Principle of DC Motor

DC MOTORS

Motor Principle

An electric motor is a machine which converts electrical energy into mechanical energy. Its action is based on the principle that when a current‒carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming's left hand rule and magnitude is given by:

F = BIl (N)

where,

B = Flux density due to the flux produced by the field winding

I = Magnitude of current passing through the conductor

l = Active length of the conductor

Constructionally, there is no basic difference between DC generator and DC motor. In fact the same DC machine can be used interchangeably as a generator or as a motor. In generator the input mechanical energy is supplied by a prime mover and develops a voltage, while in a DC motor input electrical energy is supplied by a DC supply and it develops a torque resulting in mechanical rotation.

Fleming's Left Hand Rule

"Keep the fore finger, middle finger and thumb of the left hand mutually perpendicular to one another. If the fore finger indicates the direction of magnetic field and the middle finger indicates the direction of current in the conductor, then the thumb gives the direction of motion (force experienced) of the conductor".

By knowing the direction of the magnetic field and the direction of the current in the conductor, the motion of the conductor can be determined by the Fleming's left hand rule.

Principle of operation of DC Motor

Consider a single conductor placed in a magnetic field as shown in Fig. 2.26 (a).

Fig. 2.26 (b) shows a conductor excited by separate DC source and carrying current particular direction. Consider it carrying a current away from an observer, but the field due to the N and S pole has been removed.

There is no movement of the conductor in the above two conditions.

Fig. 2.26 (a) Conductor in a magnetic field; (b) Flux produced by current carrying conductor

Fig. 2.27 (c) shows the current carrying conductor placed in magnetic field. It is clear that the direction of flux above the conductor are same, hence the field due to the current in the conductor supports the main field above the conductor but opposes the main field below the conductor.

Fig. 2.27 Interaction of two fluxes and force experienced by the conductor

The result is to increase the flux density in the region directly above the conductor and to reduce the flux density in the region directly below the conductor. It is found that a force acts on the conductor, trying to push the conductor downwards as shown by the arrow.

If the current in the conductor is reversed, the strengthening of flux the conductor, and the conductor will be pushed upwards. (Fig. 2.28)). (Fig, 2.28)),

Fig. 2.28 Direction of rotation of conductor

But, practically, a DC machine will have multiple conductors and each conductor (as explained above) will be experiencing a force F=BIl Newton. These forces collectively produce a driving torque which sets the armature rotating. The machine is then said to be motoring.