Inductive/Capacitive Inrush 2

You could look at in a transients program such as ATP or another EMTP program. What you're likely to find is that the capacitor "inrush" is a very high frequency current that will ring down in 1/4 to 1/2 of an electrical power system cycle. Then you have many power system cycles of much improved power factor as the motor starts. The capacitor transient is so short it is typically ignored; many thousands, if not millions, of PF correction caps successfully connected between starter and motor shows that the transient is far less obnoxious than the affects of starting the motor without the caps.

I am not sure of motors,but we transformer engineers know that capacitors will not improve the no-load power factor of transformers and also will not reduce inrush current much.This is because of the appreciable % of various harmonic currents present in the no-load current. The capacitor will compensate only the fundamental component.Once it is taken care of,the harmonics will predominate and there total will be appreciable.

Thank you for the comments,

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

They will add and subtract multiple times over the first 3-6 ms after both the motor and the capacitor are energized. If the capacitors are on the service side of the motor starter, as suggested in the referenced thread, then the capacitor will discharge into the motor inductance and you'll have similar effects plus the risk of having too much capacitance on the system if there aren't enough motors running.

The transformer inrush is very different from the induction motor 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
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Half full - Half empty? I don't mind. It's what in it that counts.

Thanks for the clarification. It makes a bit more sense now.

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

DistCoop said:

There's a lot to learn, so I may be doing it a while

Click to expand...

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]

Added a crude presentation showing some of the aspects:



Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

You are showing that if the amount of capacitance added to bring a running motor's power factor from operating (less than one) to one (what you call PFC current), that this will not effect blinking lights at starting. I agree with that.
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?

The blink in the lights comes from voltdrop. If the blink is say 1.5 seconds, you need something near the load to supply the power when the voltdrop occures. When a large load starts, the volt drop on the supply line increases and lowers the votlage on the user end and thus the blink. If you need to stop the light from blinking, you need to have a supply device (capacitor) that will maintain the voltage by draining its stored energy during the low voltage period. Then you don't get the blink as the votlage does not drop. I hope you understop that.

It's AC, squeeky. AC cannot be stored in a capacitor over several periods. DC, OK - but AC, no.

Also, such big capacitors will start doing things to the contacts, as you already mentioned.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

The switching of capacitor and the inductor are dealt in the switching transient phenomena. These are the study of voltage amplification due to the excitation of the L-C circuits. This phenomenon should not be confused with power system behavior for the transient conditions. The power system transient condition falls under the category of transient stability analysis. This will be generally performed with power system study softwares, such as EDSA, CYME, ETAP. The switching transient study will be performed using softwares such as EMTP, ATP etc.

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.