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Inspecting Permanent Magnets

When needs to inspect permanent magnets arise, there are a few different options.  This article discusses these options.

Inspecting the Base Material

The magnetic properties of the base material is inspected on a lot basis.  This is done by running a second quadrant hyterises curve, also known as the demagnetization curve or simply BH Curve.  The test is destructive; therefore, only a sample is checked this way.  Typically, one or two curves are run for a single manufacturing "heat lot".  The results allow confirmation that the correct material properties were achieved prior to processing the material into a customer specified shape.

Inspecting the Magnets After Magnetizing

The intent is to confirm that the magnets were magnetized correctly, and for a secondary confirmation of the correct material being used.  There are two methods of inspection available: measuring the flux density or measuring the total flux.

  1. Flux Density

    Measuring the flux density is done using a device called a Hall Probe and a Gaussmeter.  The hall probe has a small chip called a hall effect sensor.  This typically has a cross-section of .180" x .180", but can be made smaller on a special order case.  The magnet pole surface must be covered by the hall effect sensor.  Flux lines eminating from the magnet cause the current in the conductor to experience a force from the magnetic field.  This force is called the Lorentz Force.  Because of this force, the charged particles, electrons travelling within the conductor, are pushed to one side, which causes a potential difference between the sides of the conductor.  By measuring this potential difference, called the Hall voltage, one can determine the amount of magnetic field.  The Gaussmeter is the instrument that converts the data into Gauss, the magnetic field.

    It is important to realize a few things regarding this type of measurement: 

    1. This measurement is a sampling. The only portion of the magnet that was measured is the area which had the Hall Effect Sensor over it.  If the magnet were larger, as the probe is moved closer to the edge we would expect the readings to increase because there are more flux lines near the edge then at the center of the magnet.
    2. Additionally, the magnet pole surface is not magnetically perfectly symetrical.