How to Test Generators.—In the operation of electrical stations, many problems dealing with the generators installed therein can be readily solved by the aid of characteristic curves, which bear a relation to the generators similarly as do indicator diagrams to steam engines.
Fig. 2,908.—General form of characteristic curves for a series dynamo. The general curve that may be expected is OA. It is obtained practically in the same manner as for the shunt characteristic curve, except that no field rheostat is employed. Commencing with no load or amperes, there will probably be a small deflection noticeable on the voltmeter, due to the residual magnetism. The other readings are taken with successive reductions of main current resistance. The curve OA thus obtained for a certain series generator is practically a straight line at the beginning, representing thereby a proportional increase of voltage with increase of current, but after a certain current is reached (about 20 amperes in this case) the curve flattens and takes a downward direction. The turning point occurs in the characteristic curves of all series generators, and it denotes the stage at which the iron magnet cores become so saturated with lines of magnetic force that they will not readily allow more to pass through them; this turning point is technically known as the point of saturation, and the current corresponding (20 amperes in this case) is called the critical current of the dynamo. The point of saturation in any given series machine is governed by the amount of iron in the magnetic circuit; its position in the curve therefore varies according to the design of the generator as does also the critical current. The value of the latter is important inasmuch as the valuable features of a series generator assert themselves only when the machine is supplying a greater number of amperes than that of the critical current, for if the series generator be worked along that part M A of the curve to the right of the point of saturation it becomes nearly self-regulating as regards current, because as the current increases the voltage drops. In the diagram in addition to the characteristic curve O A, which may more definitely be called an external characteristic curve on account of representing the conditions external to the generator, there is shown a total characteristic curve, O C B. The latter curve represents the relation between the current and the total voltage developed in the armature, and may be plotted from the external characteristic curve if the resistance of the armature between brushes and the resistance of the series field winding be known. For example, assume these combined resistances amount to .6 ohm. At 30 amperes there would be required 30 × .6 = 18 volts to force this current through the armature and field windings. At 30 amperes the external pressure is 65 volts, as shown by the curve O A; the total voltage developed for 30 amperes is, therefore, the external voltage plus the internal voltage or 65 + 18 = 83 volts. Plotting 83 volts for 30 amperes will give one point for the external characteristic curve of this machine, and by determining in like manner the total voltages developed for six or eight different currents over the scale, sufficient data will be at hand for plotting and drawing in the curve O C B.
In steam engineering, a man who did not fully understand the method of taking an indicator diagram would be considered not in touch with his profession, and in electrical engineering the same would be true of one ignorant of the method of obtaining characteristic curves.
The necessary arrangement or connection of the generator from which it is desired to obtain a characteristic curve, consists in providing a constant motive power so that the machine may be run at a uniform speed, and when the field magnets of the generator are separately excited the field current from the outside source must also be maintained constant, preferably by a rheostat connected in the field of the auxiliary exciting machine. It is also necessary in every case that means be provided for varying the main current of the generator step by step from zero to maximum. This may best be done by employing a water rheostat, as shown in [fig. 2,909].
Ques. What instruments are needed in making a test of dynamo characteristics?
Ans. A voltmeter, ammeter, speed indicator, the usual switches and rheostats.
Ques. How is the apparatus connected?
Ans. It is connected as shown in [fig. 2,910].