---
title: "Capacitor start and run motor principle"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:capacitor-start-run-principle"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/capacitor-start-run-principle
site: BCT Engineering
---

# Capacitor start and run motor principle

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2071 Chaitra, 7 marks
- 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 Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/capacitor-start-run-principle
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Explain the operating principle of capacitor start and run single phase induction motor.

## Short answer

This motor uses two capacitors: a large starting electrolytic capacitor (with centrifugal switch) for high torque, and a smaller running capacitor always in circuit. At start, both parallel to achieve ~90° phase difference; after switch-off, the run capacitor maintains near two-phase balance, giving good efficiency and high overload capacity, used in refrigerators.

## Answer

A **capacitor-start capacitor-run (two-value capacitor) motor** uses two capacitors with the auxiliary winding: a large one for high starting torque and a small one that remains in circuit while running, so the motor works as a nearly balanced two-phase motor at all times.

#### Construction

```text
 AC o----+------------------------+
         |                        |
      Main wdg                 Aux wdg
         |                        |
         |                 +------+------+
         |                 |             |
         |               Cs (start,    Cr (run,
         |               electrolytic) oil-paper)
         |                 |             |
         |               switch S        |
         |                 |             |
 o-------+-----------------+-------------+
```

- Main and auxiliary windings $90^\circ$ apart in space; the auxiliary winding is **permanently connected**.
- **Starting capacitor $C_s$** (large, electrolytic, about 10-15 times $C_r$) in series with a centrifugal switch.
- **Running capacitor $C_r$** (small, oil-filled/paper, continuous rating) always in circuit.

#### Operating principle

1. **At start:** $C_s$ and $C_r$ are in parallel, giving a large capacitance. The auxiliary current leads $V$ so that $I_a$ and $I_m$ are about $90^\circ$ apart; with nearly equal mmfs, a strong rotating field is produced and starting torque $T = KI_mI_a\sin\alpha$ is high.
2. **At about 75 % of $N_s$:** the centrifugal switch removes $C_s$.
3. **Running:** only $C_r$ remains. Its value is chosen so that at full load the two winding currents are still nearly $90^\circ$ apart with balanced mmfs. The backward field is almost eliminated, so the field is nearly uniform and rotating, as in a balanced 2-phase motor.

```text
 T  |\  start (Cs + Cr)
    | \___
    |     \    switch opens
    |      \__________ run (Cr only)
    |                 \
    +------------------\---- speed
                       Ns
```

#### Advantages

- High starting torque and the best running performance of the single-phase motors.
- Better power factor and efficiency, quiet and smooth running (very little double-frequency pulsating torque at full load).
- Higher overload capacity.

#### Applications

Refrigerators, air-conditioners, compressors, pumps, and other loads needing high starting torque and continuous quiet operation.

## Check yourself

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

1. In a capacitor-start capacitor-run motor, which capacitor stays in circuit while running?
   A. The small running capacitor Cr
   B. The large starting capacitor Cs
   C. Both
   D. Neither
   Answer: A. Cs is cut out by the centrifugal switch; Cr is permanently connected. (Key point: Two capacitors)

2. At start the capacitance in the auxiliary branch is:
   A. Cs and Cr in parallel, giving large capacitance
   B. Cr alone
   C. Cs and Cr in series
   D. Zero
   Answer: A. The large capacitance gives a high Ia and about 90 degree phase difference, hence high starting torque. (Key point: Starting condition)

3. With only Cr, the motor runs as:
   A. Nearly a balanced two-phase motor with the backward field almost eliminated
   B. A shaded-pole motor
   C. A reluctance motor
   D. A DC motor
   Answer: A. Cr is chosen so currents are about 90 degrees apart with balanced mmfs at full load. (Key point: Running performance)

4. A major advantage over a capacitor-start motor is:
   A. Better power factor, efficiency and quiet running without pulsating torque
   B. Lower cost
   C. No capacitor needed
   D. Lower starting torque
   Answer: A. Balanced two-phase operation avoids double-frequency pulsating torque. (Key point: Advantages)

## Close variants set in other exams

- [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×

## 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
- [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
- [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
- [Single stack stepper motor principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-stack-stepper-motor) — Electric Machines II, chapter 3, asked 2×

## 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
- [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
- [Split phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asplit-phase-im) — Electrical Machine, chapter 7