The electric motor
The engine is the distinctive element of a motorcycle and determines its character and image: twin-cylinder, single-cylinder, 4-cylinder in line, V-shaped, L-shaped, electric. In the case of an electric bike, the function of the motor is to convert the electrical energy supplied by the battery into mechanical energy to move the motorcycle.
The components used to rotate the shaft of the electric motor are the stator and the rotor; the first is fixed while the second is integral with the shaft and rotates with it. The rotor is rotated by the magnetic field generated by the alternating current flow passing through the stator windings. The motion of the rotor is transmitted to the pinion by different types of final transmission: through a gear, a belt, a gearbox. There are also cases in which the pinion is fixed directly on one end of the motor shaft.
Electric motors can be divided according to various criteria; the main one is the way they are fed:
• DC motors (Direct Current): powered by direct current
• AC (Alternating Current) motors: powered by alternating current
DC motors were the first to be invented and have the advantage of being very cheap and easy to control but wear out quickly and have a low power density. AC motors, on the other hand, are constructively more complex and difficult to control, but they manage to reach very high efficiencies (>97%) and very high torque values.
In the automotive and motorcycle sector, the permanent magnet synchronous AC motor is currently the most used type due to its high efficiency and reliability: its acronym is PMAC (Permanent Magnet AC Motor).
Another type of electric motor is the reluctance one (SynRM), characterized not only by its high energy efficiency but also by its reliability and low maintenance requirements.
The element that supplies current to the motor is the battery; however, this provides direct current while the motor needs alternating current to generate the magnetic field in the stator. The inverter performs the conversion function from direct current to alternating current. Here we do not want to go into the details of the operating principle of an electric motor, but we prefer to describe how this component characterizes the bike and the riding style. For those wishing to learn more about how an electric motor works and how it is built, it can be found in this section (link).
An endothermic engine is characterized by a power and torque curve, which increases as the rpm varies up to a maximum and then decreases. Given that at high rotation speeds, the torque reduces a lot, to accelerate even more, a gearbox is coupled to the engine; the effect obtained is represented in this graph, where I, II, III and IV indicate the gears engaged.
Conversely, an electric motor has a power curve that increases with the number of revolutions for most of the range of use until it stabilizes at high revolutions. On the other hand, the torque curve has its maximum already at zero revs and remains constant for a large part of the range of use, then decreases to a minimum at maximum revs.
The torque curves of the electric motor and the internal combustion engine have opposite trends with important effects on the behaviour of the motorbike. Referring to MotoE, coming out of a slow corner, therefore, at low engine revs, the bike's torque is high even at the minimum rotation of the throttle. On the other hand, in fast corners, the electric motor works in the area where the torque value has dropped, and the throttle can almost be fully twisted, which is hazardous, for example, with a Moto2, which, in the same situation, has the engine at maximum torque.
One more important effect of the motor on the riding style of an electric motorcycle is the handling. An endothermic engine is characterized by considerable moving masses, whose gyroscopic effect acts to reduce the handling of the bike. This can be reduced by varying the engine configuration but cannot be fully eliminated. In an electric motor, the only rotating mass is that of the rotor and is extremely little. As a consequence, its gyroscopic effect of very little too. For this reason, an electric motorcycle is much easier to handle than a motorcycle with an endothermic engine of similar weight.
Electric motors also have a unique feature: regenerative capacity. When braking, the rear wheel reduces motor rotation, reversing its operating principle. The engine, therefore, works as a generator, creating a flow of current to the battery, which is thus recharged.
Introduction to Electric Motorcycles
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