Fig. 1,404.—Revolving field of Fort Wayne alternator equipped with amortisseur winding. The object of this winding is to check any tendency toward hunting when the alternator is to be run as a synchronous motor, either for rotary condenser or power service. The amortisseur winding consists of heavy copper bars, placed around and through the pole faces and short circuited at the ends by heavy copper rings; it serves as a starting winding to bring the rotor up to speed as an induction motor, and also serves as a damping device to neutralize any tendency toward "hunting" caused by variation in speed of the generator supplying the current.
Alternators which produce a smooth current wave and are maintained at uniform speed by properly designed governors, operate fairly well in parallel, but are not entirely free from hunting, and other means are provided to overcome the difficulty.
When heavy copper flanges, called dampers, are put over the polar projections or copper bars laid in grooves on the pole face and short circuited by connecting rings (called amortisseur winding), the powerful induced currents which are produced when the alternators get out of step tend to quickly re-establish the phase relation.
Fig. 1,405.—Westinghouse field with amortisseur or "damper" winding for 75 kva. and larger belted alternators, which prevents hunting and reduces eddy currents in the pole pieces. The copper bars of the amortisseur cage winding are arranged in partially closed slots in the pole pieces.
Two examples of a field provided with amortisseur[4] winding are shown in figs. 1,404 and 1,405.
[4] NOTE.—Amortisseur windings are often erroneously called "squirrel cage" windings on account of similarity of construction. The latter term should be reserved for its proper significance as being the name of the type of armature winding generally used for induction motors, the name being suggested by the resemblance of the finished armature to the wheel of a squirrel cage. A comparison of figs. 1,405 and 1,746 will show the distinction. In a squirrel cage winding there is a large number of bars uniformly spaced; an amortisseur winding consists of a comparatively small number of bars, usually unevenly spaced, that is they are divided into groups with considerable space between the groups, as in fig. 1,405, and less pronounced in fig. 1,404. The bars are short circuited by rings the same as in squirrel cage winding.
Fig. 1,406.—Diagram of monocyclic system, showing monocyclic armature and transformer connections. The monocyclic system is a single phase system primarily intended for the distribution of lights with an incidental load of motors. The lighting load is entirely connected to one single phase circuit, and the motors are started and operated from this circuit with the assistance of the teazer wire. The long coil indicates the main winding of the armature, which is similar in its arrangement and size to the ordinary armature winding of a single phase alternator. The short coil which connects at one end to the middle point of the coil above mentioned, and at the other to a third collector ring is called the "teazer" coil. Its use is to generate a pressure in quadrature with that of the main coil. This pressure is combined with the main pressure of the alternator by transformers, so as to give suitable phase relations for operating induction motors. In the diagram the voltage has been assumed to be 2,080 volts, and the voltages marked to correspond with the generated pressure. The coils of the alternator armature are connected, as shown, to two main leads and to a teazer wire. Between each end of the main coil and the end of the teazer coils, a resultant pressure is generated. These resultants are about 12 per cent. larger than half the main pressure. They also have a phase difference.