Transcriber's Notes:
Underscores "_" before and after a word or phrase indicate _italics_
in the original text.
Equal signs "=" before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Misprints in the table SAVING DUE TO HEATING THE FEED WATER, Pg. 1936
have been corrected, they are:
Init. Temp. Pressure Old Value New Value
130 40 .0954 .0934
200 40 .0900 .0999
210 40 .1000 .1010
230 100 .0117 .1012
In the original text, there are two Fig. 2769's and two Fig. 2770's.
The second of each has had an "a" suffix added, i.e. 2769a and 2770a.
On line 7397 the word "impedence" was corrected to "impedance".
Superscripts are shown as ^{d} and subscripts are shown as _{d}, where
"d" is an integer.
Inconsistent spelling and hyphenation has been left as in the original.
THE THOUGHT IS IN THE QUESTION
THE INFORMATION IS IN THE ANSWER
HAWKINS ELECTRICAL GUIDE NUMBER EIGHT
QUESTIONS ANSWERS & ILLUSTRATIONS
A PROGRESSIVE COURSE OF STUDY FOR ENGINEERS,
ELECTRICIANS, STUDENTS AND THOSE DESIRING TO
ACQUIRE A WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE by HAWKINS AND STAFF
THEO AUDEL & CO. 72 FIFTH AVE. NEW YORK.
COPYRIGHTED, 1915, BY THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE No. 8
=WAVE FORM MEASUREMENT= 1,839 to 1,868
Importance of wave form measurement--=methods=:
step by step; constantly recording--=classes of
apparatus=: wave indication; _oscillographs_--=step
by step methods=--Joubert's; four part commutator;
modified four part commutator; ballistic galvanometer;
zero; Hospitalier ondograph--=constantly recording
methods=: cathode ray; glow light; moving iron; moving
coil; hot wire--=oscillographs=--moving coil type;
construction and operation; production of the time scale;
oscillograms--falling plate camera; its use.
=SWITCHBOARDS= 1,869 to 1,884
=General principles=: diagram--small plant a.c.
switchboard--=switchboard panels=; =generator
panel=; diagram of connections--simple method of
determining bus bar capacity--feeder panel--diagrams of
connection for two phase and three phase installations.
=ALTERNATING CURRENT WIRING= 1,885 to 1,914
Effects to be considered in making
calculations--=induction=; self- and mutual; mutual
induction, how caused--=transpositions=--inductance
per mile of three phase circuit, table--=capacity=;
table--=frequency--skin effect=; calculation;
table--=corona effect=; its manifestation;
critical voltage; spacing of wires--=resistance of
wires--impedance--power factor=; apparent current;
usual power factors encountered; example--=wire
calculations--sizes of wire--table of the property of
copper wire--drop=; example--current--example covering
horse power, watts, apparent load, current, size of wire,
drop, voltage at the alternator, and electrical horse power.
=POWER STATIONS= 1,915 to 1,988
Classification--=central stations=; types: a.c.,
d.c., and a.c. and d.c.; reciprocating engine vs.
turbine--=location of central stations=; price of
land; trouble after erection; water supply; service
requiring direct current--=size of plant=; nature
of load; peak load; load factor; machinery required;
example; factors of evaporation; grate surface per
horse power--=general arrangement of station=;
belt drive with counter shaft; desirable features of
belt drive; conditions, suitable for counter shaft
drive; location of engine and boilers; the steam pipe;
piping between engine and condenser; number and type
of engine; superheated steam; switchboard location;
individual belt drive; direct drive--=station
construction=--=foundations=--=walls=--
=roofs=--=floors=--=chimneys=;
cost of chimneys and mechanical draft; high chimneys
ill advised--=steam turbine=; types: impulse
and reaction; why high vacuum is necessary; the
working pressure--=hydro-electric plants=--water
turbines; types: impulse, reaction--=isolated
plants=--=sub-stations=; arrangement; three phase
installations; reactance coils in sub-stations; portable
sub-stations.
=MANAGEMENT= 1,989 to 2,114
The term "management"--=selection=; general
considerations--=selection of generators=;
efficiency of generators; size and number;
regulation--=installation=; precautions;
handling of armatures; assembling a machine; speed
of generators; calculation of pulley sizes; gear
wheels--=belts=; various belt drives; horse
power transmitted by belts; velocity of belt; endless
belts--=switchboards=; essential points of difference
between single phase and three phase switchboard wiring;
assembling a switchboard; usual equipment.
=Operation of Alternators--alternators in parallel=;
synchronizing; lamp methods; action of amortisseur winding;
synchronizing three phase alternators; disadvantage of
lamp method--=cutting out alternator=; precautions;
hunting--=alternators in series=.
=Transformers=; selection; efficiency; kind of oil
used; detection of moisture; drying oil; regulation;
transformers in parallel; polarity test--=motor
generators=; various types and conditions requiring
same--=dynamotors=; precautions--=rotary
converters=; objections to single phase type; operation
when driven by direct current, by alternating current; most
troublesome part; efficiency; overload; starting; starting
current.
=Electrical measuring instruments=; location;
readings; station voltmeters; points relating to
ammeters; attention necessary; usual remedies to correct
voltmeter--=how to test generators=; commercial
efficiency; various tests.
=Station Testing:= resistance measurement by "drop"
method--methods of connecting ammeter voltmeter and
wattmeter for measurement of power--=motor testing:=
single phase motor--three phase motor, voltmeter and
ammeter method; two wattmeter method; polyphase wattmeter
method; one wattmeter method; one wattmeter and Y box
method--three phase motor with neutral brought out; single
wattmeter method--temperature test, three phase induction
motor--=three phase alternator testing:= excitation
or magnetization curve test--synchronous impedance
test--load test--three phase alternator or synchronous
motor temperature test--=direct current motor= or
=generator testing:= magnetization curve--(shunt)
external characteristic--direct current motor testing;
load and speed tests--temperature test, "loading back"
method--=compound dynamo testing=: external
characteristic, adjustable load--=transformer testing=:
external characteristic, adjustable load--=transformer
testing=: core loss and leakage or exciting current
test--copper loss--copper loss by wattmeter measurement
and impedance--temperature--insulation--internal
insulation--insulation resistance--polarity--winding
or ratio tests.
