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I'm using the following BLDC motor as part of a project to generate power:

http://www.rcdude.com/Cobra-C-3520-18-Brushless-Motor-p/c-3520-18.htm

I'm able to spin this motor to about 3000 RPMS which gives me about 5 Volts. This is to be expected given that the motor is rated at 550 KVs. I'm using a three-phase rectifier with a smoothing cap to convert this to DC power. What I don't understand is the current produced. I'm getting about 33mA which is to low for my application. Essentially I would like to spin a small PC fan with this power. I'm able to do it but at a very low speed given that there isn't enough current flowing from the generator. What I would like to understand is why there is so little current flowing at those RPMS. I would at least expect it to be 100mA or 200mA, but this is just speculation on my part. Apologies for my noobness. Any help is appreciated. Thanks.

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    \$\begingroup\$ How exactly did you measure that current? What current did you measure, did you e.g. short out the rectified DC, connect a load resistor or use the fan as a load? \$\endgroup\$ – jms Feb 24 '16 at 22:54
  • \$\begingroup\$ I used a PC fan (12V 0.25 A) as load on the output of the rectifier and measured this current with a multimeter. \$\endgroup\$ – Joel Pou Feb 25 '16 at 0:15
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    \$\begingroup\$ How fast will the fan spin if you connect it to some other 5V source, for example, a USB port (half an amp)? 12V fans tend to spin slowly if at all when connected to 5V. I just tested one with a bench supply and can see load current similar to what you have reported. \$\endgroup\$ – Oleg Mazurov Feb 25 '16 at 0:24
  • \$\begingroup\$ I powered this fan with a benchtop psu and at 5V I'm getting about 100mA which is why I would think that the current would be higher. \$\endgroup\$ – Joel Pou Feb 25 '16 at 0:38
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You think that the problem is in the motor; that the motor cannot supply more than 33 mA. This isn't correct, the problem is that your 12V fan is not able to function at such a low voltage and thus does not draw much current. Remember: current isn't "pushed" by the source, it's drawn by the load. If you were to short out the rectified output and measure the short circuit current with a multimeter, I guarantee you will see several amps flowing trough (possibly blowing the multimeter fuse, destroying the diodes and/or overheating the motor coils in the process).

A standard silicon rectifier diode will drop 0.7 V when conducting. As a rectifier bridge has two such diodes always in series with the load, the diodes will steal 1.4 V from whatever meager voltage your motor puts out.

You can rectify this by using schottky diodes, which have a much lower voltage drop (about 0.2 V), and using a lower RPM/V (KV in radio control terms) motor. For example, a 140 RPM/V (KV) brushless gimbal motor should put out 21.5 V AC at 3000 RPM, which can then be rectified and dropped down to a nice stable 12V or 5V with a buck converter.

If you want to stick with your motor for whatever reason, and you have the necessary programming skills, you could also write a custom firmware for a commercially produced ESC (RC brushless motor driver) to act as a synchronous rectifier instead. The ESC MOSFETs will drop next to no voltage when conducting and they are already arranged in a full bridge configuration, granting much better efficiency than any diode. You could even boost the motor voltage while rectifying by pulse width modulating the MOSFETs, doing exactly what the ESC normally does but in reverse.

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  • \$\begingroup\$ I know that the fan needs 12V to draw more current but I made tests with a benchtop psu and at 5V I'm getting about 100mA. This is why I'm trying to figure out why. Also, I'm using schottky diodes to rectify. I bought this motor from hobbyking: hobbyking.com/hobbyking/store/… It has a KV of 149 but the shaft has too much inertia and its too heavy for the coupled motor to spin which results on excessive heat on windings. Why are these shafts so heavy?? Is it because of the bearings or permanent magnets? \$\endgroup\$ – Joel Pou Feb 25 '16 at 0:43
  • \$\begingroup\$ Magnetization losses in the iron core, friction in bearings and eddy currents all play a role in how much friction the motor generates with no load. These losses are mostly related to motor size and pole count, and both of your motors are enormous and way overkill for generating a few watts as an alternator, with the aerodrive one being specified for over 2 kW! Why do you insist on using motors so powerful for powering a <4 W fan? \$\endgroup\$ – jms Feb 25 '16 at 0:51
  • \$\begingroup\$ Lol, I was just using the fan to test the output of the smaller motor. My real purpose is to have a steady current flow on the output to charge li-ion batteries with them. Thanks for the reply. That was puzzling me. \$\endgroup\$ – Joel Pou Feb 25 '16 at 1:10
  • \$\begingroup\$ How much power do you want to generate? As low RPM/V motors designed for low power (<300W) are a rare beast, I think that you should consider using brushless gimbal motors, rewinding a ~200W class motor for a lower KV (more turns of thinner wire), using gearing to increase RPM or even boosting the motor output with three equal ferrite core transformers. What is your power source, a gasoline engine? Some guy on an exercise bike? \$\endgroup\$ – jms Feb 25 '16 at 1:22
  • \$\begingroup\$ Hey jms, thanks for the replies. I really appreciate the help. My aim is to generate a minimum of 30 Watts (30 V @ 1A). Why do you suggest a BLDC gimbal? Would you suggest one in particular under these ratings? \$\endgroup\$ – Joel Pou Feb 25 '16 at 1:29

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