Unbalanced VFD input currents 3

[edison123]

A 100 HP, 300 RPM motor driven by VFD has such a widely unbalanced input currents as below (the VFD output currents to the motor are balanced).

For (RYB) phase sequence
Line voltages (Vab Vbc Vca) 421.0 417.1 415.2 V
Phase voltages (Va Vb Vc) 241.5 242.5 239.6 V
Line Currents (Ia Ib Ic) 10.3 10.0 2.2 A
Power 3.1 KW

Two phases were interchanged on the VFD input side and the current pattern did not follow the lead change as below.

For (RBY) phase sequence
Line voltages (Vab Vbc Vca) 417.1 412.7 409.5 V
Phase voltages (Va Vb Vc) 238.6 240.4 236.5 V
Line currents (Ia Ib Ic) 10.3 10.1 2.1 A
Power 3.1 KW

Why such a variation of input currents and why the current in Ic shows the lowest even after sequence change ?



Muthu

http://www.edison.co.in

 

[edison123]

This is happening in all the drives in the plant and the drives are of different ratings (250 to 400 A) and the different OEM's (Schneider, Siemens, ABB, etc.)

Another important point. This current unbalance issue vanishes when the VFD's are fed from the client's captive power (diesel generator). The problem occurs only when the drives are powered by the supply from the utility.

Muthu

http://www.edison.co.in

 

[edison123]

Of course, the DG voltages are well balanced.

Could the unbalanced voltages from the utility be the issue here ? If so, why the current in C phase did not change even after interchanging two phases ?

Could improper neutral grounding on the utility side also be a problem ?

Muthu

http://www.edison.co.in

 

[waross]

The VFD inputs are rectifiers and only clip the tops of the voltage wave forms. They are extremely susceptible to low voltages on one phase.
If you have any three phase battery chargers they may also show a similar pattern.
Solution; Check the taps on your main transformer. Check the incoming voltages. Try to balance the incoming voltages.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Thanks Bill. How much of unbalance is really the "unbalance" ? In this case, it seems to be about 2% and nothing can be done about it from the utility side. Can't the drive handle this unbalance by itself ?

Also, if voltage unbalance is the issue, shouldn't the low current follow the phase when two phases are exchanged ?

Muthu

http://www.edison.co.in

 

[itsmoked]

Any chance one of the six rectifiers are blown? Possibly on more than one VFD due to some past event.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[CJCPE]

For a VFD with a rated input current of 200 amps or so, 8 amps of unbalanced input current with an unloaded drive is not likely anything to be concerned about. What are the drive input currents when the motor is running closer to full speed / full load?

CJC

http://www.vfdriveinfo.com

 

[DickDV]

You can get these current imbalances when the phase to ground voltage is not balanced or, even worse, if the input power is floating with respect to ground.

In the case of floating power, if the drive is CE rated, the internal noise filter is trying to force the network to balance and all kinds of unexpected currents can result.

Most CE rated drives require that the internal noise filter be modified or disconnected from ground when operating on a floating system.

 

[Marke]

The effect of supply voltage imbalance on input curent is much worse on VFDs that do not have either AC Line Reactors or DC Bus Chokes.
If the VFDs do not have either reactors, add them and the voltage imbalance will reduce.

Best regards,

Mark Empson
L M Photonics Ltd

 

[LionelHutz]

In both cases, Vab is the highest voltage so those 2 phases are sourcing the current. The DC buss capacitors in the drive are large enough to ride through the time when phase C would normally source some current due to the small load on the VFD.

So, put some load on the motor and re-test. The capacitor ripple current will increase and phase C will begin to source current.

The drive is built to support a 100hp motor = 75kW and likely about 145A @ 415VAC - you are running at 3.1kW and 10A which is well below it's ratings.

 

[edison123]

Thank you all. I'll ask the client to do more study under various load conditions.

Muthu

http://www.edison.co.in