CHAPTER LXIII
WAVE FORM MEASUREMENT
The great importance of the wave form in alternating current work
is never denied, though it has sometimes been overlooked. The
application of large gas engines to the driving of alternators
operated in parallel requires an accurate knowledge of the wave form,
and a close conformation to a sine wave if parallel operation is
to be satisfactory. It is also important that the fluctuations in
magnetism of the field poles should be known, especially if solid
steel pole faces be used.
If an alternator armature winding be connected in delta, the
presence of a third harmonic becomes objectionable, as it gives
rise to circulating currents in the winding itself, which increase
the heating and lowers the efficiency of the machine.
That the importance of having a good wave form is being
realized, is proved by the increasing prevalence in alternator
specifications of a clause specifying the maximum divergence
allowable from a true sine wave. It is however perhaps not always
realized that an alternator which gives a good pressure wave on
no load may give a very bad one under certain loads, and the
ability of the machine to maintain a good wave form under severe
conditions of load is a better criterion of its good design than
is the shape of its wave at no load.
The question of wave form is of special interest to the power
station engineer. Upon it depends the answer to the questions:
whether he may ground his neutral wires without getting large
circulating currents; whether he may safely run any combination
of his alternators in parallel; whether the constants of his
distributing circuit are of an order liable to cause dangerous
voltage surges due to resonance with the harmonics of his
pressure wave; what stresses he is getting in his insulation due
to voltage surges when switching on or off, etc. It has been
shown by Rossler and Welding that the luminous efficiency of the
alternating current arc may be 44 per cent. higher with a flat
topped than with a peaked pressure wave, while on the other hand
it is well known that transformers are more efficient on a peaked
wave. Also the accuracy of many alternating current instruments
depends upon the wave shape.
In making insulation breakdown tests on cables, insulators, or
machinery, large errors may be introduced unless the wave form at
the time of the test be known. It is not sufficient even to know
that the testing alternator gives a close approximation to a sine
wave at no load; since if the capacity current of the apparatus
under test be moderately large compared with the full load current
of the testing alternator, the charging current taken may be
sufficient to distort the wave form considerably, thus giving
wrong results to the disadvantage of either the manufacturer or
purchaser.
[Illustration: FIG. 2,583.--General Electric simultaneous record of
three waves with common zero.]
The desirability of a complete knowledge of the manner in which the
pressure and current varies during the cycle, has resulted in various
methods and apparatus being devised for obtaining this knowledge.
The apparatus in use for such purpose may be divided into two general
classes,
1. Wave indicators;
2. Oscillographs.
and the methods employed with these two species of apparatus may be
described respectively as,
1. Step by step;
2. Constantly recording.
that is to say, in the first instance, a number of instantaneous
values are obtained at various points of the cycle, which are plotted
and a curve traced through the several points thus obtained. A
constantly recording method is one in which an infinite number of
values are determined and recorded by the machine, thus giving a
complete record of the cycle, leaving no portion of the wave to be
filled in.
[Illustration: FIG. 2,584.--General Electric simultaneous record of
three waves with separate zeros.]
[Illustration: Figs. 2,585 and 2,586.--Oscillograms (from paper by
Morris and Catterson-Smith, Proc. I. E. E., Vol. XXXIII, page 1,023),
showing _how the current varies_ =in one of the armature coils of
a direct current motor=. Fig. 2,585 was obtained with the brushes
in the neutral position, and fig. 2,586 with the brushes shifted
forward.]
The various methods of determining the wave form may be further
classified as:
{ Joubert's method;
{ Four part commutator method;
{ Modified four part commutator method;
1. Step by step { Ballistic galvanometer method;
{ Zero method;
{ By Hospitalier ondograph.
[Illustration: FIG. 2,587.--=Oscillogram= by Bailey and Cleghorne
(Proc. I.E.E., Vol. XXXVIII), =showing= _the sparking pressure or
pressure between the brush and the commutator segment at the moment
of separation_. The waves fall into groups of three owing to the fact
that there were three armature coils in each slot.]
{ cathode ray;
{ by use of various types { glow light;
2. constantly recording { of =oscillograph=, { moving iron;
{ such as { moving coil;
{ hot wire.
[Illustration: FIG. 2,588.--Various wave forms. The sine wave
represents a current or pressure which varies according to the sine
law. A distorted wave is due to the properties of the circuit, for
instance, the effect of hysteresis in an iron core introduced into a
coil is to distort the current wave by adding harmonics so that the
ascending and descending portions may not be symmetrical. A peaked
wave has a large maximum as compared with its virtual value. A peaked
wave is produced by a machine with concentrated winding.]
=Joubert's Method.=--The apparatus required for determining the
wave form by this step by step method, consists of a galvanometer,
condenser, two, two way switches, resistance and adjustable contact
maker, as shown in fig. =2,589=.
The contact maker is attached to the alternator shaft so that
it will rotate synchronously with the latter. By means of the
adjustable contact, the instant of "making" that is, of "closing"
the testing circuit may be varied, and the angular position of the
armature, at which the testing circuit is closed, determined from
the scale, which is divided into degrees.
A resistance is placed in series with one of the alternator
leads, such that the drop across it, gives sufficient pressure for
testing.
=Ques. Describe the method of making the test.=
[Illustration: FIG. 2,589.--Diagram illustrating Joubert's =step by
step method= of wave form measurement.]
