Hello, I am currently working on making a hybrid power system using IC engine and generator/ alternator/ motor for a UAV (VTOL capability - quadcopter and fixed wing combo) we are designing for MEng project. I have found some information on alternator by Sullivan and other company/forums. I have also looked into using motors to generate elec but not sure as how I would be able to generate sufficient power. Current chosen motor spec - 42amp draw continuous and rated at 850watts each. takes 4-5s lipo. Any suggestions??
HERE is a previous discussion about the idea..
I could be interested in this, do you guys have a budget for this project?
If we built a lightweight APU maybe from one of my small nitro engines, using a lipo pack and brushless that both starts the engine and is used as a alternator.
If the nitro engine is tuned right it could be relitivlty efficient, however I have been playing around with a nitro engine running on a Hydrogen Gas mix... I think if we can make a lightweight fuelcell we would be in business
Thankyou for your reply. Greatly helped
I'm new to DIY Drones but I've logged quite a few hours looking into this concept for my own quadcopter power endeavorers. The math I had checked out, but barely. Don't expect a heavy lifter. The fixed wing may buy you some flight time though. Some motors and generators have incredible peak efficiencies but thats the peak and not the average. I hunted all over and came to the conclusion that you should expect 50% of the power created by the engine to be available at the props in no battery equilibrium. To enplane in over simplified terms 70% efficiency at the generator times 70% efficiency at the motor is 49% total. In reality these numbers could be a little higher or lower and you also have losses in the ESC (but I don't think that much). Eventually I went with single engine direct drive which was a mechanical nightmare but o-so-much power (90%+ efficiency).
To start off your power design pick an engine that has about twice the power you think you will need (convert horse power to Watts). Keep in mind that rated horse power is what the engine manufacture decides to stamp on the engine and should not be considered to be exact. Now to ask gasoline or nitro engine? List goods and bads of the systems in general and for specific engines you think will suit your purpose.
For your generator I would suggest a brushless motor just like all the other hobby grade components you will be using. There is a clever way to use 6 diodes rectify the power coming from the three leads with minimal electric penalty (http://www.instructables.com/answers/Windmill-Generator-why-wont-a-...). The instructables discussion is kinda long and has some useful stuff in it but you really just need the picture partway down with 6 diodes drawn on it. As far as which motor to get? get something with as low of a kV(motor constant) rating as possible. It is easy to step voltage down and will even let you get away with an undersized generator as higher voltage means less required amps which means less damaging heat.
Brushless motors come in much greater variety then engines so you may need to try 2 or three generators before finding something can convert your power efficiently and is light weight. If you have the luxury of sizing the rest of your airframe after this life would be easier.
Another better thought, if you relayed on half to 3/4 battery power to VTO&L and fixed your foreword flight prop directly to the engine and in-line with the generator you could get by with much less of a generator if you even still feel the need to have a generator. Best of both worlds.
A couple of things to consider:
A permanent magnet motor used as an alternator doesn't have a "field" winding to regulate the voltage. As a result, the output voltage will be almost directly proportional to the RPM, so if you want a constant voltage to the motors you will have to keep the engine at a fixed RPM. If the engine doesn't hold a nearly constant RPM with varying load, you will need a governor. You COULD use a buck regulator to keep the voltage constant, but good luck in finding a suitable one!
Even the best 100PIV/60A Schottky bridge rectifiers have a voltage drop of about 1.7V, so if you are producing 40A, you will create 68 Watts of heat in the rectifier. The only way to avoid this is to use active (FET) rectification.