Environmental Engineering Reference
In-Depth Information
feeding the tail lamps in an automobile would experience up to a 5% voltage droop
under steady state loading. Depending on the chassis return path integrity, it is
possible for this value to increase with ageing.
As an example, suppose the tail lamps require 40 W of power to be delivered at
a load voltage of 13.5 V
dc
. In order to meet a less than 5% line drop, the source
voltage (i.e. alternator regulated output voltage) must be 14.2 V
dc
. The harness
resistance is thereby constrained to be less than the value given in (8.17):
1
h
R
c
¼
R
L
h
ð
8
:
17
Þ
V
L
2
P
L
1
h
R
c
¼
h
According to (8.17) for the example noted, the cable resistance would have to
be less than 0.24
W
.
Example 1:
Electrical distribution system in a vehicle is sized so that voltage drops
at load points do not introduce more than a 5% droop. Calculate the maximum
branch wire resistance for each of the following electrical loads given a power law
relationship for load power sensitivity to system voltage of 13.5, 14.2 and 15.1 V:
a
U
2
U
1
P
2
¼
P
1
(a) Headlamps:
P
1
= 120 W;
U
1
= 12.8 V and
a
= 1.6
(b) HVAC blower fan on high setting:
P
1
= 300 W;
U
1
= 13.5 V;
a
= 2.4
(cage fan)
(c) Cooling fan motor:
P
1
= 600 W;
U
1
= 13.5 V;
a
= 2.6 (axial flow fan)
Solution:
All the calculations of branch wire resistance will assume a line drop of
5% voltage. For the three cases of system voltage stated, Table 8.6 summarizes the
interim calculations and wire resistance maximums for headlamps (a). The reader is
requested to apply the same methodology to (b) and (c) using the stated power law.
Table 8.6 Solution for headlamps in Example 1
System voltage (V)
13.5
14.2
15.1
Load power (W)
120.37
130.52
144
Load current (A)
9.386
9.675
10.04
Wire resistance (m
W
)
71.9
73.4
75.2
The branch circuit wire must therefore be sized for the lowest system voltage
in order to meet the criteria of
<
5% voltage loss.