Ans. For current wave measurement switch No. 1 is placed on contact
F, and for pressure wave measurement, on contact G, switch No. 2 is
now turned to M and the drop across the resistance (assuming switch
No. 1 to be turned to contact F) measured by charging the condenser,
and then discharging it through the galvanometer by turning the
switch to S. This is repeated for a number of positions of the
contact maker, noting each time the galvanometer reading and position
of the contact maker. By plotting the positions of contact maker as
abscissæ, and the galvanometer readings as ordinates, the curve drawn
through them will represent the wave form.
[Illustration: FIG. 2,590.--=Four part commutator method= of wave
form measurement. The contact device consists of two slip rings and
a four part commutator. One slip ring is connected to one terminal
of the source, the other to the voltmeter, and the commutator to the
condenser. By adjusting R when a known direct current pressure is
impressed across the terminals, the voltmeter can be rendered direct
reading.]
[Illustration: FIG. 2,591.--=Modified four part commutator method=
of wave form measurement (Duncan's modification). By this method
one contact maker can be used for any number of waves having the
same frequency. Electro-dynamometers are used and the connections
are made as here shown. The moving coils are connected in series to
the contact maker, and the fixed coils are connected to the various
sources to be investigated, then the deflection will be steady and
by calibration with direct current can be made to read directly in
volts.]
The apparatus is calibrated by passing a known constant current
through the resistance.
[Illustration: FIG. 2,592.--Diagram illustrating the =ballistic
galvanometer method= of wave form measurement. =The test may be made=
as described in the accompanying text, or in case the contact breaker
is belted instead of attached rigidly to the shaft, it could be
arranged to run slightly out of synchronism, then by taking readings
at regular intervals, points will be obtained along the curve without
moving the contact breaker. If this method be used, a non-adjustable
contact breaker suffices. =In arranging the belt drive= so as to run
slightly out of synchronism, if the pulleys be of the same size, the
desired result is obtained by pasting a thin strip of paper around
the face of one of the pulleys thus altering the velocity ratio of
the drive slightly from unity.]
=Ballistic Galvanometer Method.=--This method, which is due to
Kubber, employs a _contact breaker_ instead of a _contact maker_.
The distinction between these two devices should be noted: A contact
maker keeps the circuit _closed_ during each revolution for a short
interval only, whereas, a contact breaker keeps the circuit _open_
for a short interval only.
Fig. 2,592, shows the necessary apparatus and connections for
applying the ballistic galvanometer method. The contact breaker
consists of a commutator having an ebonite or insulating segment
and two brushes.
_In operation_ the contact breaker keeps the circuit closed
during all of each revolution, except the brief interval in which
the brushes pass over the ebonite segment.
The contact breaker is adjustable and has a scale enabling its
various positions of adjustment to be noted.
=Ques. Describe the test.=
[Illustration: FIGS.. 2,593 and 2,594.--=Two curves= _representing
pressure and current respectively of a rotary converter._ Fig. 2,593,
pressure wave V, fig. 2,594 current wave C. These waves were obtained
from a converter which was being driven by an alternator by means of
an independent motor. The rotary converter was supplying idle current
to some unloaded transformers and the ripples clearly visible in the
pressure wave V, correspond to the number of teeth in the armature of
the rotary converter.]
Ans. The contact breaker is placed in successive positions and
galvanometer readings taken, the switch being turned to F, fig.
2,592, in measuring the current wave, and to G in measuring the
pressure wave. The results thus obtained are plotted giving
respectively current and pressure waves.
=Ques. How is the apparatus calibrated?=
Ans. By sending a constant current of known value through the
resistance R.
=Zero Method.=--In electrical measurements, a zero method is one _in
which the arrangement of the testing devices is such that the value
of the quantity being measured is shown when the galvanometer needle
points to_ =zero=.
In the zero method either a contact maker or contact breaker may be
used in connection with a galvanometer and slide wire bridge, as
shown in figs. 2,595 and 2,596.
[Illustration: FIG. 2,595.--Diagram illustrating zero method of wave
measurement with _contact_ =maker=. The voltage of the battery must
be at least as great as the maximum pressure to be measured and must
be kept constant.]
=Ques. What capacity of battery should be used?=
Ans. Its voltage should be as great as the maximum pressure to be
measured.
=Ques. What necessary condition must be maintained in the battery?=
Ans. Its pressure must be kept constant.
=Ques. How are instantaneous values measured?=
Ans. The bridge contact A is adjusted till the galvanometer shows no
deflection, then the length AS is a measure of the pressure.
The drop between these points can be directly measured with a
voltmeter if desired.
=Ques. How did Mershon modify the test?=
Ans. He used a telephone instead of the galvanometer to determine the
correct placement of the bridge contact A.
[Illustration: FIG. 2,596.--Diagram illustrating zero method of wave
measurement with _contact_ =breaker=. The voltage of the battery must
be at least as great as the maximum pressure to be measured and must
be kept constant.]
=Ques. How can the instantaneous values be recorded?=
Ans. By attaching to the contact A, a pencil controlled by an
electro-magnet arranged to strike a revolving paper card at the
instant of no deflection, the paper being carried on a drum.
=Hospitalier Ondograph.=--The device known by this name is a
development of the Joubert step by step method of wave form
measurement, that is to say, the principle on which its =action is
based=, consists in _automatically charging a condenser from each
100th wave, and discharging it through a recording galvanometer, each
successive charge of the condenser being automatically taken from a
point a little farther along the wave._
[Illustration: FIG. 2,597.--Diagram of Hospitalier ondograph showing
mechanism and connections. It represents a development of Joubert's
step by step method of wave form measurement.]
As shown in the diagram, fig. 2,597, the ondograph consists of
a synchronous motor A, operated from the source of the wave form
to be measured, connected by gears B to a commutator D, in such a
manner that while the motor makes a certain number of revolutions,
the commutator makes a like number diminished by unity; that is to
say, if the speed of the motor be 900 revolutions per minute, the
commutator will have a speed of 899.
