Bus Regulation
21
The derivative term senses a rapid rise in the bus voltage and activates the bus
regulator prior to actually reaching the bus voltage regulation set point Vreg. The
derivative term is important since it minimizes overshoot in the bus voltage when
bus regulation begins thereby attempting to avoid an over-voltage 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.
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
[Bus Reg Mode A], parameter 161 is the mode normally used by the drive unless
the “Bus Reg Md B” digital input function is used to switch between modes
instantaneously, in which case [Bus Reg Mode B], parameter 162 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 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.
Important:
These faults are not instantaneous. Test results have
shown that they can take between 2-12 seconds to occur.