AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 40

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

Freescale Semiconductor, Inc.
Application Note
In the event of a system fault, it is important to note that the software
services the event in a timely manner. The software accommodates two
fault inputs, overcurrent and overvoltage.
Overcurrent — In case of overcurrent, the external hardware provides
a rising edge on the fault input of the microcontroller’s FAULT2 input
through the circuit described in
Figure
4. This signal disables all motor
control PWM’s outputs (PWM1 through PWM6) and sets general fault
flag Gf_flag. The Over_current_flag is set in the failure register
used by the PC-Master control interface.
Overvoltage — The sensed dc-bus voltage is compared with a limit
within the software. In case of overvoltage, all motor control PWM
outputs are disabled by the software setting bits in the microcontroller’s
PWM control register (PCTL1) and setting a bit in the general fault flag
Gf_flag. The Over_voltage_flag is set in the failure register used
by the PC-Master control interface. The overvoltage fault is set only if the
motor is braking (in generator mode). In motor mode (the motor supplies
power to the load), if the overvoltage occurs, PFC is disabled; the
overvoltage failure is not detected and the motor is not blocked. It should
be noted that the PFC output voltage operates very close to 400 volts,
which is the overvoltage limit. The regulation overshoot would cause the
overvoltage failure when PFC is running. Therefore, the overvoltage is
blocked when PFC is running.
If any of these faults occur, the fault LED will flash. The system remains
disabled until the fault is cleared by switching the START/STOP switch
to the STOP position and then to the START position or the fault can be
cleared by the PC-Master by setting the ERROR_CLEAR_PMFLG bit in the
Motor_Ctrl control register. After the switch START/STOP is set to
START, the motor will restart.
The processes described earlier are implemented in a single state
machine and are illustrated in
Figure
24,
Figure
25, and
Figure
26.
The general state diagram incorporates the main routine entered from
reset and eight interrupt states. The main routine includes the
initialization of the microcontroller and a software timer for the control
algorithm timebase. The interrupt states provide calculation of the speed
of the motor, overcurrent fault handler, PWM generation process, zero-
AN1857
40
MOTOROLA
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