4
vacon • 46 Active Front End (AFE)
Local contacts: http://drives.danfoss.com/danfoss-drives/local-contacts/
4.14 Pre-Charging circuit
The Active Front End unit requires an external pre-charging circuit. The purpose of the pre-charging
unit is to charge the voltage in the intermediate circuit to a level sufficient for connecting the Active
Front End unit to the mains. The charging time depends on the capacitance of the intermediate
circuit and the resistance of the charging resistors. The technical specifications of our standard pre-
charging circuits are shown in Table 26. Pre-charging circuits are suitable for 380-500 Vac and 525-
690 Vac.
The Active Front End unit must not be connected to mains without pre-charging. In order to ensure
the correct operation of the pre-charging circuit, the input circuit-breaker or contactor, as well as
the pre-charging circuit contactor, must be controlled by the Active Front End unit. The input circuit-
breaker or contactor as well as the pre-charging circuit contactor must be connected as shown in
Appendix 74.
If the capacitance of the intermediate circuit in the system exceeds the values shown, contact your
nearest distributor.
The example shown in Appendix 74 uses a spring-return switch. The switch has positions 0-1-
START. The spring returns the switch from position START to position 1. To start the pre-charging,
the switch is turned from position 0 via 1 to START. When pre-charging starts, the switch can be
released and it returns to position 1. No other control measures are required. The Active Front End
application controls the main contactor of the system with Relay Output RO2, see Appendix 76. When
pre-charging of the intermediate circuit is ready the main contactor will be closed. The status of the
main contactor is monitored via digital input (Default is DIN4). As a default the main contactor
monitoring is ON but it can be set OFF with parameter. The main contactor should not be possible
close without pre-charging.
To open the main contactor, simply turn the switch to 0. The contactor should not be opened under
load. Opening the contactor under load will shorten its service life.
NOTE! Wirings that are used for connecting the pre-charging circuit to the intermediate circuit has
to be double-insulated (example: NSGAFÖU 1.8/3kV (IEC), NSHXAFÖ 3kV (IEC Halogen free),
MULTI-STANDARD SC 2.2 (UL)).
NOTE! Enough space must be reserved around the resistors to ensure sufficient cooling. Don’t place
any heat sensitive components near the resistors.
Table 26. Capacitance Min and Max value for Pre-charging circuit
Enclosure
size
Resistance
Capacitance
Min Max
FI9 2x47R 4950 µF 30000 µF
FI10 2x20R 9900 µF 70000 µF
FI13 2x11R 29700 µF 128000 µF
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