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 Section 3. METERS AND SUPPORT DEVICES
1. INTRODUCTION. Accurate and reliable data can be obtained using either
permanent or portable meters. Permanent meters are generally installed at
service entrances and at points of large power demand. To perform a power
demand survey, it may be necessary to monitor many additional locations within
a system. It may not be economical to install permanent meters at each
location, so portable units are used. The cost of portable meters is higher
than permanent meters but their accuracy is generally very good.
2.
METERING DEVICES.
2.1 Watthour Meter. Consumption of electrical energy is measured with either
a mechanical or electronic watthour meter.
2.1.1 Mechanical Watthour Meter. A mechanical watthour meter is a small,
precision built, induction motor. It consists of a rotating disk,
electromagnet, permanent damping magnet, bearings, geartrain, and dial
assembly. The electromagnet is wound with a voltage coil and a current coil.
When the meter is energized, a small current flows through the voltage coil,
connected across the lines, and full load current flows through the current
coil. The current in these coils sets up magnetic fields, or magnetic flux
that induce small currents (called eddy currents) in the disk. These eddy
currents set up their own magnetic field. The combination of fluxes, or
magnetic fields, interact with one another in such a manner that the disk is
forced to rotate in a positive direction. With voltage and current coils
energized, the disk gathers momentum and spins faster. Therefore, a braking
device to retard disk speed is provided. The braking device is a permanent
magnet through which the disk rotates.
2.1.2 Electronic Watthour Meter. An electronic watthour meter consists of a
current sensor, kilowatt transducers electronic totalizer, and mechanical or
electronic digital display. Sensor, transducer, totalizer, and display
combinations are available for readings of watthours to megawatthours. As
with most electronic power monitoring equipment, the cost is high. Most
electronic kWh q eters are used in conjunction with computerized energy
management systems.
2.1.3 Watthour Meter Selection Criteria. Some facts that should be known or
considered before purchasing and installing a meter are as follows:
q Type
of service: single-phase network, 3-wire; 3-phase, 3- or 4-wire;
wye (Y) or delta ( ∆ )
q Circuit
voltage: line-to-line or line-to-neutral, depending on q eter
connections. Determine if requirement exists for PT with current
limiting primary fuse.
9-12
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