Finding the right brush-less Direct Current (BLDC) motor to use as generator

Since I got my Honda GX25 motor, the task of finding the right generator to make the most efficient and compact generator for the batteries in the robot has lingered somewhere in the back of my mind.

First I thought about using an car alternator for this, but have since found perhaps an even better alternative, namely high-end radio controlled helicopter motors. It turns out alternators weigh a lot and are clunky and heavy, while BLDC motors from modern RC helis are not.

So how would I go about finding the correct BLDC for my project? I got some help here and ended up with the following:

  • Lead-acid batteries have optimal charging voltage of 13.8 to 14.4 volts.
  • The GX25 motor has optimal RPM (RPM with highest torque) @ 6000 RPM.
  • The GX25 motor has an effect of around 0.7kW.
With this as input finding a motor for our project is easily calculated as follows:
  • Desired kv (rpmvolt) = 6000 rpm/14.4 volt ~= 417 kv
  • Desired continuous effect is above 700 W. We will go with at least the double to put the least amount of strain on the generator.
  • Desired continuous amp is 700/14.4 ~= 48 A
(disregarding all the voltage losses etc).

Next I just chuck these parameters into hobbyking's motor finder to get some alternatives, for example the Turnigy SK3 5055-530kv with the following specs:

Turns: 12TVoltage: 5~8S LipolyRPM/V: 430kvInternal resistance: 0.019 OhmMax Loading: 70AMax Power: 1750WShaft Dia: 6.0mmBolt holes: 25mmBolt thread: M4Weight: 378gMotor Plug: 4mm Bullet Connector

I must investigate this further to ensure that I don't go buying one of these motors that wont work because of some parameter I did not think of.


Big push for the generator project.

I decided to make a push for the GX25 generator project.

First I removed the clutch from the GX25. I will be crafting some kind of frame for the generator assembly and also a converter plate to go on the shaft of the motor.

Next I prepared to make the optical tachometer circuit for the ECU to judge motor speed precisely. I had some luck pointing my manual tachometer to  black/white tape on the makeshift plate mounted on shaft. Still TODO: I still need a way to pump initial fuel into cold engine (manual pump), and actuating the choke might be handy in cold weather. But after some new intel (se bottom) I think I have found a way to make the generator act as starter motor as well, eliminating the added complexity of a separate engagement mechanism and separately controlled starter motor. Even better, I get to keep the recoil starter for debugging purposes (I already put it back on). Fun fun.

Next I put an ad out where I proclaim that I want any excess generators or alternators that people may have lying around. This resulted in me getting in contact with a fellow who had a 140AMP 12VDC 2005 mod Volvo V75 alternator for cheap, which I bought.

Next, I put an ad out where I proclaim that I want any excess electircal motors, which has resulted in a few emails from people who want to get rid of motors. So far I didn't get any motors though.

Next, I went dumpster diving, or more correctly, electrical appliance recycling plant scavenging. I will keep the location secret because, oh-boy was there a lot to be found!

3.5 HP 180V  7.5 AMP 5200 RPM Threadmill motor

Threadmill motor driver circuit

ACME Screw linear actuator from threadmill.

One of 2 washing machine motors.
And last but not least, I put a question on electronics stackexchange asking what motor I would be looking for to make my generator. I got an excellent answer that I am definitely going to pursue next. In the same fell swoop I asked a question about how to measure the torque vs. RPM curve for my motor collection. Turns out there is something called "prony brake".