Synchronous Machines - Types, Operation, Phasor Diagram, Torque and Power

SYNCHRONOUS MACHINES
Synchronous machine at steady state rotates at a constant speed known as synchronous speed. DC
excitation is used to feed field windings of the rotor while ac current flows in armature or stator windings
while permanent magnet is used to produce field flux for permanent magnet synchronous
machine(PMSM). As a generator, mechanical power is converted into ac power while as a motor, it
transforms ac electrical power to mechanical power. The machine has cylindrical rotor and stator which
are coaxial and made of highly permeable material. Voltage is induced in the armature winding.
They are two types of rotors namely; salient pole
rotors for low speed application and non-salient
pole for high speed application. Synchronous
generators of salient poles used in hydro generates
tens or hundreds of megawatts.
Operation of synchronous generator.
DC excitation current supplied to the rotor produces
magnetic flux that cuts winding of the stator when
rotor is engaged with the prime mover. Rotating magnetic field then induces 3-phase voltage to the stator
windings described by set of equations below
van ( t ) =Em cos ( ωt ) , vbn ( t ) =Em cos ( ωt−1200 ) ,∧vcn ( t ) =Em cos ( ωt−2400 ) ,
The excitation voltage given in rms is as shown below
Ef =4.44 f ∅f N Kw = √ 2 πN ∅f f Kf =K ∅f ω
The synchronous speed of the generator is given by ns= 120 f
P rpm
Open-circuit characteristics (OCC) of synchronous generator.
Experimentally, when stator’s terminals are open, voltage rises
linearly with increasing field current I f until magnetic saturation
point where field current increases in non-linear relationship with
terminal voltage as shown below.