Emerson PACSystems RSTi-EP - Feedback Energy in DO Modules; Calculation of Feedback Energy

User Manual Section 3
GFK-2958L May 2021
Configuration 34
3.8 Feedback Energy in DO Modules
With digital output modules, power is fed back through the channels when inductive loads
are switched off. The respective permissible breaking energy is noted in the technical data
of the DO modules. Depending on the switching frequency, the breaking energy leads to
additional energy loss in the output module.
If the maximum permissible feedback energy for a module is exceeded, the module shuts
down temporarily.
Note: Feedback energy can be prevented by installing external freewheeling protection. With it, the same
switching rate can be achieved with an inductive load as with a resistive load.
3.8.1 Calculation of Feedback Energy
The feedback energy for a digital output module can be calculated as follows:
P =
Σ
all channels ½ ni * Li * Ii²
where
P
=
Feedback energy of the module
n
i
=
Switching cycles of Channel i in 1/seconds
E
i
=
Feedback energy when shutting off Channel i during a shutdown procedure
I
i
=
Current through the load connected to Channel i
L
i
=
Inductance of the load connected to Channel i
Once the maximum permissible feedback energy E
max
is reached, the module shuts down.
Figure 16: Maximum Feedback Energy Varies with Ambient Temperature