. American telephone practice . ason why it is not well to use dynamotors when only a highpotential direct current is available for driving it is that in suchcases there is always a liability that the insulation between the pri-mary and secondary windings of the machine will break down under 36 562 AMERICAN TELEPHONE PRACTICE. the stress of the greater electromotive force, thus impairing the ac-tion of the machines. Of course where the motor generator is usedthe two armatures are entirely separate, and this danger does notexist. It may be said in general that the dynamotor is not anefficient m
. American telephone practice . ason why it is not well to use dynamotors when only a highpotential direct current is available for driving it is that in suchcases there is always a liability that the insulation between the pri-mary and secondary windings of the machine will break down under 36 562 AMERICAN TELEPHONE PRACTICE. the stress of the greater electromotive force, thus impairing the ac-tion of the machines. Of course where the motor generator is usedthe two armatures are entirely separate, and this danger does notexist. It may be said in general that the dynamotor is not anefficient machine for any but ringing purposes in a telephone ex-change under any conditions. When used for charging muchtrouble is had due to inefficient regulation. The fact that a singlefield serves for both the generator and the motor side of the machinemakes it impossible to regulate the two sides independently. Inasmuch as one or the other of the ringing machines are, in amodern exchange, driven during twenty-four hours of each day,. FIG. 403.—SMALL DYNAMOTOR FOR RINGER. these machines become the most available for driving the busy-backand tone-test attachment already referred to in connection with A dynamotor for ringing purposes adapted to be driven bydirect current is shown in Fig. 404, and this has permanently at-tached to it a busy-back of obvious construction. Until recently trouble has been experienced in applying alter-nating current motors to the direct driving of ringing reason for this was that all alternating motors available ran atsuch high speed that when direct-connected to ringing machinesthe frequency of the alternating current generated thereby was toogreat for properly actuating the bells. As a result of this direct-con-nected machines were not used in such cases, the ringing generator POWER PLANTS FOR COMMON BATTERY SYSTEMS. 563 being belted to an alternating current motor in such manner as torun at lower speed. A ringing machine equipped
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