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.

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