PHASE CONVERSION 213 jj
mate magnitude for medium values of energy, and then apply \'<'•
only to the active energy-storing structure, under the assumption, |i
that during every energy cycle (or half cycle of alternating cur- I\
rent and voltage), the entire energy is returned and stored again. fc
While this is the case with capacity and inductance, when using !*'
momentum for energy storage, as flywheel capacity, the energy |
storage and return is accomplished by a periodic speed variation, \ •
thus only a part of the energy restored, and furthermore, only |'
a part of the structural material (the flywheel, or the rotor of |'4
the machine) is moving. Thus assuming that only a quarter *!,
of the mass of the mechanical structure (motor, etc.) is revolving, ;}
and that the energy storage takes place by a pulsation of speed of ^ f
1 per cent., then 1 kva. at 60 cycles would require 600 c.c. of >
material, at 40c. 'I
Obviously, at the limits of dielectric or magnetic field strength, ;* «
or at the limits of mechanical speeds, very much larger amounts 4;
of energy per bulk could be stored. Thus for instance, at the " ^ '
limits of steam-turbine rotor speeds, about 400 meter-seconds, ['«/
in a very heavy material as tungsten, 1 c.c. of material would ^
store about 200 kva. of 60-cycle energy, and the above figures ^
thus represent only average values under average conditions. §
126. Phase conversion is of industrial importance in changing || |
from single-phase to polyphase, and in changing from polyphase 1'
to single-phase. ft'
Conversion from single-phase to polyphase has been of con- f
siderable importance in former times, when alternating-current f
generating systems were single-phase, and alternating-current ||;
motors required polyphase for their operation. With the prac- k
tically universal introduction of three-phase electric power
generation, polyphase supply is practically always available for
stationary electric motors, at least motors of larger size, and
conversion from single-phase to polyphase thus is of importance
mainly:
(a) To supply small amounts of polyphase current, for the
starting of smaller induction motors operated on single-phase
distribution circuits, 2800 volts primary, or 110/220 volts
secondary, that is, in those cases, in which the required amount
of power is not sufficient to justify bringing the third phase to
the motor: with larger motors, all the three phases are brought
to the motor installation, thus polyphase supply used.
(6) For induction-motor railway installations, to avoid the