single-phase synchronous motor and a three-phase synchronous generator with common stator and rotor. It had two independent windings: The outer winding is a single-phase synchronous motor. The motor takes the power from the overhead line. The inner winding is a three-phase (or variable-phase) synchronous generator, which provides the power for the three- (or more) phase traction motors. Single-phase supply The direct feed from a standard electric network makes the system less complicated than the earlier systems and makes possible simple recuperation. The single-phase feed makes it possible to use a single overhead line. More overhead lines increase the costs, and restrict the maximum speed of the trains. Speed control The asynchronous traction motor can run on a single RPM determined by the frequency of the feeding current and the loading torque. The solution was to use more secondary windings on phase converter, and more windings on motor different number of magnetic poles. Types A rotary phase converter (RPC) may be built as a motor-generator set. These completely isolate the load from the single-phase supply and produce balanced three-phase output. However, due to weight, cost, and efficiency concerns, most RPCs are not built this way. Instead, they are built out of a three-phase induction motor or generator, called an idler, on which two of the terminals (the idler inputs) are powered from the single-phase line. The rotating flux in the motor produces a voltage on the third terminal. A voltage is induced in the third terminal that is phase shifted from the voltage between the first two terminals. In a three-winding motor, two of the windings are acting as a motor, and the third winding is acting as a generator. Because two of the outputs are the same as the single phase input, their phase relationship is 180°. This leaves the synthesized phase to be +/-90° from the input terminals. This non-ideal phase relationship requires a slight power de-rating of motors driven by this type of phase converter. Also, since the third, synthesized phase is driven differently from the other two, its response to load changes may be different causing this phase to sag more under load. Since induction motors are sensitive to voltage imbalance, this is another factor in de-rating of motors driven by this type of phase converter. For example, a small 5% imbalance in phase voltage requires a much larger 24% reduction of motor rated power. Thus tuning