Unlike DC motors, AC motors operate only on AC electricity. The frequency of the AC electricity can be made very precise, thus the speed of the AC motor rotation can be set precisely. DC motors lack this feature and its rotation is only controlled by the magnitude of the current input. However principally both the AC and DC motor uses a rotor and stator component to convent electrical energy into rotational kinetic energy. The rotor of the AC motor is usually the squirrel cage cylinder. It is an assembly of parallel conducting rods (either aluminium or copper) connected to end rings on either side. This entire cage is then encased in a laminated iron armature, this helps intensify the magnetic field and reducing heating effects. Whereas the DC rotor is simply a coil mounted onto an axle, encased in an armature. The AC stator consists of number of pairs of coils each with an iron core. These coil are mounted onto the frame of the motor, which surrounds a cylindrical space where is able to set up a rotating magnetic field. The AC induction motor uses this rotating magnetic field in stator component to induce a current in the rotor cage. This is achieved since there is clearly a relative motion between the conducting rods and the magnetic field and flux cutting the conductor is changing with time, thus by Faraday’s law of induction, a current will be set up. This current then interacts with the stator magnetic field to spin the rotor. This interaction comes about from Lenz’s law. The law says the induced current will oppose the change that created it. The change was the spinning of the magnetic field; hence the induced current must create a motor effect that provides a torque for the squirrel cage to spin in the direction of the rotation fields, thus attempting to stop the change that initiated the induction. However the motor must do work so that the rotor never catches up to the magnetic fields, or else the relative motion would disappear and induction will be lost. The difference between the rotor and magnetic angular velocities is termed the slip speed. This method is very different to operation of DC motors, where current is directly feed to the rotor coils, and in AC the current that drives the rotation is induced in the rotor. Hence the AC motor utilises induction and the motor effect, while the DC motor uses just the motor effect. The AC motor has several advantages over its DC counterpart. The AC motor has less moving parts and so experiences less wear and tear and consequently less maintenance and repair, reducing cost. DC motor has too many moving parts, the sparking between the commentators and brushes causes energy lost. AC motors loses energy, as induction also takes place in nearby conductors. DC motors do not have such problem.
