EEP 3243 Sync Mach

8/9/2019 EEP 3243 Sync Mach

1/42


2/42

Synchronous Machines


3/42

Synchronous Generator 3 phase synchronous generators are the largest energy converters and the primary source of all the electrical energy we consume.It has 3 windings a-1, b-2 and c- 3 are placed at 120 o to

each other.

3


4/42

C ont.W hen the magnet is rotated at constant speed, thevoltages induced in the 3 windings have the sameeffective value, but the peaks occur at different times.

4


5/42

P ower Output of 3 P hase Generator

5


6/42

C ont.T he instantaneous power supplied to each resistor is composed of the

power wave that surges between zero and a maximum value P m.

If we add the instantaneous powers of all 3 resistors, we discover that

the resulting power is constant with the magnitude of 1.5 P m.

6


7/42

N umber of P olesT he # of poles on a synchronous generator dependsupon the speed of rotation and the frequency wishto produce.T

he alternator frequency is given by,

7


8/42

Main Features of The Stator

8

It is composed of a cylindrical laminated core containing a set of slots thatcarry a 3 phase lap winding.


9/42


10/42

Main Features of the Rotor S ynchronous generator are built with 2 types of rotors; salient-pole rotors and cylindrical rotors.S alient-pole rotors are usually driven by low-speed

hydraulic turbines, cylindrical rotors are driven by high-speed steam turbines.

10


11/42

Salient-pole rotor Low speed rotors always possess a large diameter to provide the necessary space for the poles.T he salient poles aremounted on a largecircular steel framewhich is fixed to arevolving vertical shaft.

11


12/42

C ont.

T he squirrel-cage winding/damper winding produces force which dampen the oscillation of the rotor when the load on the generator changes

suddenly.Damper winding also tends to maintain balanced 3 phase voltage between the lines when the linecurrents are unequal due to unbalanced load condition.

12


13/42

C ylindrical rotor S team turbine generators possess either 2 ( 36 00 rpm)or 4 poles (1800 rpm).T he high speed of rotation produces strong centrifugal forces which impose anupper limit on the diameter of the rotor (example max 1.2mfor 36 00 rpm).

13


14/42

Field Excitation and ExcitersT he DC field excitation of a large synchronousgenerator is an important part of its overall design,because the field must ensure a stable AC terminal

voltage and system stability while the load changes.In order to attain it, 2 DC generators are used; mainexciter and pilot exciter. S tatic exciters that involve norotating parts also employed.

14


15/42

C ont.In order to attain it, 2 DC generators are used; main exciter

and pilot exciter. S tatic exciters that involve no rotating parts also employed.T he main exciter feeds the exciting current to the field of thesynchronous generator by way of brushes and slip-rings.

15


16/42

C ont.T he exciting current regulated manually or automatically by control signals produced by the pilot excited.During serious disturbance which produce a sudden voltagedrop across, the excites must then react very quickly to

keep the AC voltage from falling.

16


17/42

C ont.B rushless excitation system have been developed toeliminate the shortcoming of slip rings and brushesreplacement.T his system consists of 3 phase, stationary field

generator whose AC output is rectified by a group of rectifiers.T he commutator (mechanical rectifier) is replaced by

an electronic rectifier.

17


18/42

Factors affecting the size of synchronous generator

G enerator's efficiency automatically improves as the power increases. (1k W has 50% efficiency, 10M W has90% efficiency and 1000M W has 99% efficiency).Power output per kilogram increases as the power increases. (1k W has 50 W /kg, 10M W has 500 W /kg)

18


19/42

C ont.Increase (tripled) the size of the machine in such a way that its linear dimensions are raised in exactly the same proportion, while keeping the same materials.

T he volume will increase by a factor of 3 3=27.T he mass and losses increase by a factor 27.

T he slots are 3 X wider and 3 X deeper as a result, the cross section of theconductors is 9 X greater means they can carry 9 X more current.E =B lv, the l has tripled and v has increased 3 X because the diameter of therotor has tripled. As result , the voltage generated increase by a factor of 9.By tripling the linear dimensions, the voltage and current increase by 9 times,this means the output of the new generator increases by 81 times.

19


20/42

C ont.T he big problem is temperature rise, when linear dimensions are tripled, the heat -dissipating surface areaof the machine increases 9 X but the losses increase 27 X.T he larger machines is bound to be hotter unless better cooling means are used.T o prevent damage to the insulating materials, thetemperature rise has to be limited to a maximum of 200 oC. As regards speed, low-speed generators are always bigger

than high-speed machines of equal power.

20


21/42

Equivalent C ircuit of An A C Generator

21


22/42

C ont.Each phase of the stator winding possesses aresistance R and a certaininductance L.Because this is an AC machines, the inductancemanifest itself as areactance X s which is aninternal impedance, just like its internal resistance,R.

22


23/42

C ont.W e can neglect the resistance, unless we are interested inefficiency or heating effects.

