MIL-HDBK-1004/10
(3)
Variable resistor rating:
R = E/I
where
E
=
80 V
I
=
3.62/2 = 1.81 A
R
=
80/1.81
R
=
44.2 ohms
Ohmic value of resistor = 44.2 - 2.68 = 41.5 ohms.
Wattage rating of resistor = (3.62) x 41.5 = 543.8 W
(Resistor should reduce anode current by one-half when fullrectifier voltage
is applied.)
The nearest commercially available resistor size that meets the
above requirements is a 750-W, 50-ohm, 3.87-A resistor. This rheostat is 10
inches in diameter and 3 inches in depth, and fairly expensive. This rheostat
will not fit into most rectifier cases. In addition, the power consumed by
the rheostat is considerable. This power creates substantial heat that may
damage components within the rectifier case unless adequate ventilation is
provided. The problems associated with using a large rheostat can be
eliminated by using a separate rectifier for the riser anodes. Although
initial cost may be slightly high, power savings will be substantial and
damage by heat will be avoided.
v)
Sizing rectifier for riser:
(1)
Requirements:
dc current output = 3.62 A
Anode circuit resistance = 2.68 ohms
dc voltage required = IR = 3.62 x 2.68 E = 9.70 V
(2) Rectifier rating.
Standard ratings for a rectifier in
this size class are 18 V, 4 A.
w) Rectifier dc rating for bowl. Voltage output as previously
determined, 80 V. Current rating is 15.2 A. The nearest commercially
available rectifier meeting the above requirements is 80 V, 16 A.
x) Wire sizes and types. All positive feeder and suspension
cables (rectifier to anodes) must be No. 2 AWG, HMWPE insulated copper cable.
To avoid complication, the negative rectifier cable (rectifier to structure)
must be the same size and type (see Figure 102).
y)
Discussion of the design:
(1) The design points out the disadvantages of achieving
corrosion control through cathodic protection without the aid of a protective
coating. When the interior of a tank is coated, the current requirement is
171
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