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