Chapter 5 – synchronous generator summary: Synchronous Generator Construction

EEEB344 Electromechanical Devices
Chapter 5
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CHAPTER 5 – SYNCHRONOUS GENERATOR
Summary:
1. Synchronous Generator Construction
2. The Speed of Rotation of a Synchronous Generator
3. The Internal Generated Voltage of a Synchronous Generator
4. The Equivalent Circuit of a Synchronous Generator
5. The Phasor Diagram of a Synchronous Generator
6. Power and Torque in Synchronous Generator
7. Measuring Synchronous Generator Model Parameters
8. The Synchronous Generator Operating Alone
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The Effect of Load Changes on a Synchronous Generator Operating Alone.
9. Parallel operation of AC Generators
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The conditions required for paralleling
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The general procedure for paralleling generators
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Frequency-power and Voltage-Reactive Power characteristics of a 
synchronous generator.
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Operation of generators in parallel with large power systems
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Operation of generators in parallel with other generators of the same size.
10.Synchronous Generator Ratings 
- The Voltage, Speed and Frequency Ratings
- Apparent Power and Power-Factor Ratings 
-
Synchronous Generator Capability Curve

EEEB344 Electromechanical Devices
Chapter 5
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1. Synchronous Generator Construction
A DC current is applied to the rotor winding, which then produces a rotor magnetic field. The rotor is 
then turned by a prime mover (eg. Steam, water etc.) producing a rotating magnetic field. This rotating 
magnetic field induces a 3-phase set of voltages within the stator windings of the generator. 
“Field windings” applies to the windings that produce the main magnetic field in a machine, and 
“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.
Generally a synchronous generator must have at least 2 components:
a) Rotor Windings or Field Windings
a.
Salient Pole
b. Non Salient Pole
b) Stator Windings or Armature Windings
The rotor of a synchronous generator is a large electromagnet and the magnetic poles on the rotor can 
either be salient or non salient construction. Non-salient pole rotors are normally used for rotors with 2 
or 4 poles rotor, while salient pole rotors are used for 4 or more poles 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. The common ways are:
a) supply the dc power from an external dc source to the rotor by means of slip rings and brushes.
b) Supply the dc power from a special dc power source mounted directly on the shaft of the 
synchronous generator.
Non-salient rotor for a 
synchronous machine
Salient rotor

EEEB344 Electromechanical Devices
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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 2 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, hence 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. 
Some problems with slip rings and brushes:
-
They increase the amount of maintenance required on the machine, since the brushes must be 
checked for wear regularly. 
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Brush voltage drop can be the cause of significant power losses on machines with larger field 
currents. 
Small synchronous machines – use slip rings and brushes. 
Larger machines – brushless exciters are used to supply the dc field current.
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 3-phase output of the exciter generator is rectified to direct 
current by a 3-phase rectifier circuit also mounted on the shaft of the generator, and is then fed to the 
main dc field circuit. By controlling the small dc field current of the exciter generator (located on the 
stator), we can adjust the field current on the main machine without slip rings and brushes. Since no 
mechanical contacts occur between the rotor and stator, a brushless exciter requires less maintenance. 
To make the excitation of a generator completely independent of any external power sources, a small 
pilot exciter can be used.
A pilot exciter is a small ac generator with permanent magnets mounted on the rotor shaft and a 3-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.
A brushless exciter circuit : A small 
3-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.

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A brushless 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.
Even though machines with brushless exciters do not need slip rings and brushes, they still include the 
slip rings and brushes so that an auxiliary source of dc field current is available in emergencies.