Note that E o and E are line-to-neutral voltages and I is the linecurrent.

23


24/42

Synchronous Motor

T he synchronous generators can operate either asgenerators or as motors.S ynchronous motors run in synchronism with therevolving field so the speed of rotation is therefore tied to

the frequency of the source.Because the frequency is fixed, the motor speed staysconstant, irrespective of the load or voltage of the 3-phaseline.

24


25/42

C onstructionS ynchronous motors are identical in construction to salient-

pole AC generator.T he winding is identical to a 3 phase induction motor, whichcarries a 3 phase lap winding.T he rotor has a set of salient poles that are excited by a DC current.T he exciting coils are connected in series to 2 slip rings, and the DC current is fed into the winding from an external exciter.Modern synchronous motors often employ brushlessexcitation.

25


26/42

C ont.T he DC current I x from the rectifier is fed directly into thesalient-pole windings, without going through brushes and sliprings.T he current can be varied by controlling the small exciting

current I c that flows in the stationary field winding of theexciter.

26


27/42

C ont.

27


28/42

C ont.T he rotor and stator have the same number of poleswhich determines the synchronous speed of themotor:

28


29/42

P rinciple of OperationIn order to understand the principle of operation of asynchronous motor, let us examine what happens if weconnect the armature winding of a 3-phase synchronousmachine to a suitable balanced 3-phase source and thefield winding to a D.C source of appropriate voltage.T he current flowing through the field coils will set upstationary magnetic poles of alternate North and S outh.On the other hand, the 3-phase currents flowing in thearmature winding produce a rotating magnetic field rotating at synchronous speed.

29


30/42

C ont.In other words there will be moving North and S outh poles established due to the 3-phase currents i.e at any location in the stator there will be a North pole at some instant of time and it will become a S outh poleafter a time period corresponding to half a cycle. (after a time = 1/2f).T hese two poles get attracted and try to maintain thisalignment and hence the rotor pole tries to follow the

stator pole as the conditions are suitable for the production of torque in the clockwise direction.

30


31/42

C ont.H owever the rotor cannot move instantaneously dueto its mechanical inertia, and so it needs sometimeto move. In the mean time, the stator pole would quickly (a time duration corresponding to half acycle) change its polarity and becomes a S outh pole.T hus the rotor will experience an alternating forcewhich tries to move it clockwise and anticlockwise

at twice the frequency of the supply, i.e. at intervalscorresponding to 1/2f seconds.

31


32/42

C ont.

As this duration is quite small compared to themechanical time constant of the rotor, the rotor cannot respond and move in any direction. T herotor continues to be stationary only.

T hus the synchronous rotor cannot start rotating onits own . S o, some special provision has to be madeeither inside the machine or outside of the machineso that the rotor is brought to near about itssynchronous speed.

32


33/42

C ont.

At that time, if the armature is supplied withelectrical power, the rotor can pull into step and continue to operate at its synchronous speed.Basically, a synchronous motor rotates because of the magnetic attraction between the poles of therotor and the opposite poles of the stator.

33


34/42

P ull-in Torque

As soon as the motor is running at close tosynchronous speed, the rotor is excited with D G current.T

his produces alternate N and S

poles around thecircumference of the rotor.

34


35/42

C ont.

If the poles at this moment happen to be facing poles of opposite polarity on the stator, a strongmagnetic attraction is set up between them. T hetorque developed at this moment is called the pull-in

torque.

35


36/42

Methods of Starting Synchronous Motor Basically there are three methods that are used tostart a synchronous motor: T o reduce the speed of the rotating magnetic field of the

stator to a low enough value that the rotor can easily

accelerate and lock in with it during one half-cycle of therotating magnetic fields rotation. T his is done by reducing the frequency of the applied electric power.T his method is usually followed in the case of inverter-

fed synchronous motor operating under variable speed drive applications.

36


37/42

C ont. T o use an external prime mover to accelerate the rotor of

synchronous motor near to its synchronous speed and then supply the rotor as well as stator. T his method isusually followed in the laboratory- the synchronousmachine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling procedure. T hen the power supply to the prime mover is disconnected so that the synchronousmachine will continue to operate as a motor.

37


38/42

C ont. T o use damper windings if these are provided in the

machine. T he damper windings are provided in most of the large synchronous motors in order to nullify theoscillations of the rotor whenever the synchronousmachine is subjected to a periodically varying load.

38


39/42

Losses and efficiency

T he flow of power through a synchronous motor,from stator to rotor and then to shaft output, isshown in below figure.

39


40/42

C ont.

As indicated in the power-flow diagram, the total power loss for the motor is given by

P loss = P scl + P core + P fcl + P f,w + P stray W

where:P scl = stator-copper lossP fcl = field-copper.lossP core = core lossP f,w = friction and windage lossP stray = stray load loss

40


41/42

C ont.

T he overall efficiency of a synchronous motor isgiven by

41


42/42

THE END

42

Documents

EEP 3243 Sync Mach