The commutator has three contacts, arranged to automatically
charge the condenser _cc'_ from the line, and discharge it through
the galvanometer E, the deflection of which will be proportional
to the pressure at any particular instant when contact is made.
In fig. 2,597, GG' are the motor terminals, HH' are connected to
the condenser _cc'_ through a resistance (to prevent sparking at
the commutator) and I, I' are the connections to the service to be
measured.
A permanent magnet type of recording galvanometer is employed.
Its moving coil E receives the discharges of the condenser in
rapid succession and turns slowly from one side to the other.
[Illustration: FIG. 2,598.--View of Hospitalier ondograph. =In
operation=, a long pivoted pointer carrying a pen and actuated by
electro-magnets, records on a revolving drum a wave form representing
the alternating current, pressure or current wave.]
The movable part operates a long needle (separately mounted)
carrying a pen F, which traces the curve on the rotating cylinder
C. This cylinder is geared to the synchronous motor to run at such
a speed as to register three complete waves upon its circumference.
By substituting an electromagnetic galvanometer for the
permanent magnet galvanometer, and by using the magnet coils as
current coils and the moving coil as the volt coil, the instrument
can be made to draw watt curves. Fig. 2,598 shows the general
appearance of the ondograph.
=Cathode Ray Oscillograph.=--This type of apparatus for measuring
wave form was devised by Braun, and consists of a cathode ray tube
having a fluorescent screen at one end, a small diaphragm with a
hole in it at its middle, and two coils of a few turns each, placed
outside it at right angles to one another. These coils carry currents
_proportional to the_ =pressure= _and_ =current= _respectively_ of
the circuit under observation.
[Illustration: FIG. 2,599.--General Electric =moving coil
oscillograph= complete =with tracing table=. The tracing table is
employed for observing the waves, and by using a piece of transparent
paper, the waves under observation appear as a continuous band of
light which can be traced, thus making a permanent record. This is
not, however, to be regarded as a recording attachment, and can not
be used where instantaneous phenomena are being investigated. =The
synchronous motor= for operating the synchronous mirror in connection
with tracing and viewing attachment is wound for 100 to 115 volts, 25
to 125 cycles, and should, of course, be run from the same machine
which furnishes power to the circuit under observation. A rheostat
for steadying and adjusting the current should be connected in series
with the motor. =The beam from the vibrator mirrors= _striking this
synchronous mirror moves back and forth over the curved glass, and
gives the length of the wave; the movement of the vibrator mirror
gives the amplitude, and the combination gives the wave complete_. An
arc lamp or projection lantern produces the image reflected by the
mirrors upon the film, tracing table or screen. For the rotation of
the photographic film, a small direct current shunt wound motor is
ordinarily used.]
The ray then moves so as to produce an energy diagram on the
fluorescent screen.
[Illustration: FIG. 2,600.--General Electric moving coil
oscillograph. =The moving elements= _consist of single loops of flat
wire carrying a small mirror and held in tension by small spiral
springs_. The current passing down one side and up the other, forces
one side forward and the other backward, thus causing the mirror to
vibrate on a vertical axis. The vibrator elements fit into chambers
between the poles of electro-magnets, and are adjustable, so as to
move the beam from the mirror, both vertically and horizontally. A
sensitized photographic film is wrapped around a drum and held by
spring clamps. The drum, with film, is placed in a case and a cap
then placed over the end, making the case light, when the index is
either up or down. The loading is done in a dark room. A driving dog
is screwed into the drum shaft, and which, when the drum and case are
in place, revolves the film past a slot. =When an exposure is to be
made=, the index is moved from the closed position, thus opening the
slot in the case and exposing the film to the beam of light from the
vibrating mirrors when the electrically operated shutter is open. The
slot is then closed by moving the index to "=Exposed=." A slide with
ground glass can be inserted in place of the film case or roll holder
to arrange the optical system when making adjustments. The shutter
operating mechanism is arranged so as to hold the shutter open during
exactly one revolution of the film drum. There are two devices
connected to the shutter operating mechanism; one opens the shutter
at the instant the end of the film passes the slot; the other opens
immediately, at any part of the film, and both give exposure during
one revolution. The first is useful when making investigations in
which the events are either recurring, or their beginnings known or
under control, and the second when the time of the event is not under
control, such as the blowing of fuses or opening of circuit breakers.]
The instrument is much used in wireless telegraphy, as it is capable
of showing the characteristics of currents of very high frequency.
[Illustration: FIG. 2,601.--General Electric =moving coil
oscillograph= _with case removed_, =showing= _interior construction
and arrangement of parts_. The oscillograph is furnished complete
with a three element electro-magnet galvanometer, optical system,
shutter and shutter operating mechanism, film driving motor and cone
pulleys, photographic and tracing attachments, 6 film holders, and
the following repair parts, for vibrators: 6 extra suspension strips;
6 vibrator mirrors; 1 box gold leaf fuses; 1 bottle mirror cement; 1
bottle damping liquid.]
[Illustration: FIG. 2,602.--Oscillogram showing the direct current
pressure of a 25 cycle rotary converter (below), and (above) the
pressure wave taken between one collector ring and one commutator
brush. The 12 ripples per cycles in the direct current voltage are
due to a 13th harmonic in the alternating current supply.]
=Glow Light Oscillograph.=--This device consists of two aluminum rods
in a partially evacuated tube, their ends being about two millimeters
apart. When an alternating current of any frequency passes between
them a sheath of violet light forms on one of the electrodes, passing
over to the other when the current reverses during each cycle. The
phenomenon may be observed or photographed by means of a revolving
mirror.
[Illustration: FIG. 2,603.--Curves by Morris, _illustrating the_
=dangerous rush of current which may occur when switching on a
transformer=. The circuit was broken at F and made again at G. The
current was so great as to carry the spot of light right off the
photographic plate due to the fact that a residual field was left in
the core after switching off, and on closing the switch again the
direction of the current was such as to tend to build up the full
flux in the same direction as this residual flux. =The dotted lines=
have been drawn in _to show how the actual waves were distorted from
the normal_.]
