Motor insulation testing - Polarization Index and loss-tangent testing

Hi Anok.

Insulation materials in electrical equipment are used between a pair of electrodes; therefore it can be expressed as capacitors. Applying AC voltage to a capacitor with insulation materials causes an energy loss due to the leak current, the dielectric polarization, and the partial discharge. Because of this, the phase of the capacitor current is more delayed than that of a zero-loss current (reactive current) flowing in an ideal insulation material. The delay angle is called "Dielectric Loss Angle", and the tangent is "Dielectric Loss Tangent" (tanδ).

The full explanation of polarization index is here (http://www.alltestpro.com/documents/AreviewofpolarizationindexandIEEEstandard43-2000_000.pdf - sorry, link no longer available)

They seem the same but they are clearly not. PI is a value due to the insulation material adopting a conductivity over time which would indicate a problem.

Tanδ values are the expression of the angular loss in a motor due to the leak current between the armature contacts. As the armature is exposed to a current, the field forces the armature to rotate. Any leakage of current results in a lower angular force.

I realise the above is simplistic but it suffices for your question.

Both values result in a loss of motor efficiency but in my opinion the later measurement of PI is more a safety related issue.

Any motor specialists please elaborate more as this is my limit of understanding but I hope you guys can increase my fill