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First of all, I apologize for anything stupid I say as I am not yet well versed in the proper technical terms and schematics.

I wish to use several identical BLDC motors that will be spun at the same speed (chain driven off same source) and used as generators rectified to DC power. I am curious if there is a simple way to safely parallel their output (combine their output current).

Since the motors are identical and spun at the same speed, I may potentially get away by using identical three phase bridge rectifiers for each motor and then paralleling the output of the rectifiers. However, I know that is not a reliable solution as manufacturing tolerances and other variables will still mean that the output from each rectifier will not be 100% equal, thus possibly causing a catastrophic situation.

If anyone has any advice on how to do this properly, I will greatly appreciate it. Or is it even possible?

Thank you.

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  • \$\begingroup\$ It should be possible if you rectify each unit separately. But if you are able to synchronize the BLDC's together so that they are "in-phase" with each other electrically, then you could just put them in parallel and use a single rectifier. This might be a difficult adjustment to make, to synch them up. \$\endgroup\$ – mkeith Jan 14 at 2:41
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Ideally you want the torque to be constant with rotation and not pulsed by pole positions , so the more motors the better with 360 phase split evenly between gear timing belt position like a V-8 with 8 motors uses the camshaft split the cycle or crankshaft position and a flywheel dampens the vibrations of torque.

Alignment of phases and sizing of a flywheel depends on your motive power source and inertia you need relative to the torque curve.

You will still need a MPT tracking system to prevent stalling, allow buildup of speed to develop torque on your power source and maintain adequate efficient RPM as power = Torque & RPM. This would be done by PWM ganging of motor FET rectified output. Each pair of motor wires will need a FET Half bridge for low performance losses. You would also have to evaluate the profile of torque vs RPM of your power source which may not be linear.

With 3 motors you can simulate 3 phase power with 6 diodes or 6 MOSFETs using current and RPM sensing to either maximum Power or limit the torque load as you see fit.

enter image description here

I have taken my stepper bridge driver system in full-step mode and turned off the power then moved it only to see the 12V fan start to move. You may also find that moving the motors by hand can generate the DC voltage back to your battery source. But I cannot guarantee your system will be as reliable or safe to do so for CMOS. I left USB power on and disabled the 12V motor power.

No load voltage may be sine but current depends on magnetic pole current commutation method used and tends to be distorted when loaded. The battery ESR, charge C ratio, gear ratios of cogged belts ( like a timing belt in a car) bridge commutation deadtime and other system variables requires a lot of design to optimize.

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Providing you use a 3 phase diode bridge for each generator, you can parallel the outputs with impunity. You should use the same model motor (with the same turns per winding/phase), mixing different motors will not work well. There will be no catastrophic results from this paralleling, simply a mismatch in current provided by each generator.

Even with the same model motors, you will have potential differences from each generator, so the current/voltage from each will not be equally balanced. However since the motors have winding resistance this will help to balance the current from each motor/generator under load.

Providing the voltage output curves of the coupled motors are within a volt of each other you could take active control of the balancing. You could (if you can tolerate the losses) put a single high side diode on the output of each 3 phase rectifier and use a P-FET to partially short it out to even up the current flows. This could be driven by an analog signal or a PWM signal.

schematic

simulate this circuit – Schematic created using CircuitLab

You could also replace the three low side diodes with a FET and control the balance using those signals, but it's more complex.

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