---
title: "Reason: 1-phase IM not self-starting"
subject: "Electrical Machine"
chapter: 7
key: "semester_4/electrical_machine:reason-single-phase-im-not-self-starting"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/reason-single-phase-im-not-self-starting
site: BCT Engineering
---

# Reason: 1-phase IM not self-starting

Electrical Machine · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2071 Bhadra, 4 marks
- Video: [Why 1ph Induction Motor is not Self Starting | why connect Capacitor in Motor| Hindi](https://www.youtube.com/watch?v=Xr0RTKxICoA) — Electrical With Abhishek Joshi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/reason-single-phase-im-not-self-starting
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7

## Question

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

## Short answer

A single-phase winding produces only a pulsating field, not a rotating one. By double revolving field theory, two equal and opposite rotating fields produce equal and opposite torques at standstill, giving zero net starting torque.

## Answer

A single-phase induction motor is not self-starting because a single-phase winding carrying alternating current produces a **pulsating field, not a rotating field**.

- The flux φ = φ_m cos ωt stays on a fixed axis and only alternates in magnitude and direction. A stationary rotor in an alternating field has an emf induced in it by transformer action only. The resulting currents and forces are equal on both sides of the axis, so no net torque appears.
- By the double revolving field theory, the pulsating field can be considered as two fields of equal magnitude (φ_m/2) revolving at synchronous speed in opposite directions.
- At standstill, the slip is 1 with respect to each field, so the rotor currents are equal and the forward torque T_f equals the backward torque T_b.
- Net starting torque = T_f − T_b = 0.

So, the rotor stays at rest. However, if the rotor is turned by hand in either direction, the field rotating in the same direction produces a bigger torque and the motor keeps running in that direction. Therefore a starting device (such as an auxiliary winding with capacitor or resistance, or shading ring) is used to produce a phase difference between two fluxes, which creates a rotating field and a starting torque.

## Check yourself

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

1. A single-phase winding fails to start the rotor mainly because it produces:
   A. a rotating field at half synchronous speed
   B. a pulsating field on a fixed axis, not a rotating field
   C. no flux at all
   D. a field with too high a frequency
   Answer: B. The flux phi_m cos(wt) alternates along a fixed axis and does not rotate, so there is no net torque on a stationary rotor. (Key point: Pulsating field)

2. At standstill the net starting torque is zero because:
   A. slip is 1 for both fields, so T_f = T_b
   B. slip is 0 for the forward field
   C. slip is 2 for the backward field
   D. the rotor current lags by 90°
   Answer: A. Equal rotor currents from both fields give T_f = T_b, so T = T_f - T_b = 0. (Key point: Equal torques at standstill)

3. If the rotor is turned by hand in one direction, then:
   A. it returns to rest
   B. the field rotating in that direction gives a bigger torque and the motor keeps running
   C. it runs in the opposite direction
   D. the backward field disappears completely
   Answer: B. After the push the slips differ and the field turning with the rotor produces the larger torque. (Key point: Effect of a manual push)

## Related in Electrical Machine

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- [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×
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