Schrage motor construction and working

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.

  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.