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I have a nitro engine that’s idling at 1600 RPM. As the title says , I also have a BLDC coupled directly to the nitro engine working as a generator. That generator is capable of producing 3.5kW of power. Here’s the diagram:

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So , generator is capable of producing 3.5kW of power at around 2000RPM. Let’s say engine is revving up and we have 3.5kW out of the generator. Up until this point , let’s assume an ideal condition and we have a constant power supply of the said 3.5kW .

My plan is to supply this power directly to my quadcopter. Is it possible to work it this way without looking at the current or voltage draw by the motors as long as all four of my motor’s Current times Voltage is less than 3.5kW ?

Voltage Spike
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DevOsource
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    The real question is, can you put this in a drone, along with the fuel and servo's required to run it, and still weigh less than the equivalent batteries/get a similar run time? – Ron Beyer Apr 07 '20 at 20:20
  • Yes , very possible. You’ll be able to achieve even longer endurance – DevOsource Apr 07 '20 at 20:38
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    I would probably consider building a small microcontroller circuit to drive a servo which controls the throttle of your engine - then use that as a governor to maintain a reasonably constant speed. Then use the same micro to control a MOSFET in a buck PSU. Set the 'governor' to aim for an output voltage from the generator that's somewhere convenient for the buck PSU, and have the PSU output controlled to produce the voltage you need for all the rest of the electronics on your drone. It'll be a little like what the marketing people are calling an "inverter generator" these days. – brhans Apr 07 '20 at 20:41
  • @brhans I think you would want to avoid trying to run all the power to the motors through a buck converter if you could. – DKNguyen Apr 07 '20 at 20:46
  • @DKNguyen - yeah - true, and the motors (and hopefully their drivers) should be reasonably tolerant of some voltage peaks and dips as the 'governor' adjusts the engine power as required ... so have the governor adjust the throttle to give approximately the correct voltage for the motors, and possibly exchange the buck for a buck-boost. – brhans Apr 07 '20 at 20:58
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    I assume you have a rectifier on the output of the BLDC being used as a generator? Because without rectifying it, it will produce a three-phase sine wave that can't directly be used for an Electronic Speed Controller (ESC). – user57037 Apr 07 '20 at 21:19
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    Typically you would have some kind of circuit to stabilize the voltage and charge a battery, then you would run the drone from the battery. – user253751 Apr 07 '20 at 21:30
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    There may be a lot to be said in favor of adding a battery as well as a generator. The battery can assist with peak loads during take off and high-g maneuvers. The generator can be sized to supply 110 percent of the power required for cruise. The extra 10 percent can be used to recharge the battery to keep it near full charge. – user57037 Apr 07 '20 at 22:28
  • I didn't know it was easy to use a BLDC as generator! – Marcus Müller Apr 08 '20 at 04:54
  • Not answering the direct question, but will this even fly in the first place? A generator is nowhere close to 100% efficient, so you will lose a LOT of the engine's power by running it as a hybrid setup. – MikeB Apr 08 '20 at 15:36
  • Please let me know your motor model number and specs, u need to get FOC board and that can will use motor to start the gas engine and after it will use the motor output. – Fakhre Alam Apr 12 '21 at 02:55

4 Answers4

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Is it possible to work it this way without looking at the current or voltage draw by the motors as long as all four of my motor’s Current times Voltage is less than 3.5kW ?

Maybe, if you plan on having the motor voltage vary wildly, then you can run the motor constant and the voltage (or RPM if there is no throttling on the motor) will vary with the output of the generator. If the voltage needs to stay constant (which is usually the case with most electronics) need some way to regulate the voltage.

Normal generators use some kind of controller (mechanical or electronic) to regulate the RPM's to keep the rotation and voltage output constant when the load changes.

Voltage Spike
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  • R/C electronics can handle large voltage swings; for example, the motor controller in front of me right now can handle anything from 22v to 96v. – skyler Apr 08 '20 at 13:39
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There is a lot of load matching that you would want to do ideally to make everything work out.

But if you can governor the nitro engine to run at 2000 RPM continuously, and if the generator really puts out 3.5kW at 2000 RPM, and the drone motors never ask for more than 3.5kW, then it should work. Knowing how drones work, with their peak load much higher than cruising load, I think it would be more efficient to utilize a DC-DC converter between the generator and the drone motors so that the nitro motor RPM can be reduced when the extra power is not needed.

But if you want to run open loop without current and voltage sensing, then you need the nitro motor to run at 2000 RPM continuously (with a governor to prevent over-speed).

user57037
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    As long as you are able to keep the nitro motor at a relatively constant speed, the ESC will have no real way of knowing that it is not connected to a battery. The voltage will be relatively constant, although it will go up and down a bit depending on the load. However, battery voltages also go up and down a bit depending on the load, so that is no different than a battery. – user57037 Apr 07 '20 at 21:16
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Your motor drivers for your quad are inherently looking at the RPM one way or another. If you can actually read out the RPM and send commands based on RPM (i.e. a governor mode) then you shouldn't really need to know the voltage or current. Of course, it would be optimal if the control algorithm on the motor driver itself examined and used the the voltage along with RPM since it would then know what to expect to happen for a given PWM duty cycle.

DKNguyen
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I realize this is a couple of years old now but I am trying something similar. I think you underestimated the engine RPM to get the output you need. I have a 58cc MVVS 2 stroke single cylinder engine from the Czech republic that puts out 7 hp and weighs 2 kg. This is a best of breed engine. The generator is a 7kw bldc used in skateboards that weighs 2 kg. I plan on having 4 1 kw motors driving 30" propellers, with 12s lion battery in parallel for peak power and backup. The problem is the motor is running at constant 6600 rpm for peak torque, so generator kv needs to be low to get desired 50v.

Greg Sorter
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    Greg Sorter - Hi, I see that you have edited your post to add a point that (at least tries to) address the question, so I will undelete it. Please read the [tour] & [help] to see info about how Stack Exchange rules differ from typical forums. Thanks. – SamGibson Aug 17 '22 at 15:18