Current discrepancy between VSD and Switchgear
Current discrepancy between VSD and Switchgear
(OP)
In our plant, we have 6kV compressor connected to VSD.
And I got some question regarding current discrepancy between VSD and Switchgear.
I found that during compressor operation, the current indicated in Switchgear (21A) is lower than in VSD(28A).
If the current in VSD is lower then I would understand since I would understand this due to current loses in VSD.
But I cannot understand that current in SWGR is lower than in VSD.
I asked VSD vendor and they answered that voltage is variable in VSD so current in VSD can be larger than in SWGR.
And they explained it with Ohm's law. For instance, 2V x 2A (SWGR) = 1V x 4A (VSD) like this.
But still I don't get it clear. By Kirchhoff's law, input current in VSD should be same as output,ideally. But considering loses in VSD, output could be less than input.
Which part am I wrong at?
Please give me explanation. Thanks.
And I got some question regarding current discrepancy between VSD and Switchgear.
I found that during compressor operation, the current indicated in Switchgear (21A) is lower than in VSD(28A).
If the current in VSD is lower then I would understand since I would understand this due to current loses in VSD.
But I cannot understand that current in SWGR is lower than in VSD.
I asked VSD vendor and they answered that voltage is variable in VSD so current in VSD can be larger than in SWGR.
And they explained it with Ohm's law. For instance, 2V x 2A (SWGR) = 1V x 4A (VSD) like this.
But still I don't get it clear. By Kirchhoff's law, input current in VSD should be same as output,ideally. But considering loses in VSD, output could be less than input.
Which part am I wrong at?
Please give me explanation. Thanks.





RE: Current discrepancy between VSD and Switchgear
RE: Current discrepancy between VSD and Switchgear
Try comparing kW in to kW out. That that will take into account the effective voltage that the VFD is outputting.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current discrepancy between VSD and Switchgear
At frequencies below the motor base speed the drive output voltage has to be lower than the motor base voltage (for now it just does, OK
RE: Current discrepancy between VSD and Switchgear
At first, I though of just simple circuit for understand. But considering VSD consists of capacitor bank, inductor, etc. which is not that simple as circuit and is considered as power converter, I realized that I shouldn't think just like (input current) = (output current).
There is still I need to look into about VSD though, I roughly got it.
Appreciate your replies.
RE: Current discrepancy between VSD and Switchgear
Keith Cress
kcress - http://www.flaminsystems.com
RE: Current discrepancy between VSD and Switchgear
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Current discrepancy between VSD and Switchgear
The VSD input only supplies the real power. The motor reactive power flows between the VSD and the motor. If you ran an unloaded motor at line frequency, the VSD input power is quite low since it's only the power required for the motor losses (copper, fan etc) and VSD losses. Yet, the motor current is mostly reactive and still a fairly high ratio of rated current (35% to 50% typically). The end result is that the VSD output current to the motor typically ends up being 10x to 20x the VSD line current. For example, the last motor I ran at 60Hz was drawing 230A from the VSD with 14A of line current. This demonstrates how bad using a simple input VxI = output VxI approximation can be.
As posted by waross, you can only compare input vs output power (kW).
RE: Current discrepancy between VSD and Switchgear
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Current discrepancy between VSD and Switchgear
RE: Current discrepancy between VSD and Switchgear
Adding to what you said, if the motor is delivering somewhere near full torque at low rotational speed then the output current will be high to develop torque, but the power throughput will be low and the input current will be correspondingly low too.
RE: Current discrepancy between VSD and Switchgear
(1800RPM - 1760RPM = 40RPM slip; 1800RPM / 4 = 450RPM 450RPM - 40 RPM slip = 410RPM) There may be some non-linearities with regard to the magnetizing or reactive current complicating this but you get the idea.
Remember the slip frequency and the V/Hz ratio.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current discrepancy between VSD and Switchgear
As examples,
Motor is at 1/2 speed and producing rated torque - input power is 1/2 of rated motor power
Motor is at 1/2 speed and producing 1/2 of rated torque - input power is 1/4 of rated motor power
Motor is at 1/4 speed and producing 3/4 of rated torque - input power is 3/16 of rated motor power
Motor is at 3/4 speed and producing 1/2 of rated torque - input power is 3/8 of rated motor power
If you wanted to get more accurate then you'd have to account for the slip, motor losses and VSD losses, and the losses vary as the speed and load vary.