PFC To Motors On VFD

Assumptions:

1. PFC=Power Factor Correction
2. VFD=Variable Frequency Drive.

The purpose of PFC is to reduce the current flowing in the feeder cable for the same load and the losses associated with the higher current. It can be justified economically on the basis of some combination of avoiding the up-sizing of cables, or of avoiding a penalty tariff being imposed by the utility company, or other possibilities.

"In general, power factor correction capacitors should not be used with AFDs."

http://www.ab.com/support/abdrives/documentation/techpapers/qanda01.htm

The VFD will inherently correct the displacement power factor seen by the system to .95 or better. So not only are they not needed, but they can/will cause more problems if not done right, which involves significant added cost and space (tuned reactors) with no significant additional benefit.

1) A motor needs reactive energy for building its magnetic field.

2) The input power from a VFD flows through a rectifier and the current in a diode rectifier is in line with the voltage over the diode rectifier. (seen in time, current and voltage flow of don’t flow at the same moment.) This means that a VFD can only take active power out of the supply grid.

3) Looking at point 1) and 2) I conclude that the reactive energy needed for the motor must come from the capacitors inside the VFD.

4) A motor with a low PF will need more reactive power to operate. So the total power (active + reactive) the flows from the VFD to the motor will be high. This power will flow through the IGBT’s and the switching losses in the IGBT’s will be higher.

5) Can I conclude that a combination VFD + motor with low PF is less efficient than the combination VFD + motor with higher PF ?