Abstract :
The parasitic currents which flow in the armature coils at the moment of commutation have a pronounced influence on the performance of all single-phase commutator motors; the paper shows how their effects may be calculated. Although the methods described are applicable to all such motors, the repulsion motor is chosen for particular treatment because it is peculiarly suitable for illustrating the principles involved. The methods are also applicable to the analysis of parasitic effects in dynamo-electric amplifiers. If the repulsion motor is tested with constant stator current and varying speed, the performance can be represented by loci of rotor open-circuit voltage, rotor short-circuit current and stator voltage. If the parasitic currents are eliminated by using very narrow brushes, the vector loci are found to be three straight lines. Tests with the normal brushes give very different results, which illustrate the influence of the parasitic brush-currents. A comprehensive theory for the motor with normal brushes is developed, which enables the performance to be expressed in terms of ten characteristic constants, all measurable. The comprehensive theory shows good agreement with the test results. The parasitic currents override non-linearities in their influence on the general pattern of behaviour of the machine. The theory given is based on the assumption of linearity, yet it gives a good insight into the performance under non-linear conditions, some examples of which are given.
