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Embedded Software and Motor Control Libraries for PXR40xx

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Function GMCLIB_SvmStd_FLT<br />

Table 4-151. Assignment of the duty-cycle ratios to motor phases (continued)<br />

pwm b t 2 t 3 t 3 t 2 t 1 t 1<br />

pwm c t 1 t 1 t 2 t 3 t 3 t 2<br />

The principle of the Space Vector Modulation technique resides in applying the basic<br />

voltage vectors U XXX <strong>and</strong> O XXX <strong>for</strong> a certain time in such a way that the main vector,<br />

generated by the Pulse Width Modulation approach <strong>for</strong> the period T, is equal to the<br />

original stator reference voltage vector U S. This provides a great variability of the<br />

arrangement of the basic vectors during the PWM period T. Those vectors might be<br />

arranged either to lower switching losses or to achieve diverse results, such as centrealigned<br />

PWM, edge-aligned PWM or a minimal number of switching states. A brief<br />

discussion of the widely-used centre-aligned PWM follows.<br />

Generating the centre-aligned PWM pattern is accomplished practically by comparing the<br />

threshold levels, pwm a, pwm b <strong>and</strong> pwm c with a free-running up-down counter. The<br />

timer counts to 1 (0x7FFF) <strong>and</strong> then down to 0 (0x0000). It is supposed that when a<br />

threshold level is larger than the timer value, the respective PWM output is active.<br />

Otherwise, it is inactive; see Figure 4-78<br />

Figure 4-78. St<strong>and</strong>ard space vector modulation technique - centre-aligned PWM<br />

Note<br />

As during the computation the irrational value of √(3)/4 is used<br />

<strong>for</strong> calculation, the correction constant is used to increase the<br />

<strong>Embedded</strong> <strong>Software</strong> <strong>and</strong> <strong>Motor</strong> <strong>Control</strong> <strong>Libraries</strong> <strong>for</strong> <strong>PXR40xx</strong>, Rev. 1.0<br />

540 Freescale Semiconductor, Inc.

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