In the design of single-phase induction motor. How many coils are present in the stator windings?
A
1
B
2
C
3
D
3 or more

Question

In the design of single-phase induction motor. How many coils are present in the stator windings?

Examoogle · Accepted Answer

3 or more

Explanation: A single-phase induction motor stator typically contains multiple coils arranged to form a distributed winding. These coils are grouped into two primary sets: the main winding (running winding) and the auxiliary winding (starting winding), requiring more than three physical coils in standard designs to approximate a sinusoidal MMF distribution.

Working Principle: The single-phase stator winding is designed to create a rotating magnetic field, which is not naturally present in a single-phase supply. By using multiple coils distributed in slots around the stator core, the motor creates a spatial displacement between the main and auxiliary windings, typically shifted by 90 electrical degrees. This configuration allows for the generation of a starting torque, as the current in the auxiliary winding is phase-shifted relative to the main winding.

Key Formulas: $	heta_{elec} = \frac{P}{2} 	heta_{mech}$ — relationship between electrical and mechanical degrees; $T_{start} \propto I_m I_a \sin(\phi)$ — torque generation in split-phase motors

Key Points: Single-phase induction motors are not self-starting without auxiliary winding.; The stator core contains many slots, and each slot can contain one or more coil sides.; Distributed windings are used to improve the waveform of the magnetic field.; The number of coils depends on the number of poles and the number of slots per pole per phase.

Advantages: Self-starting capability (with aux winding); Robust design and simple maintenance

Disadvantages: Lower power factor compared to 3-phase motors; Requires auxiliary components like centrifugal switches or capacitors

Applications: Domestic fans and refrigerators; Small industrial pumps and blowers

Additional Info: A single-phase stator is almost never a single coil, as this would result in a highly pulsating and inefficient magnetic field.. Options A, B, and C are incorrect because a single coil or just two coils would be insufficient to create the required flux distribution for efficient motor operation.

Answer: D is correct — The stator winding of a single-phase induction motor consists of a distributed arrangement of multiple coils to create a rotating magnetic field equivalent.

Exam Tip: Remember that in AC machines, 'distributed windings' are always preferred over 'concentrated windings' to minimize harmonic content in the air-gap flux.

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