Single Phase Motors
The speed control of universal motor used for sewing machines is by

Centrifugal mechanism
Friction
Tapping the field
Varying the resistance

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Single Phase Motors
The rotor slots, in an induction motor, are usually not quite parallel to the shaft because it

improves the efficiency
helps the rotor teeth to remain under the stator teeth
improves the power factor
helps in reducing the tendency of the rotor teeth to remain under the stator teeth

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