Inductive/Capacitive Inrush
Inductive/Capacitive Inrush
(OP)
All,
A thread titled "Blinking Lights" came up in the Electric motors, generators & controls engineering forum. There was some disagreement over whether a power factor capacitor in parallel with a motor would result in increased or decreased inrush.
Looks like the consensus was decreased due to the subtraction of inductive and capacitive currents. I wonder if anyone could elaborate on that. I guess I thought they did add in the transient domain. In steady state, the average of several cycles of ELI and ICE result in a subtractive effect. This is the average current (a phasor).
Looking at an instantaneous current however, it requires so much energy to build a magnetic field and so much to build an electrostatic field. The inductor and capacitor look like a short, and so much current will flow depending on the source voltage and effective impedance until it is not a short.
Somewhere in that paragraph is a misunderstanding. I would appreciate it if someone could point out where that is.
A thread titled "Blinking Lights" came up in the Electric motors, generators & controls engineering forum. There was some disagreement over whether a power factor capacitor in parallel with a motor would result in increased or decreased inrush.
Looks like the consensus was decreased due to the subtraction of inductive and capacitive currents. I wonder if anyone could elaborate on that. I guess I thought they did add in the transient domain. In steady state, the average of several cycles of ELI and ICE result in a subtractive effect. This is the average current (a phasor).
Looking at an instantaneous current however, it requires so much energy to build a magnetic field and so much to build an electrostatic field. The inductor and capacitor look like a short, and so much current will flow depending on the source voltage and effective impedance until it is not a short.
Somewhere in that paragraph is a misunderstanding. I would appreciate it if someone could point out where that is.






RE: Inductive/Capacitive Inrush
RE: Inductive/Capacitive Inrush
RE: Inductive/Capacitive Inrush
davidbeach, the inrush will occur at multiple inrush frequencies depending on where you're looking in the circuit, if I understand correctly. But does this mean that the inductive and capacitive currents are of opposite polarity? It seems to me until steady state is reached, an oscillatory current transient will occur as a result of both the inductance and capacitance, but there is not one current that cancels another. So wouldn't the total current then increase? I'm not suggesting it is a problem as you've commented on the times involved, but from a strictly circuit theory standpoint, does their combination results in a single increased current (as opposed to that in the absence of the capacitor)?
prc, that is an interesting comment. Thank you
I'm not trying to be argumentative, just trying to make sense of it all. I appreciate the guidance
RE: Inductive/Capacitive Inrush
RE: Inductive/Capacitive Inrush
The transformer inrush is mostly a question of where on the sine you connect the primary and also in what state of saturation the core was left when you switched off. Transformer inrush usually has a large DC Component that decays in between 10 and 100 cycles.
The motor inrush is not at all caused by magnetic saturation. It is the result of the rotor bars acting as a short circuit before the motor and load with its inertias are up and running. The current is usually a nice sine without harmonics.
So, please, do not try to apply transformer problems to motor problems. It brings confusion and false conclusions more than it brings understanding. Physics, text-books and math, plus lots of experience is needed. Easy analogies (my favourite hate object is the beer and froth analogy) will never bring understanding. Sorry for the rant.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Inductive/Capacitive Inrush
I agree Skogsgurra with the analogy rant. One of my aggravations is analogies and models... it seems that so many people don't really understand what's happening because "you can think of it like this." I'm guilty of that myself from instruction (I'm currently trying to understand imaginary power without the use of any model), but it seems most of the explanations you find will use analogies or will try to explain the full system using terminology from those analogies.
There's a lot to learn, so I may be doing it a while
RE: Inductive/Capacitive Inrush
Excellent attitude, if I may be permitted to say so; I was taught [forced?] to think that way many years ago, and it continues to serve me well.
As for analogies as hate objects: in addition to "beer/foam," were any of you ever subjected to "two horses coupled to one drawbar pulling a plow" I realize this one goes way, way back...but it drove me nuts nonetheless!
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Inductive/Capacitive Inrush
http://gke.org/presentationer/files/PFC%20and%20Mo...
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Inductive/Capacitive Inrush
The question is if the capacitance added is say 5 to 10 times the PFC value and is only applied during starting, will that stop the blinking lights?
RE: Inductive/Capacitive Inrush
RE: Inductive/Capacitive Inrush
Also, such big capacitors will start doing things to the contacts, as you already mentioned.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Inductive/Capacitive Inrush
switching transient phenomena are normally sub-cycle or modulated full cycles. A transformer inrush would remain for say 5 to 6 cycles. But they are full of harmonics.
However, the motor starting current and voltage dip problems are not related to this. It is the voltage dip during the motor acceleration. This is a dynamic phenomena. The capacitor is required to absorb the reactive power mainly during this period to stabilize the bus voltage.
The motor switching and transformer switching are comparable, as far as switching inrush is concerned. When the capacitors are connected to the motor terminals, as a thumb rule the capacitor rating shall be less than 90% of the no load current of the motor. This usually takes care of the transient voltages.
Unless the system is vulnerable for resonance, no other study is necessary for the application of capacitor to the motor terminals.
Hope this is throwing some new light on the subject issue.