Tímarit Verkfræðingafélags Íslands - 01.06.1950, Page 6

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TÍMARIT V.F.l. 1950
would be regarded as suitable for a basis of acceptance
or rejection. However, as an adjunct to the usual
mechanical and metallurgical tests, it has proved extre-
mely valuable and affords much information not pre-
viously accessible.
MAGNETIC SHEET STEEL TESTER
One of the great difficulties facing the designer of
electrical machines has always been the accurate estima-
tion of core loss, particulary when hot-rolled magnetic
steel sheet is used. This is so because it is almost impossi-
ble to achieve uniformity in the sheets, and the variations
Fig 3.
Mercury-Sealed Controlled-Atmosphere Annealing
Furnaces for Magnetic Sheet Steel Punchings.
in loss from sheet to sheet may well be as great as from
one batch to another. Even if a test piece is clipped off
a comer of each sheet, it is difficult to obtain satisfactory
results, but the problem has now been solved by the
building of a device which measures the loss in the
complete sheet and which is so arranged that it is an
economic proposition to pass all the sheets through it.
This whole-sheet tester is operated by unskilled labour
and, irrespective of the thickness of the sheet, indicates
the loss per kilogram directly on a wattmeter. As the
sheet leaves the tester, this measured loss is auto-
matically stamped all over it in indelible ink, so that it
appears on the smallest portions into which the sheet
may subsequently be punched, thus making it possible
to build up a machine or transformer core from lamina-
tions of which the true loss is definitely known.
To relieve the punching strains and to restore the
magnetic qualities of the sheet at the edges of the slots,
there is an increasing tendency to anneal core lamina-
tions after punching in a controlled atmosphere which
has a suitable decarburising effect and at a temperat.ure
which encourages grain growth. Two typical furnaces
for this purpose are shown in Fig. 3. A load of small
motor stators, which have just been annealed, can be
seen in the foreground, and in the background the outer
casing of another furnace is being lowered over the gas-
tight inner bell in which the load is enclosed.
STRAIN GATJGES
In building up machine cores it is essential that the
laminations be clamped with the correct pressure, to
avoid either loose cores or excessive core loss. This can
be done, for example, by compressing the laminations
by hydraulic jacks to a predetermined value as shown on a
suitably calibrated pressure gauge in the hydraulic system
and then tightening up the core bolts, but it is difficult
to ensure that the core does not slacken slightly when
the jacks are released. If strain gauges are used, however,
the true final pressure can be measured accurately,
because the gauge shows the actual amount by which
the core bolt has stretched in applying the pressure,
and the pressure can then be readily calculated from
the known stress/strain properties of the metal from
which the bolts are made.
A strain gauge consists of a very fine wire cemented
to the core bolt and connected in an electric circuit
which enables its resistance to be measured. As the bolt
stretches, the wire is stretched correspondingly and the
amount of this extension can be accurately measured
by the change in its resistance.
The wire is mounted on a piece of thin paper, so that
it can be conveniently handled, the paper also serving
as an insulator; and, in order to give the largest poss-
ible reading, a long length of wire, folded backwards
and forwards to give several parallel sections, is used.
To correct for temperature, a second identical element
is connected in another arm of the resistance-measuring
bridge, this element being attached to the core bolt at
right-angles to the first one, thus ensuring that both
elements are at the exact temperature of the bolt, but
that only one is subjected to strain.
PERMANENT MAGNETS.
In few branches of electrical engineering have more
spectacular advances been made than in the manufac-
ture of permanent magnets. This will be obvious when
it is stated that a modern permanent magnet measur-
ing 30X5X5 millimetres and weighing 6 grams will sup-
port no less than 21 kilograms. That is to say, it can
be used to lift about 3500 times its own weight.
In the past, permanent magnets were invariably made
of carbon steel, which consisted of about 99% iron, with
a high carbon content. Modern magnets contain only
about half this percentage of iron, and every endeavour
is made to keep the carbon content to a minimum. Fur-
thermore, modern magnetic alloys contain quite a high
proportion of non-magnetic material. For example, a
typical alloy contains 50% iron, 25% cobalt and 15%
nickel, which are all magnetic, but the remaining 10%
is composed of copper, aluminium and other non-magne-
tic metals. A minor disadvantage of all these modern
magnets is that, being hard and brittle, they must be
made in shapes which can be produced by casting and
grinding.
As a result of their much increased power, permanent
magnets are nowadays used for a most extensive range
of applications. For example, machine tools are often
fitted with permanent magnet chucks, which will hold
the work as firmly as a mechanical chuck, without any
danger of the work flying off the machine if the electric
supply fails, as could happen with the electro-magnetic
chucks sometimes used formerly.