Зображення сторінки
PDF
ePub

of 98.2 per cent at full load of 350 k.w. each, and the rotary converters of 990 k.w. each gave 96 per cent efficiency at full load, making a combined efficiency of 94.2 per cent. In comparing this with a system using alternating current throughout, it should be remembered that the latter would require two sets of transformers to bring down the pressure from 6600 volts to that used in the lamps. In other words, a set of static transformers would be substituted for the rotary converters, so that the combined efficiency would be 98.2 x 98.2 = 96.4 instead of 94.2 per cent. On the other hand, it would not be possible to use direct current arc lamps or motors, storage batteries, or any electrolytic apparatus in connection with a purely alternating current system. Furthermore, losses and the difficulties of regulation, etc., caused by inductance, capacity, and low-power factor would occur with alternating current in the distributing conductors and house-wiring. In most practical cases these would more than offset the additional loss of 2.2 per cent in the rotary converters.

A form of oil circuit-breaker used in the very high-tension (6600 volts) circuits of this system is illustrated in Fig. 172. It consists of a magnet operating the valve of a pneumatic cylinder the piston of which raises or lowers a metal cross-head carrying three wooden rods that extend down into three cells, each containing the switching apparatus for one phase of the circuit. A section through one of these cells is shown in Fig. 172, the others being the same, and separated from each other by 4-inch brick partitions to act as barriers and prevent arcing between the switches. The actual circuit-breaking parts are connected to the movable rods and are submerged in oil. This type of circuit-breaker is rated at 10,000 volts and 800 amperes per phase.

In connection with the above-described system of three-phase transmission and direct current distribution, it is common practice to employ double current generators, rectifiers, and frequency changers, which will now be explained under their respective headings.

Double Current Generators. — Since a rotary converter is provided with a direct current commutator and with alternating current collecting rings connected to its armature winding (page 97), it may be employed as a generator if driven by an engine or other source of power, and polyphase or direct currents or

both may be obtained from it. In some plants these machines are used as converters at one time and as generators at other times. They may be run as polyphase generators to supply energy at a distance through step-up transformers, and can also be utilized to charge storage batteries with direct current, these two functions being performed at different hours of the day or at the same time, if desired. When so used they are termed double current generators. The use of these machines in the stations of the Chicago Edison Company is described in The Electrical World and Engineer, May 19, 1900, which also contains complete illustrations and description of the three-phase transmission and direct current distribution system employed on a very large scale by that company.

Rectifiers. — This name is given to those forms of apparatus in which single or polyphase alternating currents are changed into direct currents by means of commutators; these machines being without field magnets or armatures. This distinguishes them from rotary converters which are complete dynamo machines. Rectifiers are much simpler, cheaper, and more efficient than converters; nevertheless, they have not been very generally introduced, chiefly on account of practical difficulties in keeping them in adjustment and avoiding sparking.

In principle a rectifier is a reversing commutator C in Fig. 174, similar to that of an open-coil arc dynamo (Vol. I. p. 332). In

[ocr errors][merged small][ocr errors][merged small]

this case a two-part commutator is represented, one segment being continuously connected to one wire M and the other segment to the other wire N of the supply circuit. These connections are not shown, but are made through a pair of brushes and rings connected respectively to the two commutator segments. The wires M N lead from a single-phase generator G through a constant current transformer T, and it is evident that the connections

is avoided. In

m

Hi

[ocr errors]

of the brushes A B will be reversed at every half revolution of the commutator. If a circuit feeding arc lamps L in series be connected to brushes A B, it is evident that the current will flow through the lamps always in the same direction, provided the connections are reversed exactly when the alternating current reverses. In Fig. 175 the alternating current A,B,C, D, F, etc., is converted into the pulsating direct current A, B, C, E, F, etc., if the reversals occur at the points C, F, H, etc. It should be noted also that the F current is zero at the instant of reversal, hence sparking is avoided. In A order that the action shall take place correctly the commutator C must revolve synchronously and in proper Fig. 175. Rectified Single-phase

Current. phase with the alternating current. For a two-part commutator the number of revolutions per second must equal the frequency, and the brushes A B must be set very carefully so that they pass from one commutator segment to another at the instant when the current is zero.

