Lightning Arresters. Lightning arresters should always be used on
both the ac input and dc output sides of rectifiers using silicon rectifying
elements. Their use on units using selenium elements is recommended in areas
where lightning strikes are frequent. The arresters on the output should have
a firing voltage greater than the rectifier output voltages.
Selenium Versus Silicon Stacks. While some old installations used
copper oxide rectifying elements, modern units use either silicon or selenium
rectifying elements. In general, silicon units are used for larger units
where their higher efficiency is more important than their lower reliability.
Ordinary selenium stacks deteriorate with time. This "aging" can
be reduced by variations in plate composition and "non-aging" stacks are
available. Aging rates are determined by operating temperatures that are a
function of current flow. The selection of a unit using selenium rectifying
elements which has a somewhat greater capacity than required will increase
stack life. The efficiency of selenium rectifying elements is a function of
operating voltage versus rated voltage as shown in Figure 41.
Silicon diodes are mounted in metal cases which are mounted on
either aluminum or copper plates to dissipate the heat generated during
operation. Silicon diodes do not age as do selenium stacks and, as shown in
Figure 42, are more efficient than selenium elements, particularly at higher
voltage ratings. Silicon rectifying elements are more subject to complete
failure from voltage surges which would only cause increased aging of selenium
stacks. Surge protection should always be used on both the ac input and dc
output of rectifiers using silicon diode rectifying elements.
Other Options. Other features listed in para. 18.104.22.168 are
available and should be selected as appropriate. In remote off-base areas,
small arms proof enclosures may be required based upon local experience.
Specifying clear anodized aluminum enclosure top coated with one clear coat of
polyurethane will reduce maintenance painting.
Rectifier Alternating Current Rating. The ac current requirement
for a rectifier can be determined based upon rectifier output and efficiency
by the following formulae:
(Edc x Idc)/F x Eac
alternating current requirement (A)
direct current output voltage
direct current output amperage
rectifier efficiency (%)
alternating current voltage (per phase)