Most power monitoring instruments synchronize their measurements to a stable 50/60Hz voltage source. The reason is to get accurate readings between the voltage and current phases. This is fine for the majority of applications, including when measuring the input of a Variable Frequency Drive (VFD), but poses a problem when attempting to measure the output of a VFD – by design, the VFD output voltage frequency varies, making it difficult for the power monitor to synchronize.

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