Why Harmonics in a three phase motor ? 2

[COMAPRO]

recently I measured the electrical parameters of a 50 HP motor that has a wye delta starter , no electronic at all and my Fluke power meter registered 60% THD-R, Why ?
Thanks in advance for any tip you can tell me on the subject

 

[electricpete]

What does the R stand for? Anyway 60% sounds extremely high no matter what, even for current.

I can only think of a few possibilities
- thd-r is some weird parameter that I don't understand.
- your meter is screwed up.
- your motor is at no-load and you have an extremely high or unbalanced voltage applied, pushing the core very very far into saturation

Then again I may just be totally missing something.

 

[jbartos]

Suggestion: Try to measure harmonics by other harmonic meter or analyzer at some other load, e.g. heater and compare it the Fluke meter readings. If not approximately the same, contact the Fluke power meter tech support.

 

[pablo02]

What other loads are served by the load side transformer? Is there a possiblity of harmonics being generated by other sources? Follow 'jbartos' suggestion to clarify the issues brought up.

 

[COMAPRO]

thank you guys for your promtly reply. the fluke meter is working fine. the only additional load that is running in parallel are the capacitors that are attached to compensate the low power factor of the motor. THD-R stands for total harmonic distorsion as % of rms total. defines amount of harmonic distortion as a percentage of the rrms value of waveforms at all frecuencies ( fundamental and harmonics ). voltage is not unbalanced. the motor was running at no load.
again what could be the reason ? harmonics created by capacitors ¿? I doubt it

 

[pablo02]

The capacitors should not be the source of the harmonics -- if the condition "electropete" mentions is not the source, then possibly it is coming in at the power source (other load banks or from the utility itself (other customers generating harmonics) -- can you measure with the 50 hp motor out of service?

I think most utility service standards do not include "quality" of power delivered unless there a special service contract provisions for such -- but if there are quality problems generated by a customer, they can and will impose corrective measures at a cost to the one creating the problems...

 

[buzzp]

Besides checking at other locations served by the same power source, check which harmonics are present(2nd, 3rd, 5th, 7th, etc). This will give you a fair idea of where to look for the source of the harmonics. If it does not appear to be coming from your plant then you can approach the utility with this data and request some power monitoring on there end for verification. If the THD is really this bad then it is definately worth tracking down and will save you some headaches later on(as well as money).
As far as power quality and harmonics, I believe IEEE519 addresses this from the users end.

 

[ScottyUK]

Is your motor delta connected? Do you have lots of dirty loads on your system? Are all the motor harmonics triplens?

 

[GordS]

I think Pete has a good point regarding saturation since you are at low load operation - especially coupled with the power factor correction capacitors which will also tend to drive motor terminal voltage up.

Also, the capacitors may be creating a resonant condition which will act as a sink for any other harmonic currents in the system.

 

[rbulsara]

What THD are you measuring, Voltage or Current?.

If it is voltage, you have a real problem and nothing much would be working right at your facility.

Assuming its the current, measuring harmoincs at no load of a motor is meaningless. You are analyzing magnetizing current which has very low (0.2 or less) power factor. 60% THD on no load means nothing. Please measure THD at full load, 75% and 50% loads which is a more meaningful analysis.


I do not know what THD the magnetizing current should read, But whatever it may be, I would not attach any singnificance to it,unless I see high THD in loaded condtion.

You may also wish to measure Voltage THD on the supply bus when the motor is off.

 

[Marke]

If the motor is operating, as ElectricPete suggests, with a high level of saturation in the iron, then the magnetising current will carry significant harmonics. If the motor is operating at close to open shaft conditions, then the current flowing will be essentially magnetising current. Now add some power factor correction, ahnd the fundemental component of the magnetising current will be significantly reduced effectively increasing the percentage distortion.
Note this assumes that you are measuring upstream of the power factor correction.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[peebee]

I agree with rbulsara: measuring %THD of current on any unloaded device is pointless. The harmonic currents won't change much, no matter what the load, but the fundemental currents obviously will change greatly, so when you put harmonics in terms of percentage of fundemental current unloaded you will always come out with a very high %THD.

It's about the same as asking why the efficiency of your unloaded motor is 0%. It doesn't (necessarily) mean that your motor sucks -- it's just a meaningless number.

 

[electricpete]

I would have thought that normal no-load thd would be lower undet normal circumstances, but here is an excercize to convert no-load THD to full load THD, using the assumptions outlined by peebee.

Let
FLA = Full load amps
INL = no-load current as fraction of FLA.
(assume INL = 25%)
Harmonics = rms of harmonic content as fraction of FLA.
THD_NL = no-load THD (given as 60%)
THD_FL = full-load THD

THD_NL = 0.6 (given)
Harmonics = THD_FL*INL = 0.6*0.25 = 0.15
THD_FL ~ sqrt(1^2 + 0.15^2)-1 = 1.011 - 1 = 0.011 ~ 1%

 

[electricpete]

I think I was wrong in starting with Harmonics = THD_FL*INL = 0.6*0.25 = 0.15. Doesn't properly capture the way that harmonics are combined with fundamental by square root of sum of squares.

Also as I remember there are two definitions of THD. One divides by the fundamental and one divides by the total. Doesn't normally make a big difference but it does when dealing with THD=60%.

I haven't got time right now to figure out a better answer. I'm pretty sure you will be surprised at how low the full-load THD will be. Anyone else want to try?

 

[electricpete]

In spite of the dramatic reduction in THD from no-load to full load, 60% at no-load still seems higher than normal to me. I will poke around to see if I can find a trace of no-load current.

 

[GusD]

COMAPRO

Harmonics are always present whenever an AC current is flowing, even without the help of electronics.Your measured THD of 60% seems extreme under any conditions.
Some motors with Low loads or no loads can have asyncronous magnectic fields causing fairly high umbalances at no load.
I don't know if the THD could get as high as 60% under those conditions.

GusD

 

[electricpete]

The voltage unbalance certainly may be playing a big role in increasing harmonics since portions of the core will go into saturation. Particularly if one or more phase voltages is much higher than nominal.

 

[COMAPRO]

Today I measured again the current harmonics on the motor under regular load and it was changing rapidly between 10 and 15 % . --the 5th harmonic was the highest one . the measurement was done at the breaker of the wye delta starter , not right at the motor terminals. The breaker is located in a control motor center where another 20 direct starters are placed and again no electronic at all like soft starters or VFD

 

[RAMConsult]

Industrial environments are inherently noisy electrically, motors are nonlinear devices and I wouldn't be so quick to rule out the capacitors. The other source of 'curious behavior' is often the use of a digital meter where the inherent damping of an analog needle is missing.

A few more facts may help: what type of motor is it, what is the rated speed, what type of load is it driving, what is the actual speed of the shaft, how balanced are the phase voltages and currents, what about the readings on other similar motors and other circuits in the plant? All of these affect the voltage and current waveforms.

 

[COMAPRO]

the motor is a standard squirrel cage three phase motor . it is running delta at 230 volts. the rpm are 1800. the readings are normal in the other motors . no unbalanced at all in current or voltage. the motor drives a stone crusher