SYNCHRONOUS MACHINES Basic principles SEE 3433 MESIN ELEKTRIK SYNCHRONOUS MACHINES Basic principles

General features Doubly excited machine Rotor – field winding – DC current Stator – Armature winding – AC supply e.g. operated as a generator Field circuit Slip rings 3- Stator terminals Prime mover

Construction Magnetic axis of rotor Salient pole A B’ If C’ Stator Magnetic axis of phase a Stator - Armature - Rotor - field Low speed operation Large number of poles e.g. application in hydroelectric

Construction Cylindrical A B’ C’ B C High speed operation A’ Magnetic axis of rotor Magnetic axis of phase a 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 Frequency of induced voltage given by: Ef known as excitation voltage

Synchronous generators Open circuit characteristic (OCC) Ef depends on: speed Flux per pole hance If 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 !

1 Synchronous motors Solved by: Frequency is slowly increased from 0 using power electronics converter

2 Synchronous motors Solved by: (Damper winding) Rotor has ‘squirrel cage’ construction At synchronous speed no current induced in the winding