What is Tanδ Test ?
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). Please refer to the technical guide explained in detail.
Tanδ value is used as an evaluation index of insulation materials, which shows the dielectric loss that is not affected by their shapes, for judging the degree of moisture absorption, dryness, contamination, void, and deterioration of insulation materials. Tanδ test is an important test item of insulation tests in electrical equipment.
The measurement is as follows: by applying AC voltage to a standard capacitor regarded as zero loss and to a test specimen, and then the respective currents are detected and compared electronically, as a result, tanδ and capacitance with the test specimen are obtained. Therefore you need a testing power supply and a standard capacitor other than a C & tanδ measuring instrument. However, with respect to measuring instruments, there are some built-in types contain testing power supplies and/or standard capacitors.
If a metal housing such as rotating machines is grounded, the measurement have to be performed on condition that the measuring electrode is grounded, for the housing is the other grounded electrode in insulation materials between two electrodes of the coil conductor and the housing. In case of the above, you should use a C & tanδ measuring set corresponding to grounded equipment so that all of the circuit elements are working at high potential side of testing supply voltage, or CT is partially used to detect measuring currents at high potential side between the equipment or the standard capacitor and the voltage source.