TM 5-811-7
R = 0.032 ohm.
(b) Find the voltage drop in riser anode feeder by
E = IR,
where I = 3.62 amperes and R = 0.032 ohm. Thus,
E = 3.62 A 0.032 ohm
E = 0.116 V.
(c) Find the voltage drop in the riser anode suspension cables for wire size No.2 AWG, 0.159 ohm
per 1000 feet rating, and estimated length 130 feet;
130 ft
0.159 ohm
R'
1000 ft
1000 ft
R = 0.02 ohm.
E = IR
where 1=3.62/2=1.81 amperes average (single current does not flow the full length of the anode string) and
R = 0.02 ohm. Thus,
E = 1.81 A x 0.02 ohm
E = 0.04 V.
(d) Find the voltage drop through riser anodes:
E = IR,
where I = 3.62 amperes and R = 2.63 ohms. Thus,
E = 3.62 A x 2.63 ohms
E = 9.52 V.
(e) Find the total voltage drop in the riser anode circuit:
ET = 0.116V + 0.04V + 9.52V
ET = 9.68 volts.
(21) Select the riser anode circuit variable resistor.
(a) Criteria for the variable resistor are the same as given for the stub anode resistor.
(b) Riser anode circuit data -- rectifier output = 80 volts, anode current = 3.62 amperes, anode
resistance = 2.63 + 0.032+0.02 = 2.68 ohms.
(c) Variable resistor rating (resistor should reduce anode current by one-half when full rectifier
voltage is applied)
r = E/I,
where E = 80 volts and I = 3.62/2 = 1.81 amperes. Thus,
R = 80 V - l.18 A
R = 44.2 ohms.
Resistor ohmic value = 44.2 ohms -2.68 ohms = 41.5 ohms.
Resistor wattage rating = (3.62 A)2 x 41.5 ohms = 543.8 W.
(d) The commercially available resistor that nearest meets the size requirements is a 750-watt, 50-
ohm, 3.87-ampere model. This rheostat is 10 inches in diameter and 3 inches deep and is fairly expensive. It
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