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

# Capacitor start motor operating principle

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2075 Chaitra, 6 marks; 2071 Shrawan, 4 marks (asked 2 times)
- Video: [Single Phase Induction Motor (Capacitor Induction Motor or AC Motor) explained](https://www.youtube.com/watch?v=FDerrQw99KU) — learnchannel
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/capacitor-start-motor-principle
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Explain the operating principle of split-phase capacitor start motor.

## Short answer

A capacitor in series with the auxiliary winding causes its current to lead, giving a phase difference near 90° from the main current. This produces a strong rotating field and high starting torque (3–4.5 times full-load). A centrifugal switch disconnects the capacitor at 70–80% speed; it is used for compressors and pumps.

## Answer

A **capacitor-start motor** is a split-phase single-phase induction motor in which a capacitor is placed in series with the auxiliary (starting) winding so that the two winding currents are nearly $90^\circ$ apart at starting, giving high starting torque.

#### Construction

```text
 AC o----+-----------------+
         |                 |
      Main wdg          Aux (start) wdg
      (Rm, Xm)             |
         |               C (electrolytic,
         |                 |  70-300 uF)
         |          centrifugal switch S
 o-------+-----------------+
       rotor: squirrel cage
```

- Main winding and auxiliary winding are placed $90^\circ$ electrical apart in the stator.
- An electrolytic capacitor $C$ and a centrifugal switch S are in series with the auxiliary winding.
- Rotor is a normal squirrel cage.

#### Operating principle

1. At start, the main winding current $I_m$ **lags** $V$ by about $40-60^\circ$ (inductive winding).
2. The capacitor makes the auxiliary current $I_a$ **lead** $V$. Choosing $C$ properly makes the angle $\alpha$ between $I_m$ and $I_a$ about $80-90^\circ$.

```text
       Ia (leads V)
        \
         \  alpha ~ 90 deg
 ---------+--------------> V
           \
            \ Im (lags V)
```

3. Two windings displaced in space by $90^\circ$ carrying currents displaced in time by about $90^\circ$ produce an almost uniform **rotating magnetic field**, as in a two-phase motor.
4. The rotating field induces rotor currents and develops starting torque $T_{st} = K I_m I_a \sin\alpha$, which is large (3-4.5 times full-load torque) since $\sin\alpha \approx 1$.
5. At about **70-80 % of synchronous speed**, the centrifugal switch opens and disconnects the capacitor and auxiliary winding. The motor then runs on the main winding alone (by double field revolving theory the forward field dominates).

#### Characteristics and uses

- High starting torque with lower starting current than resistance split-phase.
- Reversible by reversing the auxiliary winding connections.
- Used for compressors, refrigerators, air-conditioners, pumps, conveyors and machine tools.

## 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 in series with the auxiliary winding:
   A. Makes Ia lead V so the angle between Im and Ia is near 90 degrees
   B. Makes Ia lag V more than Im
   C. Increases the speed above Ns
   D. Stores energy for running
   Answer: A. Im lags V while Ia leads V, giving a phase angle near 90 degrees. (Key point: Role of capacitor)

2. Starting torque is T = K Im Ia sin(alpha). It is high because:
   A. sin(alpha) is close to 1
   B. alpha is near 0
   C. Ia is zero
   D. The switch is closed
   Answer: A. With alpha near 90 degrees, sin(alpha) is about 1, giving 3-4.5 times full-load torque. (Key point: Starting torque)

3. At about 70-80 % of synchronous speed, the centrifugal switch:
   A. Disconnects the capacitor and auxiliary winding
   B. Connects a second capacitor
   C. Reverses the motor
   D. Disconnects the main winding
   Answer: A. The motor then runs on the main winding alone. (Key point: Centrifugal switch)

4. A rotating field arises because:
   A. Windings 90 degrees apart in space carry currents about 90 degrees apart in time
   B. A DC current flows in the rotor
   C. The windings are in the rotor
   D. The supply is three-phase
   Answer: A. This is the same as two-phase operation. (Key point: Rotating field production)

## Related in Electric Machines II

- [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×
- [Not self starting and permanent split capacitor motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Anot-self-starting-psc-motor) — Electric Machines II, chapter 3
- [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
- [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

## Related in other subjects

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