If the current is turned on and the armature is then speeded up by giving it a couple of sharp twists, or winding a string around the shaft and then pulling it as one would spin a top, the motor will continue to revolve at a good rate of speed.
FIG. 48.—A well known Three-pole Battery Motor.
It may prove easier to start the motor if the armature is speeded up before the current is turned on. As soon as a good speed is reached, turn on the current and the armature should continue to run.
Commercial induction motors are self starting, and are provided with a hollow armature, which contains a centrifugal governor. When the motor is at rest or starting, four brushes press against the commutator and divide the armature coils into four groups. After the motor has attained the proper speed the governor is thrown out by centrifugal force and pushes the brushes away from the commutator. At the same time a metal ring is pressed against the interior of the commutator, short circuiting all the sections and making each coil a complete circuit of itself.
It would be very difficult to provide a small three-pole toy motor with such a governor and short-circuiting device in order to make it self-starting.
It is however possible to accomplish this in another way, by a very simple device.
This consists in providing the armature with another set of coils for use in starting only. The brushes are allowed to remain on the motor but are only used for starting. The leads of the armature winding are removed from the commutator and are all connected together. Then two or three layers of wire are wound over the coils to form new coils which are similar to the old ones but smaller.
FIG. 49.—Showing how a Three-pole Motor may be provided with "Starting Coils" and connected to form an Experimental Induction Motor.