Lecture 7
Synchronous Generator
I. Construction
Rotor field windings are
excited by a DC supply
through slip rings
Rotor poles
Non-salient pole
•
Consisting of a three-phase winding
•
•
Cylindrical in shape having slots on the rotor surface where field
windings are embedded.
Used for two- and four-pole rotors.
Prime movers are generally gas or steam turbines (with high
speeds usually 1500 RPM to 3000 RPM).
Salient pole
•
•
•
Non-salient pole
Salient pole
Projected Poles where field windings are wrapped
around.
Used for rotors with four or more poles.
Prime overs are a hydro turbine or a combustion engine
which have low or medium speeds (usually 100 RPM to
1500 RPM).
II. Operation Principle
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•
•
In a synchronous generator, a rotor magnetic field is produced either by designing the rotor as a permanent magnet or by applying
a dc current to a rotor winding to create an electromagnet.
The rotor of the generator is then turned by a prime mover, producing a magnetic field, within the machine, rotating in same
direction of rotor rotation.
This rotating magnetic field induces a three-phase set of voltages within the stator windings of the generator. The magnitude of
the voltage induced in a given stator phase is
Synchronous generators are by definition synchronous, meaning that the electrical frequency of the produced voltage is locked
in or synchronized with the rate of rotation of the magnetic fields in the machine, hence related to the rotation speed of the
generator rotor (nm= nsyn)
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 rpm. To generate 50-Hz power
in a four-pole machine, the rotor must turn at 1500 rpm.
III. Induced voltage at the stator (EA)
The rms induced voltage at any phase of this three-phase stator
𝐸𝐴 = 2 𝜋 𝑇𝑝ℎ 𝜑𝑓𝑠𝑒 = 4.44𝑇𝑝ℎ 𝜑𝑓𝑠𝑒
Since
𝑓𝑠𝑒 =
𝑃
𝑓
2 𝑠𝑚
Hence, 𝐸𝐴 = 2 𝜋𝑇𝑝ℎ 𝜑
Hence,
𝐸𝐴 = 𝐾𝜑𝜔𝑚
𝑃
𝑓
2 𝑠𝑚
where
And
𝜔𝑚 = 2𝜋𝑓𝑠𝑚
𝐾=
𝑇𝑝ℎ 𝑃
2 2
•
•
•
•
Tph is the number of stator turns per phase
P is the machine number of poles.
fse is the stator electrical frequency
fsm is the stator mechanical frequency
• ω is the angular electrical frequency (electrical
radian/sec)
• ωm is the angular mechanical frequency
(mechanical radian/sec)=2Пnm/60
The synchronous generator induced voltage and in turn its terminal voltage can be controlled by changing
the field current via changing the external field voltage or by inserting a resistance in the field circuit
IV. Equivalent circuit
The internal generated voltage induced in one phase of a synchronous generator ( EA ) is not usually the per phase
terminal voltage of the generator (VT)due to a number of factors;
1. When a synchronous generator's rotor is spun, a voltage is induced in the generator's stator windings. If a load is attached to the
terminals of the generator, a current flows producing a magnetic field in the stator. This stator magnetic field distorts the original
rotor magnetic field, changing the resulting phase voltage. This effect is called armature reaction (XAR)
2. The self-inductance of the stator coils.(XA)
3.The resistance of the stator coils. (RA)
The armature reaction effects and the self-inductance in the machine are both
represented by reactances, and it is customary to combine them into a single reactance,
called the synchronous reactance of the machine (Xs)
Equivalent circuit per phase of a three-phase synchronous generator
V. Voltage Regulation
where Vnl is the no-load voltage of the generator (=EA) and Vfl is the full-load voltage of the generator.
• A synchronous generator operating at a lagging power factor has a fairly large positive voltage regulation
• A synchronous generator operating at a unity power factor has a small positive voltage regulation
• A synchronous generator operating at a leading power factor often has a negative voltage regulation
EA
Generator Terminal characteristic with a 0.8 PF lagging load
EA
Generator Terminal characteristic with a 0.8 PF leading load
VI. Power Flow Diagram
𝐍𝐨𝐭𝐞; 𝑷𝒅𝒆𝒗−𝒎𝒙
𝟑𝑬𝑨 𝑽𝑻
=
𝑿𝑺
VII. Synchronous Generator Operation
The behavior of a synchronous generator under load varies greatly depending on the power factor of the load and on
whether the generator is operating alone or in parallel with other synchronous generators
I. THE SYNCHRONOUS GENERATOR OPERATING ALONE
An isolated synchronous generator supplying its awn load independently of other generators is very rare.
Such a situation is found in only a few out-of-the-way applications such as emergency generators
II. THE SYNCHRONOUS GENERATOR OPERATING IN PARALLEL
For all usual generator applications, there is more than one generator operating in parallel to supply the power
demanded by the loads
To achieve the match between the oncoming generator and the
running system, the following paralleling conditions must be met:
1. The rms line voltages of the two generators must be equal.
2. The two generators must have the same phase sequence.
3, The phase angles of the two a phases must be equal.
4. The frequency of the oncoming generator, must nearly equal
to the frequency of the running system.
VII. Applications
Large synchronous generators are used to generate bulk power at steam, hydro & nuclear power
stations.
• Discuss the construction of synchronous generator
• Discuss the theory of operation of synchronous generator
• State how to control synchronous generator’s terminal voltage
• Draw the equivalent circuit of synchronous generator
• Show, with the aid of figures, how the synchronous generator performance differs with loads of different power
factor
• Draw power flow diagram of synchronous generator
• State the conditions of parallel operation of synchronous generators
• State some application of synchronous generator