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Thursday, November 27, 2014
PF and DF
The PF of insulation is defined as the ratio of watt loss to total charging volt-amperes, or the cosine of the angle q between total current vector (IT) and the impressed voltage vector. It is a measure of the energy component (resistive component) of the charging current. The DF is defined as the ratio of the watt loss to charging amperes, or the tangent of the angle d between the total current vector and the capacitive current vector. The angle d is the complementary angle of the PF angle q. Although, the charging volt- amperes and watt loss increase as the volume of insulation being tested increases at a given test voltage, the ratio of watt loss to the volt-amperes (PF or DF) remains the same regardless of the volume of insulation tested. Therefore, the basic relationship of PF or DF eliminates the effect of the volume of insulation that is, the size of the electrical equipment or apparatus tested. This simplifies the problem of establishing normal insulation values for most types of electrical equipment.