--- title: "Capacitor start capacitor run motor characteristics" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:cscr-speed-torque" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/cscr-speed-torque site: BCT Engineering --- # Capacitor start capacitor run motor characteristics Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2075 Asoj, 6 marks - Video: [Capacitor Start Capacitor Run Motor | Construction, Working, Phasor, Torque-Speed Curve & Uses|| BEE](https://www.youtube.com/watch?v=PWZ8qJU4fco) — Learning with Chirag - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/cscr-speed-torque - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Explain the operating principle and speed-torque characteristics of single phase capacitor start capacitor run induction motor with suitable diagram. ## Short answer A CSCR motor has main and auxiliary windings 90° apart; a large start capacitor and small run capacitor are paralleled. At start both capacitors give high starting torque; at ~70–80% speed a centrifugal switch removes the start capacitor, leaving the run capacitor for efficient near-two-phase operation. Speed-torque curve: high start torque, then a drop at switching, followed by smooth running. ## Answer A **capacitor-start capacitor-run (CSCR)** motor, also called a two-value capacitor motor, is a single-phase induction motor that uses one large capacitor for starting and a smaller capacitor that stays in circuit while running. #### Construction - Stator with a **main winding** and an **auxiliary winding** displaced $90^\circ$ electrical in space. - Auxiliary branch has two capacitors in parallel: - **Starting capacitor** $C_s$ (large, electrolytic, short-time rated) in series with a centrifugal switch - **Running capacitor** $C_r$ (small, oil-filled paper, continuously rated) - Squirrel-cage rotor. ```text o-------+------------+ | | Main Auxiliary winding winding | | 1-ph | +----+----+ supply | | | | C_s C_r | | | | CS | | +----+----+ o-------+------------+ (rotor: squirrel cage) ``` #### Operating principle 1. At starting both capacitors are in circuit. The large total capacitance makes the auxiliary current $I_a$ **lead** $V$, while $I_m$ lags $V$. The angle $\alpha$ between them is close to $90^\circ$. 2. Two currents displaced in time and space produce a **rotating magnetic field**, so the motor starts. Starting torque $T_{st} = K I_m I_a \sin\alpha$ is high (about 3–4.5 times full-load). 3. At about 70–80% of synchronous speed the centrifugal switch disconnects $C_s$. 4. The running capacitor keeps the two windings working as an almost balanced **two-phase motor**, so the backward field is small during running. ```text I_a (with C_s + C_r) ^ | alpha ~ 90 deg ---------+-------------> V \ \ v I_m ``` #### Speed–torque characteristic - High starting torque from start to switching speed (upper curve, both capacitors). - At the switching point the torque drops to the lower curve (only $C_r$). - Running characteristic is smooth, like a two-phase motor; full-load slip is small. ```text Torque ^ ___ C_s + C_r | \___ | \__ switch opens | |\ | C_r only | \_ |___________|____\_____> speed 0 75% Ns ``` #### Advantages and applications - High starting torque, better power factor and efficiency, quiet running, smooth torque (less 100 Hz pulsation). - Used in refrigerators, air-conditioners, compressors, pumps and conveyors. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement about the two capacitors in a CSCR motor is correct? A. Both are small oil-filled and continuously rated B. C_s is large electrolytic with a centrifugal switch; C_r is small oil-filled and stays in circuit C. C_r is electrolytic and switched out D. Only one capacitor is used Answer: B. The large electrolytic C_s is short-time rated and switched out; the oil-filled C_r is continuously rated. (Key point: Auxiliary branch capacitors) 2. The starting torque is proportional to: A. I_m I_a cos(alpha) B. I_m I_a sin(alpha) C. I_m + I_a D. V / f Answer: B. T_st = K I_m I_a sin(alpha), maximum when alpha is near 90 degrees. (Key point: Starting torque expression) 3. What happens on the speed-torque curve at the switching speed? A. The torque jumps up B. The curve drops from the C_s + C_r curve to the C_r-only curve C. The motor stops D. The curve becomes flat Answer: B. After C_s is removed the motor follows the lower running-capacitor curve. (Key point: Speed-torque curve) 4. Why is the backward field small during running? A. Windings act as a nearly balanced two-phase motor due to C_r B. The rotor is open C. The supply is DC D. The capacitor is shorted Answer: A. A balanced two-phase supply gives an almost purely forward rotating field. (Key point: Running behaviour) ## Close variants set in other exams - [Capacitor start and run motor principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-run-principle) — Electric Machines II, chapter 3, asked 2× ## Related in Electric Machines II - [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2× - [DFRT, starting methods and capacitor start-run motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adfrt-starting-methods-csr) — Electric Machines II, chapter 3 - [PSC and capacitor start capacitor run motors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Apsc-cscr-motors) — Electric Machines II, chapter 3 - [Single phase IM principle, torque-slip and starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-principle-torque-slip) — Electric Machines II, chapter 3 - [Starting line current of capacitor start motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-line-current) — Electric Machines II, chapter 3 - [Shaded pole motor principle and characteristics](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashaded-pole-characteristics) — Electric Machines II, chapter 3 ## Related in other subjects - [Short note: Capacitor start and run motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Acapacitor-start-run-motor-note) — Electrical Machines, chapter 7, asked 2× - [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5 - [Torque equation of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atorque-equation-induction-max-condition) — Electrical Machines, chapter 5 - [Capacitor for single-phase motor pf correction](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Asingle-phase-motor-capacitor-40a-0-7) — Basic Electrical Engineering, chapter 7