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The auxiliary winding of Split-phase Single-Phase Induction Motor has
High Resistance and Low Reactance
Low Resistance and High Reactance
High Resistance & Reactance
Low Resistance & Reactance
High Resistance and Low Reactance
The auxiliary (or starting) winding of a split-phase single-phase induction motor is designed with high resistance and low inductive reactance. This is achieved by using thin wire with fewer turns compared to the main winding. Consequently, the current in the auxiliary winding leads the current in the main winding, creating the necessary phase difference for a rotating magnetic field.
The auxiliary (or starting) winding of a split-phase single-phase induction motor is designed with high resistance and low inductive reactance. This is achieved by using thin wire with fewer turns compared to the main winding. Consequently, the current in the auxiliary winding leads the current in the main winding, creating the necessary phase difference for a rotating magnetic field.
tanϕa=RaXa — Phase angle of Auxiliary Winding current
tanϕm=RmXm — Phase angle of Main Winding current
α=ϕm−ϕa — Phase difference between Main and Auxiliary currents
Ts∝IaImsinα — Starting torque of the motor
Single-phase induction motors are not self-starting because a single-phase AC supply produces a pulsating magnetic field rather than a rotating one. By splitting the single phase into two phases using main and auxiliary windings with different R/X ratios, a phase angle difference α (typically 30° to 40°) is created between their currents. This produces a revolving magnetic field that generates the starting torque.
Auxiliary winding consists of thin copper wire placed in the upper part of the stator slots.
Main winding consists of thick copper wire placed deep in the stator slots (High reactance, Low resistance).
A centrifugal switch disconnects the auxiliary winding when the motor reaches approximately 75% to 80% of synchronous speed.
Low cost and simple construction
Reliable and easily available
Low starting torque (1.5 to 2 times full load torque)
High starting current
Small fans and blowers
Centrifugal pumps
Washing machines
Small machine tools
| Feature | Auxiliary Winding | Main Winding |
|---|---|---|
Wire Thickness | Thin wire (High Resistance) | Thick wire (Low Resistance) |
Inductive Reactance | Low (Xa) | High (Xm) |
Slot Placement | Top of stator slots | Bottom of stator slots |
Operation | Temporary (Disconnects at 75% speed) | Continuous (Stays connected always) |
Option B describes the Main Winding of a split-phase induction motor, which has low resistance and high reactance.
Option C and D are incorrect as both windings must have contrasting R/X ratios to create a phase shift α>0.
A is correct — The auxiliary winding has high resistance and low reactance to create a phase shift between the main and auxiliary currents necessary for producing starting torque.
Remember that to maximize starting torque Ts∝sinα, the phase angle α between currents should be close to 90°. Split-phase motors only achieve α≈30°−40°, whereas Capacitor-start motors reach α≈80°−90° providing much higher starting torque.