The electric motor is a device which converts electrical energy to mechanical energy. There are mainly three types of electric motor.
All of these motors work in more or less same principle. Working of electric motor mainly depends upon the interaction of magnetic field with current. Now we will discuss the basic operating principle of electric motor one by one for better understanding the subject.
Working of DC Motor
Working principle of DC Motor mainly depends upon Fleming Left Hand rule. In a basic DC motor, an armature is placed in between magnetic poles. If the armature winding is supplied by an external DC source, current starts flowing through the armature conductors. As the conductors are carrying current inside a magnetic field, they will experience a force which tends to rotate the armature. Suppose armature conductors under N poles of the field magnet, are carrying current downwards (crosses) and those under S poles are carrying current upwards (dots).
By applying Fleming’s Left hand Rule, the direction of force F, experienced by the conductor under N poles and the force experienced by the conductors under S-poles can be determined. It is found that at any instant the forces experienced by the conductors are in such a direction that they tend to rotate the armature.
Again, due this rotation the conductors under N-poles come under S-pole and the conductors under S-poles come under N-pole. While the conductors go form N-poles to S-pole and S-poles to N-pole, the direction of current through them, is reversed by means of commutator.
Due to this reversal of current, all the conductors come under N-poles carry current in downward direction and all the conductors come under S-poles carry current in upward direction as shown in the figure. Hence, every conductor comes under N-pole experiences force in same direction and same is true for the conductors come under S-poles. This phenomenon helps to develop continuous and unidirectional torque.
Working of Induction Motor
Working of electric motor in the case of induction motor is little bit different from DC motor. In single phase induction motor, when a single phase supply is given to the stator winding, a pulsating magnetic field is produced and in a three phase induction motor, when three phase supply is given to three phase stator winding, a rotating magnetic field is produced. The rotor of an induction motor is either wound type or squirrel cadge type. Whatever may be the type of rotor, the conductors on it are shorted at end to form closed loop. Due to rotating magnetic field, the flux passes through the air gap between rotor and stator, sweeps past the rotor surface and so cuts the rotor conductor.
Hence according to Faraday’s law of electromagnetic induction, there would be a induced current circulating in the closed rotor conductors. The amount of induced current is proportional to the rate of change of flux linkage with respect to time. Again this rate of change of flux linkage is proportional to the relative speed between rotor and rotating magnetic field. As per Lenz law the rotor will try to reduce the every cause of producing current in it. Hence the rotor rotates and tries to achieve the speed of rotating magnetic field to reduce the relative speed between rotor and rotating magnetic field.
Working of schronous motor
In synchronous motor, when balanced three phase supply is given to the stationary three phase stator winding, a rotating magnetic field is produced which rotates at synchronous speed. Now if an electromagnet is placed inside this rotating magnetic field, it is magnetically locked with the rotating magnetic field and the former rotates with the rotating magnetic field at same speed that is at synchronous speed.