Chapter 6 : Alternating Current Generators

    

                                                            Figure 6.1 AC generators

    An electric generator, or dynamo, is a device which converts mechanical energy into electrical energy. The simplest practical generator consists of a rectangular coil rotating in a uniform magnetic field. The magnetic field is usually supplied by a permanent magnet.

Figure 6.2 Left-hand generator rule

     This rule shows the relationship between the direction of the conductor is moving, the direction of the magnetic field, and the resultant direction of the induced current flow.When the thumb is pointed in the direction of the conductor's motion, and the index finger is pointed in the direction of the flux, the middle finger will point in the direction of the induced electron flow. The rule is also applicable when the magnet, instead of the conductor, is moved.
     There are 2 types of alternating current generators, they are Stationery Field Synchronous AC Generator and Rotating Field Synchronous AC Generator.

1. Stationery Field Synchronous AC Generator 

In a stationary field generator, the stator in the form of fixed permanent magnets ( or electromagnets fed by DC) provides the magnetic field and the current is generated in the rotor windings. This type is usually of relatively small kilovolt-ampere capacity and low-voltage rating. It resembles a dc generator in appearence, except that it has slip rings instead of a commutator.

Figure 6.3 Stationary-field single-phase ac generator

    2.Revolving-Field three-phase AC Generator

          The revolving-type of ac generator simplifies the problems of insulating generated voltages, which are commonly as high as 18,000 to 24,000 VA revolving-field ac generator has a stationary armature called a stator. The three-phase stator winding is directly connected to the load without going through slip rings and brushes. The revolving-field ac generator uses a brushless exciter system in which a small ac generator mounted on the same shaft as the main generator is used as an exciter. The ac exciter has a rotating armature.The output of the armature is rectified by solid-states diodes, which are also mounted on the main shaft. The rectified output of the ac exciter is fed directly by means of insulated connections along the shaft to the rotating synchronous generator field. The field of the ac exciter is stationart and is supplied from a separate dc source

Sources :

http://farside.ph.utexas.edu/teaching/302l/lectures/node90.html
Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.172-173