Allen-Bradley 8720MC, User Manual, Page: 58 / 128

58 Rockwell Automation Publication 8720MC-RM001K-EN-P - September 2018
Chapter 5 Connect the 8720MC-RPS Unit
8720MC-RPS Unit Adapted to Large Capacity Capacitors
8720MC-RPS units charge to capacitors with a single-phase, full-wave
rectification circuit through the built-in precharge/discharge resistor. When
the load capacitors have large capacitance, it takes more time for charging, and
the wattage of the internal resistor becomes insufficient. When the load
capacitance is large, disable the built-in precharge/discharge resistor and
connect an external resistor or external circuit for precharging/discharging.
Calculate the rated wattage of the external precharge/discharge resistor to be
connected to the outside in accordance with the following formula, depending
on the total capacitance of all the capacitors including the built-in capacitor.
When an external precharge/discharge resistor is connected between PR1 and
PR2, change the set value of the following parameters, if necessary.
• Precharge/discharge time (F.014)
• Wattage of precharge/discharge resistor (F.015)
Table 18 - Maximum Applicable Capacitance Value and Minimum Resistance Value
External Resistor Case
Most applications are successfully integrated using the internal precharge
resistor provided in the 8720MC-RPS unit. For instances where there is a large
amount of load capacitance, caused by connecting several drives to a single
8720MC-RPS unit, use an external precharge resistor. To determine the total
capacitance C[F], add the capacitance for the applicable 8720MC-RPS unit, as
determined from row 1 of
Table 18 to the sum of the drive capacitance.
• For the 8720MC-RPS065 unit, if the total capacitance is above 7,000
μF, but less than 110,000 μF, an external resistor connected to PR1 and
PR2 is required.
Table 18 provides the minimum resistance value for the
external precharge resistor (20 ohms for the 8720MC-RPS065 unit).
• For the 8720MC-RPS190 unit, if the total capacitance is above 25,000
μF, but less than 165,000 μF, an external resistor connected to PR1 and
PR2 is required.
Table 18 provides the minimum resistance value for the
external precharge resistor (10 ohms for the 8720MC-RPS190 unit).
TIP The minimum precharging/discharging cycle is three minutes.
Attribute 8720MC-RPS065 Value 8720MC-RPS190 Value
Capacitance of the built-in capacitor 1960 μF 5600 μF
Built-in resistor (resistance value/wattage)
7000 μF
(22 Ω/120 W) 25,000 μF
(10 Ω/400 W)
External resistor (minimum resistance value) connected
to PR1 and PR2 110,000 μF
(20 Ω) 165,000 μF
(10 Ω)
External resistor (minimum resistance value)
See
Figure 30 and Figure 31 for example diagrams.
220,000 μF
(4.7 Ω) 495,000 μF
(1.5 Ω)
Rated wattage [W] = 17,000 • C[F]
Assumes surge resistivity of J[J] = 28,000 • C[F] is provided