--- title: "Split phase induction motor" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:split-phase-im" source: https://www.bctengineer.com/q/semester_4/electrical_machine/split-phase-im site: BCT Engineering --- # Split phase induction motor Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2075 Baisakh, 8 marks - Video: [split phase induction motor | split phase induction motor animation |single phase induction motor|L7](https://www.youtube.com/watch?v=JlGkspjSIMI) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/split-phase-im - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Explain the operating principle and characteristics of split phase induction motor with neat sketches. ## Short answer A split-phase motor has main and auxiliary windings 90° apart. The high-resistance auxiliary winding creates a 30–40° phase shift, producing a weak rotating field for starting. At 75–80% speed a centrifugal switch disconnects the auxiliary winding. Starting torque is 1.5–2 times full-load; used in fans, blowers, and small appliances. ## Answer A split-phase induction motor is a single-phase induction motor with two stator windings, the main (running) winding and the auxiliary (starting) winding, placed 90° apart in space, connected in parallel to the supply and designed to give a phase difference between their currents so that a rotating field is created at starting. #### Construction ```text Supply ----+---------------+ | | Main winding Aux winding (thick wire, (thin wire, many turns, fewer turns, low R, high X) high R) | | | Centrifugal switch S +---------------+ ``` - **Stator:** laminated core with the two windings. The main winding is of thick wire, occupying the bottom of the slots, and has high inductance. The auxiliary winding is thin wire with high R/X, kept at the top of the slots. - **Rotor:** squirrel cage. - **Centrifugal switch** in series with the auxiliary winding. #### Operating principle 1. At starting both windings are connected. The main winding current I_m lags V by about 60° to 80° because of its high reactance. The auxiliary winding current I_a lags V by only about 20° to 30°, because of its high resistance. 2. The phase difference between I_m and I_a is about 30° to 40°, and the windings are 90° apart in space, so a weak elliptical rotating field results. The starting torque is T_st ∝ I_m I_a sin α (α = angle between currents). 3. The rotor accelerates in the direction of the rotation of the field. At about 75 to 80 % of synchronous speed, the centrifugal switch opens and the auxiliary winding is disconnected; the motor runs as a single-phase induction motor on the main winding alone. ```text Phasors (V as reference): Ia lags V by about 25° Im lags V by about 65° angle between Ia and Im = about 40° (not 90°), so the starting field is elliptical and weak ``` #### Characteristics - Starting torque: 1.5 to 2 times the full-load torque; starting current: 6 to 8 times full-load current (high). - Torque-speed: torque rises to the pull-out value at about 80 % of N_s; after the switch operates, the curve follows the main-winding curve. - Speed regulation is good: nearly constant speed, with 3 to 5 % speed drop. - Power factor is about 0.6 to 0.7 lagging; efficiency is low to medium. - Direction of rotation can be reversed by interchanging the terminals of either the main winding or the auxiliary winding (not both). - **Rating:** 1/20 to 1/2 kW (fractional kW). - **Uses:** fans, blowers, washing machines, grinders, small lathes, centrifugal pumps, office equipment. - **Limitation:** low starting torque and high starting current, so it is not used where heavy loads start. The capacitor-start motor is better. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a split-phase motor produce a rotating field at starting? A. The two windings carry currents out of phase (I_m lags about 65°, I_a about 25°) and are 90° apart in space B. Both windings carry identical currents C. The auxiliary winding has a capacitor D. The main winding is disconnected Answer: A. The main winding is highly inductive and the auxiliary winding highly resistive, so their currents differ in phase, and with the 90° spatial displacement this gives a rotating field. (Key point: Phase difference of currents) 2. On what does the starting torque of the split-phase motor depend? A. T_st is proportional to I_m I_a sin(alpha), alpha being the angle between the currents B. T_st is proportional to I_m + I_a C. T_st is proportional to cos(alpha) only D. T_st does not depend on the currents Answer: A. Torque is proportional to I_m I_a sin(alpha), and alpha is only about 30° to 40° here, so the starting torque is modest. (Key point: Starting torque expression) 3. What happens at about 75 to 80 % of synchronous speed? A. The centrifugal switch opens and the motor runs on the main winding alone B. The main winding is disconnected C. The capacitor is added D. The supply is reversed Answer: A. The centrifugal switch cuts out the auxiliary winding and the machine continues as a single-phase induction motor. (Key point: Switch operation) 4. Which statement about the split-phase motor is correct? A. Starting torque 1.5 to 2 times full load with a high starting current (6 to 8 times full load) B. Starting torque 5 times full load with a low starting current C. Power factor near unity and very high efficiency D. Rating of several hundred kW Answer: A. It has a low starting torque and a high starting current, so it suits light loads such as fans and blowers (fractional kW). 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