Synchronous Machines

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Synchronous Machines
BEE2123 ELECTRICAL MACHINES
Muhamad Zahim Ext 2312 A1-01-06 zahim_at_ump.edu.my
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Contents

• Synchronous Generator
• Construction
• Principle of Operation
• Equivalent Circuit
• Power Flow
• Synchronous Generator Operating Alone
• Parallel Operation of Synchronous Generator
• Synchronous Motor
• Equivalent Circuit
• Torque Speed Characteristic
• Effect of Load and Field Current Changes
• Synchronous Motor and Power Factor Correction
• Starting Methods of Synchronous Motor

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Introduction

• Synchronous machines are AC machines that have a
  field circuit supplied by an external DC
  source. In a synchronous generator, a DC current
  is applied
  to the rotor winding, which produces a rotor
  magnetic field.
• The rotor of the generator is then turned by a
  prime mover (mechanical torque which forces the
  rotor to turn), producing a rotating magnetic
  field within the machine. This rotating magnetic
  field induces a voltage within the stator
• windings of the generator.
• Synchronous motors reverse this process. The
  essential feature that makes synchronous machines
  different from other electrical machines is that
  its synchronous link between stator and rotor
  magnetic fields. Because of that there is a fixed
  relationship between rotor speed and the
  frequency of induced EMF in the stator.

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Introduction

• Another advantage that makes synchronous
  machines different from other
• machines is that varying its field excitation can
  vary its power factor of operation.
• This property makes it to be useful for the
  Industry, which is always operating at low
  lagging power factor (motor inductive load). So
  part of the load is handled by synchronous
  machine whose field is adjusted such that it is
  operating at leading power factor to improve the
  overall power factor to nearly unity.

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Introduction

• Synchronous Machines
• Synchronous Generators A primary source of
  electrical energy largest
  (energy converter).
• Synchronous Motors Used as motors as well as
  power factor compensators
  (synchronous condensers).

• Asynchronous(Induction) Machines
• Induction Motors Most widely used electrical
  motors in both
  domestic and industrial applications.
• Induction Generators Due to lack of a separate
  field excitation,
  these machines are rarely
  used as generators

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Introduction

• There are numerous reasons for such an
  inside-out construction of a synchronous
  generator, some of which are listed below.
• Most synchronous generators are built in much
  larger sizes than their dc counterparts. An
  increase in power capacity of a generator
  requires thicker conductors in its armature
  winding to carry high currents and to minimize
  copper losses.
• Since the output of a synchronous generator is of
  the alternating type, the armature conductors in
  the stator can be directly connected to the
  transmission line. This eliminates the need for
  slip rings for ac power output.
• Since most of the heat is produced by the
  armature winding, an outer stationary member can
  be cooled more efficiently than an inner rotating
  member.
• Since the induced emf in the armature winding is
  quite high, it is easier to insulate it when it
  is wound inside the stationary member rather than
  the rotating member.

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Introduction

• Two terms commonly used to describe the windings
  on a machine are field winding and armature
  windings. In general, the term field winding
  applies to the windings that produce the main
  magnetic field in a machine and the term armature
  winding applies to the windings where the main
  voltage is induced.
• The magnetic poles on the rotor can be of either
  salient or nonsalient construction. The term
  salient means protruding or sticking out and
  a salient pole is a magnetic pole that sticks out
  from the surface.
• Synchronous machines are AC machines that have a
  field circuit supplied by an external DC source.
  In a synchronous generator, a DC current is
  applied to the rotor winding, which produces a
  rotor magnetic field.

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Introduction

• As generators they can be quite large, rated a
  few hundred MV A, and almost all power generation
  is through these machines. Large synchronous
  motors are not very common, but can be an
  attractive alternative to induction machines.
  Small synchronous motors with permanent magnets
  in the rotor, rather than coils with DC, are
  rapidly replacing induction motors in automotive,
  industrial and residential applications. since
  they are more efficient and lighter.
• Synchronous generators are built with two types
  of rotors
• Salient-Pole Rotor Driven by low-speed
  hydraulic turbines (btw 50 and 300 rpm). always
  possess a large diameter to provide necessary
  space for the poles.
• Cylindrical Rotor (non-salient) Driven by high
  speed steam turbines (3600 rpm) are smaller and
  more efficient than low-speed turbines.

