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Phase rotation is always counterclockwise (internationally adopted convention) and indicates multiples of 30 degree lag for low voltage winding using of the high voltage winding as the reference. Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°. According to the IEC60076-1 standard, the notation is HV-LV in sequence.
A three-phase motor is more compact and less costly than a single-phase motor of the same voltage class and rating, and single-phase AC motors above 10 hp (7.5 kW) are uncommon. Three-phase motors also vibrate less and hence last longer than single-phase motors of the same power used under the same conditions. [32]
On the inside, the configuration of the wire windings makes it different. The stator portion of the resolver houses three windings: an exciter winding and two two-phase windings (usually labeled "x" and "y") (case of a brushless resolver). The exciter winding is located on the top; it is a coil of a turning (rotary) transformer.
Transformation of three phase electrical quantities to two phase quantities is a usual practice to simplify analysis of three phase electrical circuits. Polyphase a.c machines can be represented by an equivalent two phase model provided the rotating polyphases winding in rotor and the stationary polyphase windings in stator can be expressed in a fictitious two axes coils.
Wiring schematic for a simplified bipolar-field Gramme-ring single-phase–to–direct-current rotary converter. (In actual use, the converter is drum-wound and uses a multipolar field.) [2] Wiring schematic for a simplified two-phase–to–direct-current rotary converter, with the second phase connected at right angles to the first [3] Wiring schematic for a simplified three-phase–to ...
A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer, which can also produce three-phase power from a two-phase input. A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order.