Endress+Hauser PROline promass 83 II2G Manual page 57

PROline Promass 83 FOUNDATION Fieldbus
Endress+Hauser
In the example the calculation formula results in the following segment current con-
sumption I , where I
SEG FDE
additional current in the event of a fault and the startup current is less than the bus cur-
rent:
6 I
I = + max. I
SEG B
= 45 mA + 0 mA + 9 mA + 0 mA
= 54 mA
The next step checks whether the current consumption I
rent limits of the fieldbus power supply. The maximum available supply current I
calculated taking into consideration the current drops caused by the source impedance
R of the fieldbus power supply and then by the effective resistance per unit of length
Q
of the fieldbus line. Also, the condition must be met that the supply voltage at the fur-
thest field device must be at least 9 V. To make the overall considerations easier it is
assumed that all the field devices FD (refer to Fig. 9) are connected at the end of the
fieldbus line.
Fig. 21: Replacement circuit diagram
In order that the available supply current I
the fieldbus line R must first be calculated:
cable
R = R x L
cable WK
44 : /km x 0.615 km
=
27 :
=
Calculation of the maximum available supply current I
When calculating the maximum available supply current it must be ensured that a min-
imum supply voltage of 9 V is applied at the field devices:
I = (U - 9 V) / (R
Seff. S
(18.4 V - 9 V) / (105 : + 27 : )
=
= 71.2 mA
Observing the conditions stated, a supply current of I
field devices on the bus segment. Since the actual current consumption of the segment
is only 54 mA, from the point of view of the functional considerations proper functioning
of the segment is ensured.
For reasons of satety the actual voltage U
culated as follows:
U = U - I
FG eff. S SEG
18.4 V - 54 mA x (105 : + 27 : )
=
= 18.4 V - 7.1 V
= 11.2 V
= 0, I = 0, since the field devices used do not consume
startup
+ I + I
FDE MOD startup
can be calculated the resistance value for
Seff.
SEG
+ R )
Q cable
FG eff.
x (R + R )
Q cable
calculated is within the cur-
SEG
of the fieldbus power supply:
Seff.
= 71.2 mA is available for the
Seff.
of the furthest field device can be cal-
0
<
is
Seff.
21