Page 239 - Rashid, Power Electronics Handbook

228 J. Espinoza

FIGURE 14.3 The half-bridge VSI. Ideal waveforms for the SPWM (m a ¼ 0:8, m f ¼ 9): (a) carrier and modulating signals; (b) switch S þ state; (c)
Switch S ÿ state; (d) ac output voltage; (e) ac output voltage spectrum; (f) ac output current; (g) dc current; (h) dc current spectrum; (i) switch S þ
current; (j) diode D þ current.

will be discussed later); (b) for odd values of the normalized around the normalized carrier frequency m and its multiples,
f
carrier frequency m the harmonics in the ac output voltage speci®cally,
f
appear at normalized frequencies f centered around m and
h f
its multiples, speci®cally, p ¼ lm k 1 l ¼ 1; 2; .. . ð14:5Þ
f
h ¼ lm k l ¼ 1; 2; 3; ... ð14:4Þ where k ¼ 2; 4; 6; ... for l ¼ 1; 3; 5; ... ; and k ¼ 1; 3; 5; ...
f
for l ¼ 2; 4; 6; .... Additional important issues are: (a) for
where k ¼ 2; 4; 6; ... for l ¼ 1; 3; 5; ... ; and k ¼ 1; 3; 5; ... small values of m (m < 21), the carrier signal v and the
D
f
f
for l ¼ 2; 4; 6; .. . ; (c) the amplitude of the ac output voltage modulating signal v should be synchronized to each other
c
harmonics is a function of the modulation index m and is (m integer), which is required to hold the previous features; if
f
a
independent of the normalized carrier frequency m f for this is not the case, subharmonics will be present in the ac
m > 9; (d) the harmonics in the dc link current (due to the output voltage; (b) for large values of m (m > 21), the
f
f
f
modulation) appear at normalized frequencies f centered subharmonics are negligible if an asynchronous PWM
p