Page 163 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 7.12
F C
A
D
H B
E
∅.52”
7.1 Electrical considerations 151
G
Tether configuration for OCROV.
A—75-ohm mini coax. Cap. 16.6 pf/ft 22-7 TC, XLPO-Foam, Alum,1 spiral T.C. shield B—3 3 3 #18 (19/30) T.C. (OR/BL/PUR)
C—2 3 3 #22 (19/34) T.C. (WH)
D—1 3 3 #24 AWG TP, (19/36) T.C.
E—1 3 3 #24 AWG TSP, (19/36) T.C. polypropylene insulation 0.010 wall
F—MYLAR tape
G—KEVLAR weave
H—0.045 Green cellular foam polyurethane
(Courtesy Outland Technologies.)
submersible. The electrical resistance of the conductors within the tether, especially over longer lengths, could reduce the vehicle power sufficiently during high-load conditions to effect operations.
The maximum tether length for a given power requirement is a function of the size of the con- ductor, the voltage, and the resistance. For example, using a water pipe analogy, there is only a cer- tain amount of water that will flow through a pipeline at a given pressure. The longer the pipe, the higher the internal resistance to movement of the water. As long as the water requirements at the receiving end do not exceed the delivery capacity of the pipe (at a given pressure), the system delivery of water will be adequate. If there were to be a sudden increase in the water requirement (a fire requiring water, everyone watering their lawn simultaneously, etc.), the only way to get ade- quate water to the delivery end would be to increase the pressure or to decrease the resistance (i.e., shorten the pipe length or increase the diameter) of the pipe. The same holds true in electrical terms between tether length, total power required, voltage, and resistance (Figure 7.13).
Ohm’s law deals with the relationship between voltage and current in an ideal conductor. This relationship states that the potential difference (voltage) across an ideal conductor is proportional to the current through it. So, the voltage (V, or universally as E) is equal to the current (I) times the resistance (R). This is stated mathematically as V5IR. Further, power (measured in watts) deliv- ered to a circuit is a product of the voltage and the current.