09-02-2012, 03:08 PM
Synchronous Machines
[attachment=17355]
introduction
The synchronous machine is an important electromechanical energy converter.
Synchronous generators usually operate together (or in parallel), forming a large power
system supplying electrical energy to the loads or consumers. For these applications
synchronous machines are built in large units, their rating ranging from tens to hundreds
of megawatts. For high-speed machines, the prime movers are usually steam turbines
employing fossil or nuclear energy resources. Low-speed machines are often driven by
hydro-turbines that employ water power for generation. Smaller synchronous machines
are sometimes used for private generation and as standby units, with diesel engines or
gas turbines as prime movers.
Synchronous machines can also be used as motors, but they are usually built in very
large sizes. The synchronous motor operates at a precise synchronous speed, and hence
is a constant-speed motor. Unlike the induction motor, whose operation always involves
a lagging power factor, the synchronous motor possesses a variable-power-factor
characteristic, and hence is suitable for power-factor correction applications.
©Encyclopedia of Life Support Systems (EOLSS)
UNESCO - EOLSS
SAMPLE CHAPTER
ELECTRICAL ENGINEERING – Synchronous Machines - Tze-Fun Chan
A synchronous motor operating without mechanical load is called a compensator. It
behaves as a variable capacitor when the field is overexcited, and as a variable inductor
when the field is underexcited. It is often used in critical positions in a power system for
reactive power control.
2. Types of Synchronous Machine
According to the arrangement of the field and armature windings, synchronous
machines may be classified as rotating-armature type or rotating-field type.
2.1. Rotating-Armature Type
The armature winding is on the rotor and the field system is on the stator. The generated
current is brought out to the load via three (or four) slip-rings. Insulation problems, and
the difficulty involved in transmitting large currents via the brushes, limit the maximum
power output and the generated electromagnetic field (emf). This type is only used in
small units, and its main application is as the main exciter in large alternators with
brushless excitation systems.
2.2. Rotating-Field Type
The armature winding is on the stator and the field system is on the rotor. Field current
is supplied from the exciter via two slip-rings, while the armature current is directly
supplied to the load. This type is employed universally since very high power can be
delivered. Unless otherwise stated, the subsequent discussion refers specifically to
rotating-field type synchronous machines.
According to the shape of the field, synchronous machines may be classified as
cylindrical-rotor (non-salient pole) machines (Figure 1) and salient-pole machines
(Figure 2).
The cylindrical-rotor construction is used in generators that operate at high speeds, such
as steam-turbine generators (usually two-pole machines). This type of machine usually
has a small diameter-to-length ratio, in order to avoid excessive mechanical stress on the
rotor due to the large centrifugal forces.
[attachment=17355]
introduction
The synchronous machine is an important electromechanical energy converter.
Synchronous generators usually operate together (or in parallel), forming a large power
system supplying electrical energy to the loads or consumers. For these applications
synchronous machines are built in large units, their rating ranging from tens to hundreds
of megawatts. For high-speed machines, the prime movers are usually steam turbines
employing fossil or nuclear energy resources. Low-speed machines are often driven by
hydro-turbines that employ water power for generation. Smaller synchronous machines
are sometimes used for private generation and as standby units, with diesel engines or
gas turbines as prime movers.
Synchronous machines can also be used as motors, but they are usually built in very
large sizes. The synchronous motor operates at a precise synchronous speed, and hence
is a constant-speed motor. Unlike the induction motor, whose operation always involves
a lagging power factor, the synchronous motor possesses a variable-power-factor
characteristic, and hence is suitable for power-factor correction applications.
©Encyclopedia of Life Support Systems (EOLSS)
UNESCO - EOLSS
SAMPLE CHAPTER
ELECTRICAL ENGINEERING – Synchronous Machines - Tze-Fun Chan
A synchronous motor operating without mechanical load is called a compensator. It
behaves as a variable capacitor when the field is overexcited, and as a variable inductor
when the field is underexcited. It is often used in critical positions in a power system for
reactive power control.
2. Types of Synchronous Machine
According to the arrangement of the field and armature windings, synchronous
machines may be classified as rotating-armature type or rotating-field type.
2.1. Rotating-Armature Type
The armature winding is on the rotor and the field system is on the stator. The generated
current is brought out to the load via three (or four) slip-rings. Insulation problems, and
the difficulty involved in transmitting large currents via the brushes, limit the maximum
power output and the generated electromagnetic field (emf). This type is only used in
small units, and its main application is as the main exciter in large alternators with
brushless excitation systems.
2.2. Rotating-Field Type
The armature winding is on the stator and the field system is on the rotor. Field current
is supplied from the exciter via two slip-rings, while the armature current is directly
supplied to the load. This type is employed universally since very high power can be
delivered. Unless otherwise stated, the subsequent discussion refers specifically to
rotating-field type synchronous machines.
According to the shape of the field, synchronous machines may be classified as
cylindrical-rotor (non-salient pole) machines (Figure 1) and salient-pole machines
(Figure 2).
The cylindrical-rotor construction is used in generators that operate at high speeds, such
as steam-turbine generators (usually two-pole machines). This type of machine usually
has a small diameter-to-length ratio, in order to avoid excessive mechanical stress on the
rotor due to the large centrifugal forces.
