---
title: "Short note: Capacitor start and run motor"
subject: "Electrical Machines"
chapter: 7
key: "semester_4/electrical_machines:capacitor-start-run-motor-note"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/capacitor-start-run-motor-note
site: BCT Engineering
---

# Short note: Capacitor start and run motor

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2078 Chaitra, 4 marks; 2068 Magh, 4 marks (asked 2 times)
- Video: [Single phase Induction Motor / Capacitor start capacitor run motor / Best video for you to learn.!😍💪](https://www.youtube.com/watch?v=WnQh-J0rRYg) — Electro Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/capacitor-start-run-motor-note
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

Write a short note on the capacitor start and capacitor run single phase induction motor.

## Short answer

Capacitor-start motors provide high starting torque via a capacitor and centrifugal switch. Capacitor-run motors permanently connect a capacitor for smooth, efficient operation. The combined type offers both high starting torque and good running characteristics.

## Answer

#### Capacitor-start motor
It has two stator windings placed $90^\circ$ apart in space: a main (running) winding of thick wire and an **auxiliary (starting) winding** of thinner wire in series with a capacitor ($C$ about 200 to 600 $\mu$F, electrolytic) and a centrifugal switch.

- The capacitor makes the auxiliary current lead the main current by nearly $90^\circ$, so the two windings together give a good two-phase rotating field.
- The starting torque is high (3 to 4.5 times the full-load torque) with a low starting current.
- At about 75 to 80 % of synchronous speed the centrifugal switch opens, removing the auxiliary winding and capacitor. The motor then runs as a single-phase motor on the main winding alone.
- Applications: compressors, refrigerators, pumps, washing machines, air-conditioners, conveyors.

#### Capacitor-run motor
A capacitor-run motor keeps the auxiliary winding and a capacitor (oil-filled paper, 2 to 20 $\mu$F) permanently in the circuit; there is no centrifugal switch.
- The motor behaves like an unbalanced two-phase motor, so it runs smoothly and quietly, with higher power factor and efficiency.
- The starting torque is low (50 to 100 % of full load), and the cost is low.
- Applications: ceiling fans, blowers, fan heaters, small pumps, and office equipment.

#### Capacitor-start, capacitor-run motor
This motor uses both capacitors: a large electrolytic **starting capacitor** that is cut out by the centrifugal switch at about 75 % speed, and a small oil-filled **running capacitor** that stays permanently in series with the auxiliary winding. It gives high starting torque as well as good running performance (high power factor and efficiency, quiet operation), and is used for compressors, refrigerators and air-conditioners.

```text
 Capacitor start:   Main ---------+
                    Aux--[C]--[S]-+-- supply
 Capacitor run:     Aux--[C]------+ (no switch)
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is true of the capacitor-start motor?
   A. Aux winding stays in circuit; low starting torque
   B. Aux winding and capacitor are cut out by a centrifugal switch at about 75 to 80 % speed; high starting torque
   C. It has no auxiliary winding
   D. The capacitor is permanent and small
   Answer: B. A large electrolytic capacitor is in the aux circuit only during starting, giving a high starting torque. (Key point: Capacitor-start features)

2. What is a feature of the capacitor-run motor?
   A. It has a centrifugal switch
   B. It has a very high starting torque
   C. It runs as an unbalanced two-phase motor with smooth running and higher pf, but low starting torque
   D. It has no capacitor
   Answer: C. The permanently connected capacitor improves power factor and smoothness but gives only 50 to 100 % of full-load starting torque. (Key point: Capacitor-run features)

3. What does the capacitor-start capacitor-run motor combine?
   A. Two shaded poles
   B. Only a running capacitor
   C. A starting capacitor with a switch and a small running capacitor, for high torque and good running
   D. Two centrifugal switches
   Answer: C. It uses both for high starting torque and good running performance. (Key point: Capacitor-start capacitor-run)

## Related in Electrical Machines

- [DFRT and making single phase motor self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adfrt-self-starting-explain) — Electrical Machines, chapter 7, asked 3×
- [Single phase motor not self-starting; DFRT](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-dfrt) — Electrical Machines, chapter 7, asked 3×
- [Single phase motor not self-starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-starting-methods) — Electrical Machines, chapter 7
- [Single phase motor working; effect of air gap](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-working-air-gap-effect) — Electrical Machines, chapter 7
- [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5
- [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

## Related in other subjects

- [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×
- [Capacitor start capacitor run motor characteristics](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acscr-speed-torque) — 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