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Lack of simple methods for predicting some of the losses in a machine in service has limited the theoretical work on additional losses in DC motors with armature current ripple to the prediction of additional copper losses. The additional core losses of a machine fed from the asymmetrical single-phase bridge are predicted from equivalent circuits which contain elements for the core. A method is developed for predicting the additional `skin effect?? losses in armature conductors. It is proposed that, by the use of generalised charts developed, the derating of a `standard?? bridge-fed DC motor at a specified operating condition can be estimated from look-up tables of additional losses measured on a standard machine.