The circle diagrams of the three-phase shunt commutator motor

A simplified mathematical theory of the circle diagrams of the three-phase shunt commutator motor is developed from the theory of the standard polyphase induction motor. Sources of error in the theory are outlined, and the theoretical circles are then compared with test-results. Formulae are developed for the slip, torque and mechanical power. The improvement in performance of the motor, which may be obtained by an unsymmetrical setting of the brushes, is described fully, and an approximate formula for the circle diagrams under these conditions is given. The capabilities of the motor as a generator are briefly discussed. In the Appendix, phenomena occurring in the operation of the motor at synchronous speed are noted, with particular reference to the effect of errors in the brush settings.