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The main winding & auxiliary winding of Split-phase Single-Phase Induction Motor have spaced ______________ apart and are connected in _______________ across the 1-Φ supply.
A) 90º mechanically, parallel
B) 90º electrically, parallel
C) 180º mechanically, series
D) 180º electrically, series
90º electrically, parallel
In a split-phase single-phase induction motor, the main winding and auxiliary (starting) winding are physically distributed in the stator slots such that they are displaced by 90º electrically from each other. Both windings are connected in parallel across the single-phase AC supply line to operate from the same voltage source.
In a split-phase single-phase induction motor, the main winding and auxiliary (starting) winding are physically distributed in the stator slots such that they are displaced by 90º electrically from each other. Both windings are connected in parallel across the single-phase AC supply line to operate from the same voltage source.
α=θm−θa≈30° to 40° — Phase angle difference between main and auxiliary winding currents
Ts∝ImIasinα — Starting torque of split-phase induction motor
θ_e = \frac{P}{2}$$\cdot$$\theta_m — Relation between electrical angle (θ_e) and mechanical angle (θ_m)
A single-phase AC supply creates a pulsating magnetic field rather than a rotating one, producing zero starting torque. By placing two windings displaced by 90º electrically and creating a phase difference between their currents (due to differing R/X ratios), a two-phase rotating magnetic field is produced at standstill, generating the necessary starting torque.
The auxiliary winding has high resistance and low inductive reactance (thin wire, fewer turns).
The main winding has low resistance and high inductive reactance (thick wire, more turns).
A centrifugal switch disconnects the auxiliary winding when the motor reaches about 75% to 80% of synchronous speed.
Low cost and simple construction.
Easily reversible by reversing connections of either main or auxiliary winding.
Low starting torque (typically 1.5 to 2 times full-load torque).
High starting current.
Small fans and blowers
Centrifugal pumps
Washing machines
| Feature | Main Winding | Auxiliary Winding |
|---|---|---|
Wire Type | Thick wire (Low Resistance) | Thin wire (High Resistance) |
Reactance Property | High Inductive Reactance (Xm) | Low Inductive Reactance (Xa) |
Operation Mode | Remains connected continuously | Disconnected after starting by centrifugal switch |
For a 2-pole machine, 90° electrical displacement is equal to 90° mechanical displacement, but for general multipole machines, spatial phase separation in stator winding design is always defined in electrical degrees.
Option A is incorrect because mechanical angle equals electrical angle only for 2-pole machines; space displacement is universally defined in electrical degrees.
Options C and D are incorrect because connecting windings in series would prevent creating the parallel current split necessary for generating phase difference, and 180° separation would yield zero net rotating magnetic field.
B is correct — Main and auxiliary windings are space-shifted by 90º electrically in the stator and connected in parallel across the single-phase line to split current phases and create a rotating magnetic field.
Remember that starting torque Ts is proportional to sinα, where α is the phase difference between currents; maximum torque occurs at α=90°, which capacitor-start motors approximate better than split-phase motors.