VFD line current and max. demand

[DrDrreeeaaa]

Hi All, I have come across this problem again and would like your opinion.

We have several (20+) large motors (>50kW anf 400V) fitted with VFD. When determining estimated maximum demand I have considered them operating at 0.9PF, while all other DOLs operate at 0.75. If I ignore the PF benefits of the VFD the load is too big for the TF.

I am not comfortable with my approach since:

1) While displacement PF quoted by the manufac. is 0.98, distortion PF will be poor. Does distortion power factor contribute to greater current draw?

2) What I have done is take motor kW and converted to Amps at 0.9PF to get the VFD line current. Will the VFD draw more than motor kW amps from the line (even if it is at a good disp. PF)

Thanks all

MB

 

[CJCPE]

Distortion power factor certainly can contribute to greater current draw. Unless the VFD has line or DC bus reactors, the RMS input current can exceed DOL motor current if the source impedance of the line is low.

 

[DrDrreeeaaa]

The VFD does have line reactors. One reactor per 7-0ff VSD at 50kW each.

 

[jghrist]

What I have done is take motor kW and converted to Amps at 0.9PF to get the VFD line current. Will the VFD draw more than motor kW amps from the line (even if it is at a good disp. PF)

This neglects both motor and VFD losses. Yes, the VFD will draw more current. Use the nameplate FLA of the drive.

 

[CJCPE]

With line reactors, the maximum line current is likely to be comparable to the motor current. If the line reactors are internal to the drive, you should find the nameplate input current is about equal to the nameplate output current. If the reactors are external, the actual input current will likely be less than nameplate, but you will probably need to use nameplate input current for branch circuit design.

 

[DrDrreeeaaa]

I don't have the nameplate FLA. The client has given me only motor kW. The client has not purchased the motors, but for his plant to be operational on time he needs to order the TF today.

The only information I have is the kW rating of the motors and which are VSD/DOL.

I have to be conservative - I will ignore VSDs, assume full load PF of 0.8, and assume 90% efficiency for kWi/kWo.

Since the client is an EE and knows alot about the process I will get him to apply a diversity factor.

 

[NCTHAI]

You can safey assume VFD losses at 6-10% of motor rating.

NC

 

[DrDrreeeaaa]

not going to as I have ignored VSDs (assumed all motors DOL 0.8 PF)
the PF benefits of the VSD may cancel out the VSD efficiency in this case