Ted,

There was another page somewhere with brush life vs input current. Its basically
an inverse exponential, which is why the motor is rated at 9HP at 50V. It is
capable of putting out higher power outputs, or of putting out 9HP at a lower
voltage, but the brush life drops dramatically at the higher currents. Looking
at the 48V chart, it appears that the motor makes 9HP at about 155A. Now looking
at the bottom of the chart is says that the brush life at 150A continuous is 500
hours. Now if we drop back to 5HP at 48V, we need about 90A. The chart says
that the brush life at 100A continuous is 2000 hours. That is about the same as the
TBO of a gasoline engine.

Another interesting feature of this motor is that the brushes are removable
as snap out unit, and can be changed in about a minute according to the literature.

A Yamaha 9.9hp is capable of making about ten HP on the max throttle setting,
but I almost never run it wide open because it makes too much noise, and things
start vibrating. Also, the higher power settings just result in the nose of the
boat rising further in the air, and the outboard trying to bury itself in the
water with an increase of only a knot or so. I'd bet that the average power
setting is more like 4-5HP.

So, for a _day_ sailer, with 15 minutes of motoring to get out of the slip, and
another 15 minutes of motoring to put it away, it appears you would need about
45AH at 48V, (180AH of 12V), X 3 for battery life, or 4 ~120AH batteries. As you
noted that is a BIG house bank. Also, as you noted, you are discharging them
at a pretty fast rate during the discharge cycle. However, if you have a DC genset
that is capable of producing 3-5KW, you can start the genset, and use it to power
the motor continuously. Also, with that big house bank, and a 3-5KW inverter you
can have some pretty snazzy AC appliances on the boat meaning that you can do
away with the alcohol stove, and power a microwave and an AC fridge. If you run
the genset, you can probably even power a small AC unit some of the time.

Oh, and another nice feature is that the motor can be driven as a generator by
the prop while you are sailing, although you may need a pretty fancy switcher
circuit to take advantage of the variable voltage.

Of course, all of this is going to weigh more than a 99Lb outboard, but you
hever get something for nothing.

Just thinking online,

Don W.



Ted wrote:
On Mon, 30 May 2005 15:35:59 -0700, Keith Hughes
wrote:


To get 15 Hp at 24 vdc assuming no losses, you would need 466.25 amps. The
motor is rated at a maximum motor current of 330 amps for 60 sec. You might
get the equivalent of 15 Hp for 5 seconds before it was toasted.

8 Hp would require 248.67 amps continuous at 24 vdc. brush life for that
current rating isn't given. What would it be? I don't know, maybe 24 hours
or less. It appears that brush life drops exponentially with current

Take another look at the page. The charts are for the same 50VDC motor
operated at three different voltages. The charts basically give you the
performance characteristics at the stated voltage - you can't
extrapolate from them.


Ahhh, thanks. What threw me were the same Hp readings for all 3. I thought I
was looking at 3 different motors.

Normally, when a motor specification is given (such as for say an MD408), it
is given ratings at 250 VDC. I thought I was looking at 3 motor specs. I
only gave it a quick look and assumed it would was like all the other motor
specs I have read but for different motors. Didn't realize it was the same
motor. The article was a far different method of presenting motor specs than
those by GE, Westinghouse, Reliance, etc. I am accustomed to reading
industrial motor specs and they all follow a similar format.

For instance older older MD type motors are ususally rated at 250 VDC. You
have to know they are rated to withstand 500 VDC and extrapolate the Hp and
RPM at 500 VDC.

Today, motors are rated typically at 500 VDC. The typical AC source to the
drive is 3 phase 460 VAC at 60Hz. Now when you go overseas to say China,
their standard voltage is 380 VAC at 50Hz. You won't get much over 400 VDC
from a 380 VAC source. In this case, we take the standard motor speed and KW
ratings and derate them for the 380 VAC source. What might require a
standard 50 Hp motor for an application here may require a standard 75 Hp
motor in China.

Older CD motors are rated at 250 VDC and can't commutate 500 VDC. They are
what they are and you can't exceed the specs.

Same thing applies to the type of AC motors. To place an AC motor on an
IGBPT drive, the motor has to be able to withstand the PWM power to the
drive. Unless the motor is built to withstand variable frequency and voltage
by Pulse Width Modulation, you can heat one up real fast. VM Star is a great
Reliance built motor designed for that pupose.

My error. Thanks for clearing it up. It had me going around in circles for a
bit.

My warmest regards,
Ted