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Trams use large amounts of power when accelerating, then coast without power for a distance before braking to a halt. The power demand is in the form of a large peak, followed by a period of no demand and sometimes, if regenerative braking is employed, a period when some of the power is returned to the system. The 'peaky' nature of these power requirements can be be seen in a chart recording of the demand on a tramway power station where several trams are operating simultaneously.
The maximum steady speed up different gradients can be assessed for trams of various power-to-weight ratios by using a chart .
To see the chart click here (Caution: large file 190 kb)
For examples of practical calculations click here
The power requirements for a tramway vehicle are given in ICS Vol 22 as around 1.5 kW per tonne. This is a very approximate figure which can vary with factors such as hills and service speeds, but can it be a useful way to obtain a rough estimate of the total energy requirements of a system.
Thus a 15 tonne tram running for 18 hours per day would be expected to use 1.5 x 15 x 18 = 405 kW hours of electricity.
For an estimate of the power costs of the proposed Bath system click here
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