VFD Input vs Output Current

Status
Not open for further replies.
B

blues

Member
Location
Nevada
I would like to know why the input current of a VFD would be differnt than the output from the VFD to say a 200hp, 3ph, 480v motor load. I can only assume it may have something to do with the harmonic content of the conductors feeding the VFD as apposed to the conductors leaving the VFD once the line reactor has filtered out the harmonics to the pump. Or am I way out in left-field on this one.

Thanks
Dan Craven
 
How about internal losses, due to the inherent inefficiencies associated with power conversion AC->DC and then DC->AC?
 
jim dungar said:
How about internal losses, due to the inherent inefficiencies associated with power conversion AC->DC and then DC->AC?
Click to expand...
Yes, there are losses. For the size of drive being considered, I wouldn't expect those losses to be above 3% of drive rating so input power is 3% higher than output.
As Petersonra has noted the input and output power factors differ. The motor might be 0.8 pf and the input typically about 0.95 depending on the arrangement.
With those figures, the input current would less than the output current however paradoxical that might seem.
 
blues said:
I would like to know why the input current of a VFD would be differnt than the output from the VFD
Dan Craven
Click to expand...

If you are referring to the vfd nameplate stating a higher input amp number than output amp number, it has been explained above.

If you are referring to the input amps being LESS than the output amps while running at less than base speed, this is due to the vfd being a power converter; power in=power out/efficiency so if you run 1/2 speed (therefore 1/2 output volts) at same load, the input amps will be 1/2 (plus the typical 3% efficiency) the output amps.
 
mike_kilroy said:
If you are referring to the vfd nameplate stating a higher input amp number than output amp number, it has been explained above.

If you are referring to the input amps being LESS than the output amps while running at less than base speed, this is due to the vfd being a power converter; power in=power out/efficiency so if you run 1/2 speed (therefore 1/2 output volts) at same load, the input amps will be 1/2 (plus the typical 3% efficiency) the output amps.
Click to expand...

Now throw in the VFD that is only input with single phase and has three phase output - the input and output currents are never the same or even close to the same.
 
kwired said:
Now throw in the VFD that is only input with single phase and has three phase output - the input and output currents are never the same or even close to the same.
Click to expand...

We've had a small AB drive hanging on the wall above the work bench of the shop for a couple years. 1 ph in 3 ph out. Only that combination. Input current of 19.6 and output of 9.6.



Sidenote:

My keyboard seems to be stuck in the strikeover mode instead of insert. How did I get it there and how do I get it back??
 
ptonsparky said:
We've had a small AB drive hanging on the wall above the work bench of the shop for a couple years. 1 ph in 3 ph out. Only that combination. Input current of 19.6 and output of 9.6.



Sidenote:

My keyboard seems to be stuck in the strikeover mode instead of insert. How did I get it there and how do I get it back??
Click to expand...
press the "insert" key on your keyboard probably located near upper right corner.
 
ptonsparky said:
Oh for crying out loud...how long have I had a computer(30 years)...how long have I "known" this?! :slaphead: (30 years minus an hour or so).

Maybe this will be my only "duh" moment of the day.
Click to expand...
Happens to us all....
But at least I have an excuse - I'm ancient and past my sell by date.
:)
 
Status
Not open for further replies.
Top