Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 60

Fig. 4.9 PWM bridge (B6) inverter circuit

Many pulse pattern generation schemes are known, fulfilling different optimiza-
tion approaches. One of the most popular ones is the pulse width modulation
(PWM). A synchronous modulation using sinusoidal reference voltages u ref and a
triangular carrier voltage u tri is illustrated in Fig. 4.10. The frequency ratio of the
triangular and reference voltage which should be an uneven integer is m f = 9inthe
example. Switch pairs connect an output branch either to the positive or negative d.c
terminals, depending on whether u ref > u tri or u ref < u tri . This is shown in the Figure
for u 10 and u 20 , resulting in the output line-to-line voltage u 12 .
The modulation ratio is defined using reference and triangular voltage ampli-
tudes:

u re f
u tri

m = (4.8)

In the region m ≤ 1 the fundamental output line-to-line voltage is a linear function

of m :

U 12 = √ 3

2 √ 2 m

· U d = 0 , 612 · m · U d for m ≤ 1 (4.9)

Fig. 4.10 Voltage waveforms
in PWM operation with
sinusoidal modulation