VFDs can be used to improve system power factor when oversized motors or motors that
operate at low power factor are used.
[...] (already quoted by you)
A motor operating directly from a sine wave supply (commonly referred to as “across-the-line” operation) will exhibit displacement between the applied voltage and resulting current. The displacement varies as the motor load changes. Typical values for displacement, Cos ?, are 0.85 for heavily loaded motors and 0.65 for moderately loaded motors. Lightly loaded motors may appear as pure inductors in which the power factor is nearly zero.
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Diode, and sometimes IGBT based rectification keeps the input fundamental displacement near unity. In other words, drive input current remains in phase with the supply voltage under all load conditions. Therefore, Cos ? is near unity (approximately 0.98) with VFD use. On the other hand, diode and IGBT rectification may result in relatively substantial current THD. A typical full wave diode bridge rectifier will result in approximately 45% current THD. This is a typical value for a VFD with DC link choke under fully loaded condition. Using 0.98 for displacement (Cos ?) and 0.45 for THD in the above equation, the resulting PF will be
approximately 0.89.
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Under light loading, drive input displacement power factor remains near unity due to the diode rectification. However, VFD input current THD will increase. It is not uncommon for THD to be near or even above 100% under lightly loaded conditions. Applying a large THD to the above equation results in substantially reduced power
factor. However, this in itself is not as bad as it may seem.
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Current THD increases partially because the harmonic component increases, but, also because the fundamental component decreases. THD increases to what appears to be a significant level, but, the actual overall harmonic current component itself is quite small as compared to the full load current. Overall, VFD input current will be nearly zero as the largely reactive current flowing into the lightly
loaded motor does not get reflected back to the supply.
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Only the real (torque producing) component is measured on the supply side. Even though drive input PF is lower due to increased harmonics at light load, the input current is also much lower. Subsequently, the low input power factor does not negatively impact the overall supply components (transformer, wiring, etc.) that are already sized to accommodate full load operation in the first place. Over sizing of supply components that is typical for low power factor systems is not required because the VFD input current is also low