Fig. 2,071.—Diagram of "Cascade" motor generator set or motor converter, as it is called in England where it is used extensively for electric railway work. In the diagram of motor armature winding, some of the connections are omitted for simplicity. The windings are Y connected, and as they are fed by wires joined to the slip rings at the right and center, the rest of the power passes to the converter windings back to rotor winding and out to the slip rings so that part of the power enters the rotor and part through the converter.
Cascade Converter.—This piece of apparatus was introduced by Arnold and La Cour. Briefly, it consists of a combination of an induction motor having a wound armature and a dynamo, the armatures being placed on the same shaft. The windings are joined in cascade, that is, in series with those of the armature of the induction motor. The line supplies three phase currents at high voltage direct to the field of the induction motor and drives it, generating in it currents at a lower voltage depending on the ratio of the windings.
Fig. 2,072.—General Electric shunt wound booster set. Sets of this class are used in railway stations to raise the pressure of the feeders extending to distant points of the system, for storage battery charging and regulation, and in connection with the Edison three wire lighting system. The design of the various sets is closely dependent upon their application. Booster sets are constructed in either series or shunt wound types and they may be arranged for either automatic or hand regulation, depending on the nature of the service required. Where there are a number of lighting feeders connected and run at full load for only a short time each day it will generally be economical to install boosters rather than to invest in additional feeder copper. It is important, however, to consider each case where the question of installing a booster arises, as a separate problem, and to determine if the value of the power lost represents an amount lower than the interest charge on the extra copper necessary to deliver the same voltage without the use of a booster.
Part of the current thus generated in the armature passes into the armature of the dynamo and is converted by the commutator into direct current as in a rotary converter, but is also increased by the current induced in the winding of the dynamo armature.
Fig. 2,073.—General Electric 9.5 kw. 1,000 R.P.M. balancer set. This is a form of direct current compensator, the units of which it is composed may be alternately motor or generator, and the secondary circuit is interconnected with the primary. Balancer sets are widely used to provide the neutral of Edison three wire lighting systems. They are also installed for power service in connection with the use of 250 volt motors on a 500 volt service or 125 volt motors on a 250 volt service. Although the balancer set may be manufactured for more than one intermediate voltage, the General Electric Company recommends the three wire system as it is simpler. The motors may be more equally divided and the reserve margin of the motor is more conservative for that class of work requiring increase in torque with decrease in speed. The voltage of the system being given, the capacity of a three wire balancer set is fixed by the maximum current the neutral wire is required to carry. This figure is a more definite specification of capacity than a statement in per cent. of unbalanced load. As designed for power work and generally for lighting service, the brushes of each machine are set at the neutral point in order to get the best results for operating alternately either as a generator or motor. Where the changes of balance are so gradual as to permit of hand adjustment if desired, a considerable increase in output is obtainable.
Ques. At what speed does the machine run?
Ans. Assuming equal numbers of pole, the armatures rotate at a speed corresponding to one half the circuit frequency.