AC GENERATORS

Compiled and presented by Doren Nedrick

What is a Generator?

►The phrase to generate means "to produce." An electric generator is a machine that produces a voltage by means of electromagnetic induction. This is done by rotating coils of wire through a magnetic field or by rotating a magnetic field-past coils of wire.

Construction of the Generator

Construction of the Generator Cont’d

► A continuous alternating voltage can be produced by rotating a coil of wire between the poles of a permanent magnet (Fig. 1). This is a simple generator.

► The coil is called the armature. The ends of the armature coil are connected to slip rings (Fig. 1) that are insulated from each other and from the armature shaft on which they are mounted.

► The brushes press against the slip rings. They make it possible to connect the rotating armature to an external circuit. The armature must be driven by mechanical force.

Factors Affecting the Magnitude of Induced Voltage

► The value of the voltage induced by generator action at any instant depends on three things:

► (1) The flux density of the magnetic field (the greater the flux density, the greater the induced voltage);

► (2) The velocity of the conduc tor motion (induced voltage increases as the velocity of the conductor increases); and

► (3) The angle at which a conductor cuts across flux lines (the greatest voltage is induced when the conductor cuts across flux lines at a 90-degree angle).

How does the angle affect the amount of voltage?

What cause the rotor to rotate

► The rotating parts of large ac generators are called rotors. They are turned by steam turbines, hydro-(water-driven) turbines, or diesel engines. These generators produce the electric energy used in our homes and in industry.

► Small ac generators are usually driven by gasoline engines. Such generators are commonly used to provide emergency power (back up generator). Alternating current generators are also called alternators.

Methods of Exciting an a.c. generator

►In the small ac generator, the excitation current needed to energize the field windings is first obtained from a battery or the output of the generator itself.

►The excitation current must be direct, but the output current is alternating. Thus, the output current must first pass through a rectifier to be changed into direct current and used as excitation current.

Methods of Exciting an a.c. generator Cont’d

►In some alternators, the rectifier circuit is located within the generator housing. In large ac generators, dc exciter generators produce the excitation current. This is mounted either on the shaft of the main generator or located nearby.

Types of AC Generator

There are two types of AC generators: the stationary field, rotating armature; and the rotating field, stationary armature. Small AC generators usually have a stationary field and a rotating armature.

Stationary Field Rotating Armature Disadvantage

►One important disadvantage to this arrangement is that the slip ring and brush assembly is in series with the load circuits and, because of worn or dirty components, may interrupt the flow of current.

Rotating-Armature Generator

► In small ac generators, the armatures are usually the rotors, or rotating parts. The rotors turn within magnetic fields produced by stationary field windings called stators. Rotors have collectors, or slip rings, in contact with carbon brushes.

SLIP RINGS

Rotating-Field Generator

►In rotating field generators, armatures are stationary. They are made of winding conductors placed in the slots of the frame assembly.

FIELD WINDINGS

FIELD POLES

COLLECTOR (SLIP) RINGS

Principal parts of a rotating field generator: Rotor

►The field windings are wound around pole pieces on the rotor assembly and they are connected to slip rings.

FIELD WINDINGS

FIELD POLES

COLLECTOR (SLIP) RINGS

(A) Armature (stator) Cont’d

►The excitation current passes to the field windings through the carbon brushes that are in contact with the slip rings. Generators of this kind are used in most large power-generating plants.

The Basic Circuit Of A Single phase A.C. Generator

► In this instance, the rotating electromagnet is supplied from an outside source with direct current (exciting current), forming a uni directional electromagnet in the rotor.

► When the rotor is rotated, this magnetic field cuts the conductor of the field and induces an alternating e.m.f. into them.

► This is termed a revolving-field a.c. generator.

3-phase Outputs

► Shows a similar arrangement but the single pair of coils have been replaced with three sets of coils, thus producing three sets of phases displaced at 120o to one another.

Frequency

►The frequency of the output depends on two factors: (a) number of pairs of poles; and (b) the speed of rotation in revolutions per second.

►F (Hz) = p (pairs of poles) x N (rev/s)

Output Voltage

►The output voltage of a generator depends on the speed of the rotor, the number of armature coils, and the strength of the magnetic field produced by the field windings.

►Power-station generators usually have output voltages of 10,000 V or more.