Fig. 298.—Distribution of magnetic lines through a ring armature. Since the lines follow the metal of the ring instead of penetrating the interior, no electromotive force is induced in that portion of the winding lying on the interior surface of the ring. There is, therefore, a large amount of dead wire or wire that is ineffective in inducing electromotive force; this is the chief objection to the ring type of armature.
Ques. What becomes of the energy of the current at reversal?
Ans. The energy of the current in the section of the winding undergoing commutation is wasted in heating the wire during the interval when it is short circuited, and as it passes on, energy must again be spent in starting a current in it in the reverse direction. There is, then, a lagging of the current in the armature coils due to self-induction.
Ques. What is spurious resistance?
Ans. This is an apparent increase of resistance in the armature winding, which is proportional to the speed of the armature, and due to the lagging of the current.
Fig. 299.—Distribution of magnetic lines through solid drum armature of a four pole machine.
Armature Losses.—The mechanical power delivered to the pulley of a dynamo is always in excess of its electrical output on account of numerous mechanical and electrical losses. Mechanical losses result from:
1. Friction of bearings;
2. Friction of commutator brushes;
3. Air friction.
The electrical losses may be classified as those due to: