In a large-capacity rotating electrical machine, an armature winding is provided with upper and lower coil pieces arranged in slots of a laminated iron core in a two-layer structure, and parallel circuits are connected in series, thereby increasing the generated voltage and machine capacity. However, as a voltage of an armature winding is increased, the thickness of the main insulation of an armature winding is increased to withstand such higher voltage. As a result, a cross-sectional area of a conducting portion is reduced, and a current density increases, so that a loss increases.
Particularly, in an indirect cooling type machine in which the armature winding is cooled from an outer side of the main insulation, as the thickness of the main insulation increases, a thermal resistance increases, and a temperature disadvantageously increases in the armature winding. For this reason, an armature winding is divided into multiple parallel circuits, thereby decreasing the voltage of the armature winding and the thickness of the main insulation, reducing the loss, and increasing the cooling capacity, while maintaining the machine capacity. Further, in an indirect cooling type large-capacity machine, it is common to increase the number of slots in the armature winding to increase a cooling cycle of the armature winding. Therefore, an armature winding having three or more parallel circuits is desirable.
If the armature winding having three or more parallel circuits is applied to a 2-pole machine, the voltages generated by each of the parallel circuits are not completely equated, and an eddy current is generated between the parallel circuits, and a loss in the armature winding increases disadvantageously.
In order to reduce such a loss cause by the circulation current, it is necessary to minimize imbalance between the voltages generated by the parallel circuits. For this purpose, a special consideration is required in placement of coils of each parallel circuit in each phase belt.
An example of improvement in the placement of coils will be described with reference to FIG. 13, which is a developed perspective view illustrating a part of an armature winding for one phase.
FIG. 13 illustrates an example of an armature winding having four parallel circuits applicable to a 3-phase 2-pole 72-slot rotating electrical machine as discussed in the U.S. Pat. No. 2,778,962 (hereinafter, referred to as “Taylor's patent”).
Although FIG. 13 illustrates a part of an armature winding for only one phase, it is obvious that the same configuration as that of FIG. 13 may be similarly applied to the other two phases shifted by 120° and 240°.
In Taylor's patent, assuming that the parallel circuits are numbered “1 to 4,” twelve parallel circuits of upper and lower coil pieces 15 and 16 of a first phase belt 17 are numbered “122121121221” sequentially from a pole center. Similarly, parallel circuits of upper and lower coil pieces 15 and 16 of a second phase belt 18 are numbered “344343343443” sequentially from the pole center. This decreases a deviation of voltage (an absolute value of a deviation from an average phase voltage) and a deviation of phase difference (a deviation of phase angle from an average phase voltage) of each parallel circuit.
To realize such a connection, in FIG. 13, fourteen jumper wires 20a per phase are provided in a connection side coil end 19a. 
Meanwhile, a technique for reducing deviations in the voltage and the phase angle between each parallel circuit is discussed in U.S. Pat. No. 2,778,963 (hereinafter, referred to as “Habermann's patent”).
In Habermann's patent, a voltage deviation between each parallel circuit is rated at 0.4% or smaller, and a phase angle deviation is rated at 0.15° or smaller. However, in Taylor's patent, the voltage deviation is rated at 0.12%, and the phase angle deviation is rated at 0°. It is conceived that these values are highly balanced and sufficiently efficient to decrease an eddy current under the same condition.
The connection method of Taylor's patent described above provides an armature winding having four parallel circuits applicable to a 3-phase 2-pole 72-slot rotating electrical machine. However, in an indirect cooling type large-capacity rotating electrical machine, it is necessary for the armature winding to have a greater number of parallel circuits. In this regard, as illustrated in FIG. 14, a connection method for an armature winding of a 2-pole 72-slot rotating electrical machine having six parallel circuits is known in the art. However, although this connection method provides an armature winding having six parallel circuits applicable to a 3-phase 2-pole 72-slot rotating electrical machine, its application is limited only to the 3-phase 2-pole 72-slot rotating electrical machine.
In the future, it is anticipated that a novel large-capacity technology is employed in the indirect cooling type large-capacity rotating electrical machine, and this may increase the number of windings to obtain a satisfactory generation voltage. For this purpose, it is desired to implement an armature winding having a greater number of slots. For example, it is desired to implement an armature winding of a rotating electrical machine having six parallel circuits applied to a 3-phase rotating electrical machine having 45 slots per pole.