Fig. 2,055.—Detail of Westinghouse commutating pole rotary converter brush, showing rack. The brush lifting mechanism and its operation is explained in fig. 2,050.
The characteristics of the regulating pole converter being novel, a detailed explanation of the principles involved is given to facilitate a clear understanding of its operation.
Consider a machine with a field structure as shown in fig. 2,056 resembling in appearance a machine with commutating poles, but with the brushes so set that one of the regulating poles adds its flux to that of one main pole, cutting the inductors between two direct current brushes. The regulating pole is shown with a width equal to 20 per cent. of that of the main pole.
To obtain definite figures, it will be assumed that the machine at normal speed, with the main poles excited to normal density, but with no excitation on the regulating poles, gives 250 volts direct current pressure. Then with each regulating pole excited to the same density as the main poles, and with a polarity corresponding to that of the main pole in the same section between brushes, the direct current pressure will rise to 300 volts at the same speed, since the total flux cutting the inductors in one direction between brushes has been increased 20 per cent.
If, on the other hand, the excitation of the regulating poles be reversed and increased to the same density as that of the main poles, the direct current pressure will fall to 200 volts, since in this case the regulating poles give a reverse pressure, that is, a pressure opposing that generated by the main poles.
Fig. 2,056.—Diagram of field of regulating pole converter illustrating principles explained in the accompanying text.
Now, if the machine be equipped with collector rings, that is, if it be a converter, this method of varying the direct current voltage from 200 to 300 volts does not give nearly as great a variation of the alternating current voltage; in fact, the latter voltage will be the same when delivering 200 volts as when delivering 300 volts direct current pressure, if the field excitation be the same.
This may be seen by reference to fig. 2,057, which is a diagram of the alternating current voltage developed in the armature windings by the two sets of poles.[10]
[10] NOTE.—In the Burnham split pole rotary converter, each pole is divided into only two sections, one larger than the other. A main shunt winding is arranged on the large sections, and a winding for providing the voltage regulation is placed on the other section. When the current is sent through this latter winding in one direction the voltage is raised, when in the other direction the voltage is lowered.