Iwatsu SY-956, Инструкция по эксплуатации

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6
each other, if each measurement device is ideally tuned. For example, it is implausible that a sample that
shows the highest core loss with one type of measurement device shows the lowest core loss with another type
of device.
In order to conduct accurate measurement, it is important to use a testing device with high measurement
accuracy. In addition, as described in (1) to (6) of

2.1, it is also important to keep the measurement conditions
constant.
■
2.3 How to find the detection limit
It was noted in the previous chapter that even when ideally tuned measurement devices, with different methods,
test the same sample under the same excitation conditions, the resulting values are never the same.
Thus, finding the detection limit of a device, as in the precision of the measurements derived from a specific
device, is of the utmost interest. There is an easy trick to find out; this trick may also work with an Epstein frame
and a SST. The trick is to take a measurement with no sample. In other words, use an air core coil that has only
air inside the winding. The coil needs to be the actual coil used for the measurement.
What to measure with an air core coil?
The easiest is the core loss P
cv
. With an ideal core coil, there should not be any core loss.
Therefore, without any samples, ideal core loss is P
cv
=0 and the area enclosed by the B-H curve is 0.
In reality, even without any samples, P
cv
is not 0, because of the eddy current loss generated on the winding of
a B coil, due to a leakage flux, or the core loss generated at the measuring yoke in the case of SSTs. As such,
P
cv
measured without any sample is the detection limit of core loss P
cv
of the particular device with the same
frequency under the same excitation conditions. That means the samples with core loss smaller than this value
of P
cv
cannot be measured.
■
2.4 Regarding air-gap compensation coils
Some Epstein frames and the ordinal SSTs are equipped with an air-gap compensation coil. This coil is meant
to cancel out the added air-gap flux between a B coil and a sample. However, this method does not cancel out
the air-gap flux correctly.
This kind of air-gap compensation coil is not attached to the location of the B coil is. As mentioned in

2.1, the
value of B changes with the position of the B coil with respect to the sample. This is because the magnetic flux
density within the sample is non-uniform throughout the sample in both an Epstein frame and a SST. Therefore,
the air-gap flux generated between the B coil and the sample is thought to differ from the air-gap flux generated
in an air-gap compensation coil that is attached away from the B coil.
In addition, an air-gap compensation coil does not take the cross-sectional area of the sample into
consideration. Because the hole size of an air-gap compensation coil is the same as that of a B coil, an air-gap
compensation coil may cancel out the leakage flux crossing an air-gap larger than the air-gap between a B coil
and the sample.
This instrument is not equipped with an air-gap compensation coil. One reason is the extreme difficulty of
equipping an air-gap compensation coil that works well with different shapes of single sheet that this instrument
can test. The main reason is the inherent problems with the ordinary air-gap compensation coils.