--- title: "PSC and capacitor start capacitor run motors" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:psc-cscr-motors" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/psc-cscr-motors site: BCT Engineering --- # PSC and capacitor start capacitor run motors Electric Machines II ยท Chapter 3: Fractional Kilowatt Motors - Asked in: 2082 Baisakh, 6 marks - Video: [Single phase Induction Motor / Capacitor start capacitor run motor / Best video for you to learn.!๐Ÿ˜๐Ÿ’ช](https://www.youtube.com/watch?v=WnQh-J0rRYg) โ€” Electro Academy - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/psc-cscr-motors - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Explain the operating principle and their applications of permanent split phase capacitor induction motor and capacitor run capacitor start induction motor with proper diagram. ## Short answer PSC motor has a single permanently connected run capacitor; it provides low starting torque but quiet, smooth running with good power factor, used in fans. CSCR motor adds a starting capacitor switched out at speed, giving very high starting torque and balanced running, suited for compressors and air-conditioners. ## Answer Both motors are capacitor motors in which a capacitor in series with the auxiliary winding makes its current lead the main winding current, so a rotating field is produced. #### Permanent split capacitor (PSC) motor ```text AC o----+-------------+ | | Main wdg Aux wdg | | | C (oil/paper, | permanently in) o-------+-------------+ no centrifugal switch ``` **Principle:** 1. One small capacitor (a few ยตF, continuous rating) stays in series with the auxiliary winding at all times; there is no centrifugal switch. 2. The capacitor makes $I_a$ lead $I_m$ by nearly $90^\circ$, producing a rotating field and starting torque. 3. The capacitor value is chosen for good **running** performance, so it is small; starting torque is therefore **low** (about 50-100 % of full-load torque). 4. While running, it behaves like a two-phase motor: quiet, smooth, with good power factor and efficiency. Speed can be varied by tapping the main winding or changing voltage, and direction is easily reversed. **Applications:** ceiling and table fans, blowers, air-circulators, room coolers, office machines, and loads with low starting torque. #### Capacitor-start capacitor-run (CSCR) motor ```text AC o----+-----------------------+ | | Main wdg Aux wdg | +---+---+ | Cs Cr | | | | switch S | o-------+-------------------+-------+ ``` **Principle:** 1. Two capacitors: a large **starting capacitor $C_s$** (electrolytic) with a centrifugal switch, and a small **running capacitor $C_r$** permanently connected. 2. At start, $C_s \parallel C_r$ gives large capacitance and phase difference near $90^\circ$ with high auxiliary current, so **starting torque is high** (about 3-4 times full load). 3. At about 75 % of $N_s$ the switch removes $C_s$; $C_r$ keeps the motor working as a balanced two-phase motor for good running performance. **Applications:** refrigerators, air-conditioners, compressors, pumps, conveyors. #### Comparison | Point | PSC | CSCR | |---|---|---| | Capacitors | One (run) | Two (start + run) | | Centrifugal switch | Not used | Used | | Starting torque | Low | Very high | | Running pf, efficiency | Good | Good | | Cost | Low | High | | Typical use | Fans, blowers | Compressors, pumps | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A permanent split capacitor motor has: A. One small capacitor permanently in series with the auxiliary winding and no centrifugal switch B. Two capacitors and a switch C. No capacitor D. A shading ring Answer: A. The single run capacitor stays in circuit at all times. (Key point: PSC construction) 2. The starting torque of a PSC motor is low because: A. The capacitor is chosen for good running performance, so it is small B. The capacitor is very large C. It has no auxiliary winding D. Its rotor has no cage Answer: A. A small capacitor gives about 50-100 % of full-load torque. (Key point: PSC starting torque) 3. CSCR motor starts with high torque because: A. Cs parallel Cr gives large capacitance and phase near 90 degrees B. Only Cr is in circuit C. The auxiliary winding is disconnected D. The speed is high Answer: A. The large capacitance gives high auxiliary current at nearly 90 degrees. (Key point: CSCR starting) 4. Which pairing of motor and application is correct? A. PSC - ceiling fans and blowers; CSCR - compressors and pumps B. PSC - compressors; CSCR - ceiling fans C. Both are only for electric clocks D. PSC - cranes; CSCR - lifts only Answer: A. PSC suits low starting torque loads; CSCR suits high starting torque loads. 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