Determining VFD Input current 5

[dpc]

If I know the output current rating of a generic 6-pulse PWM VFD, is there a good rule of thumb for determining the input current (rms) to achieve this output into a typical induction motor?

I'm having some difficulty getting this data for a particular vendor and I just need to get close. I'm assuming this should be fairly typical for all vendors?

I understand there will be significant 5th and 7th harmonic currents.

Thanks,

Dave

 

[ScottyUK]

Have a look at Page 9 (actually the 10th page), specifically Fig. 6.

The use of a DC link choke can modify the harmonic spectrum - at least one OEM (ABB) is using this principle in their design and making it into a bit of marketing exercise. Might be good enough for what you need though.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

Should have said that you can only get real power flow with current and voltage of the same harmonic order, so if you have a reasonably sinusoidal voltage then you can use a first order approximation that the active power transfer only takes place at fundamental frequency.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

Scotty, I think you forgot to post a link.

 

[ScottyUK]

Gaahhrrrgh! Why oh why can't we edit our posts??

http://www.gambica.org.uk/pdfs/G54_1_guide_5th_ed.pdf

Thank you David, and apologies to everyone else.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[ozmosis]

dpc
This is an extract of information to determine input current on a VFD/VSD. It uses watts instead of horses btw..

Basic info:
Inverter Input Power= x/Efficinecy[Inv] * Efficiency[Mtr]
Motor Input Power= x/Eff[Mtr]
Shaft Power = x kW
Motor Input VA= Motor input power/Motor PF
Inverter Input power=Motor Input power/Inverter PF
Inverter Input VA=Inverter input power/Inverter PF
Inverter Input Current= (Inverter Input VA * root3)/(kVac *3)

Example:
Typical 7.5kW 4 pole motor power factor (PFM) = 0.82
Typical 7.5kW 4 pole motor efficiency (EffM) = 0.87
Typical 7.5kW inverter power factor (PFI) = 0.95
Typical 7.5kW inverter efficiency (EffI) = 0.97
Assume 400Vac

Motor input power= 7.5/0.87= 8.6kW
Motor input VA= 8.6/0.82=10.5 kVA
Motor inout current= (10.5/root3)/(0.4*3)=15.2A
Inverter input power= 8.6/.97=8.9kW
Inverter input VA=8.9/0.95= 9.4kVA
Inverter input current=(9.4kVA *root3)/(0.4 *3)= 13.5A

This often confuses people when we say the input current to the Inverter is lower than the motor current but the inverter current is dependent on the electrical power required by the motor, the supply voltage, the efficiency of the inverter, and the input power factor of the inverter.
A critical factor here is the PF of the inverter. Depending on the type of harmonic mitigation in the VFD will determine the PF.
As already mentioned, VFD's with internal DC link chokes (ABB, Danfoss to name a couple) will have a lower harmonic distortion and therefore high total PF (typically 0.9~0.95) whereas drives that have no DC link choke or no AC line reactor will have a total PF in the region of 0.6~0.7. This will have an impact on the input current.

 

[dpc]

Thanks everyone. The drive's power factor was the part I was struggling with. Ozmosis' explanation helps a lot.