Variable speed drive Power Factor

Power factor is a measurement of reactive power. Reactive power is the VA used to establish the magnetic field in a motor. For permanent magnet motor, most time you see pf being unity, which means all VAs are used to generate torque (minus the loss, of course). Under the condition of field weakening operation though, you will see the PM motor gets less than unity pf because some VAs are used to counter the perm magnet field. In induction motor, the pf indicates the portion of VAs used to generate the field comparing to the VAs that generate torque (and loss), which is related to the v/hz in scalar controller or Id in vector control. So under light load condition, you can improve the pf of an induction motor by lowering the field a bit.

The motor power factor does not make a big difference other than giving an idea of loading. In regards to the variable speed drive fixing power factor everyone is mostly correct. A variable speed drive will always show a DPF (Displacement Power Factor) near unity. DPF is what a utility company traditionally monitors and is the measure of lag looking at current and voltage of power at 60hz or fundamental only. The true power factor on a VFD will change with loading etc and includes current and voltage inclusive of harmonics.

Variable speed drive makes assumptions about the motor, and these assumptions affect the relationship the drive controls between voltage applied, frequency and current. For example, a simple volts/Hz drive assumes a motor requires a certain voltage at a certain frequency. In a synchronous or PM motor this parameter can change the PF from lagging (normal) to leading. In an induction motor there is an optimal relationship for a given load. By looking at PF you can adjust the drive parameter to improve the operating PF of the motor. Those who tell you this is set at the factory are quite wrong. The optimum value is established at motor manufacturer, but there are an infinite number of sub-optimal values.