A dc machine can be used as a motor or generator, the construction of the two machines is similar.
The Working Principle of a Dc Motors
An electrical motor is equipment that turns electrical energy into mechanized energy. The motor works on the principle that when a current carrying conductor is placed in a magnetic field a mechanical force is developed. The route of the mechanical push is provided by the Fleming’s left-hand rule. The magnitude of this force is given by the formulae F=BIL. B- is the magnetic flux density, I-current applied to the armature, L-the length of the conductor in the magnetic field?
Fleming's left hand rule state that; if the first three fingers of the left hand are stretched out mutually perpendicular to each other such that the first finger points in the direction of magnetic field and the second finger points in the direction of the current then the thumb factors in the way of the pressure experienced by the conductor.
The DC motor works in such a way that when current is set up in the armature winding and then a magnetic field is provided by an electromagnet or by use of permanent magnets, the armature experiences a mechanical force. This mechanical force causes the armature to rotate.
The commutator is segmented to achieve a uniform directional torque, which would otherwise keep changing every time the direction of the conductor movement is reversed. This is the principle of operation of a DC motor.
It is a significant rule of Mother Nature that for energy conversion to occur there must be something in opposition. Generators face the opposition that is offered by magnetic drag. Motors face an opposition known as back emf when energy is being changed from electrical to mechanical energy.If mounted with gearbox, this is termed as a Gearmotor (German: Getriebemotor)
According to Faraday's law of electromagnetic induction, whenever a conductor cuts magnetic field lines a current is induced in the conductor. Therefore, in dc motors, when the armature rotates cutting the magnetic field a current is induced in a way in a way that it opposes the armature current (Ia). The magnitude of the back emf can be given by the EMF equation of dc generators.
Importance of back EMF
The magnitude of the back emf is directly proportional to the speed of motor rotation. If we consider a case where the load of the motor is reduced, the torque required is consequently reduced compared to the already existing current torque. The speed of the DC motors increases because of excess torque. However, since the speed is directly proportional to back emf, it will reduce the armature current. Reduced armature current gives a reduction in the armature speed, which now becomes sufficient for the load.
If we now consider a case where the motor is suddenly loaded, the load causes a decrease in the armature speed hence small back emf. This allows more armature current, which increases the speed, which now satisfies the load requirement. In other words, back emf makes a dc motor a ‘self-regulating’ machine.