One of those annoying electrical questions

On 31/12/15 12:52, Fredxxx wrote:

Generally a PF correction capacitor is there to bring the LC resonant frequency to 50Hz. That's how you get the zero phase shift.
without amplification, it can't be unstable.
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On Thu, 31 Dec 2015 12:52:08 +0000, Fredxxx wrote:

For a typical induction motor (if there is such a thing) the field current/reactive current remains pretty static. It's the *load* current component of total motor current that varies according to the amount of work the motor's doing. The PF improves as the motor becomes fully loaded.

True, but the 50Hz is fixed, so you're basically reinforcing my original point here.

AIUI, "perpetual motion" requires no external energy input which is not the case here. Oscillators require an in-phase energy pulse to keep them running.
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On 31/12/15 13:48, Cursitor Doom wrote:

MM. That's true, but only on account of the way PF is defined. What yiou have, with a motor, amounts to a fat inductor in parallel with a variable resistor, representing the variable mechanical load.
What te PF corrector does, is make a 50Hz tuned circuit out of it to offset the inductance and bring the phases back in line.
So the PF correction capacitor size is constant as this the inductance.. Its the Q that varies as the 'resistance' part varies.

In electrical terms, you need amplification. And te thing would ring at near enough 50hz, which is not an issue
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On 31/12/2015 13:48, Cursitor Doom wrote:

On this occasion I am going to have to disagree with you. http://myelectrical.com/notes/entryid/251/induction-motor-equivalent-circuit
might give the impression that reactive current will be heavily dependant on slip.
I accept slip could be close to zero, but there has to be some slip to get an output torque.

The oscillator drive is simply the mains. It will keep "running" as long as mains is applied. Any apparent oscillatory waveform after the supply is removed will decay very quickly.
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On Thu, 31 Dec 2015 15:25:42 +0000, Fredxxx wrote:

circuit

Urgh! NYE!! I have to confess I was "very, very drunk" :-D when I came home this morning and read your rejoinder, so you'll have to excuse me as I'm still not even 50% normal yet, but until I recover completely, please take a butcher's at this:
http://www.the-power-factor-site.com/motorcurrent.html
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On 01/01/16 11:24, Cursitor Doom wrote:

Well it says in rather more detail exactly what I was trying to explain.
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On Fri, 01 Jan 2016 11:35:03 +0000, The Natural Philosopher wrote:

Perhaps you too were "very, very drunk" at the time? BTW, here's the cultural reference for those who may have missed it:

https://www.youtube.com/watch?v=1zeTkNrMBIw

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On 01/01/2016 11:24, Cursitor Doom wrote:

The article is a very simple model of a motor of unknown type and the article seemingly sole purpose is to demonstrate a (real) load current and out of phase magnetising current. For the average electrician it is probably all they need to know.
Most motors in the kw region tend to be squirrel case induction motors, which exhibit a reactive component that is dependant on load.
The article uses the term "magnetising current" which is more commonly a term associated with transformers and not induction motors. It's a shame the article doesn't inform us for what type of motor this would be constant. I can't tell from the photo.
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On Fri, 01 Jan 2016 14:07:55 +0000, Fredxxx wrote:

I agree. It's bloody annoying having so many different types of motor and so many variations in the windings of each. The guy obviously knows what he's talking about as his company makes its revenue from PF correction, but a bit more detail on the precise motor type under discussion would have been handy for sure.
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On 01/01/16 14:07, Fredxxx wrote:

I see you did not actually read the article, or understand it.
They do not.
The reactive components do not vary with load. How could they? Load is by definition in phase current not 'wattless' current.
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On 01/01/2016 17:15, The Natural Philosopher wrote:

Hmm. I agree that the reactive components within an induction motor do not change dramatically, however the effect of slip ie the difference between rotating field and rotor speed does significantly the effect of how a reactive component is seen supply side.
I really suggest you look at various models of the induction motor and understand them before saying that the reactive component of current is constant.
I have already given one link: http://myelectrical.com/notes/entryid/251/induction-motor-equivalent-circuit
and there are many more websites that give a good overview of induction motors.
If there is anything you don't understand then I would be happy to try and explain it to you here.
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On 01/01/16 21:07, Fredxxx wrote:

That link doesn't work, and when IU actually found the the page on that site, it confirmed exactly what I said.
The reactive comnponents do not vary.

