In my last article about maximum power point tracking (MPPT) controllers I showed a standard method for exploiting the energy coming from a variable source such as a wind turbine and charging a battery. The generator I used was a stepper motor Nema 17 (used as generator) because they are cheap and available everywhere. The big advantage of steppers motors is that they produce high voltages even when spinning slowly.

In this article I present a controller specially designed for low power brushless DC motors (BLDC). The problem with these motors is that they need to spin fast in order to produce an exploitable voltage. When spinning slowly, the induced voltage is so low that it sometimes even does not allow diode conduction and when it does, the current is so low that almost no power pass from the turbine to the battery.

This circuit does at the same time the redresser and the boost. It maximizes the current flowing in the generator's coil and this way, the power can be used even at low speed.

This article does not explain how to make the circuit but if you are interested, check the last article.

Step 1: The Circuit

As in the last article I use a micro-controller Attiny45 with the Arduino IDE. This controller measures the current (using the R1 resistor and the op-amp) and the tension, calculate the power and modify the duty cycle on the three switching transistors. These transistors are switched together without regard for the input.

How is that possible?

Because I use a BLDC motor as generator, the tensions at the BLDC's terminal are a three-phase sinus: Three sinus shifted by 120° (cf. 2nd image). The good thing with this system is that the sum of these thee sinus is null at any time. So when the three transistors conduct, three current flood in them but they cancel each other in the ground (cf. 3rd image). I chose MOSFET transistors with a low drain-source on-resistance. This way (here is the trick) the current in the inductors is maximized even with low voltages. No diodes are conducting for the moment.

When the transistors stop conducting, the inductor current has to go somewhere. Now the diodes start to conduct. It can be the top diodes or the diodes inside the transistor (check that the transistor can handle such current) (cf. 4th image). You may say: Ok but now it is like a normal bridge rectifier. Yes but now the voltage is already boosted when the diodes are used.

There are some circuits using six transistors (like a BLDC driver) but then you need to scope the voltage in order to know which transistors have to be turned on or off. This solution is simpler and can even be implemented with a 555 timer.

The input is JP1, it is connected to the BLDC motor. The output is JP2, it is connected to the battery or the LED.

Step 2: The Setup

In order to test the circuit, I made a setup with two motors mechanically connected with a gearing ratio of one (cf. image). There are one small brushed DC motor and one BLDC used as generator. I can choose a voltage on my power supply and assume that the small brushed motor behave approximatively as a wind turbine: Without breaking torque it reaches a maximum speed. If a breaking torque is applied, the motor slows down (in our case the relation torque-speed is linear and for real wind turbines it is usually a parabole).

The small motor is connected to the power supply, the BLDC is connected to the MPPT circuit and the load is a power LED (1W, TDS-P001L4) with a forward voltage of 2.6 volts. This LED behave approximatively like a battery: if the voltage is below 2.6, not current enter the LED, if the voltage try to go above 2.6, the current is flooding and the voltage stabilizes around 2.6.

The code is the same as in the last article. I already explained how to load it in the micro-controller and how it works in this last article. I slightly modified this code in order to make the presented results.

Step 3: Results

For this experiment, I used the power LED as a load. It has a forward voltage of 2.6 volts. As the tension is stabilized around 2.6, the controller only measured the current.

1) Power supply at 5.6 V (red line on the graph)

generator min speed 1774 rpm (duty cycle = 0.8)
generator max speed 2606 rpm (duty cycle = 0.2)
generator max power 156 mW (0.06 x 2.6)

2) Power supply at 4 V (yellow line on the graph)

generator min speed 1406 rpm (duty cycle = 0.8)
generator max speed 1646 rpm (duty cycle = 0.2)
generator max power 52 mW (0.02 x 2.6)

Remarque: When I tried the BLDC generator with the first controller, no current were measured until power supply tension reached 9 volts. I also tried different gearing ratios but the power was really low compared to the presented results. I can not try the opposite: Branching the stepper generator (Nema 17) on this controller because a stepper does not produce three phase sinus voltage.

Step 4: Discussion

Non linearities are observed because of the transition between continue and discontinue inductor conduction.

Another test should be conducted with higher duty cycles to find the maximum power point.

The current measurement are clean enough to let the controller working without need for filtering.

This topology seems to work properly but I would love to have your comments because I am not a specialist.

Step 5: Comparison With the Stepper Generator

The maximum extracted power is better with the BLDC and its controller.

Adding a Delon voltage doubler can lower the difference but other problems appeared with it (The voltage during high speed can be bigger than the voltage battery and a buck converter is needed).

The BLDC system is less noisy so there is no need to filter the current measurements. It allows the controller to react faster.

Step 6: Conclusion

Now I think I am ready to continue with the nest step which is: Designing wind turbines and make on site measurements and finally charge a battery with the wind!