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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