I think its you that needs to understand
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On 02/01/2016 10:31, The Natural Philosopher wrote:

Do you not accept that the reactive current changes with load?
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On 02/01/16 12:02, Fredxxx wrote:

Well that is not exactly what I said.. Power factor does, because that's a ratio of reactive to resistive.
No analysis, including the one you incorrectly linked to, that I have found on the net has reactive elements varying with slip.
Of course reactive current may vary, but not the value of the inductive elements.
Which means that you don't need to vary the capacitor values with load to get a 50Hz tuned circuit, which was the point.
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On 02/01/2016 13:02, The Natural Philosopher wrote:

We seem to finally agree that reactive current does vary with load, then one must infer the inductive element as seem by the supply must vary in sympathy even if the inductive components themselves do not vary significantly. Hence my earlier question surrounding your knowledge of "slip" and models of induction motor.

?? You just agreed that reactive current can vary. That statement just doesn't make sense.
I agree that a fixed capacitor will have a specified lead current but perhaps this is the root of your misunderstanding in your belief that a fixed single sized capacitor will ensure a pf of unity over a motor's load characteristic from stall to no load?
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On 02/01/16 13:54, Fredxxx wrote:

Look, I spent 20 bloody years of my life designing circuits with L C and R components in them. Without active components any generalised collection of LCR has a resonant frequency that does not change on account of the variation in the R. Lord knows it took the invention of valves and transistors to create the wah-wah pedal from a variable resistor, and that works by 'amplifying' a capacitor value.
You have baffled yourself with bullshit.
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On 02/01/2016 13:02, The Natural Philosopher wrote:

Its pretty much implied - since changing load will change the slip.
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On 02/01/16 15:56, John Rumm wrote:

THE LOAD IS NOT A REACTIVE ELEMENT
shheesh

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On 02/01/2016 16:14, The Natural Philosopher wrote:

I don't recall anyone saying the load is a reactive component.
Most models have a reactive element in series with the load, which is dependant on slip. http://myelectrical.com/notes/entryid/251/induction-motor-equivalent-circuit
There is also a parallel reactive component, yes. But we're talking of a series component as per X2 in the above link's diagrams.
If you have an inductor and resistor in series, can you accept the reactive current will vary with value of resistor? ______ _____ ------|__L___|-----|__R__|------
If you have been working in this field for as long as you have, there is a fighting chance you might accept this hypothesis.
If you claim you can't see this webpage there are many others: http://www.openelectrical.org/wiki/index.php?title=Induction_Motor_Model http://powerelectronics.com/motion-systems/motors-efficiency-and-adjustable-speed-drives http://www.openelectrical.org/wiki/index.php?title=Induction_Motor_Torque http://homepages.engineering.auckland.ac.nz/~kacprzak/PE2.html https://en.wikipedia.org/wiki/Circle_diagram http://www.transtutors.com/homework-help/electrical-engineering/ac-machines/equivalent-circuit-of-induction-motor.aspx http://www.intechopen.com/books/induction-motors-modelling-and-control/induction-motors-with-rotor-helical-motion
These all have a common theme. A load resistor representing the resistance of the rotor, and an series inductor plus other components that serve only to confuse you.
If after all these years of designing Rs, Ls and Cs in circuits you should should be able to understand the consequence of the supply current to a nominally fixed inductor and a variable series resistor. Even if you have not seen an induction motor in your life you should be able to see that the reactive part of the supplied current with be highest when R is 0ohms, and tending to zero and R tends towards infinity.
Sheesh, and you claim to have worked in this field. How long ago???
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On 02/01/2016 16:14, The Natural Philosopher wrote:

Agreed, and so?
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