AN1910 Freescale Semiconductor / Motorola, AN1910 Datasheet - Page 13

• Manufacturer Part Number: AN1910
• Description: 3-Phase AC Motor Control with V/Hz Speed Closed Loop Using the DSP56F80X
• Manufacturer: Freescale Semiconductor / Motorola

6.1.5 DC-Bus Voltage Ripple Elimination
Note:
The volt per hertz ramp is defined by following parameters:
The ramp profile fits to 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
The voltage ripple elimination process eliminates the influence of the DC-bus voltage ripples to the
generated phase voltage sinewaves. In fact, it lowers the 50 or 60Hz acoustic noise of the motor.
Another positive aspect due to this function is the generated phase voltage. It is independent of the
level of DC-bus voltage. So, the application is well adaptable in worldwide power supply system.
The process is performed by the mcgenDCBVoltRippleElim function, converting 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 the phase
voltage (in voltage scale)and half of DC bus voltage (in voltage scale) which is defined by the
following formula:
The modulation index is specific to a given 3-phase generation algorithm and in the case of the
application, it is 1.27.
The first chart in
amplitude) is counter-modulated in order to eliminate the DC-bus ripples. The second chart delineates
the duty cycles generated by one of the 3-ph wave generation functions. The third chart contains
symetrical sine-waves of the phase-to-phase voltages actually applied to the 3-phase motor.
•
•
•
Base point - defined by f
Boost point- defined by V
Start point - defined by V
The result of the modulation index is based on the third harmonic injection PWM technique.
3-Phase AC Motor Control with V/Hz Speech Closed Loop
Figure 6-3
Freescale Semiconductor, Inc.
For More Information On This Product,
m
i
demonstrates how the Amplitude (in scale of generated sine wave
base
=
start
boost
Go to: www.freescale.com
------------------------ -
1
-- - u
2
U
(usually 50Hz or 60Hz)
at zero frequency
and f
phasemax
1
DCBus
boost
=
------ -
2
3
Software Design
(EQ 6-1.)
13