Page 140 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
138 Chapter Four

Figure 4.11 Approximation to the waveform of current taken by a switched mode power
supply.

Figure 4.12 The waveform of Fig. 4.11 without the triplen components.

tertiary winding. The load-generated triplen harmonic currents will
flow in the transformer secondary. Core fluxes will cause triplen cur-
rents to circulate in the delta winding, and these triplen currents will
neutralize those in the secondary; significant triplen voltages will not
therefore appear at the primary terminals.
If the triplen harmonic currents are removed from the waveform of
Fig. 4.11, the shape is changed to that shown in Fig. 4.12 which
includes the following harmonic components:

Harmonic order (n) 1 5 7 11 13
Magnitude (%) 100 60 30 12 8

The new waveform is far from ideal but it is a considerable improve-
ment; the peak current has been reduced by one-third, the period of
conduction has been extended to the complete half cycle, and the rms
value has been reduced by 19 percent.

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