I can say from 20+ years experience in vector control of magnetizing and torque producing current that this reactive current change based on load does NOT happen in 230v or 460v 3 ph motors. just does not.
i have seen for 20 years that there is NO significant change in reactive current portion of vector sum current in a 230/46v motor in the range of 1-200hp. none. I can produce years of scope picture of reactive current vs real current that shows this to be true from no load to full load. [/quote]
I assume that you (like me) are talking about a 3-phase motor fed from constant frequency, constant voltage. (not vfd).
In that case, you are mistaken in thinking the reactive current is constant with load. It is not the case.
As Siemens stated, the reactive component of current will increase on the order of 200% from no-load to full-load, give or take 60% or so.
so not sure how to react to this siemens article.... before the quoted section shown, it did say "To understand how to compensate for the poor power factor of a motor, we need to look at the components of the motor current. The real-power producing work is done by the resistive component of the current, which varies with the load on the motor. The reactive current of the motor consists of two components. The first is the magnetizing current that establishes the magnetic flux in the core which allows the motor to function. The magnetizing current is essentially constant regardless of load.
The second component of reactive current is the leakage reactance current, and this component varies according to the load on the motor. The leakage reactance current is relatively small, so that the total reactive current is relatively constant (compared to the kW variation) over the range of motor no-load to motor full-load."
I do not know how to turn this relatively constant comment in 140-260% they go on to mention - I wonder if perhaps that is a typo and should have been 14-26% or something? or since it is "relatively small" this 140-260% turns out in reality to be as they say, so small as to be ignorable?
You don’t need to invent a factor of 10 typo in two different places to understand what the authors meant.
It makes sense as is. Let’s review the statement in question. I’m going to add some emphasis, and I’m going to clip the
entire paragraph (including the two sentences immediately following the ones you focused on).
The leakage reactance current is relatively small, so that the total reactive current is relatively constant
(compared to the kW variation) over the range of motor no-load to motor full-load. For a range of medium
voltage machines sampled, the ratio between full-load reactive current and no-load reactive current varied
from 140-260% (depending on machine design, speed, and voltage). For perspective, the ratio between
full-load kW and no-load kW is of the order of 4000%!
Notice that the “
relatively constant” is compared to the kw variation (otherwise there is no reason to include the parenthetical “compared to the kW variation). This is further confirmed by the fact that the remaining two sentences of the same paragraph go on to discuss both reactive current variation and kW variation. So yes, I think you’ll agree the purpose of the statement is to contrast the vars variation to the watts variation (although they certainly could have said it better) and yes I think you’lll also agree that the watts variation is much more than the vars variation.
But you don’t have to take Siemens word for it...
Let’s look at some cold hard data.
Look at the same 20hp 575vac motor data sheet I linked above, repeated here for convenience:
I have attached an excel spreadsheet analysing the data provided in the datasheet. You are welcome to double check my calculations, but I included cross check columns that prove to me that my numbers are correct.
From my spreadsheet, here is the punchline for this 20hp 575 volt motor:
HP VARS
0 5963
5.01 6321
10 6988
15 8316
20 10256
25 12644
Notice, the vars are not constant as you said, but increasing with load as Siemens said.
And the ratio full-load to no-load vars is 10256/5863 = 172%, well within the 140-260% range we were told by Siemens to expect.
I really hope you are convinced by now, the numbers don’t lie.
You are not the first one to assume reactive current component is approximately constant with load. It is much more the case for transformers than for motors, so sometimes the transformer thinking creeps into the motor world where it doesn’t belong. Several members made the same incorrect assumption of constant vars with load, including myself to a certain extent at beginning of the following linked thread. By the end, I had shown it was not the case.
thread237-262325
The thread is longwinded, but I would direct your attention to my post dated 11 Jan 10 10:21 where the vars expression Q = I^2*X is used to demonstrate that the vars consumed in the leakage reactances at full load are approximately equal to the vars consumed in the magnetizing branch (which as you know is ~ load independent). That is the same thing as saying that the total vars doubles from no-load to full load. i.e:
No-load vars ~ magnetizing vars (no significant leakage vars present at no-load)
Fulll load vars = magnetizing vars + leakage vars
Full load vars = 2*magnetizing vars (since we showed magnetizing=leakage).
Full load vars = twice no-load vars
The circuit parameters in that post are certainly typical among what I have seen and if you study them it is not in the least surprising that vars can double from no-load to full load. If you have another set of equivalent circuit parameters in mind, it can easily be analysed.
=====================================
(2B)+(2B)' ?
