Renard Numbers and Standardization of Equipment Ratings
Have you ever thought why electrical equipments are rated
for certain voltage and KVA ratings. Why 630 KVA of transformer is taken in
place of 600 kVA of transformer. Have you gone for buying some screw for your
home or office and find that only specific sizes are available mostly like 1.00
mm, 1.60 mm, etc.
To know all these, may we discuss one of the ways of standardization
technique proposed by French Army Engineer, col Charles Renard in 1870, which
is adopted today by many standardization bodies including IEC.
To choose exact product dimensions in any Industry, it
should have preferred numbers or dimensions. The benefits of using preferred
numbers as dimensions can be manifold. It increases the compatibility chances
between objects designed by same industry, if the preferred number system is
followed by that Industry. This also minimizes the number of different sizes to
be manufactured as well as number of tests to be carried out for final product
release. Redesigning efforts can also be minimized once only standard ratings
are manufactured across the industry. It has impact on Industry cost also, as a
customer buying same rating items for different installations may have to go
for reduced inventory of spares.
Some of us might have studied progression series of
numbers. One such series of numbers is geometric series. In geometric series,
ratio of any two consecutive numbers in the series is constant, known as common
ratio. For example, 3, 6, 12,24 are in geometric progression because each of 6/3,
12/6, 24/12 is 2, which is common ratio.
Renard numbers are always in geometric progression, which
means the next term in the series can be calculated through multiplication with
the common ratio.
Now, as we know there can be infinite number of geometric
series between any two given numbers. So, what is the first term or number of
Renard series and what is the common ratio. But before that, let us know that
there are multiple number of Renard Series. Nth Renard series RN has
common ratio of (10)1/N.
If means if we talk of R5, common ratio will
be (101/5). If first term is a, next term will be a x (101/5),
second will be a x (101/5) x (101/5) and so on. So, five
Reynard numbers shall be obtained between a and 10 a by using R5.
R5 from 1 to 10 are calculated as 1.00, 1.60, 2.50, 4.00,
6.30. Now, refresh that the motors are rated as standard to be of 100 kW, 160
kW, 250 kW, 400 kW, 630 kW and so on. Actually electrical and mechanical
standard ratings are based on Renard Numbers for IEC system mostly.
But what is magical about it. Why only such numbers are
adopted. There are a lot of research works going on the optimization through
Renard numbers but one thing is clear the Renard number has gone along way into
our industrial system for standardization and we have to learn and adapt it for
better.
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