Smart exercise bike computer. Part #5: Experiments with power generation

Good day!

Two weeks ago I promised to continue my project when a parcel from Element14 arrives to me. The package is still on the way, but I think that this is not an excuse to stop the project. The fact is that a good idea came to me: even if I have to stop the development of software, nevertheless, I can do no less interesting secondary tasks of my project.

So, today I want to talk about the issue that was discussed in the comments to one of my previous articles [link] – about the generation of electricity by the muscle strength of a rider.

Object of research

For my current experiment, I decided to use an DC motor with permanent magnets, which I had for a long time of no use. As you know, this machine can work both in the motor and in the generator mode as well.

First, try to use this motor for its intended purpose. Below is a diagram of the experimental setup.

The motor that I decided to use is rated at 15 V. There was only one unit in my home workshop that could provide such a voltage. If you are interested in learning more about it, I offer you an article in which I quote its construction [link].

I conducted series of experiments and calculations:

1. I measured the resistance of the wire: Rc = 1,1 Ohm.

2. With the closed key S1, I determined the voltage Um* = 14.9 V, and also the current Im = 102 mA.

3. Further, based on the results of the experiment, I determined the voltage on the motor brushes  Um, for this I needed to determine the equivalent resistance of the motor Rm:

Rm = Um* / Im – 2*Rc

Rm = 14,9 / 0,102 – 2*1,1 = 143,9 Ohm

Um = Rm * Im

Um = 143,9 * 0,102 = 14,7 V

4. Knowing the current and voltage I determined the motor power on idle:

Pm_1 = Um * Im

Pm_1 = 14,7 * 0,102 = 1,50 W

5. I repeated steps 2-4 for the short circuit mode and got the power Pm_2:

Pm_2 = 5,19 W.

The numbers obtained can be interpreted as follows:

1. If we put the motor in generator mode, then when working on a low load (high resistance), we can get a voltage of ~15V, consuming only 1.5 W of power, which is quite small compared to the 98 W of load of our exercise bike.
2. When the generator is operating at a high load (low resistance), 1.5W of power will be expended to overcome the moment of motor resistance (this motor is simply dissipated).
3. Operation in short circuit mode is invalid. In this way, I will conduct further experiments with a calculated electrical load of not more than 1-2 W.

Power generation

For a low load experiment, I connected a load resistor Rn = 10 kOhm to my motor as follows:

Experiments with a high load, I conducted at a resistance of Rn = 100 Ohm.

To pair the electric machine with the exercise bike, I installed a collet chuck on the motor shaft, in which in its turn was inserted a Dremel [model 407] nozzle designed for sanding. The fact is that if you remove the skin from this nozzle, it is a rubber wheel, ideal for connecting an electric machine with an exercise bike.

I reduced the test results to the table:

Battery charging

The results of the experiment upset me a little, because the maximum voltage that I managed to get by "muscle power" is 6.0 V. Under load and at a speed of 30 km/h, the voltage dropped down to 1.4 V. This voltage can not be used to feed battery, because charging it requires a voltage of 3.3 – 4.2 V (for 1S LiPo).

We have three possible solutions to this problem:

1. Considering that the FRDM-KW41Z board consumes very little power, you can do nothing and simply connect the generator to the battery through a resistor and a Zener diode at 4.2 V. It is necessary to select a resistor in such a way that the voltage on battery is at least 3.5 V.
2. You can solve the problem with a DC-DC converter, for example using the [LM2621] chip.
3. The most correct solution is to use a multiplier (aka step-up reducer). If we increase the engine speed by only 3 times, we are supposed to be able to get stable 4.2 V at a load of 100 Ohm (i.e. 0.18 W). A further increase in RPM will allow us to enter the normal mode for my electric machine and get the calculated 1-2W.

I went the first, easiest way – and now I have though a small and low-power, but simple and reliable system of autonomous power supply!

Conclusion

Today I managed to hold though a secondary but very important experiment. I managed to perform one of the important parts of my plan (note – the Maximum plan!), I created a self-contained power supply system for an exercise bike computer.

Will I use this system in real training after the contest is over? This is a question, the answer to which can only be obtained having completely finished the hardware part of my exercise bike computer. For sure, I can say only one thing – smart exercise bike computer, this is one of my few hobby projects that I definitely will not abandon after the end of the contest! In any case, I believe in it.

Thanks for reading and have a nice day!