FLUXMETER DEFINITION BASICS AND TUTORIALS

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.

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