Fig. 389.—Conductor, lying in a magnetic field and carrying no current; the field is not distorted whether the conductor be at rest or in motion.
Ques. Why does less current flow when the motor is running than when standing still?
Ans. Because the motor, on account of its rotation acts as a dynamo and thus tends to set up in the circuit a reverse electromotive force, that is, an electromotive force in opposite direction to the current which is driving the motor.
Ques. What is the real driving force which causes the armature of a motor to rotate?
Ans. The propelling drag, that is, the drag which the magnetic field exerts upon the armature wires through which the current is flowing, or in the case of deeply toothed cores, upon the protruding teeth.
The Propelling Drag.—In [fig. 389] is shown the condition which prevails when a conductor carrying no current is placed in a uniform magnetic field. The magnetic lines pass straight from one pole to the other. The field is not distorted whether the conductor be at rest or in motion, so long as there is no flow of current. This represents the condition in the air gap of a motor or dynamo, when no current is flowing in the armature.
Ques. What happens when a current flows in the conductor of [fig. 389].
Ans. It sets up a magnetic field of its own as shown in fig. 390.
Ques. What is the effect of this magnetic field?
Ans. It distorts the original field ([fig. 389]) in which the conductor lies, making the magnetic lines denser on one side and less dense on the other as in [fig. 390].