|Jan 06, 2002, 06:08 AM|
Joined Jun 2001
Aeronutz DC/DC Converter for a 1 Amp motor toy plane.
A converter works on Watts, electrical power. It is not a magic machine, it uses power to convert voltages. This is seen as its efficiency. Here is an example of a converter with a 1 Amp output and 5v running at 90 percent efficiency, we just calculate the input Watts thus 100/90 = 1.11 x 5 Watts = 5.55 Watts. You can check by calculation back again, 5.55 Watts x 90 per cent efficiency = 5 Watts.
Then Watts = Volts x Amps.
Tip!!! donít ask how the convert actually works!
Input from cells Output to motor
5.55 Watts 90% efficient converter 5 Watts=5v x 1 Amps
The voltage of the cells varies, but for example 5.55 Watts would be:
2.5v and 2.22 Amps
1.8v and 3.08 Amps
1v and 5.55 Amps
So, itís a simple fact that as the voltage of the cells drops through the flight, the current consumed goes up. When the cells are flat, the current it very high - not what a flat battery wants!!
Using 5 Watts for output power you need big capacity cells too otherwise 50mA cells wonít last long, therefore you have heavier cells and a heavier plane, which requires more flight power. which would probably require servos - which are heavier AND use 120 mA of power! Doh !!
So, from the five Watt model we can see we need to have a battery with higher voltage and a motor with lower voltage lets try it and see how it looks.
One Amp. Model revision A
Input from cells Output to motor
4.44 Watts 90 % efficiency converter 4 Watts = 4v x 1 Amp
4.44 Watts is:
3v and 1.48Apms
2.5v and 1.77 Amps
1.8v and 2.46 Amps
1v and 4.44 Amps
That looks a bit better. If we use a Lithium Ion or Polymer cell it has a higher voltage than a NiCD.
But can they take high pulsed loads? Lithium cells canít go completely flat like Ni CDís.
If we try and design a plane using the 4 Watt model above what would the plane be like?
Well I guess it would be like a geared Kenway on four cells. 1.2v x 4 = 4.8v x 0.83 Amp = 4 Watts.
This is similar to my Albatross, which flew very well. Me thinks - why bother with a DC/DC converter then! The four cells Albatross worked ok, if it ainít broke donít fix it!
Designing like this is cheap and quick, so lets try a smaller plane that might be better for use with a converter, we have nothing to loose!
Smaller motors are much less efficient, so it would be a good idea to use a coreless motor, the only small ones available are the Maxon 075 W rated at 2.4v and pager motors. Brushless motors are small but need heavy circuits! We really need a pager motor with a manufactures data sheet so we can test the motor and use it as a bench mark, then we can guess the performance of other pager motors from there. Anyway! We do know that a pager likes lots of volts so we will hit it will 5v (not a good idea as shown earlier!) and we will take a guess at its current, say, 300 mA (0.3A) + 20mA for the control system (Z Tron infra red with 200 Ohm coils using two proportional channels) which is 320mA x 5v = 1.6 W assuming a converter efficiency of 90% means the input to the converter will need to be 1.7 W (100/90x1.6)
Lets try a couple of 50 mA NiCDís as we can easily buy these and we have experience of using them.
The voltage into the converter would vary from between 2 x 1.4v = 2.8v down to 0.5 x 2 = 1v.
So 1.7 W divided by 2.8v = 0.607 Amps (607 Ma) and 1.7 W divided by 1v = 1.7 Amps.
So the two Ni Cds are going to be progressively loaded up, which means they will die faster and faster.
Not very helpful, but then starting at 600mA is not too bad so we could get a couple of minutes flying time? The plane would have a light weight motor but you have the extra weight of the converter - allow about 1 gram. We could save some weight by using 70 mA NiMH but these are slightly lower volts. How about if we try three 70 mA NiCDs? We would have about the same weight as two Ni CDís but lots of volts - we saw earlier that lots of volts into the converter is good news.
