|A to B Magazine, U.K. Millennium Special Edition
A to B Magazine Editors: David and Jane Henshaw, Somerset, England
Our resident technology expert interprets the Ex-Bike concept
|For some time electric bike power units have utilised cast-offs from elsewhere - rumor has it that the popular Zap motor was originally designed for remote operation of irrigation water valves. But when I first viewed the advance information on Rabbit Tools Electric Wheel some months ago, I guessed we were dealing with something very different, and so it appears.
Broadly speaking electric motors can be divided into two classes - those operating from a direct current (DC) supply, and those utilising an alternating, or AC, supply. DC motors - and almost everything in the bicycle field comes into this category - tend to be cheaper, heavier and cruder in their operation. The main problem is that they only operate efficiently within a fairly narrow speed range, for technical reasons that space does not permit me to dwell upon. Suffice to say, if a DC motor is forced to turn slowly, it draws a great deal of current from the supply, and without some form of protection, it will overheat and fail. Thus, for the sake of the motor, most electric bicycles only provide assistance above a set speed. So when power assistance would be most useful - for starting and hill-climbing - there is little or no power available.
In marked contrast, the Birkestrand motor is an AC device (24 volt, 300 watt, 3-phase, for the technologists), so despite its compact size (4-1/2 x 5) and light weight (5-1/2lb) it should provide plenty of power from low speed. Rabbit claim an overall speed range of 10 to 1, so the motor could - for example - be geared to work efficiently (90-95% according to the manufacturers) at any speed between 1-1/2 mph and the legal limitiof 15 mph. Batteries, of course, only provide a direct current, so some quite complex electronics are required to produce a variable frequency AC supply for the motor, but this conversion to alternating current brings a number of useful theoretical advantages.
The best way to envisage the operation of this sort of device, is to imagine the electric current creating a magnetic field that spins around within the motor, dragging the output shaft around and propelling the machine. If the shaft rotates slower than the magnetic field - due to the extra resistance of a hill for example - a current is drawn from the battery. If the shaft rotates faster, as when cycling dow hill, a current is returned to the battery. This regenerative braking occurs with most DC motors, but as the effect usually only comes into play at high speed, its little more than a scientific curiosity for cyclists. With the Electric Wheel system, the braking effect should be available right down to walking pace, recharging the battery and saving wear and tear on the brakes.
|The QC-250 charger replaces a water-bottle and is claimed to recharge any battery in an hour.|
|DC supply combined with AC motor technology has been used very effectively on modern commuter trains in South East England, and on the Eurostar train, which shares the same tracks, but the engineers suffered many problems with electrical interference to signalling systems. One hopes the new Birkestrand machine will not have a similar effect on traffic lights!
Once one has made a commitment to the complex electronics required to produce a variable alternating current, one might as well incorporate other gadgetry. Birkestrand claims that the Ex-Bike computer can be programmed to sample battery temperature, making short bursts of extra-high power permissable, and it can be programmed to cut off at any desired maximum vehicle speed. Other features include a choice of maximum power presents, cruise control, and reduced get you home power when the batteries approach exhaustion. All very sophisticated indeed.