The Diseases Of Electrical Machinery

8 THE DISEASES OF of the necessary suppression of the sp3.rking with the unusually weak fields. If we regulate these machines still lower, down to about 10 volts, as the author many times has had to do, it is impossible to prevent heavy sparking, due to the influence of the armature currents upon the weak magnetic field. It may be here remarked that the care of these machines should be given only to experienced and conscientious hands. In many cases it will also be well to render un- necessary any considerable weakening of the field by providing the booster with a two-speed pulley, so that with low voltages one can also work at a slow speed and with a stronger magnetic field. This course might often be pursued, and above all in those cases where the booster is driven by a belt from the main machine. Excitation of Booster Fields.—It should here be remarked that a booster should always be provided with separate excitation of the field magnet. Separate excitation makes the magnetism of the machine more stable, and is in any case much to be preferred for machines where the voltage must be regulated down to a very low value. As is well known to practical men (and explained by the theory) a shunt machine can only be regulated down to about one-third of the normal voltage. From this point downwards the voltage becomes unstable, i.e. the machine is inclined to lose its voltage altogether. With separate exci- tation this is completely prevented. For this reason