AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 38

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
38
NOTE:
The calculation is based on the wave table stored in the MCU’s ROM.
The table describes either a pure sine wave or sine wave with the third
harmonic added. The second case is often preferred because it allows
generation of a first harmonic sine voltage equal to the input ac line
voltage. Because of quarter wave symmetry, only one quadrant of the
wave period is stored in the table. The wave values for other quadrants
are calculated from the first one. The format of the stored wave table
data is from 0x00 (for 0 voltage) up to PWM modulus/2 (for the 100
percent voltage). Thus, the proper data scaling is secured.
It is important to note that 50 percent PWM (or 50 percent of PWM
modulus loaded to the corresponding PVAL registers) corresponds to
the zero phase voltage. But in the wave table, the ZERO phase voltage
corresponds to the number 0x00. Therefore, the fetched wave value
from the table must be added to the 50 percent PWM modulation for
quadrants 1 and 2 or subtracted from the 50 percent PWM modulation
for quadrants 3 and 4 (see point 5 of the process description that
follows). Thus, the correct PWM value is loaded.
The input parameters of the PWMCALC.C are:
The output parameters of the process are:
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Freescale Semiconductor, Inc.
For More Information On This Product,
Table increment Table_inc that is used for the wave pointer
update
Amplitude of the generated inverter voltage
PWM value for phase A — PVAL1 register
PWM value for phase B — PVAL3 register
PWM value for phase C — PVAL5 register
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MOTOROLA
AN1857