A rotary phase converter, abbreviated RPC, is an electrical machine that converts power from one polyphase system to another, converting through rotary motion. Typically, single-phase electric power is used to produce three-phase electric power locally to run three-phase loads in premises where only single-phase is available. Operation A basic three-phase induction motor will have three windings, each end connected to terminals typically numbered (arbitrarily) as L1, L2, and L3 and sometimes T1, T2, T3. A three-phase induction motor can be run at two-thirds of its rated horsepower on single-phase power applied to a single winding, once spun up by some means. A three-phase motor running on a single phase cannot start itself because it lacks the other phases to create a rotation on its own, much like a crank that is at dead center. A three-phase induction motor that is spinning under single-phase power applied to terminals L1 and L2 will generate an electric potential (voltage) across terminal L3 in respect with L1 and L2. However, L1 to L3 and L2 to L3 will be 120 degrees out of phase with the input voltage, thus creating three-phase power. However, without current injection, special idler windings, or other means of regulation, the voltage will sag when a load is applied. Power-factor correction is a very important consideration when building or choosing an RPC. This is desirable because an RPC that has power-factor correction will consume less current from the single-phase service supplying power to the phase converter and its loads. A major concern with three phase power is that each phase be at similar voltages. A discrepancy between phases is known as phase imbalance. As a general guideline, unbalanced three-phase power that exceeds 4% in voltage variation can damage the equipment that it is meant to operate. History At the beginning of the 20th century, there were two main principles of electric railway traction current systems: DC system 16⅔ Hz single phase system These systems used series-wound traction motors. All of them needed a separated supply system or converters to take power from the standard 50 Hz electric network. Kandó synchronous phase converter Kálmán Kandó recognized that the electric traction system must be supplied by single-phase 50 Hz power from the standard electric network, and it must be converted in the locomotive to three-phase power for traction motors. He created an electric machine called a synchronous phase converter, which was a