Friday, May 20, 2011

RE: [Electric Boats] High efficiency at low rpm AND low power

 

Almost….the X-axis is Torque in units of Inch-Pounds.  With permanent magnet motors, the torque and current are essentially proportional (note the line relationship) and so you can confirm this.  Also you can calculate Kt and Kb from that line.

 

Kt = dT/dI ~ 200 in-lb / 180 amps = 1.11 in-lb/amp

Convert that to N-m/amp and you then also have Kb in units of volts/rad/sec.

 

-mt

 

From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of F Neil Simms
Sent: Friday, May 20, 2011 6:47 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] High efficiency at low rpm AND low power

 

 

Can you guys confirm I'm reading the performance charts on the bigger motors properly?

I am interpreting them to indicate that, even if I run them at as high a motor rpm as possible (ie, with reduction that allows that), at low power/current levels the efficiency will be very low.

Here's the chart for a Mars ME0708:

http://www.cloudelectric.com/v/vspfiles/assets/images/me0708data.pdf

The plot for efficiency starts at 10% and doesn't approach 90% until halfway across the chart.

And I'm assuming the X-axis on the chart is amps (why don't they label it)?

I knew I should have become an electrical engineer rather than a software geek :)...

--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> John,
>
> Good info. However, Neil want's to be able to run at 8.5kts with the same motor, batteries and prop as well as long distance. Like you, I can give advice on how to achieve either one of those goals, but not both with the same system.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>

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