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Construction of a synchronous generator

In a synchronous generator, a DC 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.

Synchronous machines theory (generator and motor) in a nutshell
Synchronous machines theory (generator and motor) in a nutshell

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.

A nonsalient two-pole rotor for a synchronous machine
Figure 1 – A nonsalient two-pole rotor for a synchronous machine

A nonsalient-pole rotor is shown in Figure 1, while a salient-pole rotor is shown in Figure 2. Nonsalient-pole rotors are normally used for two- and four-pole rotors, 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) A salient six-pole rotor for a synchronous machine. (b) Photo of a salient eight-pole synchronous ntachine rotor showing the windings on the individual rotor poles. (e) Photo of a single salient pole front a rotor with the field windings not yet in place. (d) A single salient pole shown after the field windings are installed but before it is mounted on the rotor.
Figure 2 – (a) A salient six-pole rotor for a synchronous machine. (b) Photo of a salient eight-pole synchronous ntachine rotor showing the windings on the individual rotor poles. (e) Photo of a single salient pole front a rotor with the field windings not yet in place. (d) A single salient pole shown after the field windings are installed but before it is mounted on the rotor.

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 DC power to its field windings.

There are two common approaches to supplying this DC power:

  1. 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 DC 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-free ly 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 regardless 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 field windings of a synchronous machine. They increase the amount of maintenance required on the machine since the brushes must be checked for wear regularly.

 A brushless exciter circuit.
Figure 3 – A brushless exciter circuit. A small three-phase current is rectified and used to supply the field circuit of the exciter. which is located on the stator. The output of the armature circuit of the exciter (on the rotor) is then rectified and used to supply the field current of the main machine.

In addition, the 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. A brushless exciter is a small AC generator with its field circuit mounted on the stator and its armature circuit mounted on the rotor shaft. The three-phase output of the exciter generator is rectified to direct current by a three-phase rectifier circuit also mounted on the shaft of the generator and is then fed into the main DC field circuit.

Photo of a synchronous machine rotor with a brushless exciter mounted on the same shaft
Figure 4 – Photo of a synchronous machine rotor with a brushless exciter mounted on the same shaft. Notice the rectifying electronics visible next to the armature of the exciter.

By controlling the small DC field current of the exciter generator (located on the stator), it is possible to adjust the field current on the main machine without slip rings and brushes. This arrangement is shown schematically in Figure 3, and a synchronous machine rotor with a brushless exciter mounted on the same shaft is shown in Figure 4 above.

Since no mechanical contacts ever occur between the rotor and the stator, a brushless exciter requires much less maintenance than slip rings and brushes.

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.

Title:Synchronous machines (generator and motor) in a nutshell by Stephen J. Chapman
Format:PDF
Size:11.7 MB
Pages:113
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Synchronous machines (generator and motor) in a nutshell
Synchronous machines (generator and motor) in a nutshell

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One Comment


  1. Samake
    Feb 23, 2021

    Je suis très enchantĂ© d’avoir la chance de lire tout sa c’est très enrichissant.
    Je vous souhaite pour ingĂ©niositĂ© aux services de l’humanitĂ©.
    Cordialement

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