Electrical Machines
Losses in a D.C. Motor
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 (Ia), Loss in interpole winding = Ia 2× Resistance of interpole winding Loss in compensating winding = Ia 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 read more
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Commutation in D.C. Motors
Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings. In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. This is illustrated in Fig. (4.10). In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation. The function of the commutator and the brush gear in a d.c. motor is to cause the reversal read more
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