---
title: "Single phase motor not self-starting; two starting methods"
subject: "Electrical Machines"
chapter: 7
key: "semester_4/electrical_machines:spim-not-self-starting-starting-methods"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/spim-not-self-starting-starting-methods
site: BCT Engineering
---

# Single phase motor not self-starting; two starting methods

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2068 Bhadra, 8 marks
- Video: [How can we make Single Phase Induction Motor self starting](https://www.youtube.com/watch?v=b9_S6KYi5jE) — Easy Way Tech
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/spim-not-self-starting-starting-methods
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

Why single phase induction motor are not self starting? Explain any two starting methods for single phase induction motor.

## Short answer

A pulsating field gives zero net torque at standstill. Split-phase method uses an auxiliary winding with high resistance for 30–40° phase shift; capacitor-start adds a capacitor in series for near 90° shift, both cut out by a centrifugal switch at about 75% speed.

## Answer

#### Why a single-phase induction motor is not self-starting
A single-phase supply to the stator winding produces only a **pulsating field** (equivalent to two equal rotating fields of $\phi_m/2$ turning in opposite directions). At standstill, the torques due to the forward and backward fields are equal and opposite, so the net starting torque is zero. A **rotating field** is needed at start, and it is obtained by using an auxiliary winding with a phase shift.

#### Starting methods
**1. Split-phase (resistance start) motor**
- The stator has a main winding (low resistance, high reactance) and a starting winding (thin wire, high resistance, low reactance), placed $90^\circ$ apart in space.
- The currents $I_m$ and $I_a$ differ in phase by about $30^\circ$ to $40^\circ$, giving an elliptical rotating field and a moderate starting torque (1.5 to 2 times full load).
- A centrifugal switch disconnects the starting winding at about 75 % of synchronous speed.
- Use: fans, blowers, washing machines, grinders.

```text
 Main (R low, X high) ----+
                          +-- supply
 Aux (R high, X low)--[S]-+
 Im lags V by large angle; Ia lags by small angle
```

**2. Capacitor-start motor**
- A capacitor is connected in series with the starting winding, so that $I_a$ leads $I_m$ by nearly $90^\circ$.
- A nearly circular rotating field is produced, and the starting torque is high (3 to 4.5 times full load) with a low starting current.
- The centrifugal switch cuts out the capacitor and the auxiliary winding at about 75 % speed.
- Use: compressors, refrigerators, pumps, air conditioners.

```text
 Main -------------------+
                         +-- supply
 Aux --[Capacitor]--[S]--+
```
Shaded-pole and permanent-split capacitor motors are other options for small loads.

## Check yourself

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

1. Why is a single-phase induction motor not self-starting?
   A. The stator is too small
   B. The forward and backward torques are equal and opposite at standstill
   C. The rotor is wound
   D. The supply voltage is too low
   Answer: B. A pulsating field equals two opposite rotating fields, giving equal and opposite torques at s = 1. (Key point: Zero starting torque)

2. In a resistance split-phase motor, how does the starting winding differ from the main?
   A. Thick wire, high reactance
   B. Thin wire, high resistance and low reactance
   C. Thick wire, low resistance
   D. Same as the main
   Answer: B. A high R/X auxiliary winding makes Ia lag V less than Im, giving about 30 to 40 degrees of phase difference. (Key point: Split-phase winding)

3. What does the capacitor do in a capacitor-start motor?
   A. Makes Ia lead Im by nearly 90 degrees for a near-circular field and high torque
   B. Reduces voltage
   C. Makes Ia lag by 30 degrees
   D. Stores energy for running
   Answer: A. A series capacitor gives nearly a two-phase field, with 3 to 4.5 times full-load starting torque. (Key point: Capacitor-start effect)

4. Why is a centrifugal switch used?
   A. To increase speed
   B. To reverse direction
   C. To start the motor
   D. To disconnect the starting winding at about 75 % speed, preventing overheating
   Answer: D. The starting winding is for short duty, so it is cut out near 75 % of synchronous speed. (Key point: Centrifugal switch)

## Related in Electrical Machines

- [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×
- [Reason: single phase motor not self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-reason) — Electrical Machines, chapter 7
- [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×
- [Synchronous motor not self-starting; starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-not-self-starting-methods) — Electrical Machines, chapter 6, asked 2×
- [Need of starter and induction motor starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-starter-methods) — Electrical Machines I, chapter 5
- [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

## Related in other subjects

- [Why single-phase IM is not self starting; methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-not-self-starting-methods) — Electric Machines II, chapter 3, asked 2×
- [1-phase IM not self-starting; two methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asingle-phase-im-not-self-starting-methods) — Electrical Machine, chapter 7, 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
- [Reason: 1-phase IM not self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Areason-single-phase-im-not-self-starting) — Electrical Machine, chapter 7