Hi, Eric, interesting info, including re belt efficiency.
I am now confused - I had some preconceived notions that it seems might
be incorrect:
Is it correct that any DC motor generally has the speed controlled by
lowering the volts?
Surely the 'given power' will be less, as the purpose of the lower volts
is to reduce the speed, therefore the amps might well remain at the same
level?
Can I assume that the numbers on this are not proportional, that the
speed will reduce more than 50% if the volts are halved?
John
2b. Re: Pros and Cons of Lower Voltage
Posted by: "Eric" ewdysar@yahoo.com ewdysar
electricboats@yahoogroups.com
Date: Sun Apr 10, 2011 9:28 am ((PDT))
Jak,
It's kind of hard to answer the question without knowing your
installation and how you expect to use the boat, but I'll make some
broad generalizations. From my perspective, there are few Pros and many
Cons.
The most obvious answer is that halving the volts, doubles the amps for
any given power. What kind of power are you looking to get from your
drive? If you're looking for only 2.5kW, then at 24V your wiring, fuses
etc. only need to handle 100 - 120A. But if your boat needs 5kW (7+hp)
then your electrical system will need to be sized for at least 225A.
Bigger wires are heavier and more expensive. Higher amp loads have a
higher potential for voltage drop in the wiring.
If you're planning on using flooded batteries, those kind of amp loads
will push you deep into Peukert's losses. Even at lower power settings,
you'll find that your battery range will be much less than you expect,
higher amp draws (regardless of the voltage) lose energy to heat as a
symptom of the battery's internal resistance. You can mitigate this by
running a larger battery bank or chosing AGM batteries. AGM batteries
have a lower internal resistance than flooded batteries, and Lithium
batteries are better still.
Aa far as just the difference of efficiency in the motor itself, you'll
find that the same motor at half the voltage will create more heat for
the same power setting (double the amps) so you'll need to address
supplemental cooling, especially at low rpm where the motor's internal
fan is much less effective.
Finally, modern cogged belts, as opposed to common smooth v-belts, are
more efficient than you might think. Some products are more than 95%
efficient at transmitting power, so your loss would be minimal.
For this type of question, I tend to favor higher voltage and I'm pretty
sure that the belted drive will probably outperform the lower votage
direct drive with that motor. But I could be wrong.
Fair winds,
Eric
1964 Cheoy Lee Bermuda 30 ketch - Serenity
5.5kW Propulsion Marine drive, 8kWh LiFePO4 battery bank
Marina del Rey, CA
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