USB Bike Generator

Step 1 BikeGen Testing

If you only want to learn how I built the USB Bike Generator and not why the BikeGen system wasn't the best, you can skip this step.

In order to set a bench mark for the efficiency of BikeGen system I decided to do some testing.  In my searching for switching regulators one of the best resources I found was this article http://www.dimensionengineering.com/switchingregulators.htm It explains how switching regulators work and how you can calculate the power lost through heat generation.  The power lost to heat can be calculated using this equation:

Power lost = (Input voltage – output voltage) * load current

So in order to find the power lost you need to know the input and output voltages as well as the load current.  Luckily, I already had the tools for this job which required 3 digital multimeters and a small drill press.  

The test setup was fairly simple, I tightened the drill press chuck onto the shaft of the stepper motor and loosely clamped the motor in a vice.  This was the same stepper motor that I have used for both the Bike Generator and and BikeGen Instructabes.  This actually works out really well for two reasons.  First the speeds that the drill press can spin are labeled and from these speeds I calculated how fast I would need to ride the bike based on how fast the generator is spinning.  So using the tire size, rim size and the diameter of the small wheel mounted on the stepper motor for the BikeGen instructable I calculated the following speeds:

Drill Press RPM            Bike Speed (MPH)
         620                                     4.7
        1100                                    8.3
        1720                                   13.0
        2340                                   17.7
        3100                                   23.4

This range of bike speeds seems very reasonable.  The second reason that using the drill press is a good representation of a bike rider is the power rating of the drill press motor.  This drill press is rated at 1/3 horse power which is roughly 250 watts, this is attainable by an average person while riding a bicycle.  

The next step was to connect 3 digital multimeters into the circuit.  I had cut some of the wires and use a few sets of alligator clips to make this happen.  Check out the picture or the Test Setup pdf to see how I connected the meters.  Basically the current meter was placed inline between the regulator and the charging circuit and the voltage meters connected after the diodes and after the regulator and were grounded at the same spot.  This measures the values I need to calculate the power lost.

Once everything was ready I plugged in the drill and turned it on.  I tested the generator at every speed the drill was capable of and found out that at the first 3 speeds (620,1100,1720 RPM) there was not enough power to consistently charge the batteries.  This was confirmed by the blinking LED on the charger.  At 2340 RPM and 3100 RPM enough power was provided by the generator to charge the batteries.  The values I measured at these two speeds are listed below:

Motor RPM                  2340       3100
Input Volts                    15.2        20.2
Output Volts                   13        12.68
Output Amps                0.29        0.28
Power Lost (W)           0.638     2.106
Output Power (W)       3.770     3.550
Total Power (W)          4.408     5.656
Efficiency                      85.5%    62.8%

The power lost was calculated from the equation I showed earlier, the other values were calculated from the following equations:

output power = output voltage x output current
total power = output power +  power lost
efficiency = output power / total power

RESULTS

So looking at these results I think there is something to be desired.  First there was not enough power to charge the batteries until 2340 RPM which is 17.7 mph on the bike.  This is a high speed and doesn't seem reasonable for a casual bike ride.  Of course this speed is attainable but it would be a lot of effort to maintain the speed for an extended period of time.  The efficiency  seems very good at 2340 RPM (85%) but it drops to 62% at 3100 RPM.  This is because the input voltage goes up at higher speeds which means more power is lost to heat.  So unless I want to ride the bike at and average speed of 18mph everywhere I ride I will not achieve the best efficiency possible.    

Comments

[traeblain]

I know this is why your previous generation motor didn't work, but I think there could have been better options than moving to a new motor (although I have not built this and don't know the ins-and-outs of your work). My first question is why not try a new wheel for the previous stepper motor? My calculations (assuming you're using a 700x23 wheel and tyre) simply using an aggressive inline skate wheel (47mm) you could move your target down to 12.9mph which is pretty good for casual riding. Not sure your target speed though. It may help to understand as I believe you got the last motor from an old printer? Where this one may cost a bit more money.

[dbc1218]

The main reason I didn't want to reuse the stepper motor from the printer was that its voltage output was to high, which caused the regulator to become inefficient. I think the goal should be to first create the voltage you need, in this case 5v, and then try to develop as much current as you can at that voltage, to get the most power and efficiency. As far as a speed target, I wanted something that would maintain high efficiency over a large speed range. I didn't want to have a really high efficiency at one certain speed because then I would have to ride at that speed all the time to get the best performance. As far as the cost goes, I spent $10 on the new stepper motor from electronics surplus.com They have that same motor on sale on-line for $10 as well.

[retasker]

Ah, but that is the whole point of using a switching regulator. I don't know what is actually in your $1.00 USB convertor, but a properly designed switching regulator would allow you to transfer power efficiently and get more current out than you put in by operating with a higher input voltage. That is, with the proper switching regulator design including an input storage capacitor, you could use a higher voltage stepper motor to generate 12V for your convertor which would then step the voltage down to what you need. You see a tiny bit of that at the higher input voltages where you get slightly more current out than you are putting in. However, as it is now, you are barely getting enough voltage to make the switching regulator necessary. Based on the output voltages you list you would actually now be better off with the linear regulator - especially at the lower speeds. I suggest you try the old stepper motor with the higher voltage output with your new switching regulator. If you do this, make sure you don't get so much voltage that you destroy the switching regulator - I don't know what the input rating is although it is at least 15V if it is designed to operate in a car and probably more. Or try your new stepper motor setup with your older linear regulator. Additionally, you could get more efficiency by using schottky diodes for your rectifiers since they have a lower voltage drop than the 1N4001 diodes (0.4V vs 0.7V each)

[dbc1218]

Using the old stepper motor will not work, I know because I fried two of the regulators during testing. The MC34063 is rated at 40V and the new stepper motor puts out 30V unloaded at 3100 rpm. I think the high load of the dead battery is what caused the voltage to stay around 7V for the new motor, but I'm not quite sure how this works. At lower loads or lower output power the voltage will go up and the input current will decrease, right? But won't a higher load cause the voltage to drop even further because it can't compensate for the load? I guess my real question is what conditions are needed for max power output and what conditions are needed for max efficiency? Are they the same or is it a trade off.