The constant current transformer T may be one of the forms already shown in Figs. 135, 136, and 137; and it will feed the lamps practically the same as if they were connected to it directly, except that the current through them will be unidirectional instead of alternating, thus allowing ordinary types of direct current arc lamps to be used. The fact that the current is pulsatory is not objectionable for arc lighting, since the standard Brush and Thomson-Houston arc dynamos produce currents of this character. Incandescent lamps may also be operated equally well with this current. A low frequency of 25 is too low for very satisfactory running of either kind of lamp, but at 40 or more periods per second both work well. The pulsating current is also applicable to the charging of storage batteries and to other electrochemical purposes ; but for the operation of the ordinary direct current motors it would be likely to cause sparking unless the pulsations were “smoothed out” by inductance, storage batteries, or other suitable means.

Rectifiers have been more generally employed in England than in America, the Ferranti type being used in a number of stations. This consists essentially of a synchronous alternating current

motor driving the rectifying commutator. A constant current transformer with movable coils somewhat similar to that shown in Fig. 135 is employed in connection with this rectifier, which is usually applied to arc lighting.

An interesting example of rectifier is that installed by Mr. W. S. Barstow in Brooklyn. The 6600 volt three-phase current from the main generating station is supplied to the primary of a con stant current transformer of the type illustrated in Fig. 135, the secondary circuit at 6600 volts is led through the rectifier, which consists simply of a three-part commutator driven by a synchronous motor, the three-phase conductors being connected respectively to the three segments of the commutator. Two brushes set diametrically opposite each other are applied to the commutator, and are connected to a series circuit of arc lamps. The standard form of Thomson-Houston commutator is employed with the usual blower attachment to suppress sparking. Since the ThomsonHouston armature has practically a three-phase winding of the Y form, the current supplied to its commutator segments is practically the same as in a three-phase rectifier. In the latter case the commutator is placed at a distance from the armature winding, and is driven by a synchronous motor. Owing to changes in the phase of a motor when variations in load, etc., occur on the circuit, the position of the brushes may not agree exactly with the points of zero current, so that sparking will occur. Since the maximum potential exists between adjacent commutator segments separated only by a small air-gap, there is a strong tendency to flashing or “ring-fire” around the commutator, thus short-circuiting a pressure of several thousand volts. This constitutes the chief difficulty in the operation of rectifiers.

The direct current obtained by rectifying a two- or three-phase current does not pulsate so much as the rectified single-phase current in Fig. 175, for the reason that in the former case two or three waves are superimposed. If, for example, we reverse all the waves below the zero line in Figs. 110 and 114, the resulting direct current would be represented by a curve obtained by summing up the ordinates at every point, the fluctuations being much less than in Fig. 175.

Frequency Changers. — As their name implies, these machines are used for changing the frequency of an alternating current. Ordinarily the object is to increase a low frequency of say 25 periods per second, which is hardly high enough for arc or incandescent lighting, to 60 periods for example, which is more satisfactory for the purpose and is also suited to the usual types of motors and transformers. The type of frequency changer made by the General Electric Company is essentially a polyphase transformer with a movable secondary. The latter consists of a secondary or armature suitably wound for any desired voltage and phase, which is mechanically revolved and acted upon by the rotating field of a polyphase primary. If the secondary is revolved in a direction opposite to that of the rotary field, obviously the frequency of the current in the secondary will be higher than the frequency of the current supplied to the primary, and vice versa. When the secondary rotates against the rotary field, it acts as a generator and requires power to drive it, and when it turns with the field the machine acts as an induction motor. Thus we see that there is a combined generator and transformer action when the frequency is raised.

The secondary may be rotated by any suitable mechanical means, the synchronous polyphase motor being ordinarily used for this purpose. By over-exciting the field of the latter, the leading current thus produced (p. 134) may be made to balance the lag caused by the primary of the frequency changer, thereby raising the power factor on the supply circuit. The output of a frequency changer when the frequency is increased is equal to the sum of the mechanical power applied to it and of the electrical imput in the primary, less the losses. The frequency of the secondary current is equal to the number of poles of the primary, multiplied by the sum of the revolutions per second of the shaft and of the field (when they run in opposite directions). If the primary has a three-phase winding and the secondary is provided with a twophase winding, the current is changed from three- to two-phase at the same time that the frequency is raised. It is evident also that the opposite change may be effected by transposing the windings.

Tests of a 200 k.w. General Electric frequency changer of this type gave the following results :

Primary wound for 6000 volts, 3 phase and 25 cycles.
Secondary “ “ 2400 - 2 “ “ 62! "

« НазадПродовжити »