Alpha Unmanned Systems

How to choose the right generator to be used in conjunction with the AUS Power Board?

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You just acquired the Alpha Unmanned Systems Power Board (PB) to make your piston engine UAV more reliable thanks to the back up battery while reducing or eliminating the battery care task. But now, which brushless generator should you choose for your UAV? Wich are the right RPMs to achieve the voltage I need?  

If you made yourself this questions continue reading this article.

First of all, we have good news for you, all electric engines can act as motors or as generators, this means that if you apply electric energy to the engine it will give you work,  but if you apply work it will give you electricity! This means  that any drone electric engine can be used as a generator thats why they can be found at very affordable prices.

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But let's focus on what's important. In brushless engines there is a constant that will give us the clue to calculate all the necessary parameters to integrate the Power Board (PB) in the UAV. This constant is the Kv, motor velocity constant or the back Electro Magnetic Force (EMF) constant, this value tells us the number of RPMs that will be achieved when applying a certain voltage and is a data that all the manufacturers will provide.

Then, having this value and knowing that we need a minimum of 19 input volts to charge the backup battery and that the maximum allowable input voltaje is 30 volts, a nice voltage value to work with would be lets say 24 volts, now assuming that the typical rpms of the propeller of a Fixed Wing SUAS are 9000 rpms we have all the necessary data to define our brushless engine. 

RPMs = Kv X Voltaje

9000 = Kv X 24 --> Necessary Kv constant is: 375

But what  happens if, for example, we can only find a 475 Kv engine, then we would need to increase the rpms of the brushless generator  to achieve  the same voltage, in that case we can use the transmission between the propeller shaft and the generator to increase the rpms as follows:

Necessary rpms are: Kv X Voltaje --> 475 x 24 = 11400

Gears/Pulleys relationship must be: 11400/9000 = 1.26 this means that the generator gear or pulley must have 1.26 times the number of tooths of the one installed on the propeller shaft, f.e. 15 one and 12 the other (12  X 1.26 = 15.12).

Now you have all necessary information to integrate your generator and Power Board on your UAV!

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