Different amp readings

S

stephen@spelectric

Member
Location
Texas
Occupation
Electrician
Good morning
Here is my question - we have 2 different industrial recycling lines. I am using a fluke 376c meter
There is a 2000a MDP 480v 4w
We have a 10 section 26 motor MCC
5 total vfd's 2 powerflex 525 and 3 powerflex 755
1 - 422hp motor
1 - 177hp motor
1 - 40hp motor
When i use my meter to read the total draw on this equipment i am reading 134amps each phase. The readings from each vfd and mcc are totaling 400+ amps.
I can read 175a on the load side of the 422hp motor vfd without a load and on the line side i read 30amps each phase. Can anyone help me out with what is going on? I know the readings we are getting on the load side are correct but why is the line side so low?? In Speaking to a engineer he said our voltage and current are out phase and the PF is our problem. We other engineers who dont seem to be concerned with the problem.
 
VFD present good displacement pf to line so line current low. Load side current high as it includes reactive component current of motor also
 
stephen@spelectric said:
I can read 175a on the load side of the 422hp motor vfd without a load and on the line side i read 30amps each phase. Can anyone help me out with what is going on? I know the readings we are getting on the load side are correct but why is the line side so low?? In Speaking to a engineer he said our voltage and current are out phase and the PF is our problem. We other engineers who dont seem to be concerned with the problem.
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Deltaforce said:
VFD present good displacement pf to line so line current low. Load side current high as it includes reactive component current of motor also
Click to expand...

Yes. Without a mechanical load, there is very little slip of the rotor relative to the rotating magnetic field from the stator. And so the current flowing between the VFD and motor is mostly just the magnetizing current, which is primarily reactive from the inductance of the motor's windings. This reactive current circulates between the motor windings and the VFD DC bus capacitor, and as a result stored energy is exchanged back and forth between them. Therefore this reactive current is not drawn from the VFD input. Instead, the input current to the VFD is mostly due to motor losses and VFD losses, and as @Deltaforce mentions, has a good displacement power factor.
 
To add to the above, in addition to the VFD 'absorbing' any displacement power factor, the VFD is acting as a step down converter.

When the motor is operated at reduced speed, the VFD supplies reduced frequency and reduced voltage. When the VFD converts the full supply voltage to the lower motor operating voltage, the effect is quite analogous to a step down transformer, higher voltage lower current in, lower voltage higher current out.

-Jonathan
 
stephen@spelectric said:
How does the hmi screen read 435amps but putting my meter on the incoming feed reads 178amps
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The hmi screen is likely reading _motor_ amps, not _input_ amps.

It is also possible that your meter is not correctly reading the input current because of the harmonic content of the current to the VFD input rectifier, but I think that is a less likely explanation.
 
winnie said:
The hmi screen is likely reading _motor_ amps, not _input_ amps.

It is also possible that your meter is not correctly reading the input current because of the harmonic content of the current to the VFD input rectifier, but I think that is a less likely explanation.
Click to expand...
Op may configure to show input and output power, powerfactors in addition to input and output currents on hmi screen. It clarify his doubt and his meter accuracy
 
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