third harmonic produced by AC drive

[tem1234]

Hi,

I have measured current of AC drive at 600V. Under an unbalanced voltage between 0.4%-1.0% approximately, the third harmonic produced by these drives is 10%!

Do you ever saw so much 3rd harmonic produced like that by a drive? 10% is pretty much and i don't know why we have such value.

I hope somebody can give me a clue about why we have such 3rd harmonic. There are filter syntonized at 5th harmonic at 4kV and other capacitor without filter at 25 kV in the same facility. I don't know if this can be related.

Thanks

 

[electricuwe]

The unbalanced voltage is the reason for the third harmonic current.
- at what load did you take your measurement ?
- what is the power rating of the drive ?
- are there inductors between line and drive input or in the drives DC-link ?

 

[tem1234]

Hi electricuwe,

I know that the 3rd harmonic is produced under voltage unbalance, but i didn't think to have that much!

i have measurement of 15 drives where the 3rd harmonic current is 10% (all in the same facility). Measurement were done directly before the drive. Drive are loaded at approximately 50% of their rating. There is no inductors.

One thing that i was asking too is that the 3rd harmonic current doesn't change that much if the voltage unbalance is 0.5% or 1%, unlike others drive which produced more 3rd harmonic current when the Uunb raise. It seems to be saturated at 10%!

Thanks

 

[sreid]

The thrid harmonic current is primarilly caused by the non-linearity of the electrical steel in the motor vs. magnetic flux level. The difference could be in the design of the motor, the quality of the electrical steel used, it's thickness, how hard it is being driven toward magnetic saturation and how good the laminations are insulated.

 

[electricuwe]

tem1234,

with drives injecting harmonic currents also the voltage waveforms gets distorted. It is very common to find trapezoidal voltage waveforms instead of sinusoidal waveforms on busses that mainly supply voltage source converters with diode frontend (as nearly all drives are today). With a voltage waveform distorted that way, current peaks are smaller than with a pure sinewave. Therefore current harmonics usually do not increase linear with their root cause.

sreid,

Since the motors are opeated via a variable speed drive, nonlinear characteristcs of the machine can be excluded as reason.

 

[sreid]

electricuwe,

You are absolutely correct, I missed the "Drives" part and assumed "Off the Line" operation. A reminder to me to read posts completely before shooting off a reply (which I have been guilty of before).