User Manual V 1-4.XX User guide

Troubleshooting 12-21
Understanding the Bus Voltage
Tracker
Bus/Brake Opts (parameter 13) also lets you select a rate, called a
slew rate, for the bus voltage tracker. The bus voltage tracker slowly
tracks changes in the actual bus voltage. If the actual bus voltage
drops 150 volts or greater below the current value of the bus voltage
tracker, the drive automatically disables modulation and enters
precharge.
Important: You should only use the bus voltage tracker if you are
having ridethrough problems. The bus voltage tracker adjusts the bus
sensitivity to ridethrough for cases where there is an unstable bus.
By changing the rate used for the bus voltage tracker, you can make
your system more or less sensitive to changes in the actual bus
voltage. For example, if your drive currently enters precharge after
the motor exits regeneration, you may need to change your slew rate.
Figure 12.2 shows an example of the filtered bus voltage reference.
Figure 12.2
Example Bus Voltage Line
At point A, the motor was in regeneration, so the value of the bus
voltage tracker slowly increased.
At point B, the motor was no longer in regeneration and the bus
voltage had dipped below the nominal range. If the drive compared
point B with point A, the drive would have seen a bus drop of 150V
and entered precharge. However, because the drive compared point B
with the bus voltage tracker, the bus drop was less than 150V and the
drive continued operating.
At point C, the bus voltage had dropped 150V and the drive entered a
precharge state.
Bus/Brake Opts provides the following options for changing the slew
rate:
If all bits are clear (0), the slew rate is 0.05V/second. If more than one
bit is set, the first bit that is set is used for the slew rate. For most
applications, the default slew rate of 0.05V/second, which is 1 volt in
20 seconds, should be appropriate.
This bit: With this text: Sets the slew rate to:
0 Slew Rate 1
10V/second. This option is the most sensitive to
changes in the actual bus voltage.
1 Slew Rate 2 5V/second.
2 Slew Rate 3 0.5V/second.
3 Slew Rate 4 0.05V/second.
4 Slew Rate 5
0.005V/second. This option is the least sensitive to
changes in the actual bus voltage.
Volts
Time
150V
Bus voltage tracker
A
B
C
Actual bus voltage