed discs of non-arcing metal. These discs are insulated
from each other, and from the central tube, which provides a
support for the apparatus and also an earth. The condenser
effect is obtained by means of the annular discs and the tube;
an adjustable spark gap, a high resistance, and a fuse all
connected in series, complete the valve.
[Illustration: FIG. 2,091.--75 light Westinghouse-Cooper Hewitt mercury
vapor rectifier constant current regulating transformer. View showing
assembly in case.]
~Buttner Valve.~--It is of the Nodon type employing a cathode
of magnesium-aluminum alloy, and probably iron or lead as
anode, with an electrolyte of ammonium borate. Buttner claims
that the borate is superior to the phosphate in that it does
not attack iron, and will keep in good working condition for
longer periods.
[Illustration: FIG. 2,092.--75 light Westinghouse-Cooper Hewitt mercury
vapor rectifier constant current regulating transformer with case
removed. The transformer is of the repulsion coil type, oil cooled
and oil insulated. It is so arranged as to give a constant secondary
current and to insulate the arc lines from the primary circuit. The
regulating transformer contains two stationary secondary coils and
two moving primary coils balanced against each other. Each secondary
coil of the 75 light regulator is wound in two parts, owing to the
use of two rectifier bulbs in series in outfits of this capacity. The
repulsion between the primary and secondary coils changes the distance
between them according to the variation of load, and the induced
current in the secondary is thus kept constant. An increase in current
causes the primary and secondary coils to separate, and a decrease in
current permits them to approach each other, until the normal balance
is restored. The moving coils are hung from sheave wheels having roller
bearings and are balanced so that they are sensitive to the slightest
impulse tending to separate them or draw them closer together. (See
figs. 1,981-2, and 2,111.) The windings are insulated for a voltage
considerably in excess of that existing in normal service. Several
taps are provided to take care of different voltages and wave forms. A
combination of taps will be found which will be suitable for any wave
form coming within the American Institute of Electrical Engineer's
limits for a sine wave. The secondary coils are also provided with taps
for 85 per cent. of normal load, so that less than normal load can be
taken care of at a good power factor. Any part of the full load can be
carried temporarily with the full load connections of the transformer,
but at permanent light loads the power factor and efficiency will be
improved by using the 85 per cent. connections. Standard regulating
transformers are wound for 6.6 and 4 amperes, and for primary circuits
of 220, 440, 1,100 2,200, 6,600 and 13,200 volts. Regulators can
be specially wound for 5.5 amperes. For three phase circuits three
regulators can be used, one on each phase, or they can be furnished
in pairs with an auxiliary auto-transformer to give a balanced load.
The regulators can be connected, in cases where the unbalancing is not
objectionable, to separate phases.]
~Mercury Vapor Rectifiers.~--The Cooper Hewitt mercury vapor rectifier,
as shown in fig. 2,093 consists essentially of a _hermetically
sealed glass bulb filled with mercury vapor and provided with four
electrodes_. The two upper electrodes are of solid material and the two
lower of mercury.
The solid electrodes are the positive electrodes; the mercury
electrodes are the negative electrodes.
The mercury pools of the two lower electrodes are not in contact when
the bulb is vertical, but the bulb is so mounted that it can be tilted
to bring these two pools temporarily in contact for starting.
[Illustration: FIG. 2,093.--Diagram of connections of
Westinghouse-Cooper Hewitt mercury vapor rectifier arc light circuit.]
The bulb contains highly attenuated vapor of mercury, which, like other
metal vapors, is an electrical conductor under some conditions. The
positive electrodes are surrounded by this vapor. Current can readily
pass from either of the solid electrodes to the mercury vapor and from
it to the mercury electrode, but when the direction of flow tends
to reverse, so that current would pass from the vapor to the solid
electrode, there is a resistance at the surface of the electrode, which
entirely prevents the flow of current.
[Illustration: FIG. 2,094.--Cooper Hewitt mercury vapor rectifier.
