The paper explains some of the problems arising when a traction motor is fed from an a.c. supply through a single-phase rectifier. The output voltage of the rectifier contains an a.c. component (ripple voltage) of twice the line frequency. It is shown how the ripple voltage depends on the commutating reactance of the rectifier circuit for various load conditions. The effect of the a.c. component (ripple current) in the motor current and the effect of the degree of smoothing necessary to reduce this ripple current on the commutation and commutator-bar voltage of the motor and on the waveform of the line current are examined. Methods whereby these effects may be reduced to a permissible value are investigated, and the influence of such modifications on the commutation and transient stability—short-circuits and interruptions—are analysed. Oscillographic and other tests carried out on a few traction motors modified in different ways are shown to confirm the theory developed. In the case of a machine with a solid frame, i.e. without laminated magnetic shunt to the compole circuit, certain d.c. commutating conditions must be fulfilled to ensure acceptable commutation when the motor current contains a ripple component.