Measurements of Harmonic Distortion

Harmonic distortion is usually expressed in Total Harmonic Distortion (THD), more specifically in Total Harmonic Distortion plus Noise (THD+N). Many harmonic testers are labeled "THD" even though their readings also contain noise (THD+N). Total harmonic distortion is the sum of the power of all frequency components except the fundamental frequency divided by the power of the fundamental frequency. With respect to magnetic components, THD is measured by applying an input signal to the transformer primary and measuring the output across a load resistor connected to the secondary winding. This is known as the stimulus-reponse method of evaluating distortion. Magnetic Metals can also measure the amplitude of the harmonic frequencies.

For modem transformer applications, THD is typically stated at a test level of -10dBm and a frequency of 600Hz. This test level corresponds to a voltage of 0.245 V across a 600 ohms impedance, which is a common impedance assumed by distortion analyzers such as HP 339A. This 600 ohms impedance is comprised of the sum of the primary winding resistance, the reflected secondary winding resistance, and the reflected load resistance (see Fig 2).