The mercury vapor rectifier as developed by Peter Cooper Hewitt for
changing alternating current into direct current is the result of a
series of careful experiments and investigations of the action going
on in his mercury vapor lamp for electric lighting used on direct
current circuits only. While many attempts have been made to produce
an alternating current lamp, up to the present time, they have been
unsuccessful. The difficulty of operating a lamp on the alternating
current circuit lies in the fact that while a current will flow freely
through it in one direction, when the current reverses the negative
electrode or cathode acts as an electric valve and stops the current,
thus breaking the circuit and putting out the light. By following
up this new electrical action, Hewitt applied the principle in the
construction of a vacuum tube with suitable electrodes, and by using
two electrodes of iron or graphite for the positive or incoming current
and one of mercury for the negative or where the current leaves the
tube, the circuits could be arranged so that a direct current would
flow from the mercury electrode and be used for charging storage
batteries, electro-chemical work or operating direct current flame arc
lamps. As shown in the figure, ~the rectifier consists essentially~
_of a glass bulb into which are sealed two iron or graphite anodes
and one mercury cathode, and a small starting electrode_. The bulb
is filled with mercury vapor under low pressure. The action of this
device depends on _the property of ionized mercury vapor of conducting
electricity in one direction only_. ~In operation~ no current will flow
until the starting or negative electrode resistance has been overcome
by the ionization of the vapor in its neighborhood. To accomplish this,
the voltage is raised sufficiently to cause the current to jump the gap
between the mercury cathode and the starting cathode, or by bringing
the cathode and starting electrode together in the vapor by tilting
and then separating them, thus drawing out the arc. When this has been
done, current will only flow from the anode to the mercury cathode, and
not in the reverse direction. In order to maintain the action, a lag is
produced in each half wave by the use of a reactive or sustaining coil;
hence the current never reaches its zero value, otherwise the arc would
have to be restarted. ~There are two kinds of losses in the tube:~ 1,
_arcing, or leakage from one anode to the other_, and 2, _the mercury
arc voltage drop_. This drop does not depend on the load, the energy
represented by the drop being converted into heat, which is dissipated
at the surface of the containing vessel. According to Steinmetz, the
limit of voltage must be very high, as 36,000 volts has been rectified.
The current output is limited principally by the leading-in wires to
the electrodes, it being a difficult problem to seal into the glass
container the large masses of metal required for the conduction of
large currents. Frequency has but little influence. The direct current
voltage ranges from 20 to 50 per cent. that of the arc supply. The
life of the valve depends somewhat upon its size, being longer in
the small sizes and never, with fair usage, less than 1,000 hours.]
[Illustration: FIG. 2,095.--Three phase mercury arc rectifier. The
rectifier bulb is provided with three positive electrodes or anodes,
a negative electrode or cathode, and a starting anode, as shown. The
three phase leads are connected to the anodes at the top of the bulb,
a branch from one phase being brought down to the starting anode, a
resistance being placed in the circuit to prevent excessive current on
account of the proximity of the two lower electrodes. Since there is
always a pressure on one of the three anodes in the right direction, a
reactance coil is not necessary. The apparatus is started in the usual
way by tilting.]
The alternating current supply circuit is connected to the two positive
electrodes as shown in the diagram, and as the electrodes will allow
current to flow in only one direction and oppose any current flow in
the opposite direction, the pulsations of the current pass alternately
from one or the other of the positive electrodes into the mercury.
As these currents cannot pass from the vapor into either positive
electrode, they are constrained to pass out all in one direction
through the mercury electrode, from which they emerge as a
uni-directional current. The positive electrodes of the rectifier thus
act as check valves, permitting current to pass into the mercury vapor
but not allowing it to pass from the vapor to the solid electrodes.
[Illustration: FIGS. 2,096 to 2,098.--Westinghouse diagrams showing
comparative efficiencies of different systems of series arc lighting.]
~Ques. What condition prevails before the bulb starts to rectify?~
Ans. There appears to be a high resistance at the surface
[Illustration: FIG. 2,099.--Westinghouse-Cooper Hewitt mercury vapor
rectifier bulb. It consists essentially of a hermetically sealed glass
bulb filled with highly attenuated vapor of mercury, and provided with
electrodes. Its operation is fully explained in the accompanying text.]
