Radar System Engineering

556 PRIME POWER SUPPLIES FOR RADAR [SEC. 141
up at a frequency of j/2. The general equation for induced voltage in
a transformer
(1)
gives, for a fixed core area,
~=k,n.Ad~,
dt
(2)
w’here k is a constant, n the number of turns in the winding considered,
@ the flux linking these turns, A the area of the core, and B the magnetic
induction in the core. For sinusoidal voltages of frequency j,
E ~= = k.n. j. A. B_. (3) ~
Thus if j is halved, B= must double for a fixed input voltage, all
other terms being constant in any given case. Since losses in the iron
are a function of B-, doubling B~.. produces an immense increase in
magnetizing current and in core loss. The core area and thus the weight
of the transformer as a whole can be decreased as f increases because of
the corresponding reduction in B~,,. By going from 60 cps to 400 cps,
savings in transformer weight of 50 per cent are attainable. Core area
cannot be much further reduced by increasing j from 400 cps to 800 cps,
as iron losses per pound for constant B-, increase more than linearly with
frequency. Weight savings of 10 per cent at most are obtainable by
this change in frequency.
Performance of rotating equipment must be considered, as well as
that of transformers. For stable performance and minimum weight
of alternators and a-c motors, 400 cps is preferable to 800 cps. (See
Sec. 14.2 on wave shape. ) The majority of existing airborne radar sets
are designed for 400-cps minimum supply frequency, and it appears that
this will continue to be the nominal standard. Variable-frequency
alternators are also used; they commonly give a 2 to 1 ratio between
minimum and maximum frequencies, depending on the speed of the
aircraft engine. Ranges often encountered are 400 cps to 800 cps,
800 cps to 1600 cps, and 1200 cps to 2400 cps. Table 14.2 shows the
power frequencies used in Allied military aircraft during the past war.
TABLE14.2.+TANDARn POWERFRECIUE~CImS
Frequency, cps
400 fixed
400 to !300variable
NM fixed
S00 to 1600variable
400 to S00 (3*) variable
1200to 2400variable
{ 1300to 2600 variable