=Moving Iron Oscillograph.=--This type is due to Blondel, to whom
belongs the credit of working out and describing in considerable
detail the principles underlying the construction of oscillographs.
The moving iron type of oscillograph consists of a very thin
vane of iron suspended in a powerful magnetic field, thus forming
a polarized magnet. Near this strip are placed two small coils
which carry the current whose wave form is to be measured.
The moving iron vane has a very short period of vibration and
can therefore follow every variation in the current.
[Illustration: FIG. 2,604.--Siemens-Blondel =moving coil type=
oscillograph. The coil is in the shape of a loop of thin wire, which
is suspended in the field of an electro-magnet excited by continuous
current. The current to be investigated is sent through this loop,
which in consequence of the interaction of current and magnetic
field, begins to vibrate. The oscillations are rendered visible by
directing a beam of light from a continuous current arc lamp onto a
small mirror fixed to the loop. The light reflected by the mirror is
in the form of a light strip, but by suitable means this is drawn
out in respect of time, so that a curve truly representing the
current is obtained. The loop of fine wire is stretched between two
supports and is kept in tension by a spring. As the spring tension
is considerable, the directive force of the vibrating system is
large, and its natural periodicity very high. The mirror is fixed in
the center of the loop, and has an area of 1 square mm. In order to
protect the loops from mechanical injury they are built into special
frames. The mirrors are of various sizes, the loop for demonstration
purposes (projection device) being provided with the largest
mirror and the most sensitive loop with a mirror of the smallest
dimensions.]
Attached to the vane is a small mirror which reflects a beam of
light upon some type of receiving device.
The Siemens-Blondel oscillograph shown in fig. 2,604, is of
the _moving coil_ type, being a development of the moving iron
principle.
=Moving Coil Oscillograph.=--The operation of this form of
oscillograph is based _on the behaviour of a movable coil in a
magnetic field_.
[Illustration: FIGS. 2,605 and 2,606.--=Oscillograms= reproduced from
a paper by M. B. Field on "A Study of the =Phenomena of Resonance=
by the Aid of Oscillograms" (_Journal_ of _E. E._, Vol. XXXII). =The
effect of resonance= on the wave forms of alternators has been the
subject of much investigation and discussion; it is a matter of vital
importance to the engineer in charge of a large alternating current
power distribution system. Fig. 2,605 shows the pressure curve of an
alternator running on a length of unloaded cable, the 11th harmonic
being very prominent. Fig. 2,606 shows the striking alteration
produced by reducing the length of cable in the circuit and thus
causing resonance with the 13th harmonic.]
It consists essentially of a modified moving coil galvanometer
combined with a rotating or vibrating mirror, a moving
photographic film, or a falling photographic plate. The
galvanometer portion of the outfit is usually referred to as the
oscillograph as illustrated in figs. 2,608 to 2,612, representing
diagrammatically the moving system.
In the narrow gap between the poles S, S of a powerful magnet
are stretched two parallel conductors formed by bending a thin
strip of phosphor bronze back on itself over an ivory pulley P.
A spiral spring attached to this pulley serves to keep a uniform
tension on the strips, and a guide piece L limits the length of
the vibrating portion to the part actually in the magnetic field.
A small mirror M bridges across the two strips as shown. The
effect of passing a current through such a "vibrator" is to cause
one of the strips to advance while the other recedes, and the
mirror is thus turned about a vertical axis.
[Illustration: FIG. 2,607.--General view of electro-magnet
form of Duddell moving coil oscillograph, showing oil bath and
electro-magnet. This instrument is specially designed to have a very
high natural period of vibration (about 1/10,000 of a second) so as
to be suitable for accurate research work. It is quite accurate for
frequencies up to 300 per second. In the figure, A is the brass oil
bath in which two vibrators are fixed; B, core of electro-magnet
which is excited by two coils, one of which, C, is seen. The ends of
these two coils are brought out to four terminals at D, so that the
coils may be connected in series for 200 volt, or in parallel for 100
volt circuits. The bolts, E,E, hold the oil bath in position between
the poles of the magnet. F,F,F (one not seen), are levelling screws;
G,G, terminals of one vibrator; H, fuse; K, thermometer with bulb in
center of oil bath.]
[Illustration: FIGS. 2,608 to 2,612.--=Vibrator= of Duddell moving
coil oscillograph and =section through oil bath= of electro-magnet
oscillograph. =The vibrator consists of= a brass frame W, which
supports two soft iron pole pieces P,P. Between these, a long narrow
groove is divided into two parts by a thin soft iron partition,
which runs up the center. The current being led in by the brass wire
U, passes from an insulated brass plate to the strip, which is led
over an ivory guide block, down one of the narrow grooves and over
another guide block, the loops round the ivory pulley O, which puts
tension on the strip by the spring N, back to the guide block again,
up the other narrow groove, and out by way of the insulated brass
plate and lead U. Halfway up the grooves the center iron partition R
is partially cut away to permit of a small mirror M, bridging across
from one strip to the other, being stuck to the strips by a dot of
shellac at each corner. The figure illustrates one type of vibrator
in which P is removable from W for ease in repairing. In type 1,
these pole pieces P,P are not removable. =The vibrators= are placed
side by side in the gap between the poles S,S of the electro-magnet,
see fig. 2,610. Each vibrator is pivoted about vertical centers, the
bottom center fitting in the base of the oil bath, and the one at
the top being formed by a screw in the cock piece Y. It can thus be
easily turned in azimuth, its position being fixed by the adjusting
screw L, a spiral spring serving to keep the vibrator always in
contact with this screw. Since each cock piece can be independently
moved forward or backward, each vibrator can be tipped slightly in
either of these directions so that complete control over the mirrors
is obtained and reflected spots of light may be made to coincide with
that reflected from the fixed zero mirror, which latter is fixed to a
brass tongue in between the two vibrators. =A plano-convex lens= of
50 cm. focal length is fixed on the oil bath in front of the vibrator
mirrors to converge the reflected beams of light. It will be noticed
that this lens is slightly inclined so that no trouble will be given
by reflections from its own surface. The normal distance from the
vibrator mirrors to the scale of photographic plate is 50 cm., and
at this distance, a convenient working deflection on each side of
the zero line is 3 to 4 cm. This is obtained with a R.M.S. current
through the strips of from .05 to .1 of an ampere according to wave
form, etc. =The maximum deflection= on each side of the zero line
should not exceed 5 cm. while the maximum R.M.S. current through the
strips should in no case exceed .1 ampere.]
