How do I connect light bulbs to an alternator on a bike?

Question

Some friends and I are working on a school project and need a bit of help. The basic premise is that we’re using a bicycle (which is kept stationary) to spin an alternator and power some lightbulbs. Here are the specs of the alternator:

We’re wondering what kind of bulbs we can use (and wattage and voltage) and how to connect them (series or parallel). We’re also worried that the output of the alternator might be too high for the bulbs, will we need resistors or anything like that?

Please let me know if there’s any other information I can provide to help. We don't know much about electrical engineering ourselves.

Any advice would be appreciated, thanks!

Answer 1

You have several problems to solve on the way to a viable solution:

1. The alternator does not begin to regulate it's output until the speed is quite high (1500 rpm or above). So you'll need to configure your gearing so you spin the alternator quite fast. Looking at your datasheet you can see your likely area of operation in the 1000-1500 rpm area.

Even at just 1500 rpm and 50% efficiency (the worst it could be) you have the potential to generate about 360 W of power and be within the bounds of human effort. Your pedal cadence is likely to be about 70-80 rpm at best so you will need a total gear ratios 18-22:1 to drive the alternator.

2. Many alternators won't start if they have no voltage on the load side (they normally have a battery, even if it's a bit flat). If there is no battery as in your scenario you need to give it some help. I'd suggest you connect something like a 1000-4000 uF capacitor across the output, this will help stabilize the rotor current and help bootstrap the alternator.

3. When it does start to regulate it wants to set the voltage to about 13.8-15 V, this is the voltage normally required to charge/top-up a lead acid battery (13.8 V fully charged). You will be able to run 12 Volt auto globes or many of the 12 V LED lights you find at this voltage without having to worry about series resistors.

4. Human power output varies. If you use a sports fit person you might be able to get 500 W easily (https://en.wikipedia.org/wiki/Human-powered_transport).

5. You will lose some power to the voltage/current required for the rotor but the alternator efficiency should be about 60% or higher at the lower rpms you'll be able to achieve. You can also get a marginal increase in the overall efficiency by removing the fan on the front of the alternator since you won't generate much heat in the alternator.

6. Don't plan on spinning the alternator much beyond about 2000 rpm as the efficiency (losses increase) drops with higher rpms.

7. You could also replace the internal 3 phase bridge with more modern (and low forward voltage) Schottky diodes and boost efficiency by another 10-15%.

Note: You could also consider that there are alternative motor types that can be used very effectively as a generator. For example if you use an RC Outrunner Brushless motor, these work well as generators. The difference here is that there is no field to power (it has magnets) and the output voltage varies almost linearly with rotational speed. You could feed one of these into a 3 phase rectifier/capacitor and use a SM DC/DC convertor to regulate back to 12 V.

Answer 2

No complete design from here, but some details that you must know:

1. Wake-up

This alternator can need a substantial RPM until its current production starts. Your paper promises "less than 2500 RPM". That can be for example 2499,5 RPM. If a normal bike wheel spins that fast, the speed is about 150 km/h. That's quite high tempo for human feet, if the gearing is the usual one, actually impossible. To start the alternator internally you need much more higher gear ratio than normal bike has.

The reason for the need of the high initial spin RPM: The alternator develops itself the needed field magnetization by rectified DC current. In the beginning there's only a weak residual magnetism in the iron material. That needs a substantial coil velocity for sufficient voltage. When the wake-up has happened, the RPM can be smaller.

You can probably reduce the needed initial RPM by connecting a strong magnet to the alternator.

2. Human power output capacity

A healthy, but totally untrained person can produce sustained (=several minutes) maybe 100 watts. That means at 12V only 8,3A current. The field magnetization inside the alternator easily eats a half of that. You have 50 watts left. You can connect parallel several low wattage light bulbs or one 50W bulb. All bulbs must be for 12 volts.

Believe: It really will be hard to keep that 50W bright. Connect at first only one indicator lamp to test. Probably you should consider to have LEDs for any no-nonsense light. They give easily several times more lumens per watt than traditional bulbs.

Answer 3

Your alternator appears to be a typical car alternator that will need to be rotated quite fast in order to properly power a typical low Wattage (36W) car lamp (e.g. 12V 4W lamp). How fast? Well, that depends on the efficiecy of the alternator & how much available power that it can provide at an RPM that you are able to generate via peddling your bicycle--with the proper gearing.

As an example, if you could pedal a 10 speed bicycle with 27" tires to a speed of 19 mph, you would only generate an output rpm of ~237 rpm. Your alternator requires more than 1000 rpm to begin to produce power. That is more than 4.2 times faster than a typical 10 speed bicycle could turn the alternator. You wouldn't be able to peddle fast enough with that gearing to achieve an electrical output that would light your bulb properly.

So, for you to pedal a bicycle to generate sufficient power to light a typical 36W low Wattage bulb properly with your alternator, you would have to have different gearing to spin the alternator faster. You could use a LED bulb (which requires/uses even less power) but you would still need to change the bicycle gearing to generate sufficient rpm for your alternator.

Alternatively, you could use a typical bicycle with a dynamo to generate sufficient power to light a bulb/s. Many old bicycles used such dynamos to generate sufficient power to generate light from older 6V bulbs.

Answer 4

What range would you like to cover?

A pro cyclist could manage a best of 5-5.5w/Kg average and more peak; a good, competitive amateur or masters’ racer can put out around 4w/Kg and an untrained person would struggle to produce 2.5w/Kg.

With gravity and weight and up-down pedal thrust can do more with spurts of power.

You will need to characterize or lab test the alternator with a DC motor and switch different load lamps in parallel 35W (headlamps). PWM is used for continuously variable loads. A scope is necessary with current shunts to compute power input and out vs RPM as tungsten lamps rise in R by 800% with temperature thus keeping current more constant as voltage rises.

Surplus or scrap yard automotive parts are cheaper than retail.

It is cheap, but you can only use <10% of the rated load, so it will be an unregulated voltage vs speed output rated by the curve given ~12V/800 RPM as a linear slope (maybe 14V/800RPM) above which the V is regulated to automotive batteries at 14.2V nominal and not 14.7V.

You need to understand that energy conversion from mechanical to electrical has Torque vs RPM profile that varies with electrical load and very few loads are linear like resistors. Torque and current are directly which is a function of RPM. If these are not matched then optimal power cannot be achieved.

Gears are linear, but bulbs, LEDs and fan loads are very nonlinear.

You need to go back to the drawing board and define your project in terms of Energy, Torque, Power, RPM and then choose a load to match your input human power source. and DO NOT START ANY DESIGN UNTIL MEASURABLE SPECS ARE WRITTEN. Lab tests may be necessary to measure DC motor input power , Alt, output power vs load vs RPM. Keep tables, put into spreadsheet and do some math and use a scope if possible.

If it were me, I try to use a car rad. fan as a load and then match impedances and power optimized with PWM load control. This way the more energy generated, the higher the fan speed and thus faster cooling rate for the user.

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