SINGLE-PHASE INDUCTION MOTOR

175

If by overload the main motor, So, drops out of step and slows
down, the slowing down of Pi starts Si, and with it the brushes,
J3o, at the proper differential speed, and so carries full torque
down to standstill, that is, there is no actual dropping out of
the motor, but merely a slowing down by overload.
The disadvantage of this motor type is the sparking at the
commutator, by the short-circuiting of primary coils during the
passage of the brush from segment to segment. This would
require the use of methods of controlling the sparking, such as
used in the single-phase commutator motors of the series type,
etc. It was the difficulty of controlling the sparking, which
side-tracked this type of motor in the early days, and later, with
the extensive introduction of polyphase supply, the single-phase
motor problem had become less important.