A dispersion test is an old test that was designed by ERA in the UK, which people now know as a recovery voltage test. The difference is that the dispersion test is performed at one charging and discharging time only, whereas recovery voltage tests are performed over a range of changing and discharging times.
It is a measurement that gives an indication of the amount of ionic polarisation in the insulation, which gives a good indication of moisture internal to the insulation.
The difference between DDF (or power factor or dissipation factor or tan delta tests) and dispersion, is that DDF tests tell you resitive losses of the whole of the insulation, including the surface of the material, which can be significant if the surface IR is low, whereas dispersion is only looking at the internal condition of the insulation, and how the molecules are polarised when an electric field is applied.
The test involves applying a dc voltage to the insulation for a specifc time, then removing the test supply and shorting the terminals of the insulation for a specific time. The short is then removed and the "recovery" voltage is measured, together with a value for the leakage time constant of the insulation.
I would think nowadays that a request for a dispersion test would be very rare, and that they would possibly mean a dielectric dissipation factor (DDF) test, or a time domain (recovery voltage) or frequency domain (IDA or PAX) spectroscopy test.
ausphil
