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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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values could be interpreted as the maximum modulation index referenced to the SPWM linear<br />

limit, defined in terms of whichever row is selected. These values of maximum modulation<br />

indices are summarized in Table 4.5. (As mentioned, k 2/3 is used in this report when<br />

working with inverters.) To be clear, these are the maximum values of the modulation indices.<br />

Table 4.5 – Maximum ma referenced to SPWM linear limit (R).<br />

peak RMS<br />

L-N L-L L-N L-L<br />

(R) SPWM linear limit<br />

VDC 1.0000 1.7321 0.7071 1.2247<br />

VBUS 0.5000 0.8660 0.3536 0.6124<br />

(T) THI linear limit<br />

VDC 1.1547 2.0000 0.8165 1.4142<br />

VBUS 0.5774 1.0000 0.4082 0.7071<br />

(F) fundamental of squarewave<br />

VDC 1.2732 2.2053 0.9003 1.5594<br />

VBUS 0.6366 1.1027 0.4502 0.7797<br />

Many of the values in Table 4.5 are used in the literature when discussing inverter performance<br />

but references often do not define the definition of the modulation index they use; in those cases<br />

the table can be used as a guide. For example, an article may state that the maximum modulation<br />

index corresponding to six-step square wave operation is 0.78. Searching the (F) entries for 0.78<br />

will show that the modulation index was defined using line-line RMS quantities referenced to the<br />

full bus voltage VBUS. When the modulation index is defined with reference to the THI linear<br />

limit the maximum modulation indices are those shown in Table 4.6.<br />

Table 4.6 – Maximum ma referenced to THI linear limit (T).<br />

L-N L-L<br />

peak RMS peak RMS<br />

(R) SPWM linear limit<br />

VDC 0.8660 1.5000 0.6124 1.0607<br />

VBUS 0.4330 0.7500 0.3062 0.5303<br />

(T) THI linear limit<br />

VDC 1.0000 1.7321 0.7071 1.2247<br />

VBUS 0.5000 0.8660 0.3536 0.6124<br />

(F) fundamental of squarewave<br />

VDC 1.1027 1.9099 0.7797 1.3505<br />

VBUS 0.5513 0.9549 0.3898 0.6752<br />

202

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