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Author Topic: IS there a way to produce a "constant Power" powertrain?  (Read 1336 times)

RipSlider

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IS there a way to produce a "constant Power" powertrain?
« on: May 23, 2008, 06:18:47 PM »

Hello all.

A friend and I are thinking about experimenting with some designs for racing boats. He's going to design them and produce the formers, I'm going to - literally - slap a bit of GRP over them, and then we quickly compare and contrast how well they perform.


In order to make comparison easiar, I'd like to take as many variables out of the equation as possible - as we'll be playing with fractions of a percentage in imporvements.

What I'd really like to to have a "standard" power train - i.e standard motor, battery, wire legths etc.

However, I also want to standardise it more than this. I'd like to add a circut that contantly supplies the same amount of power to the motor.

For example, the motor is set to recieve 50 watts of power ( suppious number from the top of my head ) and it never supplies any more or any less than this - whether the prop is in the water, out of the water or any other factors are in play.

I *think* that this will need to involve some form of feedback loop. Does anyone know if such a component exists? if not, does anyone know how I would go about designing such a thing?


Thanks muchly

Steve
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Shipmate60

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Re: IS there a way to produce a "constant Power" powertrain?
« Reply #1 on: May 23, 2008, 06:37:12 PM »

Steve,
That could be a bit self defeating.
Some hulls just might require more power to get on the plane and less when on the plane.
Limiting the power available would reduce the overall effectiveness of the test.
If it was set so high it wouldn't be used there is no point.
OR
If it is set so that some set ups will be OK and others "trip" the preset you cant then get a full appreciation of the hull.
Might not the total duration at or as close as reasonable at full power be a better indication?

Bob
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malcolmfrary

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Re: IS there a way to produce a "constant Power" powertrain?
« Reply #2 on: May 23, 2008, 06:41:15 PM »

Way way back when I had an interest in model railway controls, I came across some controller circuits that basically reacted to the motor rotating by monitoring the back emf pulses from the rotating motor.  These were based on FET input op-amps, and were nothing like what I was interested in, so got dropped from the scheme of things.
The best arrangement would probably be along the lines of having excess voltage available at the battery, and feed the motor through a "constant current" circuit.  These are more properly called "current limiting" circuits.  The thing is, the voltage at the motor will vary with load and speed.  For an active component, a big, knobbly transistor and heatsink would be needed.
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RipSlider

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Re: IS there a way to produce a "constant Power" powertrain?
« Reply #3 on: May 23, 2008, 08:19:00 PM »

Malcom.
Thank you. Well... kind of. Having just had a quite read about the constant current devices, it seems they are not the simplest of things. Grrr.... I *hate* anything to do with transistors - can understan how the u take a +ve and a -ve and some how run them into THREE legs....

hmmm..... what you describe there sounds very similar to the way brushless motors are controlled. I wonder if they would be a bette rplace to start?


Bob.

The hulls I'll be playing with are racing hulls rather than displacement hulls. One of the critical factors which limits speed is the drag from the water - the "perfect world" is just having the prop in the water and nothing else.

Rather than mess about with PIC's and sensors, in my head ( and this may be completely wrong ) an easy way to measure drag is to mount the same power train into multiple  (non-displacement ) hulls and then measure the speed that this fixed amount of power produces. Dividing one by the other will get me an (*exceptionally rough*  analouge to Hull drag. I think.

Or am I wrong?


Thanks guys

Steve
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Shipmate60

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Re: IS there a way to produce a "constant Power" powertrain?
« Reply #4 on: May 23, 2008, 09:51:52 PM »

Steve,
As I see it, it is necessary to use exactly the same power train in each hull.
But even there you could have problems with some hulls favouring a surface piercing or submerged prop.
My problem with your idea is the power consumption.
The way you are looking to test will always give a theoretical figure. This is because you will never know if the current limiter is actually limiting the power to the motor.
Do you intend each hull to weigh exactly the same too, with the same balance point etc.
How will you decide the optimum angle of attack for the prop, or the drag caused by the rudder.
Straight line speed isn't everything either, if the boat spins out on turns it wont be a useful race boat.
This will ensure that you will need almost lab conditions as wind and wave resistance will have to be a constant IE 0. One gust of wind or another models wake will negate the readings.
If you use the actual run times you might not be able to get exact figures per hull, but you will get a comparison between them.
Over several runs this will average itself out.
It is rather like the real ships theoretical speed, but this is always checked against the measured mile BOTH Ways.
In reality it doesn't really matter if one hull is 0.01% more efficient in theory, for fast boats it will only matter how the boat goes in a race.

Bob
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BarryB

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Re: IS there a way to produce a "constant Power" powertrain?
« Reply #5 on: May 28, 2008, 09:59:26 AM »

Hi
As Malcomfrary mentioned earlier, model trains can benefit from a form of "constant power".
I say "a form of" as it is not so much constant power, but an attempt to achieve constant SPEED - a bit like putting your foot down on the accelerator when going up a hill.

Using a PIC, I have succeeded in reading the back EMF in the OFF period of the PWM - any changes detected means my PIC program increases OR decreases the PWM power cycle to maintain equal speed uphill and downhill.
I call this PIC chip: #703
There is also a selectable "flywheel" effect to give the feeling of mass when entering or leaving stations.
I bought a lot of '00' train stuff on eBay just to experiment - I should put it back on again I guess. :-\

Although this item #703 does not feature, my page on speed controls may be of interest.
http://www.kleefeld.freeserve.co.uk/model/barry/pic01/rc_speed.htm

I wrote this message in case it gives RipSlider any food for thought in terms of constant speed rather than constant power.

Barry
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