The Diseases Of Electrical Machinery

....'..... "" ■ ELECTRICAL MACHINERY. 7 Such motors with variable speed must always be specially calculated and designed with regard to sparking, so that they can withstand the weakening of the field. The author has published—in the Elektrotechnischen Anzeiger 1902, in the Handbuchder Elektrotechnik, vol. ix. part 2, and in other places— some artides on the speed regulation of continuous- current motors. We may here draw attention to the connection of motors (in three wire systems) to cir- cuits that work at 110 volts and 220 volts, and also to the use of motors with two windings and two com- mutators, which may be connected as desired either in series or in parallel. Both methods allow of 100 per cent, variation in speed. Booster.—In this respect the working of boosters, such as are used with accumulator batteries, is very unsatisfactory. Boosters are built to generate about 50 volts in order to be able, in series with an ordi- nary 110 or 220 volt generator, to charge a battery of 60 cells. When commencing charging, when the battery is half discharged, the booster should not generate more than 20 volts. The charging is there- fore commenced with a total pressure of 130 volts. One may consider how very difficult it is to build a dynamo which with constant speed will stand a regu- lation of the voltage to less than 50 per cent, of its normal value, while generating the full load current all the time. These machines, therefore, must be specially calculated with the greatest care in respect