Figure 2-24. Spinning-top test.
a. The "top" is a flat metal disk with a small hole drilled near one edge (figure
2-24A); it is placed above an x-ray film or cassette and made to spin while a short
exposure is made. In fully rectified current of 60 cycles, there are 120 pulsations a
second (with a half-wave rectifier there are 60): x-rays are produced by each of these
pulsations. The developed film will show a dot for each pulsation occurring during the
given exposure time. If all four rectifiers are operating properly, there will be one dot for
each 1/120 second of exposure used (60-cycle AC).
b. For example, for an exposure obtained in 1/10 second, 12 dark dots should
show on the film (figure 2-24B); if only 6 dots show (figure 2-24C) there were only 6
pulsations in 1/10 second, indicating that only half of the transformer output is reaching
the x-ray tube because one or more of the rectifiers requires replacement. The
spinning-top method may also be used to establish the accuracy of the timer.
One and two-pulse voltage patterns are produced by x-ray generators designed
to operate on single-phase AC. Generators that operate on three- phase AC produce
6-pulse and 12-pulse tube voltage patterns. Figure 2-25 shows some characteristics of
the four patterns as well as the alternating current form. Also shown is the voltage
ripple, which is the difference between the peak and minimum voltage. Two-pulse tube
voltage, for example, drops to zero value after each peak. Consequently, two-pulse
ripple is 100 percent. On the other hand, 6-pulse and 12-pulse voltages do not drop to
zero value. In 6-pulse systems, the voltage only drops a small amount before the next
pulse has reached the same amplitude, so 6-pulse ripple is 13.5 percent. For 12-pulse,
the voltage ripple is a mere 3.4 percent.