34 Rockwell Automation Publication DRIVES-AT005D-EN-P - May 2022
Chapter 3 Shared DC Bus Configuration (Piggyback)
4. The minimum rated AC output power for the PowerFlex 755TL/TR AC drive must be at least 70.6 kW heavy duty (HD).
• If your application requires regeneration, select a PowerFlex 755TR regenerative AC drive. If regeneration isn’t required, consider a
PowerFlex 755TL low harmonic non-regenerative AC drive. If you’re uncertain, select a PowerFlex 755TR regenerative AC drive.
•Use Table 25 on page 108
to select the PowerFlex 755TL/TR AC drive.
The minimum AC output power rating 480V AC PowerFlex 755TL/TR AC drive is a 20G…D156 frame 6 with heavy duty (HD) AC output power
rating of 100 HP (75 kW).
5. Verify that the chosen PowerFlex 755TL/TR AC drive is capable of precharging the connected inverter/drive DC bus capacitance.
• The PowerFlex 750-Series internal DC bus capacitance data is taken from Table 9 on page 66
in Appendix A.
• The PowerFlex 755TL/TR AC drive internal and maximum external capacitance data is taken from Table 25 on page 108
in Appendix C.
The PowerFlex 755TL/TR AC drive must be applied to the highest power AC motor. In this example, Motor 2 (45 kW) would be connected to the
PowerFlex 755TL/TR AC drive.
The total external capacitance (1905 µF) is greater than the PowerFlex 755TL/TR AC drive maximum external capacitance (880 µF).
The chosen PowerFlex 755TL/TR AC drive is NOT capable of precharging the connected DC bus capacitance. Choose a higher rated
PowerFlex 755TL/TR AC drive and repeat the sizing process.
6. Use the tables in Appendix A
to determine the rated DC input currents for each external connected inverter/drive. Size the DC power cables/
bus bars feeding each external inverter/drive for the rated DC input current.
In this example, the total rated DC amps for the external connected inverter/drives is, 23.3 A DC + 8.1 A DC = 31.4 A DC.
The chosen PowerFlex 755TL/TR AC drive isn’t an acceptable shared DC bus solution for this example. Select a different rating and repeat the sizing
process.
Advanced Sizing of the PowerFlex 755TL/TR AC Drive
The sizing calculations require the following system information:
• Motor nameplate power rating
• Motor nameplate efficiency
• Product family and catalog numbers for each inverter/drive connected on the common DC bus.
• Inverter/drive overload duty ratings normal duty (ND) or heavy duty (HD).
• Application power requirements and polarity for each motor during the acceleration, steady running, and deceleration phases of operation
1. Convert all motor powers to kW (kW = HP x 0.746).
2. In the following steps, PMotor is the motor power required for the application, not the nameplate power of the motor (unless the application
power is unknown). If the application is absorbing power, use positive values. If the application is regenerating power, use negative values.
3. Determine the total AC output power that is required during acceleration.
• For Motoring Loads: PDrive = PMotor / Motor Efficiency
• For Regenerating Loads: PDrive = PMotor * Motor Efficiency
Section
Name Drive Product Cat. No.
Drive Rated Output
Power (kW)
DC Bus Capacitance
(µF)
(1)
(1) See Appendix A and Appendix C for internal and maximum capacitance values.
Rated DC
Input Amps Notes
Motor 1 PowerFlex 755 20G…D027 11.0 1200 23.3 frame 3, HD
Motor 2 PowerFlex 755TL/TR 20G…D156 75.0 9200 – frame 6, HD shared bus supply
Motor 3 PowerFlex 755 20G…D011 5.6 705 8.1 frame 2, HD
PowerFlex 755TL/TR Max External Capacitance 880 µF
Total External Capacitance
(2)
(2) The calculated total external DC bus capacitance does not include the PowerFlex 755TL/TR AC drive internal bus capacitance.
1905 µF
Total DC Bus Capacitance 11105 µF
Total DC Rated Amps 31.4 A DC
Precharge NOT OK
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