--- title: "Short note: Capacitor starting 1-phase IM" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:capacitor-start-1ph-im-note" source: https://www.bctengineer.com/q/semester_4/electrical_machine/capacitor-start-1ph-im-note site: BCT Engineering --- # Short note: Capacitor starting 1-phase IM Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2068 Magh, 4 marks - Video: [Single Phase Induction Motor (Capacitor Induction Motor or AC Motor) explained](https://www.youtube.com/watch?v=FDerrQw99KU) — learnchannel - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/capacitor-start-1ph-im-note - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Write a short note on capacitor starting of 1-Ø induction motor. ## Short answer A capacitor in series with the auxiliary winding gives nearly 90° phase shift between currents, producing a nearly circular rotating field and high starting torque (3–4.5 times full-load). A centrifugal switch removes the capacitor and winding at about 75–80% speed. Used for compressors, pumps, refrigerators. ## Answer A capacitor-start motor is a split-phase single-phase induction motor in which a capacitor is connected in series with the auxiliary (starting) winding, to give a larger phase difference between the main and auxiliary currents. ```text Supply ---+--- Main winding -------------+ | | +--- Aux winding --- C --- S ---+ S = centrifugal switch ``` **Working** - Main winding: high inductance, current I_m lags V. - Auxiliary winding in series with an electrolytic capacitor (50 to 400 μF, short-time rating): its current I_a leads V when the capacitor is selected correctly. - The phase difference between I_a and I_m becomes nearly 90°, with the windings 90° apart in space, so the starting field is almost a circular rotating field. - At about 75 to 80 % of N_s the centrifugal switch opens and removes the auxiliary winding and capacitor. The motor then runs as a single-phase motor on the main winding. **Characteristics** - High starting torque: 3 to 4.5 times full-load torque, with a lower starting current than a split-phase motor. - Torque ∝ I_m I_a sin α, where α is nearly 90°, which gives maximum starting torque. - Direction of rotation is reversed by interchanging the auxiliary winding terminals. **Uses:** compressors, refrigerators, air conditioners, pumps, conveyors, and loads that need good starting torque (up to about 6 kW). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a capacitor-start motor, the capacitor is connected: A. in series with the auxiliary winding and the centrifugal switch B. in parallel with the main winding C. in series with the rotor D. across the supply terminals only Answer: A. The capacitor is in series with the auxiliary winding and switch, so I_a can lead V. (Key point: Capacitor in the auxiliary circuit) 2. What phase relation gives the high starting torque? A. I_a and I_m in phase B. I_a leading V while I_m lags V, giving nearly 90° between them C. Both leading V by 90° D. I_a lagging I_m by 5° Answer: B. Torque is proportional to I_m I_a sin(alpha) and alpha near 90° gives the maximum, with a nearly circular field. (Key point: Phase angle between currents) 3. Which is correct for a capacitor-start motor? A. Starting torque 3 to 4.5 times full load at a lower starting current than split-phase; reverse by interchanging auxiliary terminals B. Starting torque below full-load torque; reversed by changing frequency C. It has no centrifugal switch and runs on both windings only D. Reversal is done by changing the capacitor value Answer: A. The capacitor-start motor gives high starting torque with lower current, and swapping the auxiliary winding terminals reverses the field direction. 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