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Motor Construction
Round Rotor Machine (non-salient pole)
The stator is a ring shaped laminated iron-core
with slots. Three phase windings are placed in
the slots. Round solid iron rotor with
slots. A single winding is placed in the slots.
DC current is supplied through slip rings.
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Motor Construction
Round Rotor Machine (non-salient pole)
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Motor Construction
Salient Rotor Machine (salient pole)
The stator has a laminated iron-core with slots
and three phase windings placed in the
slots. The rotor has salient poles excited by
dc current. DC current is supplied to the rotor
through slip-rings and Brushes
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Motor Construction
Salient Rotor Machine (salient pole)
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Motor Construction
Operation concept
The field winding is supplied with a DC current
-gt excitation. Rotor (field) winding is
mechanically turned (rotated) at synchronous
speed (ns). The RMF (rotating magnetic field)
produced by the field current induces voltages in
the outer stator (armature) winding.
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DC Power Supply

• DC current must be supplied to the filed winding
  on the rotor. There are two common approaches to
  supplying this DC power
• From external DC source ? use slip rings and
  brushes (small synchronous machines)
• From special DC power source mounted directly on
  the shaft. (large synchronous machines)

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Small Synchronous Machines

• Slip rings and brushes create a few problems when
  they are use d to supply DC power to the field
  windings.
• Brushes must be checked for wear regularly ?
  increase maintenance
• Despite, slip rings and brushes are used on small
  synchronous machines.

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Large Synchronous Machines

• Brushless exciters are used to supply DC field
  current.
• Brushless exciters is a small AC generator with
  its field circuit mounted on the stator and its
  armature circuit mounted on the rotor shaft.
• A 3 phase current is rectified and used to supply
  the field circuit of the exciter (on stator).
• The output of the armature circuit of the exciter
  (on rotor) is then rectified and used to supply
  the field current of the main machines.
• To make the generator completely independent, a
  small exciter is included in the system.
• A pilot exciter is a small AC generator with
  permanent magnets mounted on the rotor shaft and
  a 3 phase winding on the stator.

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Block diagram of a large synchronous generator
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Speed of rotation of a synchronous generator

• Operation concept
• The rate of rotation of the magnetic fields in
  the machine is related to the stator electrical
  frequency
• Where fe electrical frequency, in Hz
• nm mechanical speed of magnetic
  field, in r/min (equal speed of rotor for
  synchronous machines)
• P number of poles
• Typical rotor speeds are 3600 rpm for 2-pole,
  1800 rpm for 4 pole and 450 rpm for 16 poles.

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The Internal Generated voltage of a Synchronous
Generator

• The magnitude of the voltage induced in a given
  stator is

• Where EA induced voltage/generated voltage
• OR
• The rms. value of the induced voltages is
• EA 4.44N BA f , (BA f)
• where
• N number of turns,
• B flux density,
• A cross sectional area of the magnetic circuit,
  
• f frequency,
• f flux per pole
• This voltage depends on the flux F in the
  machine, the frequency or speed of
• rotation and the machine construction. The
  simpler form is

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Equivalent Circuit of A Synchronous Generator

• The voltage EA is the induced voltage produced
  in one phase of a synchronous generator. EA is
  not usually the voltage that appears at the
  terminals of the generator. The only time EA is
  the same as the output voltage Vf of the phase
  when there is no armature current flowing in the
  machine (during no load).
• There are many factors that cause the difference
  between EA and Vf including the resistance of the
  armature coils, the self inductance of the
  armature coils, and the distortion of the air-gap
  magnetic field by the current flowing in the
  stator, called armature reaction.
• With two voltages present in the stator
  windings, the total voltage in a per phase
  circuit is just the sum of the induced voltage EA
  and the armature reaction voltage EX.

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Equivalent Circuit of A Synchronous Generator
A simple circuit

• We realize that the three phases of a
  synchronous generator are identical
• except for phase angle. It is very important to
  know that the three phases have the same voltages
  and currents only when the loads attached to them
• are balanced. If the machiness loads are not
  balanced, more complicated
• techniques of analysis are required.

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Equivalent Circuit of A Synchronous Generator

• You observe the DC power source supplying the
  rotor field circuit. The figure also shows that
  each phase has an induced voltage with a series
  XS and a series RA. The voltages and currents of
  the three phases are identical but 120 apart in
  angle.
• The three phases can be either Y or ? . If they
  are Y connected, then the terminal voltage VT is
  related to the phase voltage by

The full equivalent circuit of a three-phase
synchronous generator
If ? connected
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Phasor Diagram

• Voltages in a synchronous generator are
  expressed as phasors because they are AC
  voltages. Since we have magnitude and angle, the
  relationship between voltage and current must be
  expressed by a two-dimensional plot.
• It is noticed that, for a given phase voltage
  and armature current, a larger induced voltage EA
  is required for lagging loads than leading loads.

