AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 26

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

Application Note
dc Bus Voltage
Divider
Individual Phase
Voltage Divider
26
The system software monitors a number of analog parameters when the
motor is running. Those parameters include the dc system bus voltage
and the three individual phase voltages. In all four cases, the high
voltage is divided down to a level within the measurable range of the
MR32’s A/D converter.
Figure 17
dc bus voltage. The signal labeled V_sense_DCB_5 is the divided down
dc bus voltage that is fed to A/D of the MR32 either directly or via the
optoisolator board when one is utilized in the system. The signal labeled
V_sense_DCB_half_15 is used as a reference for the individual phases
A, B, and C zero cross detection circuits.
In a similar fashion to the dc bus sensing circuit shown in
individual phase A, B, and C voltages are divided down to match the
input level of the A/D.
divider for phase A. There are additional circuits for voltage monitoring
and zero cross detection of phases B and C. This technique allows
sensing of the back-EMF from the motor. The individual phase voltage
signals are fed to separate A/D inputs of the microcontroller either
directly or via the optoisolator board if it is utilized in the system. The
signal shown is labeled BEMF_sense_A (BEMF_sense_B or
BEMF_sense_C, depending on the particular phase signal). An
additional function of the circuit shown in
crossing of phases A, B, and C. Note that the inverting input of the
DCB_Cap_neg
DCB_Cap_pos
Earth_GND
Freescale Semiconductor, Inc.
For More Information On This Product,
is a schematic of the voltage divider used for monitoring the
Go to: www.freescale.com
Figure 17. dc Bus Voltage Sensing Circuit
C213
10 nF/3000 V
C214
10 nF/3000 V
Figure 18
C208
22 nF/
630 Vdc
shows a schematic of the voltage
+
C209
470 F/400 V
274 k –1%
R224
274 k –1%
Figure 18
R225
4.87 k –1%
R226
is to detect zero-
274 k –1%
R227
Figure
GNDA
MOTOROLA
R230
6.81 k –1%
R228
6.81k –1%
R229
255 –1%
17, the
AN1857