« НазадПродовжити »
flux, or other conditions of one with respect to the other. In other words, the ratio of conversion, that is, the relation between the primary and secondary voltages, is practically constant, no matter how much the speed or flux may be varied. To be sure the difference of potential between the secondary brushes may be decreased by introducing resistance in the primary circuit, but this merely has the effect of reducing the available voltage supplied to the motor. The amount of this reduction is the drop = IR, in which / is the primary current and R the resistance inserted. A corre-' sponding decrease in voltage is produced in the secondary circuit, but the ratio of conversion as measured at the brushes remains substantially unchanged. Resistance put in the secondary circuit will have a similar effect in decreasing the available potential, but in either case the loss of energy is considerable, its value in watts being PR. The so-called "regulation" is also seriously interfered with; that is, the available secondary voltage varies greatly with changes in the load, because any alteration in the current has a corresponding effect on the drop IR. Such a vari
1 Fig. 69. Motor-Dynamo.
ation in pressure would usually
be very objectionable; in electric lighting, for example, the voltage would fall as more lamps were added in parallel.
In order to secure independence of action between the motor and dynamo portions of a rotary transformer, the two armature windings should be carried by separate cores, each being acted upon by its own field magnet. This allows the field of the dynamo to be independently regulated, in order to vary the voltage generated. In fact, any of the well-known methods of dynamo regulation may be employed. For example, compound or over-compound winding applied to the dynamo field will give a constant or a rising pressure, with increase of current in the secondary circuit. In these cases the separate armatures may be mounted upon the same shaft, with only one pair of bearings, in the form shown in Fig. 69, or the two machines may be arranged upon the same base with an intermediate bearing, as represented in Fig. 70. If desired two entirely distinct machines may be belted or directly coupled together. In fact, almost any motor and dynamo may be employed in this way, provided the former has sufficient power to drive the latter, and the mechanical connection is arranged to give the proper speeds.
It is evident that the motor of a rotary transformer may be designed to operate with an alternating current, and the dynamo to generate a direct current, or vice versa, in order to convert alternating to direct currents, or the converse.
Still another type is the rotary converter, in which the same armature winding performs both the motor and dynamo functions. A simple form of this machine, shown in Fig. 71, consists of a ring armature, diametrically opposite points of the winding being respectively connected to two collecting rings. When the armature is supplied with direct current in the usual way by the brushes + and —, it will revolve as a motor, and an alternating current may be obtained from the brushes A and B. This action can be easily understood when it is considered that, at the time indicated, the outer collecting ring is connected to the top or + point of the winding, and the inner ring to the bottom or — point of the winding, hence the current tends to flow from the brush B to the brush A; but when the armature has turned through 180 degrees, or half a revolution, these conditions will be exactly reversed, and the current tends to flow from A to B. Thus it is seen that an armature having only a single winding may be fed with a direct current, and will give out an alternating current. The ratio between the primary and secondary voltages is practically fixed in this form of converter, since the maximum value of the alternating E.M.F. is equal to the voltage of the direct current, as is evident from the diagram. With a true sine wave the effective value of the alternating E.M.F. is 0.707 of the maximum E.M.F. If these machines are used
the alternating Current by Fig. 71. Alternating-Direct Current Converter.
which they are to be operated.
These machines are capable of exciting their own field magnets by the direct current which they generate.
By tapping the direct' current winding at three or four points, machines are made for generating or utilizing three- or two-phase alternating currents respectively.
The actions of these machines are brought out in a paper by Professor R. B. Owens, and in the discussion which followed.*
Direct Current Transformer Systems of Distribution. — The usual arrangement of rotary transformers in electrical distribution is that represented in Fig. 72, being analogous to the ordinary alternating current system with static transformers. The current produced by the main generator G is carried to the machines by the conductors A and B, to which the motor portions M of the
* Trans. Amer. Inst. Elec. Eng., July, 1897.
rotary transformers are connected in parallel. These motors are provided with shunt wound field coils that may be connected to the primary or to the secondary circuit, consequently the machines run at a practically constant speed. The dynamo portion D of the transformers are connected to the secondary circuits which supply the lamps, etc., Z, as indicated. The field magnets of these dynamos may also be fed by the main circuit AB, or they may be selfexcited by shunt or compound winding.
This system has the following advantages and disadvantages compared with the alternating current system. Rotary transformers are more complicated, cost more, require more attention, and are less efficient than static transformers. But it has been shown that they may be compound or over-compound wound, in order to supply a uniform or rising voltage, which is not practicable with static transformers. Furthermore, it is generally found that rotary
transformers are easily taken care of, and rarely get out of order. In many cases their use may be desirable or necessary, as, for example, in electrolytic, chemical, or metallurgical work, in arc lighting, in connection with storage batteries, or for other purposes for which direct currents are converted from alternating, or vice versa.
Rotary transformers may also be arranged as illustrated in Fig. 73, the motor parts M being all connected in series with the main generator G, and the dynamo elements D of the transformers being connected to the lamps, etc., L. If the current is kept constant (the generator G having a regulator like a series arc dynamo), and the motors M are simple series-wound machines, they will exert a certain torque, or turning effort, which will be constant. It follows, therefore, if the dynamos D are also series wound, that each will generate a certain current which will be constant. If lamps or other devices designed for that particular current are connected in series on the secondary circuits, the dynamos D will always maintain that current, no matter how many lamps there may be. When lamps are added, the resistance of the local circuit is raised, and the current in it decreases, so that the dynamo increases its speed until it generates sufficient E.M.F. to produce practically the same current as before. Hence this constitutes a system which is self-regulating, when lamps, etc., are cut in or out of the secondary circuits. No harm results even when the secondary is short-circuited, since only the normal current can be generated. But if the secondary circuit is opened, then the machine will race, and probably injure itself by centrifugal force, because the torque of the motor M has its full value, and there is no load upon the dynamo D. To guard against this danger, some automatic device should be provided to short-circuit the field or armature of the motor when its speed or counter E.M.F. rises above
a certain point. Another way to operate such a system would be to use motors M, with governors that maintain a constant speed for all loads, in which case the dynamos D should be shunt or compound wound, to feed lamps, etc., in parallel at constant potential.
Motor Dynamos as "Boosters" and Compensators. — The machines described in the present chapter for use in converting direct currents from one voltage to another are also applicable as "boosters" in feeder regulation, and as compensators in three- and fivewire systems of distribution. The motor-dynamo illustrated in Fig. 70 is well adapted to being employed as a "booster" in Fig. 39, for example. The left-hand machine could be driven as a shunt-wound motor by current obtained from the main generator D (Fig. 39), and the right-hand machine (Fig. 70) would serve as the "booster" R or 5 to raise the voltage in the feeders A or B (Fig. 39). The double machine shown in Fig. 70 or in Fig. 68