Phasor diagram of a synchronous generator at
unity power factor (Resistive Load).
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Phasor Diagram
Phasor diagram of a synchronous generator at
lagging factor (Inductive Load).
Phasor diagram of a synchronous generator at
leading factor (Capacitive Load).
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Power Relationships

• Not all the mechanical power going into a
  synchronous generator becomes
• electrical power out of the machine. The
  difference between input power and
• output power represents the losses of the
  machine. The input mechanical power
• is the shaft power in the generator.

Pout
Pconverted (Pm)
Pin (Motor)
Stray losses (Pst)
Core losses (Pc)
Copper losses (Pcu)
Rotational losses (Pr)
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Power Relationships
The power converted from mechanical to electrical
is given by
Where ? is the angle between EA and IA.
If the armature resistance RA is ignored (XS gtgt
RA), Therefore
Substituting this equation into Pout, gives.
Where ? is the angle between EA and VT.
The induced torque can be express as.
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Power Angle Characteristics

• The P(d) curve shows that the increase of power
  increases the angle between the induced voltage
  and the terminal voltage.
• The power is maximum when d90o
• The further increase of input power forces the
  generator out of synchronism. This generates
  large current and mechanical forces.
• The maximum power is the static stability limit
  of the system.
• Safe operation requires a 15-20 power reverse.

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Efficiency
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Voltage regulation

• As the load on the generator increases, the
  terminal voltage drops. But,the terminal voltage,
  must be maintained constant, and hence the
  excitation on the machine is varied, or input
  power to the generator is varied. That means, EG
  has to be adjusted to keep the terminal voltage
  VT constant.

? Voltage Regulation, V.R
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Example
A 240 V, 50 Hz, 4-pole, Y-connected synchronous
generator has a per-phase reactance of 0.2 O
(ignore armature resistance). At full-load, the
armature current is 50 A at 0.83 lagging power
factor. Also at full-load, the friction and
windage loss is 1.2 kW, and core loss is 1.1 kW.
The field current is initially adjusted so that
the terminal voltage is 240 V at no load, after
which it is kept constant. Assume phase voltage
VS /0. i. What is the speed of rotation of
the generator? ii. What is the terminal/generated
voltage of the generator if it is operated at
full-load rated current at 0.83 lagging power
factor? iii. What is the efficiency of the
generator when it is operating at full-load rated
current at 0.83 lagging power factor? iv. What is
the voltage regulation?
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Parallel Operation of AC Generators

• The generation of electric power, its
  transmission and its distribution must be
  conducted in an efficient and reliable way at a
  reasonable cost with the least number of
  interruptions.
• As the demand for electric energy can fluctuate
  from a light load to a heavy load and vice versa
  several times during the day, it is almost
  impossible to operate a single alternator at its
  maximum efficiency at all times.
• A single alternator cannot ensure such a
  reliable operation owing to the possibility of
  its failure or a deliberate shut-off for periodic
  inspection. Therefore, a single alternator
  supplying a variable load cannot be very
  efficient, cost-effective and reliable.
• To overcome this problem, it becomes necessary
  to generate electric power at a central location
  where several alternators can be connected in
  parallel to meet the power demand.
• When the demand is light, some of the
  alternators can be taken off line while the other
  alternators are operating at their maximum
  efficiencies.
• As the demand increases, another alternator can
  be put on line without causing any service
  interruption.

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Parallel Operation of AC Generators

• The following requirements have to be satisfied
  prior to connecting an alternator to the infinite
  bus (connection line).
• The line voltage of the (incoming) alternator
  must be equal to the constant voltage of the of
  the infinite bus.
• The frequency of the incoming alternator must be
  exactly equal to that of the infinite bus.
• The phase sequence of the incoming alternator
  must be identical to the phase sequence of the
  infinite bus.

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Power System Operation

• In a network several hundred synchronous
  generators operate in parallel.
• Each generator operates with the same speed.
• The load increase is achieved by increasing the
  input power, that increases the power angle d.
  The speed remain constant.
• The power angle must be less than 90 degrees.
  The load should be 30-20 less than the maximum
  power (d 90o).