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

# Reason: single phase motor not self-starting

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2071 Bhadra, 4 marks
- Video: [WHY SINGLE PHASE MOTOR ARE NOT SELF START?](https://www.youtube.com/watch?v=RHHhHedG4-g) — SN TECHNICAL
- 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-reason
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

Give reason: Single phase induction motors are not self starting.

## Short answer

Single-phase supply produces a pulsating field, which by double field revolving theory resolves into two equal counter-rotating fluxes. At standstill, forward and backward torques cancel, giving zero net torque, so the motor cannot start.

## Answer

A single-phase induction motor is not self-starting because a single-phase supply gives only a **pulsating** magnetic field and no rotating field.

1. The stator winding carries an alternating current, so the flux $\phi = \phi_m\cos\omega t$ only alternates in magnitude along a fixed axis; its axis does not rotate.
2. This flux can be treated as two equal rotating fluxes of $\phi_m/2$, rotating at $N_s$ in opposite directions (double field revolving theory).
3. At standstill the slip is 1 for both fields. The forward and backward fields induce equal rotor currents and produce **equal and opposite torques**:
$$
T_{start} = T_f - T_b = 0
$$
4. With zero net torque the rotor cannot start. If it is given a push in either direction, the torque in that direction becomes greater than the opposing torque, and the motor accelerates and keeps running.

So a starting arrangement (auxiliary winding with phase shift, capacitor, or shaded poles) is needed to produce a rotating field at start.

## Check yourself

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

1. What type of field does a single-phase supply produce in the stator?
   A. A rotating field
   B. A pulsating field on a fixed axis
   C. A constant DC field
   D. A field rotating at 2Ns
   Answer: B. The flux phi_m cos(wt) only changes in magnitude along a fixed axis. (Key point: Nature of the field)

2. What is the starting torque and why?
   A. Large, since both fields add
   B. Zero, as Tf and Tb are equal and opposite at s = 1
   C. Zero, because the rotor current is zero
   D. Equal to Tf
   Answer: B. T = Tf - Tb = 0 at standstill. (Key point: Net torque)

3. What is required to start the motor?
   A. A rotating field at start, from an auxiliary winding with phase shift, a capacitor, or shaded poles
   B. Higher supply voltage
   C. Higher frequency
   D. A bigger rotor
   Answer: A. A starting arrangement creates the rotating field needed for torque. (Key point: Remedy)

## 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×
- [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
- [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

- [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
- [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