SEE 3433
MESIN ELEKTRIK
SYNCHRONOUS MACHINESBasic principles
General features
Doubly excited machine
Rotor – field winding – DC current
Stator – Armature winding – AC supply
Prime mover
e.g. operated as a generator
Slip rings
3- Stator terminals
Field circuit
Magnetic axis of rotor
Magnetic axis of phase a
Salient pole Construction
A
A’
B
B’
C
C’If
Rotor - field
Stator- Armature -
• Low speed operation
• Large number of poles
• e.g. application in hydroelectric
Magnetic axis of rotor
Magnetic axis of phase a
Construction
A
A’
B
B’
C
C’
Cylindrical
• High speed operation
• Small number of poles
• e.g. application in steam turbines
Salient rotor
Stator under construction
Synchronous generator – non-salient pole
Synchronous generators
Field current in rotor produce sinusoidal flux in airgap
Rotating filed produced when rotor rotates
Rotating field induced 3 voltage in 3 phase windings on stator
Similar to induction machine, the RMS of induced voltage per phase is
Ef = 4.44 f N Kw
Ef known as excitation voltage
Frequency of induced voltage given by:
Synchronous generators
Ef depends on:
• speed
• Flux per pole hance If
Open circuit characteristic (OCC)
• Exhibit saturation as flux in core saturated
Synchronous generators
Application in power system:
Synchronous motors
Stator terminals connected to 3 supply – producing rotating magnetic flux
However, rotor won’t be able to rotate or start:
Due to inertia, rotor cannot catch-up with the fast rotating field !
1Synchronous motors
Solved by:
Frequency is slowly increased from 0 using power electronics converter
Synchronous motors
Solved by:
2Rotor has ‘squirrel cage’ construction
At synchronous speed no current induced in the winding
(Damper winding)