SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Similarly, when the modulation index is defined with reference to the fundamental obtained in
squarewave mode the maximum modulation indices are those shown in Table 4.7.
Table 4.7 – Maximum ma referenced to fundamental obtained in squarewave mode (F).
L-N L-L
peak RMS peak RMS
(R) SPWM linear limit
VDC 0.7854 1.3603 0.5554 0.9619
VBUS 0.3927 0.6802 0.2777 0.4810
(T) THI linear limit
VDC 0.9069 1.5708 0.6413 1.1107
VBUS 0.4534 0.7854 0.3206 0.5554
(F) fundamental of squarewave
VDC 1.0000 1.7321 0.7071 1.2247
VBUS 0.5000 0.8660 0.3536 0.6124
Finally, when the modulation index is defined with reference to the magnitude of a base SV the
maximum modulation indices are those shown in Table 4.8. This reference is used in the
literature so it is presented here, but some caution is due in its interpretation. The modulation
index is the ratio of [the amplitude of the fundamental component of the output voltage
(equivalently, the magnitude of a magnitude-invariant SV that represents the combined action of
the same)] to [the amplitude of the fundamental component of some other important waveform
(or the magnitude of its SV)]. But the six-step basis space vector represents a fixed (and
imbalanced) state of the inverter; it is not a space vector that represents a sinusoidal set, thus it is
somewhat abstract to use its magnitude as the reference in the definition of modulation index.
However, it is a valid concept and is frequently encountered in the literature.
Table 4.8 – Maximum ma referenced to magnitude of six-step basis space vector (S).
peak RMS
L-N L-L L-N L-L
(R) SPWM linear limit
VDC 0.7500 1.2990 0.5303 0.9186
VBUS 0.3750 0.6495 0.2652 0.4593
(T) THI linear limit
VDC 0.8660 1.5000 0.6124 1.0607
VBUS 0.4330 0.7500 0.3062 0.5303
(F) fundamental of squarewave
VDC 0.9549 1.6540 0.6752 1.1695
VBUS 0.4775 0.8270 0.3376 0.5848
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