Synchronus Generator, Quizzes of Electromagnetism and Electromagnetic Fields Theory

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Electric Machinery Fundamentals

Rizwan Khan

University of Engineering and Technology, Lahore

1

• Short Circuit Characteristics
• The Short Circuit Ratio
• The Alone Operation
• The Parallel Operation

Contents

Synchronous Generator Construction

• 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.

Field and Armature Windings

• The rotor of a synchronous generator is essentially a large electromagnet. The magnetic poles on the rotor can be of either salient or non-salient construction.
• The term salient means "protruding" or "sticking out," and a salient pole is a magnetic pole that sticks out radially from the shaft of the rotor. On the other hand. a non-salient pole is a magnetic pole with windings embedded flush with the surface of the rotor.
• A non-salient pole rotor is shown in Figure 4 - 1. Note that the windings of the electromagnet are embedded in notches on the surface of the rotor.

Salient and Non-salient Construction

• A salient-pole rotor is shown in Figure 4 - 2. Note that here the windings of the electromagnet are wrapped around the pole itself, instead of being embedded in notches on the surface of the rotor. Non-salient 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.

Salient and Non-salient Construction

Rizwan Khan (Lecturer - UET LHE) 11 ( (aJ (cJ FI GURE 4- (a) A salient six-pole rotor for a synchronous machine. (b) Photograph of a salient eight-pole synchronous machine rotor showing the windings on the individual rotor poles. (Courtesy of General Electric Company.) (c) Photograph of a single salient pole from a rotor with the field SYNCHRONOUS GENERATORS 193 (bJ (dJ windings not yet in place. (Courtesy afGeneral Electric Company.) (d) A single salient pole shown after the field windings are installed but before it is mounted on the rotor. (Courtesy of Westinghouse Electric Company.) de power to its field windings. There are two common approaches to supplying this de power:

1. Supply the de power from an external de source to the rotor by means of slip rings and brushes.

• 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 de 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.

Through Slip Rings and Brushes

• 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. 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.

Through Slip Rings and Brushes

Through Brushless Excitor

On larger generators and motors, brushless exciters are used to supply the de 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 mo unted on the rotor shaft. The three-phase output of the exciter generator is rectified to di- rect current by a three-phase rectifier circuit also mounted on the shaft of the ( generator, and is then fed into the main de field circuit. By controlling the small de 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 4-3, and a I I I I Exciter Exciter armature v<..A.-V '-.AJVV I I :L I I I I II I I T hree-phase rectifier r- Synchronous machine Main Field --- 1 ------------- -+- --- -----------+ --------- - --- - -- I 1 Excite, (^) r I 1 r----- Three-phase Three-phase input (low cun ent) FIGURE 4- fie ld : output r" Main armature 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 circui t of the exciter (on the rotor) is then rectifie d and used to supply the field current of the main machine.

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

Through Brushless Excitor

Through Pilot Excitor

so " 196 ELECTRIC MACHINERY FUNDAMENTALS 1 1 1 Pilot exciter Pilot exciter field Permanent magnets Exciter Exciter armature : ! Three- T phase rectifier : 1

Synchronous generator Main field I I I : output I I I 1 1 1 I 1 1 1 1 1 1 Three- phase rectifier Lrvv-v-,. Pilot exciter armature FIGURE 4- RF 1 1 Exciter field 1 1 I 1 1 1 Mum armature A brush less excitation scheme that includes a pilot exciter. The permanent magnets of the pilot exciter produce the field current of the exciter, which in turn produces the field current of the main machine.

The Speed of Rotation