Electric motors and battery combinations...>>>

"Marty S." wrote in message
...


I would like to know if I
get longer run time and/or more power with a 50lb 12volt motor or a
similar
sized 24volt motor? Or, would I get a better weight-power-endurance
combination getting a 36volt, 100+ lb motor??

The full answer is not so simple. A partial answer that makes a bunch of
assumptions that will be correct in most cases would be that the higher
voltage system will be a lot more efficient.

If you look just at the motor, the ability to turn electrical power into
torque is a function of the number of amps flowing through the wire and the
number of turns that the wire makes around the motor pole. If you have the
luxury of allowing a large physical size on the motor then you can trade off
number of turns for amps and use just about any voltage you want. When you
want the motor to be compact, it is usually easier to have a low voltage,
low turns, high amperage design.

If the motor was the only consideration in the equation that would be the
end of the disscussion. Unfortunately, you also need to consider the wiring
to the motor, the controls and the batteries that feed the motor. All of
these items favor lower current, higher voltage designs. While the physics
of the motor can trade off voltage for current, the losses incurred in the
wiring and controls will be effected only by the current, and will have no
"trade off". If you double the current, then you need to double the wire
size or you will lose twice as much energy into heating the wire.

The biggest single impact on efficiency will be the rate at which current is
drawn from the battery. Deep cycle batteries are typically rated at a "20
hour" rate or lower. At a higher draw, the battery will supply less. A
typical battery, say a "Trojan" 27 series, is rater at 105 amp-hours at the
20 hour rate (5.25 amps for 20 hours). If you increase the current demand up
to the 5 hour rate you will only get 79 amp-hours (this information is from
the Trojan Battery web site). In general, the higher the current, the less
efficient the battery will be.

For maximum run time you will want the lowest current draw, hence the higher
voltage system.




Also, would I get more run time from 2 batteries (1 to run, and one as a
spare, i.e., "refuel") with a 12volt motor, or 2 batteries running
simultaneously on a 24volt motor?

Again, maximum run time will occur when the current that the battery sees is
at a minimum. You will get a lot more run time if you run two batteries in
parallel than if you ran one until it was dead and then switched to the
other.


Rod McInnis