Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 48)

48 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 1 Drive Configuration
The DB current seems as if it is decreasing toward the end of the decel. This is
just a result of the sweep time of the oscilloscope and instrumentation. After all,
it’s not known as “Ohm’s Suggestion.” The point is evident that the DB transistor
is pulsing through the decel.
Option 3 “Both DB 1st”
If [Bus Reg Mode
n] is set to 3 “Both DB 1st” Both regulators are enabled, and
the operating point of the Dynamic Brake Regulator is lower than that of the Bus
Voltage Regulator. The Bus Voltage Regulator setpoint follows the “DB Turn
On” curve. The Dynamic Brake Regulator follows the “DB Turn On” and turn
off curves. For example, with a DC Bus Memory between 650 and 685V DC, the
Bus Voltage Regulator setpoint is 750V DC and the Dynamic Brake Regulator
turns on at 742V DC and back off at 734V DC.
It is possible that the drive can react differently between Flux Vector mode and
Sensorless Vector mode. The important thing to remember here is that in SV
control, the drive does not use the value entered into P426 [Regen Power Lmt]. If
left at default (-50%) and the decel is such that it creates a large amount of regen
power, the drive again attempts to protect the resistor.
Consider the plots below.
Option 4 “Both Frq 1st”
If [Bus Reg Mode
n] is set to 4 “Both Frq 1st” Both regulators are enabled, and
the operating point of the Bus Voltage Regulator is lower than that of the
Dynamic Brake Regulator. The Bus Voltage Regulator setpoint follows the “Bus
Reg Curve 2” below a DC Bus Memory of 650V DC and follows the “DB Turn
Off ” curve above a DC Bus Memory of 650V DC (
Table 4 ). The Dynamic Brake
Regulator follows the “DB Turn On” and turn off curves. For example, with a
DC Bus Memory at 684V DC, the Bus Voltage Regulator setpoint is 742V DC
and the Dynamic Brake Regulator turns on at 750V DC and back off at 742V
DC.
Figure 6 shows that upon a stop command the bus voltage rises immediately to a
point where the DB transistor turns on briefly bringing the voltage down to a
point where the bus regulator can regulate the bus by adjusting the output
frequency (speed).