There are different kinds of motors as there are dynamos. Series motors are used in hoists, cranes, railways, etc., where it is necessary to start with a full load and where the automatic regulation of speed is not necessary. Shunt motors are used when automatic regulation is desired. In starting any direct current motor, it is necessary to put a considerable resistance in series with the armature. Otherwise, the very low resistance of the armature would permit the flow of an enormous current, which would blow fuses or overheat armature coils and cause excessive sparking. As the machine increases in speed this resistance is cut out by using a starting box with each motor.

Fig. 91.