UM0324 datasheet(98/105 Pages) STMICROELECTRONICS

98 / 105 page

Additional information

UM0324

98/105

A.2

Selecting PWM frequency for 3 shunt resistor configuration

Beyond the well known trade-off between acoustical noise and power dissipation,

consideration should be given to selecting the PWM switching frequency using the AC IM

IFOC software library.

As discussed in Section 4.2.5 on page 43, depending on the PWM switching frequency, a

limitation on the maximum applicable duty cycle could occur if using three shunt resistor

configuration for current reading. Table 2: PWM frequency vs maximum duty cycle

relationship on page 51, summarizes the performance of the system when the software

library is used in conjunction with STR750-MCKIT hardware.

Note:

The MB459 board is an evaluation platform; it is designed to support different motor driving

topologies (PMSM and AC induction) and current reading strategies (single and three shunt

resistors). Therefore, the figures given in Table 2 on page 51 should be understood as a

starting point and not as a best case.

Moreover, to keep the synchronization between TIM0 and PWM peripherals, it is always

necessary to finish executing the IFOC algorithm before the next PWM period begins as

shown in Figure 49.

Figure 49.

AD conversions for three shunt topology stator currents reading and

IFOC algorithm execution when REP_RATE=1

Given that the raw execution time of the IFOC algorithm is around 27.5µs and that other

delays (such as the time necessary to enter ADC ISR) have to be considered, this limits to

about 12.5 kHz the maximum IFOC algorithm execution rate when using REP_RATE =1.

However, no limitations occur in the typical range of PWM frequencies when using

REP_RATE=3

The following table summarizes the performance of the system for different PWM

frequencies.

UM0324 Datasheet(PDF) 98 Page - STMicroelectronics