Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 44)

44 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 1 Drive Configuration
The derivative term senses a rapid rise in the bus voltage and activates the bus
regulator prior to actually reaching the bus voltage regulation setpoint V
reg
. The
derivative term is important because it minimizes overshoot in the bus voltage
when bus regulation begins thereby attempting to avoid an overvoltage fault. The
integral channel acts as the acceleration or deceleration rate and is fed to the
frequency ramp integrator. The proportional term is added directly to the output
of the frequency ramp integrator to form the output frequency. The output
frequency is then limited to a maximum output frequency.
Bus Regulation Modes
The drive can be programmed for one of five different modes to control the DC
bus voltage:
• Disabled
• Adjust Frequency
• Dynamic Braking
• Both with Dynamic Braking first
• Both with Adjust Frequency first
P372 [Bus Reg Mode A] is the mode normally used by the drive unless the “DI
BusReg Mode B” digital input function is used to switch between modes
instantaneously, in which case P373[Bus Reg Mode B] becomes the active bus
regulation mode.
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 occurs if the speed reaches [Max Speed] + [Overspeed
Limit]. If this condition is unacceptable, take action to 1) limit supply voltages
within the specification of the drive and, 2) limit fast positive input voltage
changes to less than 10%. If this operation is unacceptable and the necessary
actions cannot be taken, the “adjust freq” portion of the bus regulator function
must be disabled (see parameters 372 and 373).
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 372 and 373). In addition, installing a
properly sized dynamic brake resistor provides equal or better performance in most
cases. Important: These faults are not instantaneous. Test results have shown that
they can take between 2…12 seconds to occur.