CHAPTER XIV
PHASE CONVERSION AND SINGLE-PHASE GENERATION
125. Any polyphase system can, by means of two stationary
transformers, be converted into any. other polyphase system,
and in such conversion, a balanced polyphase system remains
balanced, while an unbalanced system converts into a polyphase
system of the same balance factor.1
• • In the conversion between single-phase system and polyphase
system, a, storage of energy thus must take place, as the balance
factor of the single-phase system is zero or negative, while that
of the balanced polyphase system is unity. For such energy
storage may be used capacity, or inductance, or momentum or a
combination thereof:
Energy storage by capacity, that is, in the dielectric field,
required per kilo volt-ampere at 60 cycles about 2000 c.c. of
space, at a cost of about $10. Inductance, that is, energy
storage by the magnetic field, requires about 1000 c.c. per kilo-
volt-ampere at 60 cycles, at a cost of $1, while energy storage by
momentum, as kinetic mechanical energy, assuming iron moving
at 30 meter-seconds, stores 1 kva. at 60 cycles by about 3 c.c.,
at a cost of 0.2c., thus is by far the cheapest and least bulky
method of energy storage. Where large amounts of energy have
to be stored, for a very short time, mechanical momentum thus
is usually the most efficient and cheapest method.
However, size and cost of condensers is practically the same
for large as for small capacities, while the size and cost of induc-
tance decreases with increasing, and increases with decreasing
kilovolt-ampere capacity. Furthermore, the use of mechanical
momentum means moving machinery, requiring more or less
attention, thus becomes less suitable, for smaller values of power.
Hence, for smaller amounts of stored energy, inductance and
capacity may become more economical than momentum, and
for very small amounts of energy, the condenser may be the
cheapest device. The above figures thus give only the approxi-
1 "Theory and Calculation of Alternating-current* Phenomena/' 5th
edition, Chapter XXXII.
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