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.

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.

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.

{ cathode ray;
{ by use of various types { glow light;
2. constantly recording { of =oscillograph=, { moving iron;
{ such as { moving coil;
{ hot wire.

=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.=

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.

The apparatus is calibrated by passing a known constant current
through the resistance.

=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.=

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.

=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.

=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._

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.

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.

The ray then moves so as to produce an energy diagram on the fluorescent screen.

The instrument is much used in wireless telegraphy, as it is capable of showing the characteristics of currents of very high frequency.

=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.

=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.

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_.

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.

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.

=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.

=Ques. What is the function of the mirrors on the vibrating vane?=

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.

=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.

=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.

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.

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.

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.

SOME OSCILLOGRAPH RECORDS