Schrage motor diagram and application

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

  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.)