I would disagree with the assertion that rpm does not drop on a PM DC motor with variable loads.
I have a handheld rpm guage, and I think I checked the rpm with and without the motor linked to the prop and it was quite different. That was a while ago, and I would be willing to double check that fact if someone were really interested.
Hans
--- On Tue, 7/28/09, Chris Baker <chris@currentsunshine.com> wrote:
From: Chris Baker <chris@currentsunshine.com>
Subject: Re: [Electric Boats] Re: Abusive and dangerous Re-Epower technical specification
To: electricboats@yahoogroups.com
Date: Tuesday, July 28, 2009, 5:01 PM
Hi Redu,
You touch on an important issue here, and it doesn't seem to be
mentioned much.
That is that RPM of a PM DC motor is proportional to volts, and the
converse, that by measuring volts, you can deduce the RPM.
And its this point that is critical for our use of electric motors on
boats.
What follows is based on my observations from recent adventures with
an electric motors, and I'm really not expert at this. So I'd welcome
commentary from those better informed than me.
I often see the idea promoted that dc motors are great because they
have maximum power available from zero revs, and throughout their
range. But in practise this turns out not to be so, and it seems to
be because of this relationship between volts and RPM.
Lets say the motor is running at 1 tenth of its maximum revs, and
using this rule of proportions, the voltage at this RPM is also 1
tenth of its maximum. So to get anywhere near its rated power, the
amps that its drawing would have to be ten times the amps that would
be needed for its rated power.
To put some real figures on it, say the motor is rated at 4kw, and at
50 volts, that would mean its drawing 80 amps. (thats 80 amps x 50
volts = 4000 watts). Say the maximum revs of the motor is 2000 rpm.
So now we can look at what happens when the motor is running at 200
RPM. Its voltage is 5, and the amps its needs to provide 4kw of
power is 800. And the reality is that the cables, fusing,
controller, brushes and so on are not rated at that, and so it simply
doesn't happen. The controller will probably limit the current to
some preset maximum, say 200 amps, and even that will only last a
short time. But even at 200 amps (lots of heat in the wires etc) the
power is back to 200x5 = 1kw.
(I wonder if the motor was directly connected to the batteries, and
not through a PWM controller, and therefore 50 volts was available
and fed to the motor, whether it would actually be capable of full
power throughout its range?)
Wouldn't it be then that the real power curve for an PM DC motor is a
straight line, starting at zero power at zero revs, and ending at
peak power at peak revs?
On the other hand, the power curve of a diesel or petrol engine
usually peaks at less than maximum revs, and so its seems that more
of its power is practically available in the mid rev range.
And thinking about props, If the prop is large, and you don't get
much slip, the dc motor could be easily overloaded at low revs, and
never be able to get the boat up to its speed potential, and never be
able to use that maximum power that is only available at maximum
revs. If the prop is relatively smaller diameter, and allows more
slip, it would then allow the motor to spin faster, and thereby get
more of the motor's potential power converted to thrust.
The combination of these factors mean its critical to have the prop
well matched to the motor.
Cheers
Chris
http://www.currents unshine.com
On 29/07/2009, at 5:57 AM, redu wrote:
>
> The key feature of any PM DC motor is, that RPM is directly
> proportional
> to supply voltage:
> - You can turn throttle to maximum without any load, and RPM = k *
> voltage. Note: No excessive revs possible! Note: Knowing this "k", you
> can see motor RPM directly from input voltage reading.
> - If there is a heavy load, you still get RPM = k * voltage.
> This means that you can increase HP simply by loading the motor
> more and
> more. Input current figure only increases accordingly. And input power
> to motor is voltage * current.
>
> Indeed, it is possible to load more heavily for instance by changing a
> gear ratio or by increasing the propeller pich,
>
> In practice there is an input ampere limit, as coils inside motor
> start
> to get heated due to their ohmic resistance, if too high current
> figures.
>
> redu
>
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