T-Pain Is On A Boat: Re: [Electric Boats] Re: AC vs DC motors

Friday, July 22, 2016

Re: [Electric Boats] Re: AC vs DC motors

WOW! Somebody really knows their stuff! Thanks for the details

On Friday, July 22, 2016 2:15 PM, "'Myles Twete' matwete@comcast.net [electricboats]" <electricboats@yahoogroups.com> wrote:

Practically, 86 vs 92% efficient may not be worth a lot to a lot of us. Why?
Let's say your cruising speed requires 2.5kw to the driveshaft.
Motor loss at 86% efficiency: 407watts
Motor loss at 92% efficiency: 217watts
Difference: 190watts

Is that significant? Absolutely it is. But let's say that you converted an outboard motor to electric and didn't bother going to a larger diameter prop. In that case, your prop efficiency might be in the 40-45% range---spending money for a higher efficiency motor is putting your money in the wrong place given you could reprop (with a lot of effort) and boost that efficiency to 60-70%.

Perhaps the better example to consider: How much slower would I expect the boat speed to be given the same Motor power driving an 86% vs 92% efficiency motor when cruising and say 2.72kw to the motor? The efficient motor will get nom. 2.5kw to the prop while the inefficient motor would deliver approx. 2.34kw, or about 160watts less mechanical power. Since we typically see motor power doubles for each knot of speed, we'd lose 1knot of speed if the power to the motor dropped to 1.25kw. The motor efficiency difference of 6% resulted in 160watts less power to the shaft or only about 1/8^th of the power loss required to slow the boat down by 1knot. Given the non-linear relationship, you'd see less than 0.1knot difference in speed between these 2 motors with exactly the same power applied in this case. You'd have to cruise all day long to see a 1 n-mi difference in range.

To many users, that difference is insignificant given their boating habits (just short jaunts) and other factors. While motor efficiency is important, focus on getting the best hull and propeller efficiencies you can afford first, then look at the next most important factors. If all else fails, slow down ½ knot. J

YMMV-

-MT

From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Friday, July 22, 2016 12:01 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: AC vs DC motors

NAH, not correct.

I looked at one of the mentioned manufacturers, and the actual data from
them was 86% for a good marine app.

((
Marine app wants low rpm, high torque, ie 1800 rpm is much better than
3600 rpm.
An ideal marine motor is 400 rpm to 1000 rpm, with a large diameter prop
(3 sizes up from comparable ice).))

The best, peak, efficiency for one case was 92%, iirc from the mentioned
manufacturers. I was impressed.

Still, Quoted-peak for one case at 92% does not mean, at all, that "they
are all" 93%.
It also does not mean that the system is 92%, for you, installed.

So a quoted-best-case of 86% might actually be about 80-82% for you,
installed, in the real world.
(Contactors create losses. Cabling. Isolation switches. Temperature. etc..)

Technically, anything brushless using a method, any method, of precisely
knowing the angular position of the motor will always be better than
relying on brushes and a fixed "advance".

AC Servo motors for example use an optical encoder, and "twisted"
laminations of fairly small angular section.
The smart part comes from knowing "in advance" where to direct most of
the power, and taking into account current load, lag, and
desired-position via "commanded-position" in the PID loop.
This allows very, very, very accurate tracking of the commanded
position, upto about 10/10.000 counts = 1/1000 of one rev, or less than
0.3 degrees max worst-case error.

On 22/07/2016 08:39, fitloose wrote:
>
> I don't why folk say brushless always beats brushed. It's simply not
> always true. The Saietta, Lynch, AGNI, and all the variants that came
> after Cedric Lynch's motor design are all around 93% efficient. I can
> show you plenty of brushless that get nowhere near that and expensive
> ones like Parker Hannafin that better. Comes down to what you want.
>
>
> John R.

--
-hanermo (cnc designs)