[Illustration: FIG. 2,100.--Westinghouse-Cooper Hewitt mercury vapor
rectifier bulb and box. The life of the bulb is materially increased
by operating it at certain temperatures and for this reason the bulbs
in the arc light rectifier outfits are immersed in oil and mounted in
the same tank with the regulating transformer. Two bulbs in series
are used with the 75 light outfit. The bulb is mounted on tilting
trunnions in a box which can be lifted out through a door in the top
of the tank without disconnecting any leads. The containing box has
contacts on the bottom so arranged that when it is lowered into place,
the bulb is automatically connected in circuit. To replace a defective
bulb it is only necessary to lift out the containing box by its handle
through the door in the top of the case and mount a new bulb in it,
after which the box can be lowered into place. It is desirable to have
at each installation, a spare bulb box in which a bulb can be kept,
connected ready for use. If this be done, it is only necessary, in case
of trouble with the bulb, to withdraw the old bulb and box and replace
them with the spare set. This avoids having the lamps out of service.]
of the mercury, which must be broken down so that the current can pass.
~Ques. What is this apparent surface resistance called?~
Ans. _The negative electrode resistance._
~Ques. What must be done before any current can pass?~
Ans. The negative electrode resistance must be overcome.
When once started the current will continue to flow, meeting
with practically no resistance as long as the current is
uninterrupted.
~Ques. What will happen if the current be interrupted even for the
smallest instant of time?~
Ans. The negative electrode resistance will re-establish itself, and
stop the operation of the bulb.
~Ques. How is the negative electrode resistance overcome?~
Ans. The bulb is tilted or shaken so that the space between the mercury
electrodes is bridged by the mercury.
~Ques. What happens when the bulb is tilted?~
Ans. Current then passes between the two mercury electrodes from the
starting transformer and the little stream of mercury which bridges
the space between the electrodes breaks with a spark as the bulb is
returned to its vertical position.
~Ques. What duty is performed by the spark?~
Ans. It breaks down the negative electrode resistance.
~Ques. What conditions are now necessary for continuous operation of
the rectifier?~
Ans. The rectifier will now operate indefinitely as long as the current
supply is uninterrupted and the direct current load does not fall below
the minimum required for the arc.
~Ques. Is the rectifier self-starting?~
Ans. After the bulb has been started a few times, as described above,
it becomes self-starting, so that under all ordinary operating
conditions it will commence to operate when the switches connecting it
with the load and the alternating current supply are closed.
~Ques. What provision is made in the Westinghouse-Cooper Hewitt
rectifier to render it self-starting?~
Ans. It is rendered self-starting by means of a condenser.
[Illustration: FIG. 2,101.--General Electric 50 light double tube
combined unit series mercury arc rectifier outfit; front view. This
unit consists of a constant current transformer, reactance, tube tank
and exciting transformer mounted on a common base; also a static
discharger and pilot lamp mounted on top of the transformer. This
arrangement makes the rectifier outfit, with the exception of the
switchboard panel, complete in itself.]
~Ques. Describe the arrangement and operation of the condenser.~
Ans. The condenser is connected between one of the positive electrodes
and a coating of tinfoil outside [Illustration: FIG. 2,102.--General
Electric 2,200 volts, 60 cycle, primary, 6.6 ampere, secondary, 75
light, double tube mercury arc rectifier outfit with automatic shaking
device, the case being removed to show parts. The constant current
transformer is air cooled. The winding which supplies energy for the
exciter transformer is located at the top of, and around the core of
the constant current transformer. The exciting transformer is mounted
on the base of the constant current transformer inside of the casing.
It supplies low pressure currents to the starting anodes of the
rectifier tube. This current establishes an auxiliary arc when the
tube is shaken, which is necessary in order to start the rectifier.
The exciting transformer is wound for 110 volts and it consumes about
200 volt-amperes. The direct current reactance is mounted on the base
of the transformer and enclosed in the same casing. It is connected
in series with the lamp load and its function is to reduce the
pulsations of the circuit to a value most satisfactory for operation.
The tube tank for holding the oil is mounted on the same base as the
transformer. It is provided with a cooling coil; a tube carrier is
provided for raising or lowering the tube in the tank. A thermometer
is provided to gauge the temperature of the oil in the tank. The
static dischargers consisting of horn gaps in series with resistance,
are connected between the anodes and the cathode in order to protect
the tubes and other apparatus from excessive electrical strains. The
horn gaps open the circuit after discharge, and in case the resistance
becomes damaged the discharge passes across the spark gap provided,
thereby shunting the resistance.] the part of the bulb containing the
mercury, and induces static sparks on the surface of the mercury which
break down the negative electrode resistance.
