ents. A permanent
magnet provides a uniform magnetic
field into which a spring-mounted mov-
ing coil is positioned. A pointer is at-
tached to the moving coil. The current to
be measured passes through the coil,
which is free to pivot. As the current
passes through the coil, it produces a
magnetic field that interacts with the uni-
form magnetic field produced by the per-
manent magnet. This interaction causes
the moving coil and pointer to rotate.
Since these movements can be precisely
calibrated, they form the basis of many
analog meters.
To eliminate the need for precise me-
chanical components and to reduce the
need for calibration and adjustment, digi-
tal electronics form the basis for much
metering done today. Analog voltage and
current signals are converted to digital
values and then displayed as numbers.
Digital technology allows one metering
device to display multiple values.
What can you meter? Both analog
and digital metering methods can mea-
sure many useful electrical quantities, in-
cluding the following.
Voltage. A voltmeter is put in parallel
with a circuit element to measure the
potential difference across that element.
To minimize the current flow through the
voltmeter, theyre often designed with
internal impedances on the order of many
megohms per volt. Voltmeters can deter-
mine relay contact and switch states while
Basic design configuration of a DArsonval meter.
the circuit is energized, easing the
troubleshooting process. A zero voltage
reading across the contacts indicates the
contacts are closed, and a non-zero volt-
age indicates theyre open.
Current. Ammeters are put in series
with a circuit element to measure the
current flowing through that part of the
circuit. To minimize the voltage drop
across the meter, ammeters are designed
with a very low internal impedance. Be-
cause of this low internal impedance,
improperly connecting an ammeter can
result in a very high current flow through
the meter, which could damage the meter
coil or electronics. Therefore, ammeters
are typically fused to protect the meter
from excessive current. Some ammeters,
known as clamp-on ammeters, use a built-
in current transformer, which can be
clamped around a conductor to facilitate
a quick and easy measurement.
Power. Since power is essentially the
product of voltage and current, you can
meter power using a voltage, or poten-
tial, element and a current element. If real
power is to be measured, the rms magni-
tudes of the voltage and current wave-
forms are multiplied together, and that
product is multiplied by the cosine of the
angle difference between the voltage and
current waveforms, which is the power
factor. If reactive power is to be measured,
the power factor is replaced by the sine
of the angle difference between the volt-
age and current waveforms.
Energy. Integrating power over time
yields energy; therefore, introducing a
time standard to a power meter makes it
possible to measure watt/hours, VAR-
hours, and volt-ampere-hours. This is
done electromechanically with an induc-
tion disk meter, which is essentially an
induction motor that rotates at a speed
proportional to the power measured by
the potential and current coils. A me-
chanical register or odometer counts the
disk revolutions. When the number of
revolutions is scaled by the appropriate
constantsbased on the meter design
its possible to determine the amount of
energy passing through the meter.
The basic principles of metering can
be used to measure other quantities like
frequency, power factor, and synchro-
nism (voltage magnitude and angle com-
parison) across an open circuit breaker.
While some of these metering methods
may be quite complex, the fundamentals
of metering remain the same.
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