--- title: "Schrage motor construction and working" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:schrage-motor-construction" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/schrage-motor-construction site: BCT Engineering --- # Schrage motor construction and working Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2070 Asar, 6 marks - Video: [Schrage motor working animation | Schrage motor working principle | Schrage motor working explained](https://www.youtube.com/watch?v=pPFPLFUBRaw) — Mind of Engineer - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/schrage-motor-construction - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Explain about the construction and working principle of Schrage motor with neat diagram along with its field of applications. ## Short answer The rotor carries a primary winding fed via slip rings and a regulating winding connected to a commutator. The stator houses a secondary winding with each phase connected between two brush sets on opposite rockers. Shifting the brushes varies the injected emf magnitude and phase for speed control and power factor correction. ## Answer A **Schrage motor** is a three-phase, **rotor-fed shunt commutator motor**. It works like an induction motor with an emf of adjustable size and phase injected into its secondary through a commutator, giving smooth speed control above and below synchronous speed and improved power factor. #### Construction 1. **Primary winding** on the **rotor** (lower slots), fed from the 3-phase supply through **three slip rings**. 2. **Regulating (tertiary) winding** on the rotor (upper slots), connected to a **commutator** like a DC armature. 3. **Secondary winding** on the **stator**; its three phases are not connected together; each phase is connected between two brushes on the commutator. 4. **Two brush sets** ($A_1A_2A_3$ and $B_1B_2B_3$) mounted on two rockers that can be moved in **opposite directions** by a hand wheel. ```text 3-ph ===> slip rings ===> ROTOR | primary wdg | | regulating wdg|--commutator | | brushes A ------+ | brushes B ---------+ | | STATOR secondary A1--[ph1]--B1 (each phase A2--[ph2]--B2 between brushes) A3--[ph3]--B3 ``` #### Working principle - Primary current sets up a field rotating at $N_s$ relative to the rotor. The rotor turns at $N$, so the field rotates relative to the stator at slip speed, and the secondary has emf $sE_2$ at slip frequency $sf$. - The commutator converts the emf of the regulating winding to **slip frequency** too, so it can be added to the secondary circuit. The injected emf $E_j$ depends on the **brush separation**. - **Brushes together (same segment):** $E_j = 0$; motor behaves as a plain induction motor (stator shorted), speed just below $N_s$. - **Brushes separated so $E_j$ opposes $sE_2$:** rotor must slip more to circulate current, so speed **falls below** $N_s$ (sub-synchronous). - **Brushes separated the other way so $E_j$ aids $sE_2$:** speed **rises above** $N_s$ (super-synchronous). - Moving both brush sets together around the commutator shifts the phase of $E_j$, giving a component in quadrature that **improves power factor** (even to leading). Approximate speed relation: $N \approx N_s\left(1 \mp \dfrac{E_j}{E_2}\right)$. #### Characteristics - Shunt (nearly constant-speed) characteristic at each brush setting - Speed range about 3:1 (e.g. 0.5 to 1.5 $N_s$) - Good power factor and efficiency at all speeds #### Applications Textile mills, printing presses, paper-making machines, fans, blowers and pumps, cranes and conveyors, cement kilns, and test beds. Inverter-fed induction motors have largely replaced them now. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a Schrage motor, where is the primary winding placed and how is it supplied? A. On the stator, directly from the supply B. On the rotor, through three slip rings C. On the rotor, through the commutator D. On the stator, through brushes on the commutator Answer: B. The primary is on the rotor and is fed from the 3-phase supply through slip rings. The stator carries the secondary winding. (Key point: Winding arrangement) 2. What is the job of the regulating (tertiary) winding with its commutator? A. To supply DC excitation to the rotor B. To produce an injected emf at slip frequency for the stator secondary circuit C. To short-circuit the rotor at starting D. To supply the stator with line frequency emf Answer: B. The commutator converts the regulating winding emf to slip frequency so that it can be injected into the secondary circuit through the brushes. (Key point: Role of commutator and regulating winding) 3. When the brush sets are separated so that the injected emf Ej aids the secondary emf sE2, what happens to the speed? A. It falls below synchronous speed B. It stays just below Ns C. It rises above synchronous speed D. The motor stops Answer: C. An aiding Ej lets current flow with less slip, so the rotor runs super-synchronously. An opposing Ej gives sub-synchronous speed. (Key point: Speed control by brush shift) 4. What is the behaviour when both brushes of a phase are on the same commutator segment? A. Ej is maximum and speed is highest B. Ej = 0 and it runs like a plain induction motor just below Ns C. Ej is maximum and the motor runs sub-synchronously D. The motor works as a DC motor Answer: B. With zero brush separation Ej = 0, the stator is effectively shorted and the machine behaves as an induction motor. 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