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Synchronous Machines
Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power
Synchronous generators are the primary source of electrical energy we consume today
Large ac power networks rely almost exclusively on synchronous generators
Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives
Operation Principle
The rotor of the generator is driven by a prime-mover
A dc current is flowing in the rotor winding 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
Equivalent Circuit_1
The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator.
The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine.
There are a number of factors that cause the difference between Ef and Vt:
The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction
The self-inductance of the armature coils.
The resistance of the armature coils.
The effect of salient-pole rotor shapes.
Three-phase equivalent circuit of a cylindrical-rotor synchronous machine
The voltages and currents of the three phases are 120o apart in angle, but otherwise the three phases are identical.
Determination of the parameters of the equivalent circuit from test data
The equivalent circuit of a synchronous generator that has been derived contains three quantities that must be determined in order to completely describe the behaviour of a real synchronous generator:
The saturation characteristic: relationship between If and f (and therefore between If and Ef)
The synchronous reactance, Xs
The armature resistance, Ra
The above three quantities could be determined by performing the following three tests:
Open-circuit test
Short-circuit test
DC test
Open-circuit test
The generator is turned at the rated speed
The terminals are disconnected from all loads, and the field current is set to zero.
Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.
It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information
Short-circuit test
Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammeters.
Record the armature current Isc as the field current is increased.
Such a plot is called short-circuit characteristic.
DC Test
The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals.
The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings
Determination of Xs
For a particular field current IfA, the internal voltage Ef (=VA) could be found from the occ and the short-circuit current flow Isc,A could be found from the scc.
Then the synchronous reactance Xs could be obtained using
Short-circuit Ratio
Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by
Parallel operation of synchronous generators
There are several major advantages to operate generators in parallel:
Several generators can supply a bigger load than one machine by itself.
Having many generators increases the reliability of the power system.
It allows one or more generators to be removed for shutdown or preventive maintenance.
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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.
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[attachment=15005]
Synchronous Machines
Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power
Synchronous generators are the primary source of electrical energy we consume today
Large ac power networks rely almost exclusively on synchronous generators
Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives
Construction
Various Types
Salient-Pole Synchronous Generator
Cylindrical-Rotor Synchronous Generator
Operation Principle
The rotor of the generator is driven by a prime-mover
A dc current is flowing in the rotor winding 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
Electrical Frequency
Generated Voltage
Equivalent Circuit_1
The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator.
The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine.
There are a number of factors that cause the difference between Ef and Vt:
The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction
The self-inductance of the armature coils.
The resistance of the armature coils.
The effect of salient-pole rotor shapes.
