| ALTERNATING CURRENTS | [997] to 1,066 |
|---|---|
| The word "alternating"—advantages of alternating current—direct current apparatus; alternating current apparatus—disadvantages of alternating current—alternating current principles—the sine—application and construction of the sine curve—illustrated definitions: cycle, alternation, amplitude, period, periodicity, frequency—commercial frequencies—advantages of low frequency—phase—phase difference—phase displacement—synchronism—"in phase"—curves illustrating "in phase" and "out of phase"—illustrated definitions: in phase; in quadrature, current leading; in quadrature, current lagging; in opposition—maximum volts and amperes—average volts and amperes—elementary alternator developing one average volt—virtual volts and amperes—effective volts and amperes—relation between shape of wave and form factor—wave form—oscillograph wave form records—what determines wave form—effect of one coil per phase per pole—single phase current; hydraulic analogy—two phase current; hydraulic analogy—two phase current distribution—three phase current; hydraulic analogy; distribution—inductance—the henry—inductive and non-inductive coils—hydraulic analogy of inductance—inductance coil calculations—ohmic value of inductance—capacity: hydraulic analogy—the farad—specific inductive capacity—condenser connections—ohmic value of capacity—lag and lead—mechanical analogy of lag—lag measurement—steam engine analogy of current flow at zero pressure—reactance—examples—choking coil—impedance curve—resonance—critical frequency—skin effect. | |
| ALTERNATING CURRENT DIAGRAMS | [1,067] to 1,100 |
| Definitions: impressed pressure, active pressure, self-induction pressure, reverse pressure of self-induction—rate of change in current strength—properties of right angle triangles—equations of the right triangle—representation of forces by wires—parallelogram of forces; the resultant—circuits containing resistance and inductance—graphical method of obtaining the impressed pressure—equations for ohmic drop and reactance drop—examples—diagram for impedance, angle of lag, etc.—circuits containing resistance and capacity—capacity in series, and in parallel—amount of lead—action of condenser—the condenser pressure—capacity pressure—equation for impedance—- examples and diagrams—circuits containing resistance, inductance, and capacity—impedance equation—examples and diagrams—equation for impressed pressure—examples and diagrams. | |
| THE POWER FACTOR | [1,101] to 1,124 |
| Definition of power factor—true watts—- apparent watts—ferry boat analogy of power factor—limits of power factor—effect of lag or lead—how to obtain the power curve—nature of the power curve—synchronism of current and pressure; power factor unity—case of synchronism of current and pressure with power factor less than unity—steam engine analogy of power factor—"wattless current;" power factor zero—examples of phase difference nearly 90 degrees—mechanical analogy of wattless current—why the power factor is equal to cos φ—graphical method of obtaining the active component—examples and diagrams—effect of capacity—diagrams illustrating why the power factor is unity when there is no resultant reactance in the circuit—usual value of power factor—power factor test—how alternators are rated; kva.—curves illustrating power factor—how to keep the power factor high—why power factor is important in station operation—wattmeter method of three phase power measurement. | |
| ALTERNATORS | [1,125] to 1,186 |
| Uses of alternators—classes of alternator—single phase alternators; essential features; width of armature coils—elementary single phase alternator—polyphase alternators—uses for two and three phase current—elementary three phase alternator—starting difficulty with single phase motors—six and twelve phase windings—belt or chain driven alternators—sub-base and ratchet device for tightening the belt—horse power transmitted by belts—best speeds for belts—advantages of chain drive; objections—direct connected alternator—"direct connected" and "direct coupled" units—revolving armature alternators; their uses—revolving field alternators—marine view showing that motion is purely a relative matter—essential parts of revolving field alternator—the terms "stator" and "rotor"—inductor alternators: classes, use, defects—hunting or surging in alternators—amortisseur windings—monocyclic alternators—diagram of connections—teaser coil—armature reaction—distortion of field—strengthening and weakening effects—superpositions of fields—three phase reactions—magnetic leakage—field excitation of alternators—self-excited alternator—direct connected exciter—gear driven exciters—slow speed alternators—fly wheel alternators—high speed alternators—water wheel alternators—construction of rotor—turbine driven alternators—construction—step bearing—alternators of exceptional character—asynchronous alternators—image current alternators—extra high frequency alternators—self-exciting image current alternators. | |
| CONSTRUCTION OF ALTERNATORS | [1,187] to 1,266 |
| Essential parts of an alternator—field magnets—methods of excitation: self-excited, separately excited, compositely excited—magneto—construction of stationary magnets—revolving field—slip rings—spider for large alternator—provision for shifting armature to give access to field—armatures—core construction—advantages of slotted core armatures—armature windings—classification: revolving and stationary windings—half coil and whole coil windings—concentrated or uni-coil winding; features; waveform—distributed or multi-coil windings: breadth of coil, partial and fully distributed coils—the Kapp coefficient—general equation for voltage—wire, strap, and bar windings—condition, governing type of inductor—coil covering—single and double layer multi-wire inductors and methods of placing them on the core—insulation—core stamping—single and multi-slot windings—- arrangement in slot of two layer bar winding—table of relative effectiveness of windings—single phase windings—advantage of half coil winding—two phase windings—shape of coil ends—three phase windings—shape of coil ends—kind of coil used with three phase windings—grouping of phases—two phase star connection—two phase mesh connection—three phase star connection—winding diagrams with star and Δ connections—three phase Δ connection—three phase winding with "short" coils—three phase lap winding star connection—three phase wave winding star connection—output of star and delta connected alternators—gramme ring armatures showing three phase star and mesh connections with direction of currents in the coils—features of star connection—characteristics of delta connection—proper ends to connect to star point—determination of path and value of currents in delta connection—points to be noted with Y connection—diagram of Y connection with return wire—chain or basket winding—skew winding—fed-in winding—imbricated winding—spiral winding—mummified winding—shuttle winding—creeping winding—turbine alternator winding: how the high voltage is obtained with so few poles; table of frequency and revolutions—turbine alternator construction—form of armature generally used—two pole radial slot field—parallel slot field—difficulty experienced with revolving armatures—how the field design is modified to reduce centrifugal force—examples of revolving fields. | |
CHAPTER XLVI
ALTERNATING CURRENTS
The word "alternating" is used with a large number of electrical and magnetic quantities to denote that their magnitudes vary continuously, passing repeatedly through a definite cycle of values in a definite interval of time.
As applied to the flow of electricity, an alternating current may be defined as: A current which reverses its direction in a periodic manner, rising from zero to maximum strength, returning to zero, and then going through similar variations in strength in the opposite direction; these changes comprise the cycle which is repeated with great rapidity.
The properties of alternating currents are more complex than those of continuous currents, and their behavior more difficult to predict. This arises from the fact that the magnetic effects are of far more importance than those of steady currents. With the latter the magnetic effect is constant, and has no reactive influence on the current when the latter is once established. The lines of force, however, produced by alternating currents are changing as rapidly as the current itself, and they thus induce electric pressures in neighboring circuits, and even in adjacent parts of the same circuit. This inductive influence in alternating currents renders their action very different from that of continuous current.
Ques. What are the advantages of alternating current over direct current?
Ans. The reduced cost of transmission by use of high voltages and transformers, greater simplicity of generators and motors, facility of transforming from one voltage to another (either higher or lower) for different purposes.
Figs. 1,206 to 1,212.—Apparatus which operates successfully on a direct current circuit. The direct current will operate incandescent lamps, arc lamps, electric heating apparatus, electro-plating and typing bath, direct current motors; charge storage batteries, produce electro-chemical action. It will flow through a straight wire or just as freely through the same wire when wound over an iron bar.