Single-phase induction motor starting characteristics: Which statement is correct for a basic single-phase induction motor (without special starting aids)?

Introduction:
Single-phase induction motors are widely used in household and light-industrial applications. Their starting behavior and speed regulation differ markedly from three-phase machines and synchronous motors. Understanding these distinctions is important for selection, control, and troubleshooting.

Given Data / Assumptions:

• Motor type: single-phase induction motor (SPIM).
• No explicit mention of auxiliary starting means (start winding, capacitor, shaded pole).
• Steady operation on single-phase supply with typical slip characteristics.

Concept / Approach:

A plain single-phase induction motor without an auxiliary starting mechanism is not inherently self-starting because a single-phase stator field alone does not produce an initial rotating magnetic field. In practice, SPIMs include auxiliary windings/capacitors or shaded poles to create a phase shift for starting. Once running, they behave like induction machines with slip; therefore they do not operate at a perfectly fixed speed (speed varies slightly with load). Reliability comparisons to “three-phase synchronous motors” are context-dependent and not a general truth.

Step-by-Step Reasoning:

Verification / Alternative check:

Standard motor theory shows that a single-phase sinusoidal field decomposes into two counter-rotating fields of equal magnitude, producing zero starting torque without asymmetry or auxiliary windings—explaining the need for starting mechanisms.

Why Other Options Are Wrong:

• is self-starting: Only when augmented with start windings/capacitors; not strictly true in the generic statement.
• operates at a fixed speed: Induction motors have slip; speed changes with load.
• is less reliable than a three-phase synchronous motor: Not a fundamental rule.
• starts by itself without any auxiliary means under all conditions: Overly absolute and incorrect.

Common Pitfalls:

• Confusing induction and synchronous operation with respect to speed constancy.
• Assuming “single-phase induction motor” implies a specific starting method without stating it.

Final Answer:

none of the above