Why Harmonics in a three phase motor ?
Why Harmonics in a three phase motor ?
(OP)
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
Thanks in advance for any tip you can tell me on the subject





RE: Why Harmonics in a three phase motor ?
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.
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
again what could be the reason ? harmonics created by capacitors ¿? I doubt it
RE: Why Harmonics in a three phase motor ?
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...
RE: Why Harmonics in a three phase motor ?
As far as power quality and harmonics, I believe IEEE519 addresses this from the users end.
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
Also, the capacitors may be creating a resonant condition which will act as a sink for any other harmonic currents in the system.
RE: Why Harmonics in a three phase motor ?
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.
RE: Why Harmonics in a three phase motor ?
Note this assumes that you are measuring upstream of the power factor correction.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Why Harmonics in a three phase motor ?
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.
RE: Why Harmonics in a three phase motor ?
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%
RE: Why Harmonics in a three phase motor ?
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?
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
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
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
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.
RE: Why Harmonics in a three phase motor ?
RE: Why Harmonics in a three phase motor ?
If your capacitor is connected at the motor terminals, could you disconnect it and then check your current again ?
RE: Why Harmonics in a three phase motor ?
The only influence of the capacitors that I know can be to increase the (fundamental) voltage. Higher voltage pushes further into saturation and causes more current harmonics.
RE: Why Harmonics in a three phase motor ?
that was exactly I was thinking.
RE: Why Harmonics in a three phase motor ?