0195136047.pdf

584 ROTATING MACHINES

(a)

jIaXs

Ia

Ef

Vt

φ

δ

(b)

jIaXs

Ia

Ef

Vt

φ δ

(c)

jIaXs

−Ia

Ia Ef

Vt

φ

δ

(d )

jIaXs

−Ia

Ia

Ef

Vt

φ δ

Figure 13.3.2Four possible cases of operation of a round-rotor synchronous machine with negligible armature

resistance.(a)Overexcited generator (power factor lagging),P> 0 ,Q > 0 ,δ >0.(b)Underexcited

generator (power factor leading),P> 0 ,Q < 0 ,δ >0.(c)Overexcited motor (power factor leading),

P< 0 ,Q > 0 ,δ <0.(d)Underexcited motor (power factor lagging),P< 0 ,Q < 0 ,δ <0.

the synchronous machine. The power-angle performance characteristics are discussed later in this

section. The dcexcitationcan be provided by a self-excited dc generator, known as theexciter,

mounted on the same shaft as the rotor of the synchronous machine.

The voltage regulation of a synchronous generator at a given load, power factor, and rated

speed is defined as

% voltage regulation=

Ef−Vt

Vt

× 100 (13.3.3)

whereVtis the terminal voltage on the load, andEfis the no-load terminal voltage at rated speed

when the load is removed without changing the field current.

EXAMPLE 13.3.1

The per-phase synchronous reactance of a three-phase, wye-connected, 2.5-MVA, 6.6-kV, 60-Hz

turboalternator is 10. Neglect the armature resistance and saturation. Calculate the voltage

regulation when the generator is operating at full load with (a) 0.8 power factor lagging, and (b)

0.8 power factor leading.