The action of the rectifier will be better understood by
reference to the diagram of current waves and impressed
pressure as shown in figs. 2,103 to 2,106.
[Illustration: FIGS. 2,103 to 2,106.--Diagram of current waves and
impressed pressure of Westinghouse-Cooper Hewitt mercury vapor
rectifier. The whole of the alternating current wave on both sides of
the zero line is used. The two upper curves in the diagram show the
current waves in each of the two positive electrodes, and the resultant
curve III represents the rectified current flowing from the negative
electrode. Curve IV shows the impressed alternating current pressure.
It is evident that if the part of the wave below the zero line were
reversed, the resulting current would be a pulsating direct current
with each pulsation varying from zero to a positive maximum. Such a
current could not be maintained by the rectifier, because as soon as
the zero value was reached the negative electrode resistance of the
rectifier would be re-established and the circuit would be broken. To
avoid this condition, reactance is introduced into the circuit, which
causes an elongation of current waves so that they overlap before
reaching the zero value. The overlapping of the rectifier current waves
reduces the amplitude of the pulsations and produces a comparatively
smooth direct current as shown in curve III. In this way the whole of
the alternating current is transformed to direct current because each
of the alternations in both directions is alternately rectified.]
~Ques. Describe a mercury vapor rectifier outfit for series arc
lighting.~
Ans. It consists of a constant current regulating transformer, a
rectifier bulb, and a control panel containing the necessary switches,
meters, etc. The transformer and rectifier bulb are mounted in the same
tank.
[Illustration: FIG. 2,107.--General Electrical rectifier tube as used
on series mercury arc rectifier. The tube rectifies the alternating
current into direct current. It consists of an exhausted glass vessel
containing one anode or positive terminal in each of the two upper
arms, two mercury starting anodes and a cathode or negative terminal of
mercury at the bottom of the tube. It is submerged in oil and supported
in a removable carrier. The tube is put into operation by slightly
shaking it. In the combined unit set, this shaking is accomplished by
an electromagnet mounted above the tube tank and operated from a pull
button switch on the panel. An automatic shaker is sometimes installed
which will automatically start the tube when the set is started, or if
its operation should become interrupted while in service. The energy
for the operation of this magnet (110 volts alternating current) is
obtained from the small auxiliary winding on the main transformer which
also supplies energy to the exciting transformer. The oil in which the
tube is placed is cooled by a circulation of water through the cooling
coils on the inside of the tube tank. The amount of water necessary
for cooling the rectifier tubes varies according to local conditions,
depending upon the temperature of the water and that of the air in the
station but under the most favorable conditions no water is required.
As rectifiers are commonly installed in steam driven stations, the
drip from the tube tanks is usually piped to the boiler supply thereby
eliminating any loss for cooling water.]
[Illustration: FIG. 2,108.--Elementary diagram of mercury arc rectifier
connections. A.A., graphite anodes; B, mercury cathode; C, small
starting electrode; D, battery connection; E and F, reactance coils; G
and H, transformer terminals; J, battery.]
~Ques. Describe the construction and operation of the mercury arc[11]
rectifier shown in fig. 2,108.~
[11] NOTE.--The terms _vapor_ and _arc_ as applied to rectifiers, do
not indicate a different principle; the Westinghouse Co. employ the
former, and the General Electric Co., the latter.
Ans. Fig. 2,108 is an elementary diagram of connections. The rectifier
tube is an exhausted glass vessel in which are two graphite anodes
A, A', and one mercury cathode B. The small starting electrode C is
connected to one side of the alternating circuit, through resistance;
and by rocking the tube a slight arc is formed, which starts the
operation of the rectifier tube. At the instant the terminal H of the
supply transformer is positive, the anode A is then positive, and
the arc is free to flow between A and B. Following the direction of
the arrow still further, the current passes through the battery J,
through one-half of the main reactance coil E, and back to the negative
terminal G of the transformer. When the impressed voltage falls below
a value sufficient to maintain the arc against the reverse pressure of
the arc and load, the reactance E, which heretofore has been charging,
now discharges, the discharge current being in the same direction as
formerly. This serves to maintain the arc in the rectifier tube until
the pressure of the supply has passed through zero, reversed, and
built up such a value as to cause the anode A to have a sufficiently
positive value to start the arc between it and the cathode B. The
discharge circuit of the reactance coil E is now through the arc A'B
instead of through its former circuit. Consequently the arc A'B is now
supplied with current, partly from the transformer, and partly from the
reactance coil E. The new circuit from the transformer is indicated by
the arrows enclosed in circles.
