Electrical Machines
Dr. Ankur Bhattacharjee
Department of Electrical and Electronics Engineering,
BITS Pilani
BITS-Pilani, Hyderabad Campus, India
Hyderabad Campus
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Module Name: Synchronous Machines
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Outline
• Synchronous machines
• Construction
• Operation
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Introduction
• Generation of E.M.F
– E.M.F can be induced (or generated) due
to relative motion between a magnetic
field and a conductor of electricity.
• DC machine
• Induction machine
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Introduction: Generation of E.M.F
• Revolving-armature
– Armature rotates through
a stationary magnetic field,
and the generated AC is
brought to the load by
means of slip rings and
brushes.
• Found only in alternators
of small power rating.
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Introduction: Generation
• Revolving-field
• The revolving-field type alternator
has a stationary armature and a
rotating magnetic field.
• The generated voltage can be
connected directly to the load
without having to pass across the
slip rings and brushes
• The voltage applied to generate
the rotating field is a small DC
stationary
voltage (called a “field excitation”
voltage)
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Introduction
• Alternators prefer ‘rotating field’ type construction with ‘stationary
armature’.
– High Voltage
– Centrifugal forces
– Easier to collect larger currents at very high voltage
• The voltage required to be supplied to the field is low (110V to 220V d.c)
– Easily supplied with the help of slip ring and brush assembly by keeping it
rotating.
– Sparking at the slip rings can be avoided
– Insulation required is less
– Field system has very low inertia.
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Synchronous generator or Alternator
The turbine converts some kind of energy
(steam, water, wind) into mechanical
energy
The synchronous generator
converts
mechanical energy from
the turbine into
electrical energy.
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Synchronous generator:
Construction
• The synchronous generator has two main
parts:
– Stator:
– Rotor:
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Synchronous generator:
Construction of Stator
Stator of a large salient pole hydro
generator;
Stator:
Carries 3 (3-phase) armature windings, AC,
physically displaced from each other by 120o
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Synchronous generator:
Construction of Rotor
Generator rotor with conductors
Rotor:
placed in the slots.
Carries field windings,
connected to an external DC
source via slip rings and brushes or
to a revolving DC source via a
special brushless configuration.
Rotor completely
assembled. COMMUNICATION
Large hydro
generator rotor
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Synchronous Generator: Rotor
• Salient-pole rotors
– Used for low speed applications (<300rpm) which require
large number of poles to achieve required frequencies
(e.g. hydro turbines)
• Cylindrical rotors
– Used for high-speed applications (steam/gas turbines).
– Minimum number of poles is 2, so for 50Hz the maximum
speed is 3000rpm.
– High speed of rotation produces strong centrifugal forces,
which impose upper limit on the rotor diameter.
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Salient Pole Type
• This is also called projected pole type as all the poles
are projected out from the surface of the rotor.
• The poles are built up of thick steel laminations.
• The poles are bolted to the rotor.
• The field winding is provided on the pole shoe.
• These rotors have large diameter and small axial
length.
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Salient Pole Type
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Smooth Cylindrical Type
• The individual rotor poles are produced using a
slotted cylindrical rotor,
–
–
–
–
–
characterized by distributed windings,
nearly-uniform air gap,
smaller rotor diameters,
used in applications requiring high machine speed and
a small number of machine poles, typically 2 or 4
poles (example - steam or gas turbine generators).
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Cylindrical rotor
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Synchronous generator: rotor
What type of rotor ?
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ALTERNATOR: Principle of operation
• The elementary 3-phase 2-pole synchronous
generator has a stator equipped with 3 coils
displaced 120o from each other;
– The stator coil/ windings are actually ‘distributed’
type.
• When the rotor is excited with D.C and rotated,
– the resultant field will also rotate so that sinusoidal
voltages are generated in the 3 stator phases,
– displaced 120o in ‘time’
– having a frequency directly related to rotor speed.
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ALTERNATOR: Principle of operation
The rotor of the generator is driven by a prime-mover
A DC supply is given to the rotor winding (field) which produces a rotating magnetic field
within the machine
The rotating magnetic field induces a three-phase voltage in the stator winding of the
generator
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Three Phase Alternator
VB
The three-phase alternator has three
single-phase windings spaced so that the
voltage induced in any one is phasedisplaced by 120 degrees from the other
two.
120O
120O
VA
120O
VC
A
A
Stator Connection
B
C
Three Phase STAR
Connected
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C
B
Three Phase
DELTA Connected
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Three Phase Alternator
• The frequency of the AC generated by an alternator
depends
– the number of poles and
– the speed of the rotor
• When a rotor has rotated through an angle so that two
adjacent rotor poles (a north and a south) have passed one
winding,
– the voltage induced in that one winding will have varied through
a complete cycle of 360 electrical degrees.
• The magnitude of the voltage generated by an alternator
can be varied by
– adjusting the current on the rotor which changes the strength of
the magnetic field.
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Three Phase Alternator: frequency
For 2-pole machine,
one complete revolution of rotor (360 mechanical degrees)
results in one complete cycle (360 electrical degrees)
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Three Phase Alternator: frequency
•A four pole alternator will
produce two electrical cycles for
each
mechanical
rotation
because two north and two
south poles move by each
winding on the stator for one
complete revolution of the rotor.
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Three Phase Alternator: frequency
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Three Phase Alternator: frequency
f is the electrical line
frequency produced by the
alternator
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Synchronous machine
Synchronous machine
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Broad picture
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Necessary
• Synchronous machine model
• Operating characteristics
• Power flow equations
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Where
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The rotor field axis is known as the direct-axis and the axis at
90° elect. from it is known as the quadrature-axis.
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distributed (and also possibly
shortpitched)
stator winding
It easily follows that the emfs produced in the
other phases of the stator would progressively
differ in time
phase by 120°.
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3-phase stator supplies a balanced load, it sets up its own mmf
vector
Far (sinusoidal and distributed in space), called the armature
reaction, rotating at synchronous speed in the same direction as
the rotor.
The magnetic circuit is now subjected to two rotating mmf
vectors
Ff and Far ,
both rotating at synchronous speed with a certain angle between
them
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BITSPilani
Pilani
BITS
Hyderabad Campus
Hyderabad Campus
Thank You
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