Pat
Published tests (by Steve Neu of Neutronics) suggest most pwm controllers for brushed motors are about 99% efficient with a 1-2 % drop off at any part throttle setting.
The highest level of motor efficiency will be achieved at 0 degrees timing.
With Brushless the situation is a little more complex. ESC's vary to a greater extent than with brushed speed controllers. Again best efficiency is obtained at 0 degrees timing.
At part throttle the efficiency drop is equal to about part percentage of throttle not used used *.7 - that is to say running at 70% throttle, 30% isn't used, the additional efficiency will drop be approx .7 of 30% = 21% so a motor which at full throttle and 0 degrees timing has peak efficiency of 85% will rin at a peak of 64% with the effect present across the operating range.
ESC's tested showed little variation at 100% throttle replicating the simpler ESC's for brushed motors, however, they degree of varied either side of the percentage given above. The answer to the obvious question as to how to estimate which would vary most, in 2006, seemed to be the more you paid the better it got.
In short the losses here are minimal.
When changing props all you need to asses is the current drawn and the voltage on the cells and boat speed over the ground by plotting these variables you will find some props achioeve more at lower speed (berthing) some at mid or higher speed cruise or peak.
As motor current drops as the rpm rise you can achieve considerable gains in economy by propping so that at 100% wot brushed motors turn at 85-87% of peak (perhaps up to 91%) rpm given by the actual voltage; Brushless 92-93%. Unfortunately brushed motors have a narrower power peak and the efficiency drops away from that peak more rapidly the further you run away from it in terms of rpm or amp draw.
The optimum system will run the motor at 100% throttle, constant rpm and vary the load to the motor using a gearbox or CVT.
Andrew Gilchrist
www.fastelectrics.com
Hi Geoff,
Perhaps there are some rules of thumb someone can give you, but PWM (pulse width modulation) creates complex signals that are difficult to measure. True RMS meters might give you voltage and current values that you can multiply together to get a feel for power, but like measuring regular sine wave AC, the subject of power factor will crop up due to inductance in the motor windings. I don't believe there are any simple forumlas that will give you accurate data. However, I am sure there is high end test equipment that can give you what you want by sampling instantaneous voltage at current over a period of time. However, I bet that equipment is expensive. I do not have any experience in that area. Can anyone else give Geoff some help?
Pat
--- In electricboats@yahoogroups.com, Geoff Vaizey <g.vaizey@...> wrote:
>
> Pat
> Â
> Thank you for such a prompt reply. What I am trying to achieve is a better idea of how much power the prop is absorbing rather than just measuring what is being used from the batteries. I have started logging watts/rpm/speed etc. to build up a series of curves for various prop sizes but have no idea yet how much is being lost through the controller, gearbox, propshaft etc. If I fit an ammeter and voltmeter to the motor terminals, is there a correction factor which will give a reasonable indication, the figure of 1.11 has been suggested but which way, up or down?
> Â
> Geoff V
>
>
> ________________________________
> From: greenpjs04 <forums@...>
> To: electricboats@yahoogroups.com
> Sent: Wednesday, 25 July 2012, 16:42
> Subject: [Electric Boats] Re: PWM output measuring
>
>
>
> Â
>
> Hi Geoff,
> You are correct that measuring at that point in the circuit is difficult. Most multimeters set to AC assume a nice clean sine wave when displaying voltage or current. Some more expensive units (eg, Fluke) have "True RMS" capabilities and do much better. However, when using PWM, what voltage or current do you want to measure? Peak? Average? If average, over what period is the average taken? When trying to diagnose a problem, you need to be able to see the waveform and an oscilloscope is needed to really know what is going on.
>
> Pat
>
> --- In mailto:electricboats%40yahoogroups.com, Geoff Vaizey <g.vaizey@> wrote:
> >
> > Gentlemen
> > Â
> > I have recently completed a small electric powered catamaran and wish to take measurements of the power consumed by the motor as well as monitoring the battery consumption with a NASA marine BM1. The motor speed is controlled by a PWM unit and I am concerned that the square pulse form of the output will give incorrect meter readings, can anybody in the group help me with correction factors?
> > Â
> > My thanks in anticipation.
> > Â
> > Geoff V
> >
>
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