Fig. 1,426.—Diagram showing lateral field between adjacent poles.
Magnetic Leakage.—In the design of alternators the drop of voltage on an inductive load is mainly dependent upon the magnetic leakages, primary and secondary. They increase with the load, and, what is of more importance, they increase with the fall of the power factor of the circuit on which they may be working. This is one reason why certain types of alternator, though satisfactory on a lighting circuit, have proved themselves unsatisfactory when applied to a load consisting chiefly of motors.
The designer must know the various causes which contribute to leakage and make proper allowance.
In general, to keep the leakage small, the pole cores should be short, and of minimum surface, the pole shoes should not have too wide a span nor be too thick, nor present needless corners, and the axial length of the pole face and of the armature core should not be too great in proportion to the diameter of the working face.
Figs. 1,427 and 1,428.—Diagram showing respectively the character of stray field between adjacent straight poles, and between adjacent poles with shoes. Across the slightly V-shaped spaces the stray field passes in lines that, save near the outer part, are nearly straight. Quite straight they would not be, even were the sides parallel, because the difference of magnetic pressure increases from the roots towards the pole ends. At the roots, where the cores are attached to the yoke, the magnetic pressure difference is almost zero. It would be exactly zero if there were not a perceptible reluctance offered by the joints and by the metal of the yoke. The reluctance of the joint causes a few of the lines to take paths through the air by a leakage which adds to the useful flux. At the tops of the cores there is a difference of magnetic pressure equal to the sum of the ampere turns on the two cores, tending to drive magnetic lines across. This difference of magnetic pressure increases regularly all the way up the cores from root to top; hence, the average value may be taken as equal to the ampere turns on one core. The stray field, therefore, will steadily increase in density from the bottom upwards. In addition to this stray field between the pole cores there is also a stray field between the projecting tips or edges of the pole shoes, as shown in fig. 1,428. In some machines the dispersion due to the pole shoes is greater than that between the flanks of the cores.
To keep the increase of leakage between no load and full load from undue magnitude, it is required that armature reactions shall be relatively small, that the peripheral density of the armature current (ampere-conductors per inch) be not too great, and that the pole cores be not too highly saturated when excited for no load.
Fig. 1,429.—Lincoln revolving field alternator. The frame has openings for ventilation, the fanning action of the pole pieces causing a current of air to pass not only over the end of the windings, as is usual with other designs, but also through ventilating slots in the windings themselves. The armature core laminations are annealed after punching and before assembling to guard against the crystalizing effect of the punching. The armature coils are form wound and insulated before being placed in the slot. There is also slot insulation which is put in the slot previous to inserting the coil. When the winding is completed, it is tested with a pressure of 4 to 10 times the normal voltage of the machine. The bearings are self-aligning. The machine is normally designed to operate at a power factor of approximately 70 per cent., which means that at that power factor, the armature and fields at full load will heat equally. If it have a higher power factor than 70 per cent. it means that the field windings will run considerably cooler than the armature windings with full load. If the power factor be lower than this, it will mean that the field windings will run hotter than the armature on full load; however, the machine is designed so that harmful heating does not occur on full load with greater power factor than 40 per cent.
The general character of the stray field between adjacent poles is shown in figs. 1,427 and 1,428 for straight poles and those having shoes.