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 5 . POLYPHASE A-C CIRCUITS. The energy delivered over a polyphase circuit is
the total energy delivered over each equivalent single-phase circuit that make
up the polyphase circuit. Energy can be measured by connecting a single-phase
watthour meter in each phase and then adding up readings of individual
meters. This is not commercially practicable because: it requires too many
meters; it takes much more time to read the meters; and it multiplies the
chance of mistakes both in reading meters and in totaling meters. The
electrical industry has developed polyphase watthour meters.
6 . POLYPHASE WATTHOUR METER. The polyphase watthour meter is a combination
of single-phase watthour meter stators that drive a rotor at a speed
proportional to the total power in the circuit. The meter consists of a
multistator motor, means for balancing the torques of all stators, a magnetic
retarding system, a register, and compensating devices. These components are
assembled on a frame and mounted on a base.
6.1 Operating Principle. The operating principle of polyphase watthour
meters, having any number of stators, is the same as single-phase watthour
meters. Torque on each stator results from current in one set of
electromagnetic coils and eddy currents induced in a disk, or disks, by
current in the other set of coils. The torques of the several stators combine
to give a resultant torque proportional to total power.
6.2 Blondel's Theorem. Because the same rules apply to measurement of both
polyphase energy and polyphase power, principal parts of single-phase watthour
meters can be combined for polyphase energy measurement, much as components of
single-phase wattmeters are combined for polyphase power measurement.
Blondel's theorem applies to measurement of energy exactly as it does to
measurement of power. A polyphase watthour meter is built with the number of
elements necessary to satisfy Blondel's theorem.
7. METER TYPES. A wide variety of meters are available to meet almost every
measuring need. Meters can be divided into two major types, indicating and
recording.
7.1 Indicating Meters. Indicating meters are used to provide a measured
value at a given moment, and show how instantaneous value changes as a
function of time. Indicating meters may have analog pointers or digital
readouts. Some meters have a built-in time delay. Many are equipped to show
average use over a 15 minute to 30 minute increment.
7.2 Recording Meters. Recording meters are used to accumulate a measured
value over a period of time. These meters use either dials such as a
kilowatthour q eter, a paper chart (either circular or strip), or magnetic tape
where readings are coded into pulses and then encoded by a computer system to
record the increase in reading between reading intervals.
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