--- title: "True/false: auxiliary winding construction" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:tf-auxiliary-winding-differs" source: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-auxiliary-winding-differs site: BCT Engineering --- # True/false: auxiliary winding construction Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2068 Bhadra, 1+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-auxiliary-winding-differs - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question State whether the following statement is true or false and justify it: Construction of auxiliary winding of 1 phase induction motor is different from that of the main winding. ## Short answer True. The auxiliary winding has thinner wire, fewer turns, and higher resistance (or a series capacitor) to shift its current phase relative to the main winding, creating a rotating field for starting torque. Without this difference, no torque would be produced. ## Answer **True.** In a single-phase induction motor (split-phase, capacitor-start or capacitor-run type), the auxiliary winding is built differently from the main winding, because it has to carry a current that is phase-shifted from the main winding current, and (except in capacitor-run motors) it is used only for starting. - **Main winding:** thick wire, many turns, low resistance and high reactance (low R/X ratio). Its current lags V by a large angle. - **Auxiliary winding in a split-phase motor:** thin wire, fewer turns, high resistance and low reactance (high R/X ratio), so its current lags V by a smaller angle. The winding carries current only for a short time, so a thinner wire is acceptable. - **In capacitor motors:** the turns and wire size of the auxiliary winding are chosen to suit the series capacitor, so that its current leads the main winding current by nearly 90°. - The auxiliary winding is placed 90° (electrical) from the main winding in the stator slots, and in split-phase and capacitor-start motors it is disconnected by a centrifugal switch at about 75 % of synchronous speed. If the two windings were identical, their currents would be in phase and the net field would still be pulsating, not rotating, giving no starting torque. The difference in construction creates the phase difference needed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The statement that the auxiliary winding is constructed differently from the main winding is: A. False, they are identical B. True, because it must produce a phase-shifted current for starting C. False, because the auxiliary winding has more turns D. True, but only on d.c. Answer: B. The auxiliary winding is built differently to give a current phase shift against the main winding. (Key point: True or false verdict) 2. Which describes the auxiliary winding of a split-phase motor? A. Thick wire, many turns, low R B. Thin wire, fewer turns, high R/X ratio C. Thick wire, low R/X ratio D. A winding with no resistance Answer: B. A high R/X ratio makes its current lag V less than that of the main winding. (Key point: Split-phase auxiliary winding) 3. What if both windings were identical? A. The motor would start with higher torque B. Their currents would be in phase, the field would stay pulsating and there would be no starting torque C. The motor would run at double speed D. The centrifugal switch would not be needed and the torque would be larger Answer: B. Without a phase difference the two windings simply produce a pulsating field. (Key point: Why identical windings fail) 4. In a capacitor-start motor, the auxiliary winding is connected: A. in series with a capacitor so its current leads the main winding current by nearly 90° B. in series with a resistor only C. in parallel with the rotor D. with no switch, running continuously at full load Answer: A. The series capacitor makes the auxiliary current lead, giving close to 90° phase displacement. (Key point: Capacitor-start auxiliary winding) ## Related in Electrical Machine - [Auxiliary winding construction differs from main winding](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aaux-winding-construction-true-false) — Electrical Machines, chapter 7 - [True/false: series motor started at no load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-dc-series-motor-start-noload) — Electrical Machine, chapter 4 - [DC series motor started at no load: true/false](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-no-load-true-false) — Electrical Machines, chapter 4 - [True/false: rotor core loss neglected](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-rotor-core-loss-neglected) — Electrical Machine, chapter 5 - [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5 - [True/false: CT secondary kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-false-ct-secondary-open) — Electrical Machine, chapter 2 ## Related in other subjects - [Split-phase motor winding currents at starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asplit-phase-starting-currents) — Electric Machines II, chapter 3 - [True/false: derivative controllers always used with others](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aderivative-always-with-others) — Control Systems, chapter 7, asked 3× - [True/False: ampere conductors and temperature rise](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-ampere-conductor-temperature) — Electric Machine Design, chapter 5 - [True/False: armature reaction and ampere conductors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-armature-reaction-ampere-conductor) — Electric Machine Design, chapter 5