Page:Popular Science Monthly Volume 92.djvu/496

 will pull the contact arm back to the start- ing position and the motor will not only- stop but will be protected against the large surge of current which would flow if, when the motor was not revolving, the full voltage were applied across the armature with no starting series resist- ance.

With the motor brought to full speed by cutting out the starting resistance, the revolutions per minute may be adjusted by use of the motor field rheostat MFR. When this resistance is all cut out, the maximum current flows through the field, and the motor revolves most slowly. Conversely, by cutting in more of the field resistance the motor field current is reduced, the field is weak- ened, and the motor speeds up. If the field is made too weak, the motor will lose power, and though it will run at very high speed when no mechanical load is thrown on it, the speed will be much reduced under load and the operation will be unsatisfactory. Thus it is evident that too much field resistance cannot be used. For a reasonable range, however, the speed may be raised by increasing the resistance in the motor fi_eld circuit.

The Generator Circuit

This brings us to the generator G of Fig. 34. The generator is usually mounted upon the same shaft as the motor, as is indicated by the dashed line extending to the right from Fig. 33, and to the left from Fig. 34. Of course the two machines then turn at the same speed, and changing the motor speed by use of the motor field rheostat alters the generator speed correspondingly.

The generator, like the motor, has two windings; one is for the field and has its terminals marked F Fi, the other is the armature with connections A A\ in Fig. 34. In the generator, however, there is no common terminal; the field wind- ings carry direct current supplied from the DC line through the generator field rheostat GFR, and the armature windings produce the alternating current which is used in the radio transmitter. The fre- quency of this output of alternating cur- rent is determined by the speed of the generator, and may be reduced by slowing down the motor through the motor field rheostat The voltage of the alternating

��current, which may be measured by con- necting an A.C. voltmeter across the armature at A Ai, is varied by changing the strength of the generator field; the stronger the magnetic field, i. e., the less resistance in the rheostat GFR, Hence the greater the field current, the higher the alternating voltage at A Ai.

Adjustment of Frequency and Voltage

In addition to the generator itself. Fig. 34 shows the power circuits of the two circuit spark transmitter of Fig. 32, or, in fact, of any sender which uses alternating current to charge a condenser. The armature or output terminals of the alternating current generator G are con- nected through the signaling key K to the primary P of the transformer T. The secondary S of the transformer is con- nected directly across the high-potential condenser C, which in turn may discharge through the wires X F to a spark-gap and inductance coil which are not shown in the figure. By varying the two field rheostats, alternating currents of any frequency and voltage within the range of the apparatus may be applied to the condenser C. The desirability of having such adjustments available will appear when their effects are described in later articles.

In some coastal commercial radio stations, and in most amateur plants, the alternating current is supplied from a central public service station at a distance. This makes it difficult to adjust the power circuits so as to give the best operation under the most efficient condi- tions, since the frequency cannot be changed and it is hard even to alter the voltage. When power is supplied in this way, and is used without conversion through rotating machinery (which gives a special and controllable generator at the wireless plant), the line conditions usually vary so much from moment to moment that it is not practicable to maintain the rather critical adjustments which give the best results. Consequent- ly, the motor-generator installation is much to be preferred.

In the next few articles the action of the high voltage and radio frequency circuits, as well as several types of spark gap, will be explained.

(To he continued.)

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