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.

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

resistance to the flow of eddy currents than a core of solid construction. The doughnut

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

Integrated Publishing, Inc. |