Техническое чтение для энергетиков. Бухарова Г.П. - 36 стр.

UptoLike

ВУЗ: 

УлГТУ | Ульяновск

Составители: 

Рубрика: 

38
class because such measurements are rarely undertaken and. generally speaking, are
used only for periodic checks upon the accuracy of primary standards. They are or
interest only to the specialist, the very large majority of the measurements made in
practice being secondary, or comparison, measurements.
INSTRUMENTS AND METERS
Component Parts of Indicating and Recording Instruments Classification.–
Electrical measuring instruments can be divided into three classes: indicating
instruments, recording instruments and integrating instruments.
Indicating instruments, such as ammeters, voltmeters and wattmeters, constitute the
largest of the three classes. These are fitted with a pointer which moves over a fixed
scale and their characteristic is that they give an immediate indication of the value of
the current, voltage or other quantity being measured. Such an instrument might
therefore be compared with, say, a weighing machine or a barometer, giving an
immediate reading of the weight or pressure existing at any instant, but making no
permanent record of such a measurement.
Recording instruments, or graphers, as they are sometimes called, instead of being
fitted with a pointer and scale, carry a pencil or pen, which presses on to a travelling
ribbon of paper, and thus makes a continuous chart or record of the values measured.
Such an instrument could be compared with the recording barometer often exhibited
in an instrument maker's window. It will be noted that these two types do not differ in
principle, since they are both used to measure the same kinds of things; but in the one
case the indication is momentary and must be read by an observer on the spot, whilst
in the other case the values are recorded on a chart for future observation and
reference.
The third group, consisting of integrating instruments or electricity supply meters,
differs fundamentally from the other two groups, since instead of indicating or
recording, these instruments add up the total amount consumed over any given
period. Thus, instead of reading, say, the current or the power flowing at any instant,
they measure the product of current and time (in ampere-hours) or of power and time
(in watt-hours), and so add up the electrical quantity or energy consumed.
An integrating instrument is, therefore, like the gas meter, which registers the
quantity of gas consumed. Instead of a pointer and scale with a limited arc of
movement, they are usually made to revolve and carry a train of gearing and a
register which counts the number of revolutions made. In such instruments, the rate
of revolution being proportional to the current (in an ampere-hour meter) or to the
power (in a watt-hour meter), the total number of revolutions is proportional to the
ampere-hours or watt-hours respectively.
All indicating instruments have three essential features: an operating force or
mechanism, a controlling force or mechanism, and a damping force or mechanism. It
must be realized that all measurement is comparison, and just as a length can be
measured by putting a foot rule against it, or a weight can be measured by balancing
it against another weight, so an electrical effect can only be measured by allowing it

Страницы