26 Rockwell Automation Publication DRIVES-AT005D-EN-P - May 2022
Chapter 2 Regenerative Bus Supply Configuration
3. Determine the total DC bus power that is required during acceleration.
For Motoring Loads: PDrive = PMotor / Motor Efficiency / Inverter Efficiency (0.97)
For Regenerating Loads: PDrive = PMotor * Motor Efficiency * Inverter Efficiency (0.97)
Calculate the DC bus power that is required during acceleration times less than 1 minute, taking advantage of the normal duty 110% for 1
minute (OL=1.1) or heavy duty 150% for 1 minute (OL=1.5) overload rating of the bus supply. If acceleration time is greater than 1 minute, use
an overload rating of OL=1.0.
PAccel = (PDrive1 + PDrive2 + …) / OL
4. Determine the total DC bus power that is required during steady-state run operation.
For Motoring Loads: PDrive = PMotor / Motor Efficiency / Inverter Efficiency (0.97)
For Regenerating Loads: PDrive = PMotor * Motor Efficiency * Inverter Efficiency (0.97)
Calculate the steady-state DC bus power that is required.
PRun = PDrive1 + PDrive2 + …
5. Determine the total DC bus power that is required during deceleration.
For Motoring Loads: PDrive = PMotor / Motor Efficiency / Inverter Efficiency (0.97)
For Regenerating Loads: PDrive = PMotor * Motor Efficiency * Inverter Efficiency (0.97)
Calculate the DC bus power that is required during deceleration times less than 1 minute, taking advantage of the normal duty 110% for 1
minute (OL=1.1) or heavy duty 150% for 1 minute (OL=1.5) overload rating of the bus supply. If acceleration time is greater than 1 minute, use
an overload rating of OL=1.0.
PDecel = (PDrive1 + PDrive2 + …) / OL
6. Compare the absolute values of the normal duty (ND) or heavy duty (HD) DC bus power that is required during acceleration, deceleration, and
steady state run (steps 3…5). Use the highest numerical value to select the bus supply in step 7.
7. Select a PowerFlex 755TM regenerative bus supply for the system AC voltage based on the normal duty (ND) or heavy duty (HD) DC output
power rating that meets or exceeds the highest calculated DC bus power from step 6. See the PowerFlex 755TM regenerative bus supply
tables in Appendix A
for DC output power ratings.
8. The DC bus capacitance per output amp ratio must meet or exceed the target µF/A ratio. Additional DC bus capacitor modules may be
required. See DC Bus Capacitance Calculation Method on page 87
for more information about DC bus capacitor µF/A ratio calculations.
9. The bus supply selected in step 7 must be able to precharge the total system DC bus capacitance. Sum the internal DC bus capacitance
values for all connected AC inverters/drives and additional capacitor modules. Exclude the internal capacitance of the PowerFlex 755TM
regenerative bus supply in this calculation.
See the tables in Appendix A
for internal DC bus capacitance values used in PowerFlex 750-Series drives, PowerFlex 755TM common bus
inverters, and Kinetix 5700 servo drives. The tables in Appendix A also show the maximum external DC bus capacitance values for PowerFlex
755TM regenerative bus supply.
If the total external capacitance is less than the PowerFlex 755TM regenerative bus supply maximum external capacitance rating, then the
PowerFlex 755TM regenerative bus supply is capable of precharging the external AC drives; otherwise, select a higher rated PowerFlex
755TM regenerative bus supply and repeat steps 8 and 9.
10. Verify that the system AC and DC bus bars and cables are sized to support the expected current. The total rated DC bus current is the sum of
all inverter/drive rated AC output currents calculated in step 8 divided by 0.9.
The PowerFlex 755TM system standard bus bars are rated for 3000 A DC and optional (-P46) 4700 A DC. All DC bus bars in the common DC
bus lineup must have the same amp rating. See PowerFlex 755TM System DC Bus Ratings on page 99
for more information about the DC bus
bar ratings and options. If the calculated total DC rated amps exceed these ratings, special design considerations are required to make sure
that the AC and DC bus bar systems are sized to handle the expected AC and DC currents. Back-to-back or center-fed enclosure lineups may
be used to satisfy these requirements. See the associated PowerFlex 755T technical data and installation manuals for more information.
Third-party enclosures with suitably rated AC and DC bus bars may also be employed to meet the AC and DC current requirements of this
example system.
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