Siemens sinumerik 840D sl Function Manual page 363

Figure 2-12
The drive and DC link pulses are deleted at specific voltage levels. This automatically
causes the drives to coast down.
If this behavior is not desired, the user can use a resistor module to divert the surplus
energy. This resistor module operates in the gray hatched area in the diagram, thus lying
below the critical voltage level.
Note
The pulse power of the resistor module is greater than the I/RF power.
Monitoring the intermediate circuit minimum voltage limit
The voltage of the intermediate circuit can be monitored according to a parameterizable
threshold (MD1634 $MD_LINK_VOLTAGE_RETRACT).
If the voltage falls below the threshold parameterized in the following axis machine data, this
can be utilized as an internal error source for retraction:
MD1634 LINK_VOLTAGE_RETRACT
This is to avoid disconnection of the drive hardware without separation of workpiece and tool
when the intermediate circuit voltage is lower than the minimum of 280V.
In addition, one/several axis/axes (useful for one axis per I/RF area) can be programmed,
whether a retraction is to be triggered when the voltage falls below the Intermediate circuit
threshold (MD 1634). A precondition for this is, that synchronous actions linking of system
variables $AA_ESR_STAT is dependent. This means that parameterized and programmed
ESR are executed if it is released via system variable $AA_ESR_ENABLE.
The power required for ESR can be supplied to the DC link by parallel, regenerative braking:
See DC link backup.
Special functions: Axis Couplings and ESR (M3)
Function Manual, 11/2006, 6FC5397-2BP10-2BA0
Voltage level of SIMODRIVE 611D DC link
Detailed description
2.7 Extended stop and retract (ESR)
143