23-01-2013, 11:38 AM
SYNCHRONOUS MACHINES
SYNCHRONOUS.ppt (Size: 4.53 MB / Downloads: 458)
Physical Description of a Synchronous Machine
Consists of two sets of windings:
3 phase armature winding on the stator distributed with centres 120° apart in space
field winding on the rotor supplied by DC
Two basic rotor structures used:
salient or projecting pole structure for hydraulic units (low speed)
round rotor structure for thermal units (high speed)
Salient poles have concentrated field windings; usually also carry damper windings on the pole face.Round rotors have solid steel rotors with distributed windings
Nearly sinusoidal space distribution of flux wave shape obtained by:
distributing stator windings and field windings in many slots (round rotor);
shaping pole faces (salient pole)
Rotors of Steam Turbine Generators
Traditionally, North American manufacturers normally did not provide special “damper windings”
solid steel rotors offer paths for eddy currents, which have effects equivalent to that of amortisseur currents
European manufacturers tended to provide for additional damping effects and negative-sequence current capability
wedges in the slots of field windings interconnected to form a damper case, or
separate copper rods provided underneath the wedges
Rotors of Hydraulic Units
Normally have damper windings or amortisseurs
non-magnetic material (usually copper) rods embedded in pole face
connected to end rings to form short-circuited windings
Damper windings may be either continuous or non-continuous
Space harmonics of the armature mmf contribute to surface eddy current
therefore, pole faces are usually laminated
Balanced Steady State Operation
Magnitude of stator mmf wave and its relative angular position with respect to rotor mmf wave depend on machine output
for generator action, rotor field leads stator field due to forward torque of prime mover;
for motor action rotor field lags stator field due to retarding torque of shaft load
Transient Operation
Stator and rotor fields may:
vary in magnitude with respect to time
have different speed
Currents flow not only in the field and stator windings, but also in:
damper windings (if present); and
solid rotor surface and slot walls of round rotor machines