88 Rockwell Automation Publication DRIVES-AT005D-EN-P - May 2022
Appendix A Electrical Ratings, Recommended Protective Devices, and DC Bus Capacitance
The data tables in this manual list the rated AC RMS output current for the drives and inverters. The following µF/A ratio calculations take the sum of
inverter/drive rated AC RMS output current to obtain the total rated AC RMS output current. Alternatively, you could sum the rated DC input current
of the inverter/drives and multiply this by 0.9 to obtain the approximate equivalent rated AC output amps used in the µF/A ratio calculations.
Common DC bus systems with PowerFlex 755TM bus supplies are designed based on a 40 µF/A ratio for 400/480V AC systems, and a 28 µF/A ratio
for 600/690V AC systems, regardless of the mixture of approved inverters that are connected to the common DC bus.
Calculation Process:
1. Sum the DC bus capacitance of all the inverter/drives connected on the common DC bus plus the bus supply internal capacitance. See the
tables in Appendix A
for the internal DC bus capacitance of each inverter and bus supply. The sum is the 'system DC bus capacitance'.
2. Sum the rated AC output current of the inverters connected on the common DC bus. See the tables in Appendix A
for the rated AC output
current of each inverter unit. Alternatively, you could sum the rated DC input amps of each inverter/drive and multiply this by 0.9 to obtain
the approximate rated AC output current. Use the Normal Duty (ND) or Heavy Duty (HD) rated current values in the tables based on your
application inverter/drive sizing. This sum is the 'system output amps'.
3. Divide the 'system DC bus capacitance' value by the 'system output amps' value. The resulting ratio is 'system µF/A ratio'.
4. The required uF/A ratio depends on the system voltage:
a. 400/480V AC, the minimum system µF/A ratio is 40 µF/A.
IF the calculated system µF/A ratio is equal to or greater than 40 µF/A proceed to step 5.
IF the calculated system µF/A ratio is less than 40 µF/A calculate the required additional capacitor bank:
Additional capacitance = [40 µF * system output amps] - system DC bus capacitance; proceed to step 5.
b. 600/690V AC, the minimum system µF/A ratio is 28 µF/A.
IF the calculated system µF/A ratio is equal to or greater than 28 µF/A proceed to step 5.
IF the calculated system µF/A ratio is less than 28 µF/A calculate the required additional capacitor bank:
Additional capacitance = [28 µF * system output amps] - system DC bus capacitance; proceed to step 5.
5. Check to make sure that the PowerFlex 755TM bus supply is capable of pre-charging the system DC bus capacitance, including any
additional capacitor banks. The bus supply internal and maximum external DC bus capacitance specifications can be found in Appendix A
.
a. IF (system DC bus capacitance + additional DC bus capacitance) is less than the bus supply maximum DC bus external capacitance then
the system design is okay. End.
b. IF (system DC bus capacitance + additional DC bus capacitance) is greater than the bus supply maximum external DC bus capacitance
then:
IF PowerFlex 755TM regenerative bus supply, choose the next higher rating and repeat steps 1…6, or
IF PowerFlex 755TM non-regenerative supply, set the precharge ramp time to a higher value. See PowerFlex 755TM Non-Regenerative
Supply, publication 750-UM100
, for information on setting the precharge ramp time.
The following are examples of DC bus capacitance calculations using this method.
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