Still using the pager and 5v with 320mA current = input to the converter 1.7 W.
Cell voltage would be 3 x 1.2 = 3.6 down to say 3 x 0.5 = 1.5v. This gives 1.7 W/3.6v = 472 mA and 1.7 W/1.5v = 1.133 Amps. Hey! That sounds a bit more reasonable.
But would a pager on 5v fly a plane that weighs about 23g? Wait a minute! My new Mig 3 weighs 24g and it has a simple geared KP00 with two 50 mA NiCDís, so once again why bother? If it ainít broke donít fix it.
Dr Chris and the French guys have been flying one cell planes with pager motors. How would that work out? It would reduce the weight so we donít need as much power for flight, We have seen earlier that it is good to have high input volts and low output volts so maybe we could reduce the volts to the motor?
Lets try 4v and say 250mA for the motor = 1 W @ 90% = 1.11 W input.
One NiCD would give 1.4v to 0.8v = 0.792 A and 1.38 Amp. That is not too bad. Dr Chrisís Tiger Moth weighs in at 12g (0.41 Oz) If we had a 50% power to weight ratio we would need to produce 6g of oomph from the pager, at 4v and 250mA that sound OK too.
How much would the airframe weigh? The little Tiger Moth is not very big, about 10Ē span so maybe 3g would be Ok, Could we make a gearbox, prop and converter and motor for 3g? Control system will weight 2g?
So now we have a basic budget for the plane. In reality we could make some parts better while other parts are worse. Dr Chris used foam for the airframe , his converter was his own design and he used a DIY, super thin carbon prop. Pager gears we can get out of servos. We could fly slower and use less power?
I think the first thing would be to test a pager to see ho much thrust we can get from 3 to 4v. If the answer is more than 6g then we could throttle back a little and that would considerably increase the flight time. Or maybe use a lighter pager and run at full power?
These numbers are all very small, but try to think of this budget at a series of percentages. To save weight what would we reduce? Flying weight is 12g and the cell is 3.7g = 30% !! So we would improve the model if we can reduce the weight of the cell, that is why it would be nice to use a Lithium polymer cell that weighs 3g. This cell would have a higher voltage too which would be really useful as we have seen above, the current load on the cell would go down considerably.
No - I donít know of one!! Not yet anyway.
|Jan 06, 2002, 10:39 AM|
I agree that our best shot of making this work is with efficient coreless motors which happen to like more volts. It could also be that we are trying to achieve the alchemist equivalent of turning lead into gold.
I have a Cloud 9 pager motor (about 1.7g after outer casing is removed) rigged up with 4.2:1 gearing, but have done only limited static tests. Also, I don't know who the manufacturer is. Mike posted a subset of his pager motor static tests, which were similar to what I got. Right now I have a Punkin bipe to finish, so probably won't get to the pager motor tests for a bit.
With a GWS 4.5x4.0 prop at this gearing I got
3.5v, 0.31amps, 5.25g thrust
4.0v, 0.32amps, 6.43g thrust
4.5v, 0.35amps, 7.45g thrust
With a GWS 5.0x3.0 prop at this gearing I got
4.0v, 0.31amps, 7.10g thrust
4.5v, 0.34amps, 8.46g thrust
These are pretty preliminary with just the gearing I had on hand, and props available. But, it's a start.
|Jan 06, 2002, 07:38 PM|
I believe your pager motor is heavily overpropped (or undergeared). As a rule of thumb you should never pull more amps than half of the stall current for the given voltage. because mechanical power goes down again at more than half of the stall current. This is valid for every motor, not only coreless.
|Jan 07, 2002, 07:49 AM|
I completely agree. I just haven't had a chance to build a gearbox with a higher ratio. I'd like to build a two stage gear box and am still figuring out how to do it. Problem is, when I get something new like pager motors I just have to try them. I've heard that these like to be geared between 10:1 and 20:1. So, I've got some work to do.
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