Why is a single-phase motor not self-starting?

A single-phase motor is not self-starting primarily because it lacks a revolving magnetic field. In order for an electric motor to start and run properly, it requires a magnetic field that rotates around the stator. Single-phase motors, however, only have a single alternating current supply, which generates a pulsating magnetic field rather than a continuous rotating one. This pulsating field cannot start the rotor moving on its own.

To compensate for this, single-phase motors often use auxiliary windings or capacitors that create a phase shift, allowing for a starting torque. Once the motor reaches a certain speed, it can continue running due to inertia and the design of the motor, but the initial start-up relies on these additional components because of the inherent limitation of the magnetic field generated by a single-phase supply.

In contrast, options that mention insufficient current, low voltage supply, and high resistance in the windings relate to motor performance but do not specifically address the fundamental issue of why a single-phase motor lacks the ability to start on its own. These factors might affect the efficiency or operational capacity of the motor, but they do not directly explain the absence of a rotating magnetic field, which is the core reason behind the self-starting issue in single-phase motors.