You should have read this before posting
Current THD increases partially because the harmonic component increases, but, also because thefundamental component decreases. THD increases to what appears to be a significant level, but, theactual 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 lightlyloaded motor does not get reflected back to the supply. The reactive current is supplied by the VFD DClink circuit (bus capacitors) and circulates within the VFD’s output section. It does not flow from thesupply side. Therefore, even though drive input PF is low with the lightly loaded condition, the VFD inputcurrent is also much lower because only the “real” current (torque producing) component is reflectedback to the supply. The resulting harmonic current is quite small even with the high THD level and doesnot represent any significant issue for the supply and associated components. Therefore, the low PFencountered with a lightly loaded VFD is not a detriment to the system and effective use of power ismaintained.
For example, a lightly loaded motor operating across-the-line will continue to draw 25-35% of ratedcurrent as magnetizing current (reactive current). This current is at nearly 90 degrees displacement fromthe applied voltage making for a very low power factor system. The supply and associated componentswill be exposed to this substantial current at low power factor. This same current will flow to the motorwhen operating from a VFD, however, the VFD input current will be near zero because this reactivemotor current is supplied solely by the VFD DC link circuit. Only the real (torque producing) componentis measured on the supply side. Even though drive input PF is lower due to increased harmonics at lightload, the input current is also much lower. Subsequently, the low input power factor does not negativelyimpact the overall supply components (transformer, wiring, etc.) that are already sized to accommodatefull load operation in the first place. Over sizing of supply components that is typical for low powerfactor systems is not required because the VFD input current is also low.
Expanding on this, motors with low power factor will exhibit a large magnetizing current (reactivecurrent). An additional benefit of using a VFD is elimination of the magnetizing component from beingreflected back on the supply. VFD input current can be less than the motor current even at 60Hzoperation. This will hold true for motors with relatively low full load power factor (less thanapproximately 0.85) and VFDs that maintain high overall input power factor (0.89 or greater.
if you had not said the motor/vfd were loaded to 100% it may make sense
if motor loaded <40%
vfd is way oversized for motor
and motor i mostly reactive
