Synchronous generators

Synchronous GeneratorsWorkshop on Basic Electrical Engineering, held at

VVCE, Mysuru, on 30-April-2016

R S Ananda Murthy

Associate ProfessorDepartment of Electrical & Electronics Engineering,

Sri Jayachamarajendra College of Engineering,Mysore 570 006

R S Ananda Murthy Synchronous Generators

Learning Outcomes

After completing this lecture the student should be able to –Describe the principle of operation of an alternator.Describe different types of construction of alternators.List the advantages of rotating field type of alternators.State the reasons for distributing armature conductors inslots.State the meaning of pitch factor, distribution factor, andwinding factor in respect of armature winding of alternators.Find the frequency of the generated E.M.F.Calculate the generated E.M.F. in the alternator taking intoaccount distribution factor and pitch factor.


Photographs of Practical Generators

Horizontal shaft type, typically driven by diesel engine.



Vertical shaft type, typically driven by water turbine.



Horizontal shaft, turbo-alternator, driven by steam turbine.


Principle of Operation of Synchronous Generator

A1

A2

B1B2

C1

C2

StationaryArmature

RotatingField Coil

D.C.Supply

S N

A1

A2

B1

B2

C1

C2

Stator

Rotor

When the poles on the rotor, driven by prime mover, move pastthe stator conductors, due to the relative motion of conductorswith respect to the poles, the magnetic flux lines are cut by theconductors and voltage is induced in them.


Salient Pole and Cylindrical Rotor Types

.

CastSteel

Frame

ArmatureCore

N

N

S S

N

S

Salient Pole Construction Cylindrical Rotor Construction

... .. ..

.... .. ..

If prime move speed is low as in case of water turbines, salientpole type is preferred. If the prime mover speed is high as incase of steam turbines, cylindrical rotor is preferred.


Advantages of Rotating Field Construction

The coil connections can be made easily and securely onthe stator than on the rotor.If the armature winding is placed on the rotor, then, threeslip rings would be required where as if the poles areplaced on the rotor, only two slip rings designed to carrylow power for the field winding are required.Transferring large armature power through brush and slipring causes them to wear out frequently which is preventedif the armature is stationary.


Advantages of Rotating Field Construction

As the generated voltage is 11 kV or above, armaturewinding requires thicker insulation which is difficult todesign if the armature is on the rotor.Field winding is lighter than armature winding andtherefore it is preferable to place it on the rotor.In very big synchronous generators, forced hydrogencooling of armature winding is employed which can beconveniently implemented if the armature is stationary.


Reasons for Using Distributed Winding

Armature winding in practical generators is uniformly distributedin many slots for the following reasons —

It is difficult to put all the conductors of a phase winding inone or two slots.Distributed winding reduces harmonics in the generatedvoltage and makes the waveform closer to sinusoidalshape.Distributed winding helps in more uniform heat distributionand cooling and thus helps in preventing insulation failuredue to excessive heat in the winding.


Photograph of Stator Winding

Stator winding of a generator in a hydro-electric power plant.


Armature Winding Coils

P Q P Q

Full Pitched Coil Short Pitched Coil

A1 A2 A1 A2

Overhang

Coil Span

CoilSide

In practical generators, short pitched coils are used to minimizethe length of overhangs and also to reduce harmonics in thegenerated voltage.


Pitch Factor (Kp) of Armature Winding

Due to short pitched coil, the magnitude of E.M.F. induced e willbe slightly reduced when compared to a full pitched coil. For ashort pitched coil, the pitch factor is defined as

Kp =Voltage generated in a short pitched coilVoltage generated in a full pitched coil

(1)

Formula for Kp is

Kp = cosβ

2(2)

where β is the angle in electrical degrees by which the coil isshort pitched as shown in the previous slide. Typically Kp isabout 0.9 in a practical generator.


Distribution Factor (Kd ) of Armature Winding

For a distributed winding, the distribution factor, Kd is defined as

Kd =Voltage induced in a distributed winding

Voltage induced in a concentrated winding(3)

Kd is given by

Kd =sin(mα/2)

msin(α/2)(4)

where α is known as slot angle and m are given by

α =180S/P

and m =S

3P

where S is total number of slots, P is number of poles. TypicallyKd is about 0.9 for a practical generator.


Frequency of Generated E.M.F.

When a conductor moves past a pair of poles, one cycle ofsinusoidal voltage is completed. If P = total number of poles inthe machine, then,

Number of cycles per revolution =P2

(5)

If N = R.P.M. of the motor, then, the rotor makes N/60 R.P.S.Hence, the frequency of the induced E.M.F. is given by

f =P2× N

60=

PN120

Hz (6)


Equation for Induced E.M.F. in an Alternator

If P = number of poles in the machine, and Φ = flux per pole,magnetic flux cut by a conductor in one revolution of the rotor= PΦ. If N is the R.P.M., then, time taken by the rotor to makeone revolution = 60/N seconds. Therefore,

Flux cut per second by a conductor =PΦ

60/N

But average induced E.M.F. in a conductor = flux cut persecond. Therefore

Average induced E.M.F. in a conductor =PΦN

60



If T = total number of turns connected in series per phase, andsince each turn will have two conductors, we haveZ = Total number of conductors in series per phase = 2T . So,

Average E.M.F. induced per phase = |Eav |=PΦN

60·2T

The air gap flux in the generator will have more or lesssinusoidal distribution. Then, the induced E.M.F. in each phasewill also be sinusoidal. For a sinusoidal waveform we have

Form Factor =|Eph||Eav |

= 1.11

where Eph = R.M.S. value of the induced voltage per phase.



Therefore, the R.M.S. voltage induced per phase is

|Eph|= 1.11× PΦN60×2T =

2.22PΦNT60

(7)

But the frequency of the induced E.M.F. is given by

f =PN120

=⇒ 2f =PN60

Substituting this in Eq. (7) we get

|Eph|= 4.44ΦfT Volts



In a practical machine the armature winding is evenlydistributed in the slots and short pitched coils are used. Due tothis, the induced E.M.F. is slightly reduced by a factor Kw whereKw = KpKd is known as the winding factor. So, the inducedE.M.F. in an actual machine is given by

|Eph|= 4.44ΦfTKw Volts (8)

Since the three coils in the armature winding of a practicalgenerator are always star connected, the line voltage at theterminals of the synchronous generator is given by

|E |=√

3×|Eph| (9)


License

This work is licensed under aCreative Commons Attribution 4.0 International License.


Synchronous generators

Engineering

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