Motor efficiency is determined by a whole bunch of factors, some of which impact on low power performance, some of which impact on performance at any power level and some of which only impact on high power performance.
What I did was choose a motor with a high power rating, but with losses that were very low at low power levels, which gave me the efficiency I was looking for.
Broadly speaking, brushless motor losses are made up of the following elements:
- Frictional loss in bearings and seals, a fairly fixed loss that is there across the power range. Picking a motor with the smallest diameter shaft commensurate with the torque delivery requirement will minimise these losses.
- Resistive loss in the motor windings, wiring, controller and battery internal resistance. The lower the motor winding resistance the lower the losses. Higher current rated motors will generally have a lower resistance than those with a lower current rating.
- Eddy current loss in motor magnetic material. This loss is proportional to the frequency with which the magnetic field reverses, so proportional to rpm. It tends to be pretty low at normal operating rpm, but can rise to high levels at high rpm with a high pole-pair count motor.
- Windage loss. This comes from the aerodynamic drag of the rotating parts and is higher for a large diameter motor and proportional to the square of rpm, so increases rapidly as rpm increases.
In my case I chose a motor very carefully to match the needs of my boat system. I used a model aircraft BLDC outrunner motor, rewired from delta to wye to reduce circulating current losses and also to lower Kv. I chose one with a 63mm rotor inside diameter, for low windage losses. It has a small 8mm diameter shaft, which reduces bearing frictional losses. It has a maximum rpm of around 7000 to 8000, but I run it at a maximum of 1500rpm, which both reduces windage losses and also losses from from eddy currents. The maximum current for this motor is around 50 to 60 amps, but I run it at between 10 and 20 amps, which keeps resistive losses low.
It's an oddball system, optimised for running a low speed riverboat, so it doesn't scale well, but it does show that if you take the time to choose carefully you can squeeze a fair bit of performance from an electric drive system, accepting the limitations we have with energy storage.
Jeremy
--- In electricboats@yahoogroups.com, "fneilss" <fneilss@...> wrote:
>
> Hey Jeremy,
>
> Thanks for your info.
>
> Regarding oversized motors, I'm trying to figure out how that's good for efficiency at low power settings.
>
> For example, the Mars ME0909 I'm looking at is rated 4.8kw continuous, and 90+% efficiency, but you have to be running at a rate of at least 500 watts to achieve that efficiency. At 250 watts it drops to something like 50%.
>
> I too am looking for high efficiency at low power settings (as low as 100 watts for my new long and skinny build), so I'm curious what motor you're using that runs at such high efficiencies and low power as you report.
>
> Thanks!
>
> Neil S.
>
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