c. A step-up transformer has a greater number of turns in the secondary winding
and increases the voltage, but decreases the current availability; a step-down
transformer has more turns in the primary winding and decreases the voltage, but
increases the current capability. In other words, the voltage in the primary winding is to
the induced voltage in the secondary winding as the number of turns in the primary
winding is to the number of turns in the secondary winding. The formula to express the
relationship between the number of turns in the windings and the voltages is:
Ep
Tp
___ = ___
Ts
Ep
where EP is the primary voltage, ES is the secondary voltage, TP is the number of turns
in the primary coil, and Ts is the number of turns in the secondary coil.
(1) A transformer does not generate power; it transfers power from one
circuit to another. The power output can be no greater than the power input, and the
power input to the primary side (Pp) of the transformer must equal the power output of
the secondary side (PS), (The assumption that the transformer is 100 percent efficient
simplifies the mathematics--actual1y, Ps is slightly less than Pp). In as much as the
power in an electrical circuit equals voltage multiplied by amperage, β = Ep x IP and
Ps = ES x IS . Since Pp = PS, EP x IP = ES x Is where IS is current in amperes in the
secondary coil, IP is current in the primary coil, ES voltage in the secondary coil, and EP
is voltage in the primary coil. This relationship can be arranged to express the following
equation:
Ep
Is
___ = ___
Es
Ip
(2) Step-up transformers are employed to obtain the high voltages needed
in diagnostic x-ray equipment. A step-up transformer can take a low voltage source (50
to 220 volts) and increase this voltage to a value of 30,000 to 150,000 volts (30 to 150
kilovolts) or more, which supplies the high voltage required to force electrons across an
x-ray tube at the high rate of speed (energy) that will produce x-radiation.
2-27. TRANSFORMER CORE TYPES
a. Closed-Core. The close-core transformer has heavily insulated coils wound
around a square metal "doughnut" (figure 2-17A). The magnetic field is provided with a
continuous path so that a relatively small amount of magnetic energy is lost. The
transformer core is laminated (made up of thin strips of steel pressed together). Each
strip or layer is insulated by paint or enamel. A laminated core offers a much higher
type of closed-core transformer is efficient and is commonly utilized in x-ray generating
equipment. Usually, the high voltage transformer is submerged in a special type of oil
to ensure maximum insulation and cooling.
MD0950
2-25