Unbalanced VFD input currents
Unbalanced VFD input currents
(OP)
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 ?
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
www.edison.co.in





RE: Unbalanced VFD input currents
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
www.edison.co.in
RE: Unbalanced VFD input currents
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
www.edison.co.in
RE: Unbalanced VFD input currents
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
RE: Unbalanced VFD input currents
Also, if voltage unbalance is the issue, shouldn't the low current follow the phase when two phases are exchanged ?
Muthu
www.edison.co.in
RE: Unbalanced VFD input currents
Keith Cress
kcress - http://www.flaminsystems.com
RE: Unbalanced VFD input currents
CJC
http://www.vfdriveinfo.com
RE: Unbalanced VFD input currents
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.
RE: Unbalanced VFD input currents
If the VFDs do not have either reactors, add them and the voltage imbalance will reduce.
Best regards,
Mark Empson
L M Photonics Ltd
RE: Unbalanced VFD input currents
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.
RE: Unbalanced VFD input currents
Muthu
www.edison.co.in