Page 336 - Power Electronics Handbook

326 D.C. link frequency changers

Table 13.13 B.rmonlf mtent da two-pulac M-direetiolul wave witb thlrd and Blth hrmoaie
euminmtion

2DIT R.M.S. voltage as percentage of d.c. supply
1 3 5 7 9 11 13 15 Total
0 75.5 0 0 22.4 36.8 27.2 2.40 15.2 100
0.02 75.4 0 0 20.3 31.1 21.0 1.65 8.92 98.0
0.04 74.9 0 0 14.3 15.7 5.10 0.15 4.69 95.9
0.06 74.2 0 0 5.58 4.61 13.1 1.85 14.4 93.8
0.08 73.1 0 0 4.21 23.5 25.3 2.39 12.3 91.7
0.10 71.8 0 0 13.2 35.0 25.9 0.41 0.00 89.4
0.12 70.2 0 0 19.7 35.7 14.6 0.45 12.3 87.2
0.14 68.3 0 0 22.4 25.2 3.41 2.03 14.4 84.9
0.16 66.2 0 0 20.9 6.90 19.8 2.33 4.69 82.5
0.18 63.8 0 0 15.4 13.6 27.1 1.16 8.92 80.0
0.20 61.1 0 0 6.91 29.8 22.0 0.74 15.2 77.5
0.22 58.2 0 0 2.81 36.7 6.76 2.18 8.92 74.8
0.24 55.0 0 0 12.0 32.3 11.6 2.24 4.69 72.1
0.26 51.7 0 0 19.0 17.7 24.6 0.89 14.4 69.3
0.28 48.1 0 0 22.3 2.31 26.3 1.02 12.3 66.3
0.30 44.4 0 0 21.3 21.6 16.0 2.29 0.00 63.2
0.32 40.5 0 0 16.4 34.2 1.71 2.11 12.3 60.0
0.34 36.4 0 0 8.26 36.2 18.6 0.60 14.4 56.6
0.36 32.2 0 0 1.41 26.8 27.0 1.29 4.69 52.9
0.38 27.8 0 0 10.8 '9.16 23.0 2.36 8.92 49.0
0.40 23.3 0 0 18.2 11.4 8.40 1.95 15.2 44.7
0.42 18.8 0 0 22.1 28.4 10.0 0.20 8.92 40.0
0.44 14.2 0 0 21.7 36.5 23.8 1.53 4.69 34.6
0.50 0 0 0 0 0 0 0 0 0

when the third and fifth, the third and seventh, or the fifth and seventh
harmonics are being eliminated, respectively.
A comparison of the harmonic tables shows that, in general,
unidirectional switching has a higher total harmonic content than
bi-directional switching, the quasi-square wave having the least.
An interesting aspect of harmonic elimination is that the amplitude of
the harmonics increases with the harmonic number, which is contrary to
that found with quasi-square waves. For bidirectional switching the ninth
harmonic is the largest when the third and fifth harmonics are eliminated,
whereas when the third and seventh are eliminated the eleventh
predominates. The same is true for unidirectional switching, but to a lesser
extent and at higher harmonic numbers. It should also be noted that for
unidirectional switching every harmonic which is an odd multiple of the
harmonic being eliminated is absent from the output waveform. Therefore
the third, ninth, fifteenth, etc. are absent, and so on. For bi-directional
switching only the two harmonics chosen are eliminated.
Generally, harmonic elimination causes the remaining harmonics to be
of much larger magnitude. Perhaps, however, it is worth noting the
phenomenal increase in the third harmonic when unidirectional switching
is used to eliminate either the fifth or the seventh harmonic. The results
given in Table 13.15 also deserve special mention. Although the fifth and