20-10-2012, 01:10 PM
Synchronous Machines
Synchronous.ppt (Size: 6.37 MB / Downloads: 212)
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
Basic parts of a synchronous generator:
Rotor - dc excited winding
Stator - 3-phase winding in which the ac emf is generated
The manner in which the active parts of a synchronous machine are cooled determines its overall physical size and structure
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
Voltage Regulation
A convenient way to compare the voltage behaviour of two generators is by their voltage regulation (VR). The VR of a synchronous generator at a given load, power factor, and at rated speed is defined as
Where Vfl is the full-load terminal voltage, and Enl (equal to Ef) is the no-load terminal voltage (internal voltage) at rated speed when the load is removed without changing the field current. For lagging power factor (PF), VR is fairly positive, for unity PF, VR is small positive and for leading PF, VR is negative.
Equivalent Circuit
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.
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
Active and reactive power-angle characteristics
The real and reactive power delivered by a synchronous generator or consumed by a synchronous motor can be expressed in terms of the terminal voltage Vt, generated voltage Ef, synchronous impedance Zs, and the power angle or torque angle d.
Referring to Fig. 8, it is convenient to adopt a convention that makes positive real power P and positive reactive power Q delivered by an overexcited generator.
The generator action corresponds to positive value of d, while the motor action corresponds to negative value of d.