The flux component of the stator current must be calculated from a mathematical model of the motor. A mathematical transform (Clarke-Park transformation) is carried out on the actual stator currents to derive a measure of the actual flux and a representation of the deviation from the desired value. The inverse transform is used to derive the corresponding error correcting signals to be applied to the input of a variable frequency inverter to generate the appropriate stator currents (amplitude, frequency and phase) to correct the error.
A passing loop (UK usage) or passing siding (North America) (also called a crossing loop , crossing place or, colloquially, a hole ) is a place on a single line railway or tramway , often located at a station , where trains or trams travelling in opposite directions can pass each other.  Trains / trams going in the same direction can also overtake, providing that the signalling arrangement allows it. A passing loop is double-ended and connected to the main track at both ends, though a dead end siding known as a refuge siding , which is much less convenient, can be used. A similar arrangement is used on the gauntlet track of cable railways and funiculars , and in passing places on single-track roads .