258 ELECTRICAL APPARATUS
For this reason, the permutator has never been introduced in
this country, and has practically vanished abroad.
2. The transformer is mounted on the revolving-motor struc-
ture, thereby revolving, permitting direct connection of its
secondary leads with the commutator segments. In this case
only the three or four primary phases have to be lead into the
rotor by collector rings.
The mechanical design of such structure is difficult, the trans-
former, not open to inspection during operation, and exposed to
centrifugal forces, which limit its design, exclude oil -and thus
limit the primary voltage, so that with a high-voltage primary-
supply system, double transformation becomes necessary.
As this construction offers- no material advantage over (3),
it has never reached beyond experimental design.
3. A lesser number of collector rings and supply phases is
used, than the number of commutator segments and synchronous-
motor armature coils, and the latter are used as autotransformers
to divide each supply phase into two or more phases feeding suc-
cessive commutator segments. Fig. 123 shows a 12-phase recti-
fying commutator connected to a 12-phase synchronous motor
with six collector rings for a six-phase supply, so that each sup-
ply phase feeds two motor phases or coils, and thereby two recti-
fier segments. Usually, more than two segments are used per
supply phase. The larger the number of commutator segments
per supply phase, the larger is the differential current in the
synchronous motor armature coils, and the larger thus must be
this motor.
Calculation, however, shows that there is practically no gain
by the use of more than 12 supply phases, and very little gain
beyond six supply phases, and that usually the most economical
design is that using six supply phases and collector rings, no
matter how large a number of phases is used on the commutator.
Fig. 123 is the well-known synchronous converter, which hereby
appears as the final development, for large powers, of the syn-
chronous rectifier.
This is the reason why the synchronous rectifier apparently
has never been developed for large powers: the development of the
polyphase synchronous rectifier for high power, by increasing
the number of phases, byepassing the differential current which
causes the sparking, by shunting the commutator segments with
the armature coils of the motor, and finally reducing the number