Figs. 402 to 410.—Diagrams showing relative direction of rotation of motors and dynamos. From figs. 391 and 392, it is seen that the direction of the current in a motor armature must be such as will increase, by the flux it produces, the intensity at the leading polar edge and decrease the intensity at the trailing polar edge. In a dynamo, the armature has to be moved by mechanical force, against a magnetic force, hence the leading polar edge is weakened, while the trailing edge is strengthened. The magnetomotive force in a motor armature is, therefore, opposed to the direction of that in a generator armature, when the direction of rotation and the direction of the field magnetomotive force are the same. Upon this depends all the relations existing between the direction of rotation of a machine when acting as a motor or as a dynamo.
Ques. What is the behavior of a shunt dynamo when used as a motor?
Ans. Its direction of rotation remains unchanged.
Ques. Why is this?
Ans. Because if the connections be such that the current supplied will flow through the armature in the same direction as when the machine is used as a dynamo, the current through the field will be reversed, since the field windings are in parallel with the brushes.
Fig. 411.—The "left hand rule" for direction of motion in motors. Place the left hand, as shown, so that the thumb points in the direction of the current, the 3rd, 4th and 5th fingers in the direction of the lines of force, then will the 2nd or forefinger, at right angles to the others, point in the direction in which the conductor is urged.
Armature Reaction in Motors.—In the operation of a motor the reaction between the armature and field magnets distorts the field in a similar manner as in the operation of a dynamo. A current supplied from an outside source magnetizes the armature of a motor and transforms it into an electromagnet, whose poles would lie nearly at right angles to the line joining the pole pieces, were it not for the fact that negative lead must be given to the brushes.
