This instrument
is an extremely stable and linear integrator that integrates the
voltage induced in a flux coupling coil or sensor, usually called a
search coil, connected to the input terminals of the fluxmeter. The
voltage, V, induced in a coil that is placed in a time varying
magnetic field is V = NAdB/dt where N is the number of turns in the
coil and A is the area of this coil.
This
relationship can be rewritten to yield òVdt = NA(DB). From this
equation it can be seen that the time integral of the voltage induced
in the coil is proportional to the change in B at its location. The
leads from the flux coupling coil should be twisted to eliminate the
effect of stray magnetic flux linking these leads and producing
errors.
The plane of
the coil should be positioned to obtain the maximum reading, which
yields the correct value. Because a fluxmeter measures the change in
B, one way that a dc measurement can be made is by turning on and off
the magnetic device under test, which causes B to go from
approximately zero to its operating level.
If the
fluxmeter is zeroed when the power to the magnetic device is off,
then the instrument will indicate the operating level of B when power
is applied. Whenever possible, the accuracy of measuring B can be
improved by reversing the applied voltage on the device under test
which causes the change of B to be twice the operating level of B.
Reversing the
applied voltage and measuring the change in B from full voltage of
one polarity to full voltage of the reverse polarity eliminates the
effects of residual magnetism. Another technique is to zero the
fluxmeter while the search coil is in a zero magnetic field and then
place the search coil in the magnetic field to be measured.
When it is used
to measure the flux produced by ac systems, the search coil is kept
stationary after positioning it to obtain the maximum reading. If the
device under test is of the permanent magnet type, it may not be
possible to turn it off.
Then a change
in B through the search coil can only be accomplished by zeroing the
fluxmeter when the coil is not in the vicinity of the device and then
placing the coil where the measurement of B is wanted.
The resulting
change in B passing through the coil, which is indicated by the
fluxmeter, is the measured B. The fluxmeter controls are organized so
as to display the average flux density across the area of the coil or
the total flux within the coil. Input dials provide for setting in
the area-turns of the coil or only the number of turns.
The indicating
meters have either analog or digital displays with readings in the
cgs system. An output signal from zero to one volt proportional to
the meter readings is also available.
These
instruments have ranges from 104 to 109 Mx turns with accuracies as
great as 1/4% for dc measurements. They will respond to input pulses
with rise or fall times (10–90%) as fast as 10 ms.
Uncompensated
thermocouple voltages, which are usually present at the instrument’s
input, are removed by adjusting a zero control for minimum drift of
the integrator. The resulting drift can be as low as 100 Mx turns per
minute.
For ac fields
these instruments will yield rms values of B for frequencies from 10
Hz to 100 kHz with accuracies as high as 1/2 of full scale. These
instruments can operate from a 50/60 Hz ac power line or internal
batteries.
No comments:
Post a Comment