Each strip of the loop passes through a separate gap (not shown
in the figure). The whole of the "vibrator," as this part of the
instrument is called, is immersed in an oil bath, the object
of the oil being to damp the movement of the strips, and make
the instrument dead beat. It also has the additional advantage
of increasing by refraction the movement of the spot of light
reflected from the vibrating mirrors.
The beam of light reflected from the mirror M is received on
a screen or photographic plate, the instantaneous value of the
current being proportional to the linear displacement of the spot
of light so formed.
With alternating currents, the spot of light oscillates to and
fro as the current varies and would thus trace a straight line.
To obtain an image of the wave form, it is necessary to traverse
the photographic plate or film in a direction at right angles to
the direction of the movement of the spot of light.
[Illustration: FIG. 2,613.--Duddell =moving coil oscillograph= _with
projection and tracing desk outfit_. The outfit is designed for
teaching and lecture purposes. =In operation=, _after the beam of
light from the arc lamp has been reflected from the oscillograph
mirrors, it falls on a vibrating mirror which gives it a deflection
proportional to time in a direction at right angles to the deflection
it already has and which is proportional to the current passing
through the oscillograph_. It is therefore only necessary to place a
screen in the path of the reflected beam of light to obtain a trace
of the wave form. Since the vibrating mirror is vibrated by means
of a cam on the shaft of a synchronous motor, which motor is driven
from, or synchronously with, the source of supply whose wave form is
being investigated, the wave form is repeated time after time in the
same place on the screen, and owing to the "persistence" of vision,
the whole wave appears stationary on the screen. The synchronous
motor with its vibrating mirror, mentioned above, is located
underneath the "tracing desk." When used in this position a wave a
few centimeters in amplitude is seen through a sheet of tracing paper
which is bent round a curved sheet of glass. A permanent record of
the wave form can thus easily be traced on the paper. A _dark box_
which is designed to hold a sheet of sensitized paper in place of
the tracing paper, can be fitted in place of the tracing desk. Thus
an actual photographic record of the wave form is obtained. If the
synchronous motor be transferred from its position underneath the
tracing desk to the space reserved for it close to the oscillograph,
the beam of light is then received on a large mirror which is placed
at an angle of about 45 degrees to the horizontal and so projects the
wave form onto a large vertical screen which should be fixed about
two and a half meters distant. Under these conditions a wave form of
amplitude 50 cm. each side the zero line may be obtained which is
therefore visible to a large audience.]
=Ques. How are the oscillograms obtained in the Duddell moving coil
oscillograph?=
Ans. In all cases the oscillograms are obtained by a spot of light
tracing out the curve connecting current or voltage with time. The
source of light is an arc lamp, the light from which passes first
through a lens, and then, excepting when projecting on a screen,
through a rectangular slit about 10 mm. long by 1 mm. wide. The
position of the lamp from the lens is adjusted till an image of the
arc is obtained covering the three (two moving, one fixed) small
oscillograph mirrors. The light is reflected back from these mirrors
and, being condensed by a lens which is immediately in front of them,
it converges till an image of the slit is formed on the surface where
the record is desired. All that is necessary now to obtain a bright
spot of light instead of this line image is to introduce in the path
of the beam of light a cylindrical lens of short focal length.
[Illustration: FIGS. 2,614 and 2,615.--Sectional view of =permanent
magnet form= of Duddell =moving coil oscillograph.= This instrument
has a lower natural period of vibration (1/3000 second) than the
type shown in fig. 2,612, and therefore is not capable of accurately
following wave forms of such high frequency, but it is sufficiently
quick acting to follow wave forms of all ordinary frequencies with
perfect accuracy. It is easier to repair, and more portable owing to
the fact that the magnetic field is produced by a permanent magnet
instead of an electro-magnet. This also renders the instrument
suitable for use on high tension circuits without earth connection,
as, owing to the fact that no direct current excitation is required,
the instrument is more easily insulated than other types.]
=Ques. What is the function of the mirrors on the vibrating vane?=
[Illustration: FIG. 2,616.--Diagram of connections of Duddell
oscillograph =to high pressure circuit.= The modification necessary
for high pressure circuit only applies to the vibrator which gives
the pressure wave and consists in adding two more resistances, R_{4}
and R_{5}. Referring to fig. 2,617, it will be seen that in case fuse
f_{2} blows, or the vibrator be accidentally broken, the full supply
voltage is immediately thrown on the instrument itself. This is not
permissible in high voltage work and therefore the resistance R_{5}
is introduced as a permanent shunt to the oscillograph vibrator.
