Booster (electric Power) - Reversible Booster

Reversible Booster

Before solid-state technology became available, reversible boosters were sometimes used for speed control in DC electric locomotives. To avoid confusion, it should be explained that it is the electrical output of the booster that is reversible, not the direction of rotation.

The motor of the MG set was connected in parallel with the supply, usually at 600 volts, and was mechanically coupled, via a shaft with a heavy flywheel, to the generator. The generator was connected in series with the supply and the traction motors, and its output could be varied between +600 volts, through zero, to -600 volts by adjusting switches and resistors in the field circuit. This allowed the generator voltage to either oppose, or supplement, the line voltage. The net output voltage could therefore be varied smoothly between zero and 1,200 volts as follows:

• Generator producing maximum opposing voltage, net output zero volts
• Generator producing zero volts, net output 600 volts
• Generator producing maximum supplementary voltage, net output 1,200 volts

To match the 1,200 volt output, the locomotive would have three 400 volt traction motors connected in series. Later locomotives had two 600 volt motors in series.

When the locomotive was working at full power, half the energy came through the MG set and the other half came directly from the supply. This meant that the power rating of the MG set needed to be only half the rating of the traction motors. Thus there was a saving in weight and cost compared to the Ward Leonard system, in which the MG set had to be equal in power rating to the traction motors.

If the power supply to the locomotive was interrupted (e.g. because of a gap in the third rail at a junction) the flywheel would power the MG set for a short period to bridge the gap. During this period, the motor of the MG set would temporarily run as a generator. It was this system that was used in the design of British Rail classes 70, 71 and 74 (Class 73 does not utilise booster equipment).