---
title: "Auxiliary winding construction differs from main winding"
subject: "Electrical Machines"
chapter: 7
key: "semester_4/electrical_machines:aux-winding-construction-true-false"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/aux-winding-construction-true-false
site: BCT Engineering
---

# Auxiliary winding construction differs from main winding

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2068 Bhadra, 1+3 marks
- Video: [Understanding electric motor Windings! | DON'T SUBSCRIBE](https://www.youtube.com/watch?v=YYQayMrK4Fo) — Sabin Civil Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/aux-winding-construction-true-false
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

State whether the following statement is true or false and justify: Construction of auxiliary winding of 1 phase induction motor is different from that of the main winding.

## Short answer

True. The main winding has low resistance and high reactance; the auxiliary winding has high resistance, low reactance (or series capacitor) to create a phase difference. They are 90° apart for a rotating field; the auxiliary is disconnected by centrifugal switch after starting.

## Answer

**True.**

**Justification:** In a single-phase induction motor (split-phase type), the auxiliary (starting) winding is constructed differently from the main winding, because a phase difference is required between their currents to create a rotating field at start.

- **Main winding:** thick wire, more turns, low resistance, high reactance; placed in the bottom of the slots. The current lags the voltage by a large angle.
- **Auxiliary winding:** thin wire, fewer turns, high resistance and low reactance (or in series with a capacitor); placed in the upper part of the slots, $90^\circ$ electrical away from the main winding.
- Because of the difference in $R/X$ ratio, the auxiliary current $I_a$ lags the supply voltage by a smaller angle than the main current $I_m$. The phase difference (30° to 40°, or nearly 90° with a capacitor) produces a rotating field and the starting torque.
- The auxiliary winding is thin and designed for short-time duty, so it is cut out by a centrifugal switch at about 75 % speed to avoid overheating.

## Check yourself

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

1. Is the statement true or false?
   A. False, they are identical
   B. True, they differ
   C. False, only the number of poles differs
   D. True, only the colour differs
   Answer: B. The auxiliary winding is built with different wire size, turns and R/X to create a phase difference. (Key point: True or false)

2. How does the auxiliary winding differ from the main winding?
   A. Thicker wire, more turns, low R
   B. Thin wire, fewer turns, high R and low X
   C. Same R/X ratio
   D. Higher reactance
   Answer: B. It is thin, with fewer turns, high R and low X, placed in the upper part of the slots. (Key point: Auxiliary winding)

3. Why is the difference needed?
   A. To reduce cost
   B. To raise the speed
   C. To make Ia and Im out of phase, producing a rotating field and starting torque
   D. To reduce the frequency
   Answer: C. The phase difference between the currents creates the rotating field needed for starting; it is cut out by a centrifugal switch. (Key point: Why the difference)

## Related in Electrical Machines

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## Related in other subjects

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