[Illustration: FIG. 2,109.--Diagram showing connections of General
Electric combined unit mercury arc ~single tube~ rectifier outfit with
remote controlled non-automatic shaking device.]
[Illustration: FIG. 2,110.--Diagram showing connections of General
Electric combined unit mercury arc ~double tube~ rectifier outfit with
remote controlled non-automatic shaking device.]
[Illustration: FIG. 2,111.--Diagram showing connections of General
Electric series mercury arc rectifier.]
[Illustration: FIGS. 2,112 and 2,113.--Diagram of current waves
showing effect of reactance coil. If the alternating current wave
could be rectified without the use of the reactance coil, the direct
current produced would consist of a series of impulses which would
rise and fall from the zero line as illustrated in fig. 2,112. The
action of the reactance coil not only maintains the current through the
tube while the supply current is passing through zero, but helps to
smooth out the pulsations of the direct current which is passing out
of the cathode terminal of the tube to the batteries, or other direct
current apparatus put in its circuit. The smoothing out effect of the
reactance is shown in fig. 2,113. It will be seen from the diagram
that the current does not drop down to zero and the pulsations of the
direct current are greatly reduced. The waves A, A, etc., are from
the positive waves of the alternating current supply, while B, B, are
from the negative waves, and together they form the rectified current,
flowing in the same direction to the external direct current circuit
shown at B in the diagram, fig. 2,108.]
~Ques. How is a mercury arc rectifier started?~
Ans. A rectifier outfit with its starting devices, etc., is shown in
figs. 2,114 to 2,116. To start the rectifier, close in order named
line switch and circuit breaker; hold the starting switch in opposite
position from normal; rock the tube gently by rectifier shaker. When
the tube starts, as shown by greenish blue light, release starting
switch and see that it goes back to normal position. Adjust the
charging current by means of fine regulation switch on the left; or, if
not sufficient, by one button of coarse regulation switch on the right.
The regulating switch may have to be adjusted occasionally during
charge, if it be desired to maintain charging amperes approximately
constant.
[Illustration: FIGS. 2,114 to 2,116.--General Electric mercury arc
rectifier outfit, or charging set. The cut shows front, rear, and side
views of the rectifier, illustrating the arrangement on a panel, of the
rectifier tube with its connection and operating devices.]
[Illustration: FIGS. 2,117 and 2,118.--General Electric mercury arc
bulbs or tubes for 200 and 10,000 volt circuits.]
~Ques. In the manufacture of rectifiers, could other metals be used for
the cathode in place of mercury?~
Ans. Yes.
~Ques. Why are they not used?~
Ans. Because, on account of the arc produced, they would gradually wear
away and could not be replaced conveniently.
In the case of mercury, the excess vapor is condensed to liquid
form in the large glass bulb or condensing chamber of the tube
and gravitates back to the cathode, where it is used over and
over again.
[Illustration: FIG. 2,119.--General Electric series mercury arc
rectifier outfit; view showing method of replacing a tube. The
illustration also shows tube carrier and drip tray.]
~Ques. In the operation of rectifiers, how is the heat generated in the
bulb dissipated?~
Ans. In small rectifier sets the heat generated is dissipated through
the tube to the air, and in large tubes such as used in supplying 40 to
60 kw. for constant current flaming arc lights operating at 4 or 6.6
amperes, the tubes are immersed in a tank of oil, and cooled similar
to the arrangement used for oil insulated water cooled transformers.
~Ques. What results are obtained with oil cooled tubes?~
Ans. In practice it is found that the life of oil cooled tubes is
greatly increased and temperature changes do not affect the ability to
start up as in the air cooled tubes.
[Illustration: FIG. 2,120.--General Electric 100 volt mercury arc
rectifier tube. A,A, anodes; B, cathode; C, starting anode; D, tube or
bulb.]
