GENERATORS

SYNCHRONOUS GENERATORS

Synchronous generators or alternators are synchronous machines used to convert

mechanical power to ac electric power. This subject explores the operation

of  synchronous generators, both when operating alone and when operating together

with other generators.

 SYNCHRONOUS GENERATOR CONSTRUCTION

In a synchronous generator, a de current is applied to the rotor winding, which

produces a rotor magnetic field. The rotor of the generator is then turned by a

prime mover, producing a rotating magnetic field within the machine. This rotating

magnetic field induces a three-phase set of voltages within the stator windings

of the generator.

Two terms commonly used to describe the windings on a machine are field

windings and armature windings. In general, the term "field windings" applies to

the windings that produce the main magnetic field in a machine, and the term

"armature windings" applies to the windings where the main voltage is induced.

For synchronous machines, the field windings are on the rotor, so the terms "rotor

windings" and " field windings" are used interchangeably. Similarly, the terms

"stator windings" and "armature windings" are used interchangeably.

The rotor of a synchronous generator is essentially a large electromagnet.

The magnetic poles on the rotor can be of either salient or nonsalient construction.

The term salient means "protruding" or "sticking out," and a salient pole is a magnetic pole that sticks out from the surface of the rotor. On the other hand, a nonsalient pole is a magnetic pole constructed flush with the surface of the rotor.

while salient-pole rotors are normally used for rotors with four or more poles.

Because the rotor is subjected to changing magnetic fields, it is constructed of thin

laminations to reduce eddy current losses.

A dc current must be supplied to the field circuit on the rotor. Since the rotor

is rotating, a special arrangement is required to get the de power to its field

windings. There are two common approaches to supplying this dc power:

I. Supply the dc power from an external dc source to the rotor by means of slip

rings and brushes.

2. Supply the dc power from a special de power source mounted directly on the

shaft of the synchronous generator.

Slip rings are metal rings completely encircling the shaft of a machine but insulated

from it. One end of the dc rotor winding is tied to each of the two slip rings

on the shaft of the synchronous machine. and a stationary brush rides on each slip

ring. A "brush" is a block of graphitelike carbon compound that conducts electricity

freely but has very low friction. so that it doesn't wear down the slip ring. If the

positive end of a dc voltage source is connected to one brush and the negative end

is connected to the other, then the same dc voltage will be applied to the field winding

at all times regard less of the angular position or speed of the rotor.

Slip rings and brushes create a few problems when they are used to supply

dc power to the fi e ld windings of a synchronous machine. They increase the

amount of maintenance required on the machine, since the brushes must be

checked for wear regularly. In addition, brush voltage drop can be the cause of

significant power losses on machines with larger field currents . Despite these

problems, slip rings and brushes are used on all smaller synchronous machines,

because no other method of supplying the dc field current is cost-effective.

On larger generators and motors, brushless exciters are used to supply the

dc field current to the machine.

To make the excitation of a generator completely independent of any external

power sources, a small pilot exciter is often included in the system. A pilot exciter

is a small ac generator with permanent magnets mounted on the rotor shaft

and a three-phase winding on the stator. It produces the power for the field circuit

of  the exciter, which in turn controls the field circuit of the main machine. If a

pilot exciter is included on the generator shaft, then no external electric power is

required to run the generator.

Many synchronous generators that include brushless exciters also have slip

rings and brushes, so that an auxiliary source of dc field current is available in

emergencies.

THE SPEED OF ROTATION OF A

SYNCHRONOUS GENERATOR

Synchronous generators are by definition synchronous, meaning that the electrical

frequency produced is locked in or synchronized with the mechanical rate of

rotation of the generator. A synchronous generator's rotor consists of an electromagnet to which direct current is supplied. The rotor's magnetic field points in whatever direction the rotor is turned. Now, the rate of rotation of the magnetic

fields in the machine is related to the stator electrical frequency by Equation :

Where    fe= electrical frequency, in Hz

n_m = mechanical speed of magnetic field, in r/min (equals speed of

rotor for synchronous machines)

P = number of poles

Since the rotor turns at the same speed as the magnetic field, this equation relates

the  speed of rotor rotation to the resulting electrical frequency. Electric power is

generated at 50 or 60 Hz, so the generator must turn at a fixed speed depending on

the number of poles on the machine. For example, to generate 60-Hz power in a

two-pole machine, the rotor must turn at 3600 r/min. To generate 50-Hz power in

a four-pole machine, the rotor must turn at 1500 r/min. The required rate of rotation

for a give n frequency can always be calculated from Equation