The losses occurring in a d.c. motor are the same as in a d.c. generator  (i) copper losses (ii) Iron losses or magnetic losses (iii) mechanical losses As in a generator, these losses cause (a) an increase of machine temperature and (b) reduction in the efficiency of the d.c. motor.
The following points may be noted:

(i) Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles (commutating poles) and compensating windings. Since these windings carry armature current (I_a),

Loss in interpole winding = I_a ²× Resistance of interpole winding
Loss in compensating winding = I_a ²× Resistance of compensating winding
(ii) Since d.c. machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses are nearly constant.
(iii) The mechanical losses (i.e. friction and windage) vary as the cube of the speed of rotation of the d.c. machine (generator or motor). Since d.c. machines are generally operated at constant speed, mechanical losses are
considered to be constant.

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