SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

(4 / ) V 1.2732V
DC DC . 32 As with the harmonics involved in armature windings and rotor
fluxes (Appendix C), the squarewave represents some type of physical maximum and if we
cannot allow harmonics we must settle for a fundamental whose amplitude is less than this
maximum. The “natural” limit for SPWM was thought to be 1.0V DC as shown. Then it was
realized that this range could be extended for certain three-phase loads by intentionally
introducing triplen harmonics into the reference signal. Most references claim this was first done
by King in 1974 [129] or Buja in 1975 [130].
The method is based on the fact that adding an amount of the third harmonic to a fundamental
sinusoid flattens the top significantly and lowers its peak value, as shown in Figure 4.19. The
fundamental of this waveform is obviously unchanged, however. The 1.0V DC limit exists because
the inverter cannot synthesize a waveform with a peak value greater than 1.0V DC .
Figure 4.19 – Adding the third harmonic reduces peak amplitude.
If the same fundamental can be produced by a signal whose amplitude is less than 1.0V DC , we
could then increase the magnitude until this output signal is at the 1.0V DC limit of the inverter, as
shown in Figure 4.20.
Figure 4.20 – Third harmonic injection can produce larger fundamental than SPWM.
32 This is the fundamental of the pole voltage, which is a squarewave. The squarewave contains a ZS
component and this component of the neutral voltage is absent in the phase voltage. Since only the phase
voltage can drive a current through the load, we should compute the fundamental from the phase voltage
waveform. In doing so one will find the same value.
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