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)