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قراءة كتاب Hawkins Electrical Guide Vol. 8 (of 10) A Progressive Course of Study for Engineers, Electricians, Students, and Those Desiring to Acquire a Working Knowledge of Electricity and Its Applications
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Hawkins Electrical Guide Vol. 8 (of 10) A Progressive Course of Study for Engineers, Electricians, Students, and Those Desiring to Acquire a Working Knowledge of Electricity and Its Applications
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.
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.
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. |
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. | ||
