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12-Volt versus 24-Volt Electric Systems
, September 2011 . . .
Hi Steve,
I enjoyed your article in
The Masthead.
A simple rule for wire size dictated by voltage drop is that for the same power de
ered, the wire cross section goes down by the inverse square of the voltage. That makes a huge case for the 42V (36V)
I found a couple of things you said to be factually incorrect. I know you are accustomed to writing for a broader audienc
where the technical details are not as important as the conclusion, but here it might hurt a bit.
page 18
First, Ohms law applies to resistive circuits, but it does not give guidance in the case of the bilge pump where a differen
motor is required to mate a specific pump to a different voltage. You have implied that a 12V pump can simply be con-
nected to 24 Volts and the current will go down as a result of Ohm's law, but that isn't the case. In fact the current will
up, the device will try to run at a much higher speed and it will burn out. (You can use the alternate lower current motor
because of conservation of energy, not Ohm's law.)
There is a lot of mythology out there about motors where folks have generalized how one motor behaves to all motors.
motors have a property that as the voltage goes up, the current goes down (over a limited range). AC motors tend to ru
near constant speed, with the speed dictated by line frequency. That means in turn that they deliver nearly constant
power by running their load at a nearly constant speed. With the same power delivered from a higher voltage, less curr
is required.
DC motors do not run at constant speed and most motors will speed up as voltage is increased and they will draw more
power and more current as a higher voltage is supplied to the same motor.
page 20
Halogen and incandescent lamps do not run cooler and last longer at higher voltage. As the voltage is made higher, th
bulb must be designed with a finer filament wire. There is a constant erosion of the heated filament that contributes to
lamp burnout, so higher voltage lamps tend to last less long. Sometimes to compensate the lamps are designed to run
a lower temperature, sacrificing efficiency to recover the lost life, but seldom to make the life longer than the low volta
brethren. The reason the automotive industry did not adopt the "42V" systems is that ordinary lamps could not be mad
to the shock requirements at the higher voltage or deliver satisfactory life. Now that we have discharge lamps and LE
lamps, the preference for 6 or 12 Volts for lighting vs. 24 or 36Volts is not longer holding.
Otherwise I enjoyed your article and it agrees with what a number of us have been saying for years.
I hope to see you at IBEX!
Best regards,
Wayne E. Kelsoe, PE
VP of Electrical Engineering
Chief Technology Officer
Blue Sea Systems
425 Sequoia Drive
Bellingham, WA 98226
Do you have a question or comment regarding
The Masthead
or anything related to boats? E-Mail
these to Norm Nudelman at
. We will post your question in the next issue
(space permitting) and we will do our best to answer you.
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