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 ofapparatus: wave indication; oscillographs—stepby step methods—Joubert's; four part commutator;modified four part commutator; ballistic galvanometer;zero; Hospitalier ondograph—constantly recordingmethods: cathode ray; glow light; moving iron; movingcoil; 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; generatorpanel; diagram of connections—simple method ofdetermining bus bar capacity—feeder panel—diagrams ofconnection for two phase and three phase installations. | ||
| ALTERNATING CURRENT WIRING | [1,885 to 1,914] | |
Effects to be considered in makingcalculations—induction; self- and mutual; mutualinduction, how caused—transpositions—inductanceper mile of three phase circuit, table—capacity;table—frequency—skin effect; calculation;table—corona effect; its manifestation;critical voltage; spacing of wires—resistance ofwires—impedance—power factor; apparent current;usual power factors encountered; example—wirecalculations—sizes of wire—table of the property ofcopper wire—drop; example—current—examplecovering 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 ofland; trouble after erection; water supply; servicerequiring direct current—size of plant; natureof load; peak load; load factor; machinery required;example; factors of evaporation; grate surface perhorse power—general arrangement of station;belt drive with counter shaft; desirable features ofbelt drive; conditions, suitable for counter shaftdrive; location of engine and boilers; the steam pipe;piping between engine and condenser; number and typeof engine; superheated steam; switchboard location;individual belt drive; direct drive—stationconstruction—foundations—walls—roofs—floors—chimneys;cost of chimneys and mechanical draft; high chimneysill advised—steam turbine; types: impulseand reaction; why high vacuum is necessary; theworking pressure—hydro-electric plants—waterturbines; types: impulse, reaction—isolatedplants—sub-stations; arrangement; three phaseinstallations; reactance coils in sub-stations; portablesub-stations. | ||
| MANAGEMENT | [1,989 to 2,114] | |
The term "management"—selection; generalconsiderations—selection of generators;efficiency of generators; size and number;regulation—installation; precautions;handling of armatures; assembling a machine; speedof generators; calculation of pulley sizes; gearwheels—belts; various belt drives; horsepower transmitted by belts; velocity of belt; endlessbelts—switchboards; essential points of differencebetween 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 oflamp method—cutting out alternator; precautions;hunting—alternators in series.Transformers; selection; efficiency; kind of oilused; detection of moisture; drying oil; regulation;transformers in parallel; polarity test—motorgenerators; various types and conditions requiringsame—dynamotors; precautions—rotaryconverters; objections to single phase type; operationwhen driven by direct current, by alternating current; mosttroublesome part; efficiency; overload; starting; startingcurrent.Electrical measuring instruments; location;readings; station voltmeters; points relating toammeters; attention necessary; usual remedies to correctvoltmeter—how to test generators; commercialefficiency; various tests.Station Testing: resistance measurement by "drop"method—methods of connecting ammeter voltmeter andwattmeter for measurement of power—motor testing:single phase motor—three phase motor, voltmeter andammeter method; two wattmeter method; polyphase wattmetermethod; one wattmeter method; one wattmeter and Y boxmethod—three phase motor with neutral brought out; singlewattmeter method—temperature test, three phase inductionmotor—three phase alternator testing: excitationor magnetization curve test—synchronous impedancetest—load test—three phase alternator or synchronousmotor temperature test—direct current motor orgenerator testing: magnetization curve—(shunt)external characteristic—direct current motor testing;load and speed tests—temperature test, "loading back"method—compound dynamo testing: externalcharacteristic, adjustable load—transformer testing:external characteristic, adjustable load—transformertesting: core loss and leakage or exciting currenttest—copper loss—copper loss by wattmeter measurementand impedance—temperature—insulation—internalinsulation—insulation resistance—polarity—windingor 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.