Page:Popular Science Monthly Volume 92.djvu/495

 quency desired is delivered from the armature terminals of the machine.

Let us consider a typical spark-sender installation such as is used aboard ship and at many land stations. Direct cur- rent electric power is provided from the engine room (or by a public service corporation), and wired to the radio station. Here it passes to a control switchboard and a motor-starter, which is associated with the direct-current motor used to drive the alternating cur- rent generator for the radio transmitter. In Fig. 33 the power circuits of such an installation are shown, reduced to their simplest form. The direct current line comes in at Li and L-z, and usually operates at a voltage of 110 or 120. As shown in the diagram, the lines lead directly to two fuse-wires Wi Wo, which serve to protect the apparatus by "blow- ing" or melting in case too much current is drawn from the line (by reason of a short-circuit or other abnormal condi- tion). From the fuses, the connection runs to a double-pole line switch LS, which is used to disconnect the entire motor circuit when the plant is not running. The motor is usually stopped by pulling this switch open. When the smtch is closed, the power is applied across the terminals of a voltmeter VM, which will always show the voltage of the direct current line if the fuses are in good condition and if the circuits are all right up to this point. From the voltmeter the

���Fig. 33 : In most modern installations an au- tomatic electrically-controlled starter is used

wires run to the motor itself, shown at the right of the diagram Fig. 33, passing through a motor field rheostat MFR and a starting box SB. The starting box illustrated is of the simple hand-con- trolled type. In most modern installa- tions an automatic, electrically-controlled

��starter is used, but the principle is the same.

Connections of the Motor

It will be noted that the motor is shown with two field terminals marked F and Fx and two armature connections A and A^. The armature connections

���Fig. 34: In addition to the generator itself the circuits of the transmitter are shown

lead to the brushes, as shown, and current applied to them reaches the low re- sistance armature windings through the rotating commutator. The field is a high resistance stationary winding connected directly across the direct current line through the terminals F Fi. It is often called "shunt field" to distinguish it from a low resistance winding called the "series field" which is sometimes used, on compound-wound motors, in series with the armature. One terminal of the field and one of the armature are usually brought together at a "common" point, as illustrated by A F in the figure.

The starting box SB contains a variable resistance which is connected in series with the low-resistance armature windings. As the contact arm is pulled over slowly, less and less of the resistance remains in the circuit, and the motor turns over faster and faster until (when the line is directly across the armature and all the starting resistance is cut out) full speed is reached. On most starting boxes the contact arm is provided with a spring tending to hold it in the initial or high-resistance position, and a small retaining-magnet which keeps it in the running position so long as the line voltage is normally high. If the current is cut off by opening the line switch, LS, or through an interruption of the power circuit outside the radio station, this retaining magnet (often called a "no- voltage release") will let go, the spring

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