# Designing a BLDC generator

I am trying to design a brushless DC generator system for portable use of charging batteries and if suitable possible hybrid power system of an aircraft.

The "BLDC" motor/generator will be connected to a 1.5KW, 2.011HP and 10,000RPM Max water cooled engine. The reason for a brusheless motor is due to its ratios of weight to power and many are water cooled to make further use of a pre-existing cooling system already in place.

The brushless motor i have found that seems to be best suited to the specification and water cooled is this Turnigy AquaStar 4084-620KV Water Cooled Brushless Motor on Hobby-King the voltage is desire is 12V DC if i use the engine specifications of 1.5kW = 1500Watts we can say 1500Watts / 12V = 125Amps this is a lot of current to handle.

The motor/generator is rated for a maximum voltage of 37 Volts and 105 Amps that is 37 x 105 = 3885w or 3.9kw which is slightly overrated for my engine, to drive this I intend to use a pulley system to drive the generator at 21,700 rpm which will generate approximately 35v (obviously kv = rpm/volt is not exact but pretty close and less when in use as a generator) I then intend to transform this to a 12v dc at 125 amps or as close as.

Obviously if anyone can improve my calculations above and design you are welcome to input but my question is what is now the best method to transform to my desired output with regulation so that engine speed is proportional to current not voltage if possible.

Idea 1: A dc - dc buck converter could be used but either multiple or a large unit would have to be used to deal with the high current and will be expensive.

Idea 2: use a 3 phase transformer to convert to 12 volts before rectification however this will not add any regulation.

Obviously the 1.5kw of my engine is mechanical power and my estimated values may get me less as losses add up through rectification, heat, copper losses ect.

• Is the generator a DC output (internal diodes) or AC output? What is the pdak-to-peak output voltage? And, how many phases? Because of the currents involved, I'm leaning toward Nov 28, 2017 at 15:25
• Generator is AC output peak to peak voltage would be the 35 volts specified above with 3 phases. Nov 28, 2017 at 16:40
• You're unlikely to achieve losses less than about 30-40% total, with around 10-15% just in electrical losses through rectification and DC-DC conversion. This would require 37 V @ 46 A to 12 V @ 125 A conversion, which would be a fairly major DC-DC conversion project all on it's own. You may have more success (simpler) using something like a 4S BLDC such as hobbyking.com/en_us/… with synchronous rectifiers and voltage to speed control. Nov 28, 2017 at 18:17
• Jack Creasey I do like that motor you pointed out may be a better alternative I would have to add a water cooling jacket around it though. This still doesn't help me on the regulation side of things I desire to have speed of the engine proportional to amperage generated. Nov 28, 2017 at 19:06
• @JackCreasey I will accept your answer of using that particular motor with a active/synchronous rectifier if you post it as an answer. Dec 5, 2017 at 11:12

You're unlikely to achieve losses less than about 30-40% total, with around 10-15% just in electrical losses through rectification and DC-DC conversion.
This would require 37 V @ 46 A to 12 V @ 125 A conversion, which would be a fairly major DC-DC conversion project all on it's own.

You may have more success (simpler design) using something like a 4S BLDC such as this one from Hobbyking with synchronous rectifiers and voltage to speed control.

A simple speed control algorithm may be to set a single voltage control point such as 13.8 V (Lead Acid batteries for example) which triggers Idle for your motor.
This would hunt at the trigger point so you could be creative to solve that:

1. Start at Idle, test battery voltage
2. Based on battery set full rate charge for a time (say 3 minutes), start to accumulate number of charge periods in you MCU.
3. At period end, lower speed (but not to idle) and measure battery voltage, decide on next period to charge if voltage at limit, then idle for a defined rest period ...then go to 1.

You could of course measure the charge current and control that, but that would be a bit more complex to implement.

One other thought is to measure the charge power (indirectly) and control that.
For example, if the coupling between the motor and BLDC was a spring coupling with two sensor points, one on the motor and one on the BLDC shaft, you get a measure of the power drawn from the BLDC by the twist in the coupling. ...just a thought.

• One thing I will add is trying to find a synchronous or mosfet rectifier capable of anything above 50 Amps already doesn't seem possible let along potential 125 amps. Feb 6, 2018 at 20:12
• @ChrisJames, it does depend on the devices you use. Sync rectification is used in some high performance alternators and there are many more devices capable of this application sphere available today. IXYS produce some great examples: ixapps.ixys.com/DataSheet/… Feb 6, 2018 at 23:18

You can use a BLDC ebike controller, most have variable " regen " control, there is one model that has real time control of the regen rate, most others are a set limit you can change but need to be stationary to do so.

I called it " proportional regeneration ", I wont go into detail to explain why I came up with the idea a few years ago, but it has to do with ebikes and using a trailer, different loads require high braking effect.

ebikes.ca sells them, the controllers, you use the throttle to increase the amount of amperage being generated in real time, unlike most other controllers, you have to program to a set limit while stationary.

I guess I should state for those that do not know, most ebike hub motor make great generator because of the windings. Easy to calculate how much voltage they produce as most are rated XX rpm per volt.

There are even water cooled BLDC hub motor's for ebikes, golf carts, trikes etc... 10,000 kw :) is not hard when dealing with ebike hub motor's.( Golden Motor and others ) Crystalyte makes some not water cooled, the Cromotor or Hubzilla, MXUS and a few others make 10kw motor's. ASI in Ontario Canada makes some nice BLDC controllers that are very configurable via software, with a slew of input options, analog and digital.

So for a wind generator, using a controller with " proportional regen " you could set one that after XXX rpm ( High wind application ) you could increase the regen and use it as a brake and not throw away that energy. Regen goes from 50% to 100% when voltage applied to throttle signal.

If you use a mechanical brake for a wind generator, you lose that energy, more parts, more wear etc....

So by increasing the regen you would slow down in high wind to avoid damage. I hope I'm making sense heh.

Using one of the " scooter " type motor's, 16 in. tire on it, you can use it with a bicycle friction drive kit, to either drive it with a engine, or use it to drive a gear system and creating a 16 to 1 drive reduction ratio to your final output. ( Stanton in USA sells bicycle friction drive kits among others ).

You can liquid cool the controller, Banggood sells some water blocks suitable, you can use 12v computer liquid cooling pumps and rads to cool controller.

Think computer fans are not strong enough, check out one called " The Tornado " from Vantec.

You can of course use a Bridge/Rectifier and then use a Solar charge controller, but personally, if for a wind application, having the " electronic braking " via the variable regen will just save you money and time in the long run.

ebikes.ca also sells a heat sink for hub motor's and a cooling ferro fluid, cooler motor and controller, longer life, less problems, less cost.

I hope this helps to give you other idea's as to how you can use an off the shelf product to get the same results.

I used a friction drive kit, Lehr Propane engine from weed wacker, BLDC kit and ebike controller to charge my ebike batteries @ 42 amps from 0 to 10 so far but my batteries cant take higher charge rate until I get better ones, like headway cells.