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Electric Sportbikes - The Silent Future
An introduction to electric sportbikes
From the December, 2010 issue of Sport Rider
By Andrew Trevitt, Alan Cathcart
Photography by Kevin Wing
With the growing emphasis on eco-friendly technologies in the automotive sector, it's no surprise that motorcycles are headed in that direction as well. The death of the internal combustion engine is still a long way off, but it appears that an all-electric solution is the replacement of choice, as opposed to the gas/electric hybrids currently offered in the automotive sector. That only makes sense, as it would be difficult to squeeze an electric motor and battery as well as a gasoline engine and fuel into a motorcycle. The electric motor has been around for ages, but it's only recently that battery technology has improved to the point that a reasonable amount of energy can be stored in a small enough package for motorcycle use.
There are three basic components to an electric bike's powertrain: The battery, motor, and a controller that essentially translates throttle position into acceleration and speed. Just as there are many types of internal combustion engines in use, there are many types of electric motors. These can be categorized as AC or DC and brushed, induction or permanent magnet. AC and DC are fairly obvious, but the other aspects require some explanation. An electric motor turns by virtue of opposing magnetic fields in the stator (the outer, "static" part of the motor) and rotor (the inner, "rotating" part). The magnetic field in the stator is generated by passing electric current through windings; in the rotor, however, the magnetic field can be created in three ways. Most DC motors use brushes to pass current into windings in the rotor. Brushed motors are relatively inexpensive, but bring about maintenance and reliability issues - the brushes can fail and need to be replaced regularly. In AC motors, the field can be induced into the rotor without any actual contact, much as a transformer works. An induction motor is typically more reliable than a brushed motor as there are fewer parts to wear or fail, but is more expensive and requires more elaborate speed controls. Finally, a permanent magnet can be utilized; the magnetic field in the rotor is created with a magnet usually made from a rare-earth material. A permanent-magnet AC motor is sometimes referred to as a synchronous motor, because the rotor turns at a specific speed. An induction motor, in which the rotor lags behind the rotation of the magnetic field in the stator, is sometimes called an asynchronous motor.
There are still other types of motors - DC motors do not necessarily have to have brushes, and some AC motors have "slip-rings" in place of brushes. With the introduction of inexpensive electronic controls, the difference between AC and DC motors is not so clear; in fact, a universal motor can run on either power source. Because electric bikes are still in their infancy, companies are still experimenting with the various technologies; many different examples of the various types of motors can be found among the bikes shown here.
Typically, a motor's output is expressed in kilowatts (kW), a measure of electrical power, and torque. For example, the Brammo Empulse has a sealed permanent AC synchronous motor rated at 40 kW with a maximum torque of 59 ft-lb. It takes 736 watts to make one horsepower, so the Empulse makes about 50 horsepower at the engine's output shaft. Note that an electric motor makes its peak torque at all rpm, from almost zero to its peak. For the Empulse, we can use the torque and power figures to calculate a maximum engine speed of approximately 4500 rpm.
All the bikes shown here use some form of lithium-based battery. Lithium has several advantages over other materials, and gives the best compromise (for now) of cost, size and weight. A typical lithium cell generates 3.6 volts compared with the 2 volts in each of the six cells of your standard motorcycle battery or the 1.2 volts of a nickel-metal hydride cell. This means fewer cells must be joined together to generate the voltage necessary to run the motor, resulting in more reliability as well as less weight and size. A lithium-ion battery is a rechargeable unit similar to what you would find in a laptop computer or cell phone; a lithium-ion polymer battery uses a solid composite electrolyte rather than a more traditional organic solvent, and the individual cells can be packaged into more elaborate shapes.
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