Quantcast Electrical disturbances (power quality)

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
f. Transistors. Transistors are three terminal,
by other customers or even by customer's own
solid state devices constructed so that the current
equipment may also affect customer's equipment.
across the base-emitter junction will control a
"Power quality" is a relatively new term used to
greater amount of current crossing the collector-
describe the quality of power (absence of voltage
emitter junction. Because a smaller current can
dips, surges, harmonics outages, frequency varia-
control a larger one, the transistor provides gain or
tion) at the user's location. Traditional measure-
amplification. Transistors vary in size, power, and
ments for reliability studies don't deal with the
voltage ratings. Some idea of the electrical values,
power quality needs of sensitive electronic equip
type, and application of the transistor in the circuit
ment. Rather, they deal with the permanent or pro-
should be known before testing is attempted.
longed outage and how to improve upon it. While
this is indeed important to sensitive loads, there is
(1) Testing. The bipolar junction transistor
increased concern for short term or momentary dis-
(BJT) is tested just like the general purpose rectifier
diode. These transistors are actually two diode junc-
turbances. In addition to voltage limits, sensitive
loads such as computers typically require the fre-
tions combined in such a way as to obtain current
control. General purpose transistor testing is done
quency to be within plus or minus .05 Hz, the rate of change of
frequency less than 1 Hz/sec, voltage waveform dis-
as follows (fig 7-5):
tortion under five percent and voltage unbalance
(a) Check the polarity of the test leads and
zero the ohmmeter.
less than three percent. For specific applications,
the power quality requirements should be obtained
(b) Determine the type of transistor to be
from the manufacturer of the sensitive equipment.
tested, that is whether it is a positive, negative,
Some of the common types of disturbances, the
positive (PNP) or a negative, positive, negative
(NPN) junction type.
symptoms, causes and effects are summarized in
(c) Set the scale multiplier to "X1" or "L0"
table 7-1.
and recheck for zero ohms.
7-4. Disturbance measurement and monitor-
(d) Test all combinations as shown in the
ing.
diagrams and tables given in figure 7-5a or 7-5b.
Conditions may be quite different at any given site,
(e) Test all combinations as shown with the
and it is desirable to obtain specific data about the
scale multiplier set to "X100" or "H1". If the values
actual situation, if possible, before considering a
shown in the tables are not obtained, the transistor
remedy. If it is an existing site, it is useful to obtain
part number should be checked to confirm its type,
any historical data which might correlate sensitive
or the unit should be replaced if the type is known
equipment operation with power disturbances. The
to be correct.
type of data includes the sensitive equipment oper-
g. Other solid-state devices. There are numerous
ating log and maintenance records, and electric util-
other types of solid-state devices used in modern
ity operating log and voltage recordings.
electronic and control systems. Most of these cannot
a. The most useful activity for any existing site is
be statically tested with an ohmmeter and ex-
to conduct a site power line disturbance study for a
pected to give meaningful results. The following is a
one or two month period-including the storm sea-
brief list of devices that yield valid test results only
son, if possible. The monitoring should be at the
in a circuit:
same point that powers the sensitive equipment
(1) Field-effect transistors (FETs), metal-oxide
and must use equipment capable of recording the
semiconductor field-effect transistors (MOSFETS),
types of transients that can affect sensitive loads.
or insulated-gate field-effect transistors (IGFETs).
b. There are several types of equipment designed
(2) Unijunction Transistors (UJTs) or Program-
to perform this monitoring function. Unlike the tra-
mable Unijunction Transistors (PUTs).
ditional strip or circular chart recorders, this equip-
(3) Analog Operational Amplifiers and Inte-
ment is capable of recording variations of voltage in
grated Circuits.
the short time periods of interest for sensitive
(4) Any class of Digital Logic Integrated Cir-
equipment, yet operate continuously for weeks at a
cuit.
time. Much of the equipment is of the digital read-
7-3. Electrical disturbances (power quality).
out type which, unfortunately, can lead to improper
Equipment with sensitive electronic circuits (digital
interpretation of the conditions at the site because
clocks, VCRs, computers, data terminals) may expe-
it cannot always distinguish between harmless and
rience memory loss, system malfunction and even
harmful disturbances.
component failure due to electrical power source
c. Much more useful monitors produce an analog
disturbances. Sags, surges and harmonics are some
recording of the disturbances with the ability to
common types of disturbances. Disturbances caused
expand the waveforms to examine them in detail.
7-7





 


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