1. CHAPTER-FIVE Synchronous Machines
4/6/2017
CHAPTER-FIVE Synchronous Machines

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A synchronous machine is an ac rotating machine whose speed under steady state condition is proportional to the frequency of the current in its armature.
The magnetic field created by the armature currents rotates at the same speed as that created by the field current on the rotor, which is rotating at the synchronous speed, and a steady torque results.
Synchronous machines are commonly used as Generators especially for large power systems, such as turbine generators and hydroelectric generators in the grid power supply.
ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

3. Synchronous Generator
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Facts
Used principally in large power applications because of their
High operating efficiency
Reliability
Controllable power factor
Rotates at constant speed in the steady state
i.e. The rotating air gap field and the rotor rotate at the same speed.
It is a doubly excited machine
i.e. Rotor poles are excited by a DC current
Stator are connected to the ac supply
It can draw leading or lagging reactive current from the ac supply system.
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DC Machines
Stator
Rotor + Vf - If N R S
The DC excitation current in the stator generates a Resultant flux.
and induced voltage in the rotor.
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Synchronous machines
Stator
Rotor + Vf - If N R S
The DC excitation current in the rotor generates a flux.
The turbine drives the rotor and produces a rotating flux
The rotating flux induce AC three phase voltage in the stator winding.
The generated power of a synchronous machine can be adjusted by controlling the magnitude of the rotor field current.
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7. Synchronous Machine Structures
Stator and Rotor
The stator is termed as stationary armature where the generated power can be easily taken out. The armature winding of a conventional synchronous machine is almost invariably on the stator and is usually a three phase winding.
2. The rotor is the rotating member of the machine
The field winding is usually on the rotor and excited by dc current, or permanent magnets.
The dc power supply required for excitation usually is supplied through a dc generator
This dc generator known as exciter, which is often mounted on the same shaft as the synchronous machine.
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Rotor structures:
There are two types of rotor structures
Salient pole rotor
For low speed applications, such as hydroelectric generators, Diesel Generator etc.
2. Non-Salient pole (cylindrical) rotor
For high speed synchronous machines, such as steam turbine generators, Gas turbine, etc
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Salient Pole rotor
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Cont’d
ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

11. Non-Salient(Cylindrical) Rotor
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16. Mechanical and Electrical Angle
Mechanical angle
Any angle Measured on the surface of a synchronous Machine is a Mechanical Angle.
Always the total Mechanical angle over the complete surface being 3600 or 2pi radian.
Electrical Angle
The Generated Voltage depends on the number of poles is called Electrical Angle.
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Contd.
Example, we consider a four pole machine. As the rotor rotates for one revolution the induced
emf varies for two cycles and hence
For a general case, if a machine has P poles, the relationship between the electrical and mechanical units of an angle can be readily deduced as ,similarly,
Where is the angular frequency of electrical radians per second and the angular speed of the rotor in mechanical radians per second.
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Synchronous Speed
The Synchronous speed defined as
It can be seen that the frequency of the induced emf is proportional to the rotor speed and this speed is called Synchronous speed
ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

19. Distributed Three Phase Windings
The stator of a synchronous machine consists of a laminated electrical steel core and a three phase winding.
The stator lamination of a synchronous machine that has a number of uniformly distributed slots.
Coils are to be laid in these slots and connected in such a way that the current in each phase winding would produce a magnetic field in the air gap around the stator.
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Contd.
Stator coils are connected to form a three phase winding. Each phase is able to produce a specified number of magnetic poles.
The windings of the three phase are arranged uniformly around the stator periphery and are labeled in the sequence that phase ‘a’ is 120o(electrical) ahead of phase ‘b’ and 240o(electrical) ahead of phase ‘c’.
Phase a
Phase b
Phase c
120o
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as' as bs
bs' cs
cs'
vas
vbs
vcs
Contd.
Stator windings are sinusoidally distributed with Ns equivalent turns and their magnetic axes are displaced by 120o.
Where
Ns: the number of turns of the equivalent sinusoidally distributed stator windings.
s: the angular displacement about stator
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22. Rotating Magnetic Fields
Magnetic Field of a Distributed Phase Winding
The magnetic field distribution of a distributed phase winding can be obtained by adding the fields generated by all the coils of the winding.
The mmf distribution along the air gap is a square wave. Because of the uniform air gap, the spatial distribution of magnetic field strength is the same as that of mmf.
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23. Magnetic Field of Three Phase Windings
The resultant mmf generated by a three phase winding becomes
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Graphical Analysis
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Contd.
The Resultant mmf wave has a constant amplitude of 3/2 FM and rotates at constant Synchronous speed
Using Per-Phase Analysis the equivalent circuit of Synchronous Generator
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26. Equivalent Circuit of Synchronous Machine
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Contd.
Consider the terminal Voltage leads the current by Φ. Determine the Generated Voltage
Consider Triangle ONM O Vt
IaRa
IaXs Eg Ia N M Φ
VtCos Φ
VtSinΦ
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28. Open- and Short-Circuit Characteristics
The Short-Circuit -Ratio (SCR) is the ratio of the field current Of ’=If1 needed to generate rated open-circuit Armature Voltage to the field current Of ’’=If2 needed to produce rated armature current
Open- and short-circuit characteristics of a synchronous machine.
Ia1
Ia2=In
The SCR exactly equals the per unit synchronous reactance so that
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