Page 325 - Power Electronics Handbook

Output voltage control 315

li
-w[w-
t-i
I
VI3
I
0
I I I
I * 1 - I
I I
I - 1

(13.11)
VB nn P

(13.12)

Equations (13.11) and (13.12) are shown evaluated in Tables 13.2-13.4
for P = 3, 5 and 7 respectively. These show that even though lower
harmonics are eliminated, the total harmonic content is large. However,
higher harmonics are much easier to remove than lower-order
harmonics, so that a comparison based on total content can often be
misleading. Perhaps the most serious disadvantage of this control method
is that the maximum output voltage is limited by the necessity of not
allowing adjacent pulses to merge. With quasi-square waves the maximum
voltage is 90% of the d.c. supply, when the output waveshape has no zero
voltage periods. In two-pulse systems the maximum fundamental
obtainable is 75.1%, 62.2% or 33.3% of the d.c., according to whether the
third, fifth or seventh harmonic is being eliminated. Therefore in a
variable-frequency drive, a 240 V, 50 Hz motor could run at a maximum
frequency of 340 X 0.333 x 50/20 = 24Hz and still remain fully fluxed (340