The resistance R_{4} is an exact duplicate of R_{2} being a 21 ohm
plug resistance box for adjusting the sensitivity of the vibrator
to an even figure. =In practice= R_{5} is usually a part of R_{1},
and in most of the high voltage resistances, two taps are brought
out near one end to serve as R_{5}. One of these taps is usually 50
ohms distant from the end terminal and the other only 5 ohms from
the end. =The use of these taps is as follows:= The large resistance
consisting of R_{1} + R_{5} is so chosen with respect to the voltage
of the circuit under investigation that the current through R_{1} is
about .1 ampere. _It should never be more than this continuously._
Then R_{4} is connected to the 50 ohm tap, and since the resistance
of the oscillograph vibrator circuit is variable from about 5 to
26 ohms by means of R_{4}, the current can be controlled through
the oscillograph from about .066 to .091 of an ampere, enabling an
open wave form to a convenient scale to be obtained. =If it now
be desired to record large rises of pressure,= such as may occur
in cases of resonance, _the height of the wave must be reduced in
order to keep these rises on the plate_. This is accomplished by
disconnecting R_{4} from the 50 ohm tap and connecting it to the 5
ohm tap, when the current through the vibrator will be from .05 to
.016 of an ampere according to whether the resistance R_{4} is in
or out of circuit. When, instead of using the _falling plate_, the
_cinematograph_ camera is being used, it becomes necessary always
to work on the 5 ohm tap since the width of the film is much less
than that of the plate, and the current must therefore be less.
=In experiments where sudden rises of voltage are expected= _it is
often advisable to keep_ R_{1} _as great as possible._ That end of
the resistance R_{1} referred to as R_{5} in the diagram should be
securely connected to the supply main and no switch or fuse used. A
switch may, if desired, be used in series with R_{1}, provided it be
inserted at the point where R_{1} joins the supply main remote from
R_{5}. It will be seen that fuses f_{1} and f_{2} are shown. Provided
that the connections are always made in accordance with the diagram,
and the vibrators are always shunted by R_{5} or R_{3} respectively,
there is not much objection to the use of these fuses, but on general
principles it is wise to avoid fuses in high tension work and
accordingly with each permanent magnet oscillograph, dummy fuses are
supplied, which can be inserted in place of the ordinary fuses when
desired. _The remark previously made about keeping both vibrators
and the frame of the instrument at approximately the same pressure
applies with additional emphasis in high pressure work._]
Ans. They simply control the direction of a beam of light in a
horizontal plane in such a manner that its deflection from a zero
position depends on the current passing through the instrument, and
it is therefore evident that the oscillograph is not complete without
means of producing a time scale.
[Illustration: FIG. 2,617.--Diagram of connections of Duddell
oscillograph =to low pressure circuit=, R_{1} is a high non-inductive
resistance connected across the mains in series with one of
the vibrators. S_{2} is a switch, and f_{2}, the fuse (on the
oscillograph in this circuit). The resistance of R_{1} in ohms
should be rather more than ten times the voltage of the circuit,
so that a current of a little less than .1 of an ampere will pass
through it. The vibrator will then give the curve of the circuit
on an open scale. (For the projection oscillograph, the resistance
R_{1} should be only twice the supply voltage, since .5 of an ampere
is required to give full scale deflection on a large screen.) =To
obtain the current wave form=, _the shunt_ R_{3} _is connected in
series with the circuit under investigation and the second vibrator
is connected across this shunt_. Here also f_{1} is a fuse, S_{1} a
switch, and R_{2} an adjustable resistance box. The switch S_{1} is
however unnecessary if the plug resistance box supplied for R_{2}
be used, since an infinity plug is included in this box. The shunt
R_{3} should have a drop of about 1 volt across it in order to give
a suitable working current through the vibrator. The resistance
R_{2} is not absolutely essential, but it is a great convenience in
adjusting the current through the vibrator. It is a plug resistance
box, the smallest coil being .04 of an ohm and the total 21 ohms.
Being designed to carry .5 ampere continuously it can be used
with any other type of Duddell oscillograph, and by its use the
sensitiveness of the vibrator can be adjusted so that a round number
of amperes in the shunt gives 1 mm. deflection. This adjustment is
best made with direct current. =It should be noted= in connecting
the oscillograph in circuit, that _the two vibrators should be so
connected to the circuit that it is impossible that a higher pressure
difference than_ 50 _volts should exist between one vibrator and the
other, or between either vibrator and the frame_. To ensure attention
to this important point, a brass strap is provided which connects
the two vibrators together and to the frame of the instrument. This
does not mean that this point must necessarily be earthed since the
frame of the instrument is insulated from the earth. It is advisable,
however, to earth it when possible.]
[Illustration: FIGS. 2,618 and 2,619.--=Two curves= _obtained with
the_ =falling plate camera= and illustrating _the discharge of a
condenser through an inductive circuit_. =When taking curve A= the
resistance in the circuit was very small compared to the inductance,
while =before taking curve B= an additional non-inductive resistance
was inserted in the circuit so that the oscillations were damped out
much more rapidly although the periodic time remained approximately
constant.]
=Ques. How is the time scale produced?=
Ans. Either the surface on which the beam of light falls may be
caused to move in a vertical plane with a certain velocity, so that
the intersection of the beam and the plane surface traces out a curve
connecting current with time (a curve which becomes a permanent
record if a sensitized surface be used); or, the surface may remain
stationary and in the path of the horizontally vibrating beam may be
introduced a mirror which rotates or vibrates about a horizontal
axis, thus superposing a vertical motion proportional to time on the
horizontal vibration which is proportional to current, and causing
the beam of light to trace out a curve connecting current and time on
the stationary surface.
=Ques. What kind of recording apparatus is used with the Duddell
oscillograph?=
Ans. A falling plate camera, or a cinematograph film camera.
[Illustration: FIG. 2,620.--Synchronous motor with vibrating mirror
as used with Duddell moving coil oscillograph. =Since the motor must
run synchronously= with the wave form it is required to investigate,
_it should be supplied with current from the same source_. The motor
can be used over a wide range of frequencies (from 20 to 120). When
working at frequencies below 40, it is advisable to increase the
moment of inertia of the armature, and for this purpose a suitable
brass disc is used. =The armature carries a sector=, _which cuts off
the light from the arc lamp during a fraction of each revolution, and
a cam which rocks the vibrating mirror_. =It makes one revolution
during two complete periods=, and the cam and sector are so arranged
that during 1½ periods, the mirror is turning with uniform angular
velocity, while during the remaining half period, the mirror is
brought back quickly to its angular position, the light being cut off
by the sector during this half period.]