~Ques. In the operation of a rectifier, name an inherent feature of the
mercury arc.~
Ans. A reverse pressure of approximately 14 volts is produced, which
remains nearly constant through changes of load, frequency, and
voltage. Its effect is to decrease the commercial efficiency slightly
on light loads.
~Ques. What is the advantage of a rectifier set over a motor generator
set?~
Ans. Higher efficiency and lower first cost.
~Ques. What is the capacity of a rectifier tube?~
Ans. 40 to 50 amperes.
~Ques. How is greater capacity obtained?~
Ans. When a greater ampere capacity is required, two or more rectifier
sets can be joined to one circuit.
The rectifier may be joined in series for producing an
increased voltage or two tubes can be connected in series in a
single set.
~Ques. For what service is a rotary converter better adapted than a
rectifier?~
Ans. For power distribution and other cases where a great amount of
alternating current is to be converted into direct current, the rotary
converter or large motor generator sets are more practical.
~Ques. For what service is a rectifier especially adapted?~
Ans. It is very desirable for charging storage batteries for
automobiles from the local alternating current lighting circuit.
When the consumer installs and operates the apparatus for his
own use and wear, there is considerable saving over motor
generator sets because a small one to two horse power motor
generator outfit has an efficiency of only 40 to 50 per cent.
while mercury vapor rectifiers will have from 75 to 80 per cent.
~Ques. What precautions should be taken in installing a rectifier?~
Ans. It should be installed in a dry place and care should be taken
to avoid dangling wires near the tube to prevent puncturing. If the
apparatus be installed in a room of uniform moderate temperature very
little trouble will be experienced in starting, while extreme cold will
make starting more difficult.
[12]~Electro-magnetic Rectifiers.~--Devices of this class consist
essentially of a double contact rocker which rocks on pivot (midway
between the contacts), in synchronism with the frequency of the
alternating current, so changing the connections at the instants of
reversals of the alternating current that a direct current is obtained.
[12] NOTE.--The Edison electromagnetic rectifier is described in detail
in GUIDE No. 4, pages 942 to 945.
Fig. 2,121 is a combined sketch and diagram of connections of a type
of _electromagnetic rectifier_ that has been introduced for changing
alternating into direct current. The actual apparatus consists of a
box, with perforated metal sides, about ten inches square and six
inches deep. This box contains the step down transformer P,S,S', and
the condensers K and K', the magnets and contact making device about
to be described being fixed on the polished slate top of the box,
exactly as shown in the figure. The transformer primary winding P may
be connected through a switch _s_ with a pair of ways on the nearest
distribution box, or to a plug connection or lamp-holder, and the
apparatus will give a rectified current of 6 or 12 amperes at 20 volts,
according to the size.
S and S' is the secondary winding of the transformer, with a tapping
_t_ midway, joining it to a series circuit containing two alternating
current electromagnets E and E', whose cores are connected by the long
soft iron yoke Y. Pivoted at P' is a steel bar SB, which is polarized
by the two coils C and C' the current being supplied by a cell A.
Fixed rigidly to SB, and moving with it, is a double contact piece CP
with platinum contacts opposite similar ones on the fixed studs CS, CS'.
CP is flexibly connected through F to one of the direct current
terminals T, to which also are joined up one coating of each condenser
K and K'.
[Illustration: FIG. 2,121.--Diagram showing essential features of
Premier Ampero electromagnetic rectifier. Details of construction and
principles of operation are given in the accompanying text.]
The other direct current terminal T' is connected to the center of the
transformer secondary at _t_; and CS and CS' are respectively joined up
to either end of the secondary winding and to the other coatings of the
condensers.
