Wednesday, February 9, 2011

Bicycle Generator (part 2)

So, with the problems of the Mark II bike, I had to adapt it again so that I could make some useful power to charge the house battery.

The Mark III bike uses a different approach to powering the stator field coil.  I got some old NiCd batteries from a dead cordless drill and laid them out on a bit of wood.
I bought and cannibalised a car power extension lead so that I could connect the battery to the generator, using a plug and socket.  The plug was handy as it has a fuse in it (so I didn't need the breaker in the photo above), and it also has a green LED that gives you some idea of when the battery is getting discharged, glowing dimly when the battery is low.  I just tied the plug to the bike handlebars, making it easy to plug in and switch on.

By charging the batteries from the solar power, and then using the stored fixed Voltage to power the stator coils, the generator can start by itself, and you get a current and Voltage output from it that is now directly proportional to how fast / hard you pedal.

At first, I tried using a 12V battery, but the stator winding is only something like 1.8 Ohms, so it took too much current.  By using a string of six NiCd cells, I could get an ideal 7.2V at about 4A going through the stator coil.  The NiCd battery lasts about 15 minutes at that rate, and this also is about as long as I can cycle continuously :D.

The bike can make anywhere from 30 to about 70V, if you pedal really fast.  The Morningstar Tristar MPPT solar charge controller can accept any Voltage up to 150V DC to charge the 24V battery, and it will convert higher Voltages to the right Voltage, stepping up the current in the process.  This means that I can pedal fast, and the bike will generate maybe 45-55V at a a low current of around 1.0-1.5A.  Most of the time I pedal more slowly and it makes about 32-35V at about 2A.  This keeps the motor brushes and windings cool, reduces the losses on the long wire run to the house and the battery still gets all the power generated (well about 95% as some is lost in the conversion).

I was a bit worried that the MPPT charge controller is not designed to work with generators (like wind turbines or bikes) as it periodically unloads the input and does a sweep to measure the maximum power Voltage of the solar array.  This could cause a wind turbine, or my bike generator, to suddenly become unloaded and over spin (with the result of me falling off the bike again :/ ).  Happily, when I tried it out, the sweeps don't happen very often, and are very short on this controller (just a fraction of a second).  The motor does over spin a little, but not so much as to be a problem because the bike has a heavy fly wheel that limits the acceleration in that split second.
To smooth out any Voltage spikes, I set a pair of output diodes and a high Voltage capacitor in the circuit so that charge from the motor is pumped into the capacitor. Then the wiring from the generator just joins the rest of the solar panel wiring network in the garden.

The first capacitor I used was a spare 50V one that was laying about in my "lab"; It got over loaded and exploded during a "sprint" :D. So I bought a new 100V rated one, and (so far) that one has survived, but now I also have a Volt meter to keep an eye on the output level.

Here is the first power trace on the main system from my bike generator. The Morningstar MSView software logger clearly shows me pedalling and generating a steady 60W of power.

To provide a nice finishing touch, I bought a couple of analogue meters from eBay. 
One is a 0-3A moving iron Ammeter for school labs and the other is actually an antique Bakelite 0-7 Volt meter.  I just worked out what extra series resistance the meter needed in order to convert it to read 0-70V instead.  This scale gives a good indication of the bike output, which is in the 30-55V range, but can surge to about 65V if the charge controller does a measurement sweep while I'm pedalling hard and the generator over spins a bit.

Now all I need is some decent leg muscles...

Tuesday, February 8, 2011

Bicycle Generator (part 1)

A while ago, I started playing with making an exercise bicycle generator.

I got an unwanted and unloved exercise bike from a car boot sale for £5 and a car alternator from another car boot sale for £10. The result was the Mark I generator bike.
The problem with this design was that it only worked at 12V and house battery is 24V, so it was only good for charging little 12V batteries or running a light in the garage. Alternators are also designed to be powered by a car engine and not a bicycle so they need to spin quite fast before they generate any power. When they do start to generate, they use quite a lot of the power to create the magnetic field required and so you waste quite a lot of your energy just making the thing "charge up". The alternator also had a Voltage regulator that limited the output to 14.5V, which was a bit too low to charge a battery at the end of a long wire, that suffered a Voltage drop from the wire resistance.

Later, at another car boot sale, I found an old washing machine motor and matching drive belt for 50p. This is then the basis of the Mark II bike.
The washing machine motor works at 230V but is a 16 pole DC motor, controlled by a microprocessor in the washing machine. Using the motor as a generator, the rotor, with its 16 pole winding on the rotor and a big stator coil generates fairly smooth DC power. It also has a smaller pulley on the rotor than the alternator, which increased the gear ratio to about 30:1, meaning I didn't have to pedal so fast to make the generator turn at the same speed.

I discovered that the laminated iron core retained some weak magnetism and this was enough to generate a small current in the rotor winding. By connecting the rotor and stator windings in series with the load, there is a positive feedback loop.

The small residual magnetic field in the core is enough to start a tiny current flowing in the rotor. This current passes though the stator coil and the load. The current flowing in the stator coil causes a stronger magnetic field to be created, which in turn causes more rotor current to be generated. The system has positive feedback, and if the output of the motor generator has a low resistance load, it can quickly cause very large currents and high Voltages to be produced. The only limit would be the input mechanical power and whether the load or the rotor windings and carbon brushes catch fire!

Below is a diagram of the circuit.
I use a push switch to start the generator by connecting a 21W 12V lamp or a 4 Ohm power resistor directly across the output and start to pedal. When the generated Voltage gets above the main battery Voltage plus the diode drop of 0.8V, the current starts to charge the battery and I can release the push button. The current going to the battery now sustains the generator stator field, and the generator will make as much power as you can supply. The generator will easily make 60-70V, so long wiring to the battery is not an issue.

The only problem is that the power and Voltage output can vary wildly, and the effort to pedal quickly gets too hard.  But if you slow down, even for a second, the battery current may stop (if the generator Voltage falls below the battery Voltage) and then the generator suddenly stops working and as you are pedalling hard, the load is suddenly removed and you fall off the bike! :D  It also runs the risk of a Voltage spike damaging anything else attached to the battery (like my inverter)... efficient but dangerous.