=Ques. Explain the operation of the falling plate camera.=
Ans. In this arrangement a photographic plate is allowed to fall
freely by the force of gravity down a dark slide. At a certain point
in its fall it passes a horizontal slit through which the beams of
light from the oscillograph pass, tracing out the curves on the plate
as it falls.
[Illustration: FIGS. 2,621 to 2,623.--Interior of cinematograph
camera as used on Duddell moving coil oscillograph =for obtaining
long records=. The loose side of case is shown removed and one of
the reels which carry the film lying in front. =The spool of film=
which is placed on the loose reel A, passes over the guide pulley
B, then vertically downward between the brass gate D (shown open in
the figure), and the brass plate C. =The exposure aperture= is in
the plate C and can be opened or closed by a shutter controlled by
the lever M. The groove in the plate C, and the springs which press
the gate D flat on the plate C, prevent the film having any but a
vertical motion as it passes the exposure slit. E is the sprocket
driving pulley which engages with the perforations on the film and
unwinds it from the reel A to reel H. Outside the case on the far
side of it is secured to the axle G a three speed cone pulley. This
is driven by a motor of about 1/7 horse power, which also drives,
through the gears shown, the sprocket pulley E. Close to the grooved
cone pulley is a lever carrying a jockey pulley L, and a brake, which
latter is normally held onto the cone pulley by a spring and so
causes the loose belt to slip. By pressing a lever which is attached
to the falling plate camera case, the brake can be suddenly released
and at the same time the jockey pulley caused to tighten the belt
onto the grooved cone pulley, so that the starting and stopping of
the film is controlled independently of the driving motor, and being
quickly accomplished avoids waste of film. =Both reels= are alike and
each is made in two pieces. =The upper reel= is loose on its axle and
its motion is retarded slightly by a friction brake. =The lower reel=
is also loose on its axle, but it is driven by means of a friction
clutch, the clutch always rotating faster than the reel so that the
used film delivered by the sprocket pulley E is wound up as fast as
delivered. K is the front face of one reel, the boss on it pushes
into the tube on the other half H, which serves not only to unite the
two halves, but also to secure the end of the film which is doubled
through J.]
The mean speed of the plate at the moment of exposure is about
13 feet per second. This speed is very suitable for use with
frequencies of from 40 to 60 periods per second. A cloth bag is
used to introduce the plate to the slide.
A catch holds the plate until it is desired to let it fall.
Inside the case, is a small motor, 100 or 200 volts direct
current, driving four mirrors which are fixed about a common axis
with their planes parallel to it.
[Illustration: FIG. 2,624.--Portion of oscillograph record taken with
cinematograph film camera, =showing the rush of current= and =sudden
rise of voltage= _at the moment of switching on a high pressure
feeder_.]
By looking through a small slot in the end of the camera into
these rotating mirrors, the observer sees the wave form which the
oscillograph is tracing out and is thus able to make sure that
he is obtaining the particular wave form or other curve desired
before exposing the plate.
[Illustration: FIG. 2,625.--Portion of oscillograph record taken with
a cinematograph film camera =showing the effect of switching off
a high pressure feeder= and illustrating the violent fluctuations
produced by sparking at the switch contacts.]
The plate falls into a second red cloth bag which is placed on
the bottom of the slide. The plates used are "stereoscopic size",
6¾" × 3¼" (17.1 × 8.3 cm.).
=Ques. For what use is the cinematograph camera adapted?=
Ans. For long records.
For instance, in investigations, such as observation on the
paralleling of alternators, the running up to speed of motors,
and the surges which may occur in switching on and off cable,
etc. The cinematograph camera fits on to the falling plate case
and by means of which a roll of cinematograph film can be driven
at a uniform speed past the exposure aperture, enabling records
up to 50 metres in length to be obtained. An interior view of the
cinematograph camera is shown in fig. 2,621.
[Illustration: FIG. 2,626.--Curves reproduced from an article by J.
T. Morris in the _Electrician_. "On recording transitory phenomena by
the oscillograph."]
[Illustration: FIG. 2,627.--First rush of current from an alternator
when short circuited, showing unsymmetrical initial wave of current,
becoming symmetrical after a few cycles. 25 cycles.]
[Illustration: FIG. 2,628.--Pressure wave obtained from narrow
exploring coil on alternator armature, indicating distribution of
field flux. The terminal voltage of the alternator is very nearly a
sine wave, 60 cycles; about 17 volts.]
SOME OSCILLOGRAPH RECORDS
[Illustration: FIG. 2,629.--The waves of voltage and current of an
alternating arc. A, voltage wave; B, current wave showing low power
factor of the arc without apparent phase displacement. 60 cycles.]
[Illustration: FIG. 2,630.--Rupturing 650 volt circuit. A, current
wave; B, 25 cycle wave to mark time scale.]
[Illustration: FIG. 2,631.--First rush of current from alternator
when short circuited, showing unsymmetrical current wave, also wave
of field current caused by short circuit current in armature. Upper
curve, armature current; lower curve, field current.]
[Illustration: FIG. 2,632.--Mazda (tungsten) lamp, showing rapid
decrease to normal current as filament heats up. 25 cycles.]
[Illustration: FIG. 2,633.--Current wave in telephone line
corresponding to sustained vowel sound "_i_," as in machine; voice
pitched at A 110.]
[Illustration: FIG. 2,634.--Carbon lamp, showing rapid increase to
normal current as filament heats up. 25 cycles.]
[Illustration: FIG. 2,635.--Short circuit current on direct current
end of rotary converter, 21,500 amperes maximum. Upper curve, direct
current voltage; lower curve, direct current amperage. Duration of
short circuit about .1 second.]