We’ve just received many of the parts for the generator, and will test them out this week! Suddenly all of the planning and fund-raising and cogitating is going to be attached to a couple of bikes with power cables coming out of them. This is a big step, so I suppose it’s time to update everybody on how this thing is going to work.
We bought our motors from ebikes.ca in Vancouver BC. There are more local options, but these come highly recommended, and there is a wealth of information and support at their website. The motors are “brushless DC motors,” which is a bit of a misnomer, as the motor itself runs on (or outputs) three phase AC. In a motor application, that AC comes from a motor controller which converts 12V DC into a variable frequency, variable voltage power signal according to the desired speed and power. Since we’re using the motors backward, taking power out instead of putting it in, we’ll need to make sense of that 3 phase AC.
The motors come in three different windings, from 6 turns to 8 turns. This is equivalent to using gears of different sizes on your bike; if you supply a given voltage, the 6 turn motor turns faster than the 8 turn motor, but with less torque. What this means to us is that the 8 turn motor will supply our target of 14V at a lower speed, meaning that we can use the low gears we’ll need for hauling cargo. However, it might be too much, and we’ll have to use super low gears to keep our voltage low. We ordered a 7 and an 8 turn motor, and we’ll figure out which one we need once we have the bike in the trainer and a load attached.
A rectifier takes the alternating output of a motor and converts both the positive and negative peaks into positive current, effectively flipping all the troughs into peaks. It works by feeding the alternating current through an arrangement of diodes (which only allow current in one direction, like a check-valve). Ours are packaged in neat little boxes with spade terminals and a must be mounted on a heat sink; the diodes don’t put out a lot of heat, but it’s enough to damage them if they can’t dissipate it.
We’re using the element from a ceramic space heater as a load. This is basically a big resistor; current has to pass through a series of special ceramic blocks that are poor electrical conductors. These heat up, normally to heat the air blown through them. This element is unfortunately a bit too strong, with a resistance of 180 ohms, but since there two elements, we can connect them in parallel, cutting that resistance down to 90 ohms. If I’d been able to find a traditional coil element, we could clamp on anywhere along the coil to adjust the resistance.
For a more visible effect, we picked up three 100Watt light bulbs. These have a resistance of 144ohms apiece, but connected in parallel we’ll get 48 ohms, which is a pretty reasonable load to power.
One of the toys we received is the Watt’s Up meter. This little guy connects in line with our power, allowing us to see just how much power, in volts, amps, and watts (just so you don’t have to compute the product). This means we can finally get a good idea of what a person puts out on our system! This is really exciting.
Alright, see you at the workshop!