Re: [PSUBS-MAILIST] More Batteries....power configuration?

[Jari Siikarla <jari.siikarla@vertex.fi>]

At 18:10 29.4.2003 -0700, Warren Greenway wrote:
>There is a huge benefit to running your thrusters at
>a higher voltage, as any electric car buf can attest!
>Coupling efficiency in motors drops as the voltage
>drops, due to the very nature of electro magnetic
>fields. We aren't talking about little changes either.
>I think Baldor, or some other similar manufacturer of
>DC motors has a published chart to demonstrate this.

Sorry if this is common knowledge, but here goes.
The major benefit in using higher voltages is the drop in
current with the same wattage.

When power is equal to current times voltage (P=U*I)
and voltage over resistor (cabling, motor wounds, lamps, etc)
is U=R*I. This is also called "voltage drop" when discussing
power lost when transferring it via cabling
(look at solar power websites for awg and voltages they use)

So replacing the U in P=U*I with equation U=R*I we get
P=R*I*I. This is the power lost (to heat) in cabling and other
resistive loads.

So doubling your voltage halves your current and drops
power losses to 4th part. You also lose smaller part of
voltage on cables so you have higher voltages left at the
end of the cable.

This is a major issue in ROV where power is sent via
cables from surface to deep seas. They use very high
voltages and convert them to more manageable voltages
at the ROV or on launching platforms. They usually
use AC for easier handling.

Jari Siikarla
Ps. When calculating losses on cabling, remember to count
"coming" and "going" cables ie. twice the distance of cable :(