[Illustration: FIG. 2,122.--Diagram of General Electric (Batten)
electromagnetic rectifier. It is desirable for light and occasional
service, where direct current is required but only an alternating
supply is available, being used for charging storage batteries,
exciting spark coils, performing electrolytic work, etc. The rectifier
consists of a step down static transformer T, by means of which the
circuit pressure is reduced to about 50 volts; also, a polarized relay
R, the contact tongue C of which moves to one side or the other in
sympathy with the alternations of the current in the primary winding P,
the secondary current induced in the winding S being thereby rendered
direct in the outer circuit. T', T' are the main terminals which are
connected to the alternating current supply through the wires W. Lamps
inserted at L are used as resistances in the primary circuit, the
reduction of the voltage already alluded to being effected by this
means. In charging storage batteries where a low pressure is required,
a lamp (or lamps) should be connected in the secondary circuit as
shown, S B being the storage battery, and L' L' the lamp resistances in
series therewith, the battery has one end of the secondary S connected
to its middle. Thus the alternating current leaving the transformer
by the wire 1, passes by flexible connection 2, to the vibrating
contact tongue C of the relay, the latter causing the currents in
either direction to flow through the two halves H, H' of the battery,
whence the current re-enters the secondary of the transformer by the
wire 3. The soft iron core of the relay is in two halves S' S' and
the armature A, carrying C, vibrates between their polar extremities.
M, M' are two permanent magnets with their like poles together at
the center C' where A is pivoted. Supposing these poles are north as
indicated, the extremities of A will be south. The south ends of M, M
being in juxtaposition with the centers of the soft iron cores S', S'
will render their extremities facing the ends of A of north polarity.
The windings on S', S' are connected in series with each other, and
in shunt with P across the main terminals T', T'. Then because of
the polarization of A and S', S', the former will vibrate rapidly in
sympathy with the alternations of the current. K is a condenser shunted
by a lamp resistance L", this being found to improve the working of
R.]
When the alternating current circuit is broken, the springs SP, SP,
carried by SB and bearing against the adjustable studs, keep SB, CS and
CS'. The apparatus thus acts also as a _no voltage circuit breaker_,
for should the supply fail, the storage battery A' under charge will be
left on open circuit.
The action of the device is briefly as follows:
Owing to the direct current in the magnetizing coils C and C' one end
of SB will be permanently of north and the other of south polarity;
and since the polarities of the poles E and E' will alternate with the
alternations of the transformer secondary current, SB will rock rapidly
on its pivot, and contact will be made by turns with CS and CS'.
The purpose of the condensers K and K' is to reduce the sparking at
these points. When contact is made at CS, the direct current terminals
T and T' are connected to the S half of the secondary winding; and
when contact is made at CS', they are connected to the S' half. Thus a
rectified uni-directional current will flow from T and T', and it may
be used to charge the battery A', work a small motor or for various
other purposes requiring direct current.
When the rectifier is used for charging storage batteries, the separate
cell A may sometimes be dispensed with, the winding C,C' being
connected to one of the cells under charge.
The rectifier is adjusted to suit the frequency of the supply circuit
by altering the distance of the poles of E and E' from the ends of the
polarized armature SB; and also by changing the tension of SP, SP by
means of the screw studs against which they bear.
HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORM PRICE $1 EACH
* * * * *
_They are not only the best, but the cheapest work published on
Electricity. Each number being complete in itself. Separate numbers
sent postpaid to any address on receipt of price. They are guaranteed
in every way or your money will be returned. Complete catalog of series
will be mailed free on request._
~ELECTRICAL GUIDE, NO. 1~
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
~ELECTRICAL GUIDE, NO. 2~
The construction of Dynamos, Motors, Armatures, Armature
Windings, Installing of Dynamos.
~ELECTRICAL GUIDE, NO. 3~
Electrical Instruments, Testing, Practical Management of
Dynamos and Motors.
~ELECTRICAL GUIDE, NO. 4~
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.
~ELECTRICAL GUIDE, NO. 5~
Principles of Alternating Currents and Alternators.
~ELECTRICAL GUIDE, NO. 6~
Alternating Current Motors, Transformers, Converters,
Rectifiers.
~ELECTRICAL GUIDE, NO. 7~
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.
~ELECTRICAL GUIDE, NO. 8~
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.
~ELECTRICAL GUIDE, NO. 9~
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
~ELECTRICAL GUIDE, NO. 10~
Modern Practical Applications of Electricity and Ready
Reference Index of the 10 Numbers.
~Theo. Audel & Co., Publishers. 72 FIFTH AVENUE, NEW YORK~
TRANSCRIBER'S NOTES
Silently corrected simple spelling, grammar, and typographical
errors.
Retained anachronistic and non-standard spellings as printed.
Enclosed italics markup in _underscores_.
Enclosed bold markup in ~tildes~.
End of Project Gutenberg's Hawkins Electrical Guide v. 6 (of 10), by Hawkins