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generator acts as a multiplying device, putting out a direct current millivolt signal proportional to
the product of the current and a magnetic field input. The magnetic field input may be developed
from a current or a voltage source. The output signal may be used to operate a direct current
voltmeter calibrated to the product units or as an input to telemetering or recording devices. The
Hall generator principle has been applied to devices for the measurement of voltage, current,
power, reactive power, power factor, and frequency. One advantage of the Hall generator type of
transducer is its small all-solid-state construction. Other advantages include its relatively high
output signal level, a high operating speed, and the output the transducer inputs.
5.2 INSTRUMENTS. This section provides descriptions of the various instruments used to
monitor power system operating conditions.
5.2.1 Ammeters. Ammeters measure the current flowing in a circuit. An ammeter or its
associated current transformer primary is connected in series with the circuit being measured.
5.2.2 Voltmeters. Voltmeters measure the potential difference between conductors or
terminals. A voltmeter is connected directly, or through a potential transformer, across the points
to be measured. For voltmeters operating on direct current circuits above 300 V, external series
resistors are commonly required.
5.2.3 Wattmeters. A wattmeter measures the magnitude of the electric power being delivered
to a load or group of loads. To indicate the product of voltage and in-phase current, the
wattmeter has both potential coils and current coils.
5.2.4 Varmeters. A varmeter measures reactive power. A varmeter operates as a wattmeter
with the current coils connected in series with the circuit and the voltage phase shifted 90
electrical degrees from the voltage across the circuit. Varmeters usually have the zero point at
the center of the scale, since reactive power may be leading or lagging. The varmeter has an
advantage over a power factor meter in that the scale is linear, and small variations in reactive
power can be read. A power factor meter may be difficult to read near unity power factor.
5.2.5 Power Factor Meters. A power factor meter indicates the power factor of the load. It is
a direct-reading instrument that will indicate the power factor of a three-phase load only if the
voltage and load are balanced on the three phases. The meter consists of both current and voltage
elements, utilizing instrument transformers where necessary. The meter indicates unity power
factor on scale center and lead or lag for any power factor other than unity. It is possible to
obtain the average power factor over a definite period, like a day, week, or month, by use of the
readings of an integrating kilowatt-hour meter and a kilovar-hour meter.
5.2.6 Frequency Meters. The frequency of an alternating current power supply can be
measured directly by frequency meters. The most commonly used type is the pointer-indicating
type.
5-3





 


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