The direction of force acting in the Conductor carrying current and placed in a magnetic field can be found by using Flemings Left Hand Rule which states that “ Stretching out the first finger, middle finger and the thumb of the left hand such that they are at right angles to each other as shown in figure, if the first finger points towards the direction of magnetic field, the middle finger points towards the direction of current, then the thumb points out the direction of motion of the conductor”.

Basically there is no difference in the construction of a dc generator and dc motor. The only difference lies in the utilization of the machine. When the dc machine converts mechanical energy into electrical energy, it is termed as dc generator whereas when the dc machine converts electrical energy into mechanical energy, it is termed as dc motor. Both work on the similar principle. Whenever a conductor is placed in a magnetic field and mechanical motion to the conductor is given, emf is generator whereas in the motor due to the input of electric current, varying field is produced and when a conductor is placed in the varying magnetic field, a mechanical force is experienced by the conductor and the conductor is pushed away from the field. This mechanical energy is utilised to rotate the armature of the d.c. motor.

For the sake of understanding the concept, consider a single polar motor as shown in Figure. The conductor is placed around the circumference of an armature such that it falls under the influence of uniform magnetic field. In Figure. The conductor does not carry any current and so the flux of the main pole does not get distorted and is in the direction shown in Figure. In the next Figure, the flux of the main pole is purposely omitted for the purpose of clear understanding of the flux produced by the current in the conductor. The current in the conductor travels from back to front and the direction of magnetic flux produced can be found by thumb rule. Fleming left hand rule states that keeping the thumb, forefinger and middle figure at right angles to each other, if the forefinger, and middle finger points out flux and direction of current, the thumb points out the direction of force experienced by the conductor.

In Figure, both the flux of the main pole and the flux produced by the current in the conductor are shown together. It will be seen that flux produced by the current in the conductor opposes the main flux on the right hand side and aid at the left hand side due to which the flux get elongated towards the left hand side of the conductor. Since the flux pattern towards left hand side of the conductor is in unstable position, it exerts a force on the conductor in the direction shown in Figure. Since the conductor is free to move over the shaft which is supported by bearings, the conductor moves giving way for the next conductor to occupy its place. As the second conductor also carries the current in the same direction as that of the first, it also experiences a force as before and moves in the forward direction giving its place for the third conductor. The cycle continues as long as the current exists in the conductor. If the direction of current in the conductor changes then the direction of force also changes. The magnitude of the force exerted is given by the equation :

F = BllSin θ Newtons

Where F is the force exerted on the conductor in Newtons

B is the flux density in Wb/m²

I is the current in the conductor in amperes

l is the effective length of the conductor in metres

Sin θ, is by which the magnitude of force depends on the position of the conductor in the magnetic field.

From the equation, it is seen that the force exerted on the conductor depends upon the flux density of the main pole and the current in the conductor. Greater the flux density of the main pole, greater is the force on the conductor and so greater is the speed of the motor. Again, greater is the current in the conductor, greater is the force on the conductor and greater is the speed of the motor.