Public Transport Systems in Urban Areas - Volume B [antikvár]

ELECTRIC BUSES PRESENT AND FUTURE TECHNOLOGY by Michael Appleyard, M.A, Manager - Vehicles Engineering Chloride Technical Ltd Manchester England INTRODUCTION Electric road vehicles have been commonplace in the United Kingdom for many decades. There are presently around 30,000 milk delivery trucks !n daily service and the success of these is due to low operating costs, high reliability and extremely long life. They are used in residential areas In the early hours when the noise from petrol and diesel vehicles is not welcomed. The distance that these vehicles can travel before the batteries need recharging is not usually a problem. The limiting factor is more often the ability of the operator to unload and carry the milk bottles to the individual premises of his customers. A typical milk float has a range of 40 km/hr. and a maximum speed of 25 km/hr. These are achieved with very simple motors and controllers, together with batteries which have traditionally been designed for mechanical strength and long life, rather than for low weight. i: PERFORMANCE DEVELOPMENT When it was decided to investigate less specialised road vehicle applications, the first priority was to obtain levels of acceleration and speed which would be compatible with the general traffic. The 'targets chosen were 65 km/hr. maximum speed and 20 seconds to accelerate from rest to 50 km/hr. These could not be achieved satisfactorily with conventional resistance controllers. Instead, a device called a "thyristor chopper" was employed to regulate the power from the battery. Unlike the milk float, the new vehicles would be driven for most of the time at speeds well below their maximum. Therefore, the voltage required at the input to the motor viou\6 usually need to be less than the battery voltage. With resistance devices, a voltage reduction Is only achieved at the expense of considerable energy dissipation. In contrast, the "chopper" circuit switches the battery 'on' and 'off several hundred times per second and achieves variations In the average motor voltage by adjusting the ratio of time 'on' to time 'off* during each switching pulse. The energy loss associated with this process is quite small. While the basic operating principle of the "chopper" is relatively simple, the actual electronic systems required to achieve it are complex. Because of this, 1» j I' r- »V