AN1911 Freescale Semiconductor / Motorola, AN1911 Datasheet - Page 13

• Manufacturer Part Number: AN1911
• Description: 3-Phase AC Motor Control with V/Hz Speed Open Loop Using DSP56F80X
• Manufacturer: Freescale Semiconductor / Motorola

6.1.4 DC-Bus Voltage Ripple Elimination
The ramp profile fits the specific motor and can be easily changed to accommodate different ones.
Process Description
This process provides voltage calculation according to V/Hz ramp. The input of this process is
generated by desired inverter frequency Omega_Required. The output of this process is
AmplitudeVoltScale that is a parameter required by DC-Bus Voltage Ripple Elimination process.
Process Description
This process eliminates the influence of the DC-Bus voltage variation (ripples) toward the generated
phase voltage sinewaves. In fact, it lowers the 50 or 60 Hz acoustic noise of the motor. Another
positive aspect is that due to this function-generated phase voltage, it is independent of the level of
DC-Bus voltage. So the application is adaptable in both American and European power supply
systems.
The process is performed by function mcgenDCBVoltRippleElim which converts the phase voltage
amplitude (AmplitudeVoltScale) to the sine wave amplitude (Amplitude) based on the actual value of
the DC-Bus voltage (u_dc_bus) and inverse value of the modulation index (ModulationIndexInverse).
The modulation index is the ratio between the maximum amplitude of the first harmonic of phase
voltage (in voltage scale) and half of the DC Bus voltage (in voltage scale), which is defined by the
following formula:
The modulation index is specific to given 3-phase generation algorithm and in this application is 1.27.
The top chart in
amplitude) is counter-modulated in order to eliminate the DC-Bus ripples. The second chart shows the
duty cycles generated by one of the 3-ph wave generation functions. The third chart contains
sine-waves of the phase-to-phase voltages actually applied to the 3-phase motor.
Figure 6-3
Indirect Power Factor Correction Using DSP56F80X
Freescale Semiconductor, Inc.
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m
demonstrates how the Amplitude (in scale of generated sine wave
i
=
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------------------------ -
1
-- - u
2
U
phasemax
1
DCBus
=
------ -
2
3
Software Design
(EQ 6-1.)
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