Allen-Bradley PowerFlex 70 Reference Manual page 59

ATTENTION: The "adjust freq" portion of the bus regulator
! function is extremely useful for preventing nuisance overvoltage faults
resulting from aggressive decelerations, overhauling loads, and eccentric
loads. It forces the output frequency to be greater than commanded
frequency while the drive's bus voltage is increasing towards levels that
would otherwise cause a fault; however, it can also cause either of the
following two conditions to occur.
1. Fast positive changes in input voltage (more than a 10% increase
within 6 minutes) can cause uncommanded positive speed changes;
however an "OverSpeed Limit" fault will occur if the speed reaches
[Max Speed] + [Overspeed Limit]. If this condition is unacceptable,
action should be taken to 1) limit supply voltages within the
specification of the drive and, 2) limit fast positive input voltage
changes to less than 10%. Without taking such actions, if this operation
is unacceptable, the "adjust freq" portion of the bus regulator function
must be disabled (see parameters 161 and 162).
2. Actual deceleration times can be longer than commanded
deceleration times; however, a "Decel Inhibit" fault is generated if the
drive stops decelerating altogether. If this condition is unacceptable, the
"adjust freq" portion of the bus regulator must be disabled (see
parameters 161 and 162). In addition, installing a properly sized
dynamic brake resistor will provide equal or better performance in most
cases.
Note: These faults are not instantaneous and have shown test results
that take between 2 and 12 seconds to occur.
PowerFlex 70/700
The user selects the bus voltage regulator using the Bus Reg Mode parameters.
The available modes include:
• off
• frequency regulation
• dynamic braking
• frequency regulation as the primary regulation means with dynamic braking
as a secondary means
• dynamic braking as the primary regulation means with frequency regulation
as a secondary means
The bus voltage regulation setpoint is determined off of bus memory (a means to
average DC bus over a period of time). The following graph and tables describe
the operation.
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
Bus Regulation
59