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Principle of Operation of D.C. Motor

7.2. Principle of Operation of D.C. Motor

The principle of motor action can he stated as follows:

“Whenever a current carrying conductor is placed in a magnetic field, it experiences a force whose direction is given by Fleming’s left hand rule”.

  • Fig. 49 illustrates this principle.

Fig. 49 (a) shows the field set up by the poles.

Fig. 49 (b) shows the conductor field due to flow of current in the conductor.

Fig. 49 (c) shows the resultant field produced when the current carrying conductor wire of Fig. 49 (b) is inserted in the air gap of Fig. 49 (a) with the axis of the conductor at right angles to the direction of the flux.

On the upper side of the conductor in Fig. 49 (c) the magnetizing forces of the field and of the current in the conductor are additive while on the lower side these are subtractive. This explains why the resultant field is strengthened above and weakened below the conductor (wire).

The above experiment shows that the wire in Fig. 49 (c) has a force on it which tends to move it downward. Thus the force acts in the direction of the weaker field. When the current in the wire is reversed, the direction of the force is also reversed, as in Fig. 49 (d). 18

Fig. 49. The principle of motor action.


The force (F) developed in the conductor is given by the relation,


F = BIl newtons


where B = flux density, T (Wb/m2),


I = current in conductor, A, and


l = exposed length of conductor, m.


Now consider the magnetic field of a D.C. motor in which there is no current in the armature conductors; the lines of force will be distributed as shown in Fig. 50. 19

Fig. 50. Distribution of lines of force in a motor due to magnetic field only.


If, now, the armature carries current, each of its conductors will produce a magnetic field which, when super-imposed on the main field, causes a distribution of magnetic lines as shown in20

Fig. 51.


Fig. 51. Distribution of lines of force in a motor, on load, due to the armature and magnetic field.


The magnetic field is said to be distorted, since the lines of force no longer follow approximately straight paths.


These lines of force have the property of tending to shorten themselves, so that they may be regarded as being in tension. Each conductor in Fig. 51 will experience a force like that exerted on a stone in a catapult. Since these conductors are embedded in slots in the armature, the latter is caused to rotate in a clockwise direction.