---
title: "Schrage motor diagram and application"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:schrage-motor-application"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/schrage-motor-application
site: BCT Engineering
---

# Schrage motor diagram and application

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2070 Chaitra, 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-application
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Draw a neat diagram of a Schrage motor. Discuss its application.

## Short answer

A Schrage motor is a three-phase rotor-fed shunt commutator motor. By moving brush rockers, a commutator injects an adjustable emf into the stator secondary, enabling smooth speed control from sub- to super-synchronous speeds and power factor improvement. Used in textile machines, printing presses, fans, pumps, and cranes.

## Answer

A **Schrage motor** is a **three-phase, rotor-fed, shunt-type AC commutator motor** whose speed and power factor can be controlled smoothly by moving its brushes. It is essentially an induction motor with a built-in frequency converter (commutator) that injects an adjustable emf into the secondary.

#### Diagram

```text
  3-ph supply
   | | |
  slip rings
   | | |
 +---------------------------+
 | ROTOR                     |
 |  Primary winding (fed by  |
 |  slip rings)              |
 |  Regulating winding ----> |
 |  commutator               |
 +---------------------------+
      brushes A1 A2 A3  (one rocker)
      brushes B1 B2 B3  (other rocker)
        |  |  |     |  |  |
        +--+--+-----+--+--+
        each stator phase is
        connected between Ai
        and Bi
 +---------------------------+
 | STATOR                    |
 |  Secondary winding        |
 |  (3 phases, open ends)    |
 +---------------------------+
```

#### Construction (in brief)
- **Rotor:** carries (i) the **primary winding**, fed from the supply through **slip rings**, and (ii) a **regulating (tertiary) winding** connected to a **commutator**.
- **Stator:** carries the **secondary winding**; each phase is connected between a pair of brushes ($A_1B_1$, $A_2B_2$, $A_3B_3$) on the commutator.
- The two sets of brushes are on two separate rockers that move in opposite directions by a hand wheel.

#### Working (short)
The primary produces a rotating field; the secondary emf is at slip frequency. The commutator converts the regulating-winding emf to slip frequency and injects it into the secondary.
- Brushes on the **same segment**: no injected emf, motor runs as an ordinary induction motor near synchronous speed.
- Brushes moved apart one way: injected emf **opposes** secondary emf, so speed is **below** synchronous.
- Brushes moved the other way: injected emf **aids** it, so speed is **above** synchronous.
- Shifting the brush axis also gives a quadrature component, improving **power factor**.

Speed range is typically about 0.5 to 1.5 times synchronous speed.

#### Applications
Where smooth, wide speed control with good pf is needed at constant load torque:
- Textile machines (ring frames, spinning)
- Printing presses and paper machines
- Fans, blowers and pumps with variable speed
- Cement kilns, cranes, hoists, conveyors and rolling mills (auxiliary drives)
- Testing beds and synthetic fibre drives

(Today these are largely replaced by inverter-fed induction motors.)

## Check yourself

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

1. A Schrage motor is a:
   A. Single-phase series motor
   B. Three-phase rotor-fed shunt-type AC commutator motor
   C. Synchronous motor
   D. Stepper motor
   Answer: B. It is a rotor-fed three-phase commutator motor with brush-controlled speed. (Key point: Schrage identity)

2. Where is the primary winding and where is the secondary?
   A. Primary on the rotor via slip rings, secondary on the stator
   B. Primary on the stator, secondary on the rotor
   C. Both on the stator
   D. Both on the rotor
   Answer: A. The rotor carries the primary (and regulating winding), the stator carries the secondary. (Key point: Construction)

3. How is speed raised above synchronous?
   A. Brushes moved so injected emf aids the secondary emf
   B. Brushes on the same segment
   C. Injected emf opposes the secondary emf
   D. Supply frequency is halved
   Answer: A. Aiding injected emf gives speeds above synchronous; opposing gives speeds below. (Key point: Speed control)

4. Schrage motors suit:
   A. Textile machines, printing presses and fans needing smooth wide speed control
   B. Traction only
   C. Clocks
   D. Stepper positioning
   Answer: A. They provide smooth speed control with good pf for constant torque loads such as textile and printing machinery. (Key point: Applications)

## Related in Electric Machines II

- [Schrage motor construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aschrage-motor-construction) — Electric Machines II, chapter 3
- [Synchronous motor features, applications and excitation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-applications-excitation) — Electric Machines II, chapter 2

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- [Armature-controlled DC motor transfer function and block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-armature-dc-motor) — Control Systems, chapter 2
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