Amps Drawn

[John Gallard]

I have just used a wattmeter which gave a reading of 16 amps at 11.4 volts (182 watts) how relevant is this reading considering the plane is straining at the leash to break free! Surely, being tethered is the greatest load the battery will meet, a reading from a flying model would be a lot more useful.

[birdy]

You are correct but only to an extent. The difference will only be an amp or two. The reason for this is because it is like a car spinning its tires; it cant get a "purchase" on the ground. The tirers will only need slightly less power to spin at a higher speed the it will at low speed.

I admit I am rubbish at explaning things, birdy.

[Chris Bott - Moderator]

Having measured thes things in flight, it is surprising just how much the system does "unload" in the air. Its hard to put a figure on because every model is different And the graphs I have, don't show which way the model was pointing when full power was in use (usually upwards!) The reading is very useful though to prove that in worst case situation (except going backwards or motor stalled) the motor battery and ESC are all working inside safe limits. What surprised me more than the unloading however, is the average current used over a whole flight. Unless you have to hold full throttle for the whole flight, then the average current is always less than full power current (obviously), but in my experience it is much less that full power current. This is due to using full power for impressive verticals, then using the models inertia or height to complete down and horizontal parts of a manoeuvre. It is the average current that determines flight time. Without on board measurement, another way to work out max flight time is to fly for a few minutes, land and charge. Check how much charge you put back in, and you can work out how much more flying time you could have. On a typical Wot4 flight (not mine this time) I have a graph showing: PACK AMPS Min=0.0 Max=44.28 Ave =15.87 POWER Min=0.0 Max=850.18 Ave=281.31 The Max would have been stationary on the grass just as throttle is whacked to full and the model starts to move. To conclude, static measurements are very important, but not the whole picture.

[Former Member]

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[Chris Bott - Moderator]

And indeed motor too if you have a 30 amp motor and yr drawing 29 amps....

[John Bunting]

In-flight power monitoring is great - if you can afford it and be bothered with it! - but a cheap and versatile method of measuring current that I've used is to make up a .001 ohm resistor, fitted in series with your battery, and measure the voltage across it when current is flowing, using a digital multi-meter (£5 or less, from Maplin) on the milli-volts range: thus a current of 10 amps would give a reading of 10 millivolts. 

The resistor is just a short length of thick copper wire, with tapping points of thinner wire soldered on, at the correct distance apart to give .001 ohms between them. I worked this out a few years ago, using several different sizes of wire, in series with a good quality ammeter as a standard, then plotting a graph and deriving an expression relating tapping-point spacing to wire thickness. As the resistance is so low, it could form part of the wiring in a model, with the tapping points accessible from the outside, so you can see the effect of using different props etc. 

I'll dig out the figures and post them here in a day or two, in case anyone is interested.

[John Gallard]

All very interesting. Re the speedo working harder at half throttle i understand that it is switching faster but am I right in saying this is not generating any heat ie it is still drawing lower amps?

[Chris Bott - Moderator]

That's an excellent idea John. Full marks for that one. I think I'd want to calibrate anything like that if I made one, and this would mean the expense of an ammeter anyway... Another easy way is to purchase a purpose built wattmeter. This one here is about £20 with postage and even has a built in servo tester so you can operate your speed controller without any radio gear. http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6553

[John Bunting]

Thanks, Chris.

Luckily I had a decent ammeter for calibration, but many digital multimeters have a current range reading up to 10 amps; maybe not highly accurate, but probably good enough for this purpose.

Here's part of an article I sent to 'Electric Flight UK', the magazine of the British Electric Flight Association (Issue No. 63, Autumn 2000).

 We need to know the tapping point spacing as a function of the wire thickness being used for the shunt. In my scrap box, I found samples of half a dozen different sizes of single-strand copper wire and connected them, one at a time, into a simple circuit which passed a steady current of 3.0 Amps through the sample. The meter leads were fitted with miniature hook clips and these were clipped on to the sample and the spacing adjusted to give a reading of 3.0 millivolts. The distance between the clips was then measured. The thickness of each wire sample was also measured with a micrometer.

For a given resistance, 0.001 Ohms in this case, the length of a wire is proportional to its cross-sectional area which, in turn, is proportional to the square of the thickness. So the tapping point spacing, for any wire thickness, is given by the expression S=kt2 , where S is the spacing, t is the wire thickness and k is a constant which was found by dividing each value of S by the corresponding value of t2 and taking the mean of the results. The value of k obtained is 1117 if you are working in inches, or 440 if in metric. For example, suppose your wire is 0.048 inches thick; squaring this gives 0.0023 and multiplying by 1117 gives you the tapping point spacing, 2.57 inches. For the same wire, but working in metric, the thickness will be 0.1219cm, so t2 = 0.0149 and multiplying by 440 gives a spacing of 6.5cm, the metric equivalent of 2.57 inches.

Cut your shunt wire about an inch longer than the spacing distance, to allow half an inch at each end for the battery and motor connections. Also avoid large blobs of solder when attaching the tapping point wires.

As well as being useful on the test bench, a shunt could also be installed in a model as part of the wiring, making the tapping points accessible on the side of the fuselage. This would make it easy to compare motor currents when trying different propellers or making other changes to the system.

[Former Member]

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[John Gallard]

Eric

 Thanks for the explanation, think I'll just go out and fly and stop contemplating my navel!

John

[Chris Bott - Moderator]

That sounds like good advice for all of us