08-10-2012, 11:59 AM
Design of an electrical drive for motorized bicycles
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Abstract.
The aim of this work is the design of an electrical drive for a motorized bicycle, characterised by new solutions for the control method and the regenerative braking system. A dynamic model of the vehicle has been realised, and the characteristics of the drive have been individuated. An effective closed-loop control strategy has been studied, adjusting the motor torque and the current in order to increase the availability range. Finally, a feasibility study of a regenerative braking system based on the supercapacitor technology has been carried out. All the components of the drive have been selected among the models available on the market. In the paper the results of the simulations are presented and other technical-economical aspects such as energy consumption and costs are also briefly discussed.
Introduction
Human-powered hybrid electric vehicles can become a key to personal transportation in an environment where atmospheric pollution must be limited, where automotive traffic congestion is severe, and where parking space in urban centres is not available [1].
According to the European Law 2002/24/CE item h point 1, a power-assisted bicycle is a two or three wheels vehicle equipped with an auxiliary electric motor whose maximum nominal power is 0.25 kW.
Electrical bicycles offer extremely efficient, pollution-free transportation for urban and suburban areas, and the addition of electric drive extends their range.
Motorized bicycles are an economic and ecological vehicle suitable for all ages; the use of a helmet is not compulsory; they will not normally require registration and taxes, licensing or operator qualification [2, 3].
In this paper, the term “motorized bicycle” is used to describe a partially motor powered bicycle, commonly known as “pedelec” (Pedal electric cycle) [3].
Electrical Drive Design
The power propelling a bicycle and rider goes mostly into overcoming wind resistance and lifting mass up hills at normal bicycle speeds [2, 3].
Bearing and tire friction are small but can equal wind resistance at very low speeds.
The electric motor torque curve is a function of road slope p, rolling friction coefficient Croll (whose value depends on the road conditions), wind speed Vw, cyclist resistance coefficient Cr, and total mass m of the bicycle-cyclist system. Equation (1) represents the equilibrium and equation (2) keeps the bluff body pressure drag and skin friction drag into account.
Choice of the Electric Motor
Two different types of motors are commonly chosen for the auxiliary drive of such vehicles: DC Brushed motors and radial flux DC Brushless motors with permanent magnets (RFPM). Mostly for its reduced size and higher efficiency a DC Brushless motor with a particular shape – a so-called hub motor - has been chosen (external rotor and internal stator).
For this work a 0.25 kW three-phase brushless motor has been selected. It has an internal stator solidal with the wheel hub and an external rotor rotating at the wheel speed. The electric machine is positioned on the front wheel for a simpler installation. The power transmission system is of the direct drive type, so that gears and coupling joints are avoided.
Regenerative Braking
The battery types chosen in this work are Nickel Metal Hydrate with a nominal voltage of 36V and a 9Ah capacity.
Since the level of assistance is strongly influenced by the battery range, the management of the battery charge and discharge phases is particularly important.
The possibility to recover the braking energy is of great interest in designing the electrical drive. The regenerative power control for electric bicycle method is a simple and low-cost solution [5].
Under appropriate conditions, the batteries can be recharged. During a deceleration or braking, an amount of kinetic energy is usually lost as friction on the wheel. The regenerative braking system allows recovering part of such kinetic energy, to be used to feed either the battery or the electrical drive.
Conclusions
A number of different aspects thrust the use of electric bicycles in different situations. These include lower energy cost per distance travelled for a single rider, savings in other costs such as insurances, licenses, registration, parking, improvement of the traffic flow, environmental friendliness, and the health benefit for the rider.
In the paper, the design of an electrical drive for a motorised bicycle is described, using commercial components available on the market.
On the basis of technical-economical consideration, the feasibility of such a system for industrial production has been analysed.