Will VFD improvement or lower the power factor

[Brian2903]

Hi all, I'd like to know if a motor installed VFD would improve or lower the power factor, and why? Thanks!!

 

[dpc]

If you are measuring the displacement power factor at the input to a PWM VFD, the power factor will be extremely high - much better than the motor. The VFD provides the vars to the motor via its inverter.

 

[Brian2903]

Since the VFD provide the VAR to the motor, the ture pf should be improve, right?

 

[CJCPE]

The input current of a VFD has harmonic distortion. In effect it is the sum of a 50/60 Hz fundamental current and harmonic currents at various multiples of line frequency. The displacement power factor is calculated as the input watts divided by the input VA where only the line frequency (fundamental) part of the current is used in calculating the VA. The displacement power factor of a VFD is about .98, so installing a VFD would improve the displacement power factor. The new power factor of an existing motor would be improved by adding a VFD and the overall power factor of the plant would be improved by adding a VFD or a VFD and motor.

The total power factor is calculated as the input watts divided by the input VA where the total RMS current including harmonic currents is used to calculate VA. The level of harmonics is determined by the source impedance at the drive input terminals. If the source impedance is low (available fault current is high), the harmonic current will be high and the power factor will be low. If the source impedance is high (available fault current low), the harmonic current will be low and the power factor will be high. The total power factor of a VFD might be lower than the power factor of a motor without a VFD.

Even if the total power factor of the VFD is lower than the power factor of a motor without a VFD, adding a VFD may not reduce the total power factor of the plant. The total power factor of the plant must be calculated as the plant watts divided by the total plant VA including harmonics currents. When harmonic current is included, the total current is the square root of the sum of the square of the fundamental current plus the square of the harmonic current.

 

[2dye4]

pick a VFD with active power factor correction.
otherwise it is likely to be a worse power factor.

 

[CJCPE]

What is often called true power factor is the total power factor including both the displacement and distortion effects. The VFD does supply the VAR to the motor, but that means that the displacement power factor is improved. That improvement will be somewhat offset by the distortion effect.

 

[jraef]

But do most utilities calculate harmonic currents when assessing power factor penalties? After all, that is usually the main reason an end user would care.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[2dye4]

For large industrial customers they do
calculate harmonics.

The reason power companies dislike current flowing out
of phase with their voltage is because it takes capacity
from the system without generating sales of watts.
So any current flowing orthoganal to the the voltage
is a loss for the utilities be it harmonics or phase shift
due to inductive loads.

I worked at a location with a large spot welder that
consumed massive current for only 13 cycles of the
power line and then idle for 1 min. The power company
did not like this machine and charged extra for the
imbalenced load.