Regenerative Braking:
Regenerative breaking is used on all trams as it is a great way to convert what would be wasted energy back into electricity and back into the system. It is especially useful in Sheffield supertrams as there are many trams on the track throughout the day and the sheer amount of stopping at starting they do means a great amount of energy will be fed back into the system. How this works and the way it is utilised will be discussed below.
The kinetic energy that the tram has whilst moving is converted back to electricity whilst breaking it is then fed through the wires connecting the tram to the overhead wires and returned to the system. This saved electricity will be used to power another tram elsewhere around the city therefore reducing both running costs and pollution something which supertram is proud of. This is done as the three phase supply creates a magnetic field around the electric motor when a current passes through this motion is produced. The regenerative breaking utilises this in its breaking. When breaking the motion will be in the opposite direction and when compared to Fleming’s left hand rule (see right) this means the current will also be reversed thus electricity will be produced rather than the system putting electricity in to create the motion.
When a car or vehicle breaks friction between the wheel and break pad is created. This creates heat energy which is wasted figures show that this can be up to 30% of the engines generated power meaning to accelerate more energy has to be fed back into the system. This greatly reduces the efficiency of vehicles. Regenerative breaking is unique ion that it uses the motor in reverse to slow the tram which, looking at the afore mentioned explanation converts what on a different vehicle would be wasted to heat energy back into electricity. Regenerative breaking for all its qualities is only 30% efficient and this is still dependant on the wiring whether it be series or parallel and also the speed in which the tram is decelerating from, as it is less efficient at lower speeds. Less fuel used will mean less pollution there fore making regenerative breaking attractive to anyone. The energy saved (using the top speed and maximum efficiency) can be shown by working out the kinetic of the tram first. ,maximum efficiency is only at relatively high speeds so I will work out the energy saved when slowing from 22.2m/s~(max speed) to 10m/s using the formula E=0.5mv2 the mass of a Sheffield supertram is 48000kg. At top speed the tram has 11,828,160J of kinetic energy. At 10m/s it has 241000J of kinetic energy. Top – new speed = 9,428,160. The system is 30% efficient so up to 2,822,448 of energy can be saved. This is not an accurate calculation as the tram will never reach max speed around the city and the efficiency is significantly lower at lower speeds. Also there will be some resistance in the circuit.
As you can see regenerative breaking has many strengths making it attractive:- it saves on electricity bills, reduces pollution making the public more appreciative to wards the company, it also on average makes the slowing down more steady and smooth increasing the comfort for passengers, and it saves on the use of fuels. It doesn’t have any weaknesses as such only improvements and limitations:- at lower speeds ( like in the city centre) efficiency is greatly reduced, using the motor in reverse cannot make the tram completely stationary so breaks still required, could fail, difficult to gain maximum connection between trams and overhead cables so not all electricity generated will be put back into the system also when the emergency break is used the regenerative breaking is bypassed altogether as the risk of it not working cannot be taken. In the future these limitations will be improved. Currently scientists are trying to increase the efficiency of the system particularly at low speeds. They are also striving to improve the connection quality between the trams and the overhead cables. Regenerative breaking is slowly being built into the car manufacturing industry and is trying to be incorporated into future designs. Such things as exercise bikes and treadmills are also being looked into as innovative new ways to increase household efficiency as energy made when pedalling the exercise bike could be used to power lights as the resistance in the pedals could be replaced with a weak motor working in reverse thus creating energy much the way that regenerative breaking does. This is someway off as the initial costs are high and the power saved low. The motor sports industry is looking into regenerative breaking as they are currently receiving a lot of abuse from the public due to the way in which they ‘waste’ great deals of fossil fuels. One proposal is in F1 to make it compulsory in a few years as the energy saved could be used to power on board computers and communication devices. A similar proposal is being lined up in touring cars and possible speedway although speedway is more difficult and many years away.
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