VFD and power factor

[bru]

Hi. Can anyone shed some light on how power factor of a system is affected by a VFD operating a motor under low load? The stator will be energized to full load amps with no rotor.
Is the power factor improved drastically due to the use of the drive?
thanks

 

[DanDel]

Are you planning on energizing a stator with a VFD to full load amps without no rotor? How does this relate to a motor under low load?

 

[moturcu]

Regarding first part of the problem (low-load motor fed by VFD):
If the motor operates in constant torque region, VFD keeps the flux constant unless special precaution is taken in the drive. So this means at low load the motor flux which is an indicator of the power factor will be the same as rated power. On the other hand active power drops due to the low load operation. As a result power factor will drop for low load operation. However, if some efficiency improvement scheme is implemented in the drive, then power factor should remain constant as you lower the load.

If the motor operates in the constant power region, flux will be reduced with incresing speed to keep V/f constant. So depending on which speed you are operating, power factor may remain constant, incerase or decrease. However if you make sure proportional flux reduction with lowering load, than you will certainly have constant or increasing power factor.

 

[bru]

Thanks for your reply.
we have to do a copper loss test on the winding of a linear induction motor. The VA required to do energize to full load current at 60Hz is too high. If we run the test at 25 cycles the test voltage will be lower for the same current. We would use DC but not enough supply available at this location without bringing in a DC drive or at least some large rectifier arrangement.

Inductive reactance will then be lower at 25Hz and capacitive reactance will increase. In short, what kind of effect will this have on the system at the supply side of the VFD? The load will have some KW for heating of the motor under test but there will be other work so most of the load is purely inductive.

 

[DanDel]

Typically, a VFD has a constant volts/hertz ratio. Depending on the size of the VFD and the impedance of the load(which won't, I'm assuming, contribute any appreciable counter emf), current flow may or may not be too high.
In general, the input side of the VFD should not be greatly affected by anything on the motor side except the amount of load itself, since the input side of the VFD is simply concerned with providing a DC supply to the output side.

 

[bru]

Thanks much, I suspected this would be the case with the supply side but I was not sure if anyone had experienced otherwise.

 

[HarryDampers]

When you refer to Power Factor, are you talking about TOTAL Power factor or displacement Power factor??

On a standard VVVF there is or can be a big difference between the two....i dont know your reasoning behind asking the question but it may be worth following through .