An AC generator or alternator is a device used to convert mechanical energy into electrical energy. The generator is based on the principle of electromagnetic induction. The pole pieces (North and South) provide the magnetic field when an electric conductor is moved through a magnetic fields, electric current will be induced in the conductor wire.
Alternating voltage may be generated by rotating a coil/conductor in the magnetic field within a stationary coil. The amount of voltage generated depends on:
i. The strength of the magnetic field.
ii. The speed at which the conductor rotates
iii. The number of turns within the magnetic field.
In order to determine the direction of current flow in a given situation, the Left Hand Rule for generators.
In a dc generator the armature rotates and the magnetic filed is staionary.Where as in A.C generator the armature is stationary and the filed system rotates.
Advantages keeping the armature stationary(AC Machine Theory )
1.The armature winding must be insulated for a high voltage
2.The armature winding are more easily braced to prevent deformation under the mechanical stresses due to short – circuit currents and centrifugal force.
3. It is easier to collect the armature current when the armature is stationary .
Damper winding; winding is useful in preventing the momentary speed fluctuations (known as hunting) Damper winding kept in slotted pole shoes of alternator.
An AC motor is driven by an alternating current. It’s need single-phase or three phase AC power to make them work. Nearly all electric motors are based on the principle of electromagnetism. The coils which produce the magnetic field are referred as “stator”, while the coils and the solid core which rotates called the”armature
TYPES OF AC MOTOR
It is called synchronous because at steady state, the speed of the rotor matches the speed of the rotating magnetic field in the stator. The rotating magnetic field of the stator in a synchronous AC motor imposes a torque on the magnetic field of the rotor, causing it to rotate steadily. Synchronous motors have the characteristic of constant speed between no load and full load.
where f is the frequency of the AC supply (Hz) and p is the number of magnetic
poles per phase.
Induction motor is classified as a squirrel cage and wound rotor motors. In the case of an induction motor, the changing field is the motion of the resultant stator field. A force is exerted on the rotor by the induced emf and the resultant magnetic field. This force tends to cancel the relative motion between the rotor and the stator field. The rotor, as a result, moves in the same direction as the rotating stator field.
When a single phase alternating current supplied to the stator winding of single phase induction motor an alternating magnetic filed produced .Such magnetic filed acting on the stationary squirrel cage rotor cannot produce the starting torque needed for the motor .
The difference between the synchronous speed and the actual speed of rotor is called slip.
Single phase induction motor are not self starting.
- Basic Electrical Engineering book M A College Of Engineering Kothamangalam
- A Textbook of Electrical Technology In S.I. Units AC & DC Machines (Vol. II)B.L. Theraja & A.K. Theraja