Energy Conversion - Synchronous Converters

7/23/2019 Energy Conversion - Synchronous Converters

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Module 2

Synchronous Converters


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AC Generator

Alternator (AC Generator) or Synchronous

Generator it is an electrical machine converting

mechanical energy to ac electrical energy.


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Principle of Operation

The principle of operation of the alternator is exactly the same as that ofthe dc generator that is there is induced emf in the stator conductors when

they cut the magnetic flux produced in its magnetic field poles. However in

an alternator it is not essential for the armature to rotate or and either the

armature or field can be made to rotate while the other is kept constant.

Nowadays the armature is kept stationary and the field rotates around it.

Rotating armature alternator Rotating field alternator


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dvantages of Revolving

Field Stationary rmature

The armature winding is more complex than the field and can

be constructed more easily on stationary armature.

The armature winding can be braced more securely in rigid

frame. It is easier to insulate and protect the high-voltage armature

winding common to alternators.

The armature winding is cooled more because the stator

case core can be made large enough and with many airpassages or air ducts for force air circulation.

The low voltage field can be constructed for efficient high-

speed operation.


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Alternator Construction

The main parts of the alternator are the following:

1. Stator. It is built up with stampings which are

insulated with paper varnish. The stator is housed in

frame which is fabricated from electrically weldedsteel plates. Slots are cut around the inner surface

which accommodates the windings.

2. Rotor. It has rotating magnetic field poles as in dc

generators which are separately excited from a dcsource known as exciter.


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Types of Rotor Construction

Two Types of RotorConstruction:

1. Salient (or projecting)

poles. It is made of cast

iron steel of good

magnetic quality. These

types of rotors are used in

low speed and medium

speed machines. These

machines have short axial

length and large

diameters. Hydropower

alternators and diesel

engine alternators are of

this type.

Types of rotors used in alternator.

(A) Cylindrical type; (B) Salient-pole type


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Types of Rotor Construction

2. Smooth-cylindrical type.

It is made of a solid steel

piece and slots and made

on the circumference of the

rotor to hold the fieldwindings. This type of rotor

is suitable for high speed

turbo alternators. This type

of rotor is cylindrical and

has a large axial lengthand small diameter

Types of rotors used in alternator.

(A) Cylindrical type; (B) Salient-pole type


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Alternator Construction (cont.)

2. Alternator Construction

a. Stationary field revolving armature

b. Revolving field stationary armature

3. Damper Windings (Squirrel Cage Windings)

Functions of Damper Windings

Useful in preventing hunting (momentary speed

fluctuations)

Provides the starting torque needed in synchronous

motor

Tends to maintain balanced 3-phase voltage under

unbalanced load conditions


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General Types of Alternator

1. Synchronous Generator. It is a generator which is driven

at constant speed (synchronous speed) and it is used in

almost all types of applications.

2. Induction Generator. It is an induction motor which runs

as a generator with a speed above synchronous speed. Its

p.f. is normally leading and usually connected in parallel

with a synchronous generator in order to supply power for

lighting loads.

3. Induction Alternator. It generates voltage at higher

frequencies (500 Hz to 10 kHz). It is used to supply power

to induction furnace in order to heat and melt the metal.


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Prime Movers for Alternator

The following are the various prime movers used foralternators:

For large AC generator

a. Steam turbine

b. Gas turbine

c. Hydraulic turbine

d. Internal combustion engine

For small AC generator

a. Internal combustion engine


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Generated Voltage in an lternator

Where:

f = frequency in Hertz

p = number of poles

nS = synchronous speed in rpm

=

= . = .

Where: E = rms voltage generated per phase

m = maximum flux per pole

N = number of turns per phaseZ = number of conductors in series per phase

kp = pitch or chording or coil span factor

kb = breadth or distribution or winding or spread factor

Frequency of Generated EMF


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Pitch FactorPitch Factor or Chording Factor or Coil Span Factorit may be

defined as the ratio of the vector sum of the induced emfsper coilto the arithmetic sum of the induced emfsper coil. It is may also be

defined as the ratio of the emfsof short pitch coil to emfsof full-

pitch coil. By formula, it is given as

=

= [ ]

=

.

Where: kp= pitch factor

q = qth harmonic (1 for fundamental or first harmonic, 3

for third harmonic, 5 for fifth harmonic and so on.

= span of coil (coil pitch) in electrical degrees


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Pitch Factor

Short-pitched winding or chorded windinga winding issaid to be short-pitched or fractional-pitched if its coils are

placed less than one pitch (180 electrical degrees) apart.

Reasons for using Short-pitched winding They save copper of end connections.

They improved the waveform of the generated emf by

reducing or totally eliminating distorting harmonics

They reduced the eddy current and hysteresis loss byeliminating high frequency harmonics.


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Breadth Factor

Breadth or distribution or winding or spread factor it isdefined as the ratio of emfsof distributed winding to emfsof

concentrated winding.

=

=

Where:kb = breadth factor

n = number of slots per pole per phase

= number of electrical degrees between adjacent slots


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Sample Problems

1. Calculate the pitch factor for the given windings:a. 36 stator slots, 4-poles, coil span = 1 to 8

b. 72 stator slots, 6-poles, coil span = 1 to 10

c. 96 stator slots, 6-poles, coil span = 1 to 12

2. Calculate the breadth factor for a 36-slots, 4 pole,

single-layer, three phase winding.

3. A 3-, 16-pole alternator has a star-connected windingwith 144 slots and 10 conductors per slot. The flux per

pole is 0.03 Weber sinusoidally distributed and the

speed is 375 rpm. Find the frequency and the phase and

line emf. Assume full-pitched coil.


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Sample Problems

4. The stator of a 3-,16-pole alternator has 144 slots andthere are 4 conductors per slot connected in two layers

and the conductors of each phase are connected in

series. If the speed of the alternator is 375 rpm,

calculate the emf per phase. Resultant flux in the air-

gap is 5 x 10-2Weber sinusoidally distributed. Assume

the coil span as 150 electrical.

5. An alternator on open circuit generates 360 V at 60 Hz

when the field current is 3.6 A. Neglecting saturation,

determine the open-circuit emf when the frequency is

40 Hz and the field current is 2.4 A.


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Sample Problems

6. A 4-pole, 3-, 50 Hz, star-connected, alternator has 60slots, with 4 conductors per slot. Coils are short-pitched

by 3 slots. If the phase spread is 60, find the line

voltage induced for a flux per pole of 0.943 Wb

distributed sinusoidally in space. All the turns per phase

are in series.

7. A 4-pole, 50-Hz, star-connected alternator has 15 slots

per pole and each slot has 10 conductors. All the

conductors of each phase are connected in series. The

winding factor being 0.95. When running on no-load fora certain flux per pole, the terminal emf was 1825 volts.

If the windings are lap-connected as in a dc machine, what

would be emf between brushes for the same speed and

the same flux/pole. Assume sinusoidal distribution of flux.


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Synchronous Motors

Synchronous Motor it is an ac machine that operates at synchronous speed

and converts electrical energy into mechanical energy using field magnets

excited with direct current. It is also called as synchronous condenser.

Characteristics of a Synchronous Motor:

It runs either at synchronous speed or not at all. It is not inherently self-starting.

It is capable of being operated under a wide range of power factors, both

lagging and leading.

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Energy Conversion - Synchronous Converters