rectifiers that could fit on the train. This in turn related to the requirement to use DC series motors, which required the current to be converted from AC to DC and for that a rectifier is needed. Until the early 1950s, mercury-arc rectifiers were difficult to operate even in ideal conditions and were therefore unsuitable for use in railway locomotives. It was possible to use AC motors (and some railways did, with varying success), but they have had less than ideal characteristics for traction purposes. This is because control of speed is difficult without varying the frequency and reliance on voltage to control speed gives a torque at any given speed that is not ideal. This is why DC series motors were the most common choice for traction purposes until the 1990s, as they can be controlled by voltage, and have an almost ideal torque vs speed characteristic. In the 1990s, high-speed trains began to use lighter, lower-maintenance three-phase AC induction motors. The N700 Shinkansen uses a three-level converter to convert single-phase AC to AC (via transformer) to DC (via phase-controlled rectifier with thyristor) to a maximum three-phase AC (via a variable voltage, variable frequency inverter using IGBTs with pulse-width modulation) to run the motors. The system works in reverse for regenerative braking. The choice of was related to the efficiency of power transmission as a function of voltage and cost, not based on a neat and tidy ratio of the supply voltage. For a given power level, a higher voltage allows for a lower current and usually better efficiency at the greater cost for high-voltage equipment. It was found that was an optimal point, where a higher voltage would still improve efficiency but not by a significant amount in relation to the higher costs incurred by the need for larger insulators and greater clearance from structures. To avoid short circuits, the high voltage must be protected from moisture. Weather events, such as "the wrong type of snow", have caused failures in the past. An example of atmospheric causes occurred in December 2009, when four Eurostar trains broke down inside the Channel Tunnel. Distribution Electric power for AC electrification is usually taken directly from the three-phase transmission system. At the transmission substation, a step-down transformer is connected across two of the three phases of the high-voltage supply and lowers the voltage to . This is then fed, sometimes several kilometres