SEE 3433 MESIN ELEKTRIK SYNCHRONOUS MACHINES Basic 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’ IfIf 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 E f = 4.44 f  N K w E f known as excitation voltage Frequency of induced voltage given by:

Synchronous generators E f depends on: speed Flux per pole hance I f 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 !

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

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