The generator-motors can either be reversible as at Dinorwig, or have a single operating direction as at Ffestiniog. The latter is only possible when the hydraulic machine modes of pump and turbine are realised in separate components. The factors affecting the choice of generator-motor are:
The specific speed of the pump-turbines dictates the generator-motor speed and many combinations are possible. The choice is usually made on economic grounds without resorting to large extrapolation from existing .
This is because the size of the caverns increases greatly with many small machines, whereas the plant costs stay reasonably constant because the optimum speed of operation decreases with unit size, leading to more expensive electrical machines.
