The Schrage Motor

THE SCHRAGE MOTOR



The Schrage Motor seems always to have exercised a peculiar fascination over those interested in electrical machinery, one reason being no doubtthat the machine possesses both slip rings and a commutator. Its analysis thus affords a convenient opportunity of recapitulating several of the basic argu¬ments and processesof unified machine theory and forms a fitting conclusion to the present book.
Essentially the Schrage motor may be looked upon as an inverted induction motor with injectedsecondary voltage. The machine has three sets of windings. The primary winding, 1, is a normal three phase winding situated on the rotor and connected directly to the supplythrough slip rings. The secondary winding, 2, is again a normal three phase winding situated now on the stator. The air gap is cylindrical or smooth, and with the secondarywindings short circuited the machine behaves exactly like a normal induction motor, which indeed in this condition it is, the only effect of the inversion being to reverse thedirection of rotation for the same phase sequence. The variable voltage for injection to the secondary is obtained from a tertiary winding, 3, situated on the rotor andfitted with a commutator. The winding itself is a normal commutator winding, but the brush gear is unusual and forms a characteristic feature of the machine. The brushes arearranged in pairs, there being one pair per phase per pair of poles. For example the machine to be tested being three phase, six pole, has nine pairs of brushes. There are twobrush rockers each of which carries one brush out of each pair, the position of the individual brushes on the rockers being fixed and sym¬metrical. The Schrage motor is simply a