--- title: "Reluctance motor construction and principle" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:reluctance-motor" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/reluctance-motor site: BCT Engineering --- # Reluctance motor construction and principle Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2079 Bhadra, 3.5 marks; 2072 Kartik, 6 marks; 2071 Shrawan, 4 marks (asked 3 times) - Video: [Reluctance motors explained](https://www.youtube.com/watch?v=jTREou5-h4s) — Garblex Home & Tech Solutions - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/reluctance-motor - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Describe the construction and principle of operation of single phase reluctance motor. ## Short answer Salient-pole rotor without dc excition starts as induction motor and then synchronizes by reluctance torque seeking minimum reluctance alignment. Runs at constant speed with low power factor. ## Answer A **reluctance motor** is a single-phase synchronous motor whose rotor has **no DC excitation**; it runs at synchronous speed because the rotor tends to align itself in the position of **minimum reluctance** to the stator field. #### Construction - **Stator:** the same as a single-phase induction motor - a main winding and an auxiliary winding (split-phase or capacitor type) to produce a rotating field. - **Rotor:** a modified squirrel-cage rotor. Some teeth are removed at symmetrical points to form **salient poles**; the number of salient poles equals the number of stator poles. The cage bars and end rings remain, so it can start as an induction motor. ```text ___________ / __ __ \ removed teeth (high | | | | | | reluctance gaps) | |__| |__| | remaining parts act as \_____________/ salient poles rotor with cage bars and cut-outs ``` #### Principle of operation 1. **Starting:** the stator rotating field induces currents in the cage bars, so the motor starts and accelerates as an **induction motor**. 2. **Pull-in:** near synchronous speed (about 75-80 %), the **reluctance torque** acts. A piece of magnetic material in a field experiences a force that moves it to the position of minimum reluctance (shortest air path). The salient poles snap into alignment with the stator field poles. 3. **Synchronous running:** the rotor locks with the rotating field and runs at $N_s = 120f/P$. The auxiliary winding may be disconnected by a centrifugal switch. 4. The reluctance torque is $T \propto \left(\frac{1}{X_q} - \frac{1}{X_d}\right)\sin 2\delta$; it is zero if $X_d = X_q$, which is why saliency is essential. #### Features - Constant speed, no slip rings, brushes or DC supply. - Low power factor and efficiency; output is about 1/3 of an induction motor of the same frame size. - Torque-speed curve shows induction motor region up to pull-in, then a vertical line at $N_s$. #### Applications Electric clocks, timers, signalling devices, recording instruments, teleprinters and other constant-speed drives. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A single-phase reluctance motor runs at synchronous speed because: A. The rotor has a DC excitation winding B. The rotor tends to align with the stator field in the position of minimum reluctance C. Its rotor has a very high slip D. The rotor has permanent magnets Answer: B. The rotor has no DC excitation; the salient rotor poles are pulled into the minimum-reluctance position and lock with the rotating field. (Key point: Principle of reluctance motor) 2. How is the rotor of a reluctance motor made? A. Smooth cylinder of hard steel B. Squirrel-cage rotor with some teeth removed at symmetrical points to form salient poles C. Wound rotor with slip rings D. Rotor with permanent-magnet poles and no cage Answer: B. Teeth are removed to form salient poles equal in number to stator poles while the cage is kept for induction starting. (Key point: Rotor construction) 3. How does the reluctance motor start and reach synchronous speed? A. It starts as an induction motor, then reluctance torque pulls it into synchronism near 75-80 % of Ns B. It starts by reluctance torque alone at standstill C. It starts from a DC supply and then switches to AC D. It is started by a hand crank Answer: A. The cage gives induction starting; near synchronous speed the reluctance torque snaps the salient poles into alignment. (Key point: Starting and pull-in) 4. The reluctance torque T is proportional to (1/Xq - 1/Xd) sin 2(delta). What happens if Xd = Xq? A. Torque is maximum B. Reluctance torque is zero, so saliency is essential C. Torque doubles D. The motor runs faster than Ns Answer: B. With Xd = Xq the bracket is zero, so there is no reluctance torque. 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