Capacitors and their function in Alternating Circuits

[ggunn]

If an ideal capacitor and inductor are connected in series, it could result in dead short circuit.

What he is saying is if they are in parallel they are also in series; you can draw a closed loop through the inductor, through the capacitor and back to the inductor. It's been a long time since my AC circuits classes, but I believe it's sometimes called a tank circuit because at some frequency it will ring like a cylindrical tank struck with a hammer.

 

[Carultch]

What he is saying is if they are in parallel they are also in series; you can draw a closed loop through the inductor, through the capacitor and back to the inductor. It's been a long time since my AC circuits classes, but I believe it's sometimes called a tank circuit because at some frequency it will ring like a cylindrical tank struck with a hammer.

I thought it was called a tank circuit, because the hydraulic analogy for a capacitor is a tank.

 

[Ingenieur]

What he is saying is if they are in parallel they are also in series; you can draw a closed loop through the inductor, through the capacitor and back to the inductor. It's been a long time since my AC circuits classes, but I believe it's sometimes called a tank circuit because at some frequency it will ring like a cylindrical tank struck with a hammer.

the resonant freq = 1/(2 Pi (sqrt LC)) Hz

what he was saying is if in series with the power source
assume L = 1 / (C (2 pi f)^2)

Xc = 1/(2 pi f C j) = -j/(2 pi f C)
Xl = 2 pi f L j ... substitue L from above, so Xl = j/(2 pi f C)
so Xc + Xl = (1/(2 pi f C)) (-j + j) = 0 Ohm
short circuit

 

[Fishn sparky]

Everyone's post have been very helpful and my question are answered. I admit I have no PHD in physics or calculus nor did I go to engineering school. I simply am a 20 year electrician with experience in residential but mostly heavy commercial. I also teach third year electrical for a local apprenticeship. This covers motors, transformers and electrical theory. To be completely transparent, while I understand how most things work, there are still areas that need more learning and understanding on my part, capacitors being one of them.

In my class I try uncover some of the layers of what we have always been told to do and provide a simple, or as simple can be, explanation to how and why things work in a more detailed analysis. Maybe I am going deeper than I need to teach the concepts of inductors and capacitors. Like I said, I am no engineer. Maybe instead of trying to provide a scientific explanation but keep it simple and stick to the basics on how things work and why we use them in specific circuits. I just want my students to leave my class knowing these concepts.


In my real world, all calculations are done and what is on the drawings is what is to be installed. Very seldom do I do service work. Some of the guys in my class may be service electricians, in which case this information will be useful but as we know with time things are forgotten or like in your 10th grade algebra class, "when will I use this information".

 

[Carultch]

Everyone's post have been very helpful and my question are answered. I admit I have no PHD in physics or calculus nor did I go to engineering school. I simply am a 20 year electrician with experience in residential but mostly heavy commercial. I also teach third year electrical for a local apprenticeship. This covers motors, transformers and electrical theory. To be completely transparent, while I understand how most things work, there are still areas that need more learning and understanding on my part, capacitors being one of them.

In my class I try uncover some of the layers of what we have always been told to do and provide a simple, or as simple can be, explanation to how and why things work in a more detailed analysis. Maybe I am going deeper than I need to teach the concepts of inductors and capacitors. Like I said, I am no engineer. Maybe instead of trying to provide a scientific explanation but keep it simple and stick to the basics on how things work and why we use them in specific circuits. I just want my students to leave my class knowing these concepts.


In my real world, all calculations are done and what is on the drawings is what is to be installed. Very seldom do I do service work. Some of the guys in my class may be service electricians, in which case this information will be useful but as we know with time things are forgotten or like in your 10th grade algebra class, "when will I use this information".

In a general sense, capacitors, inductors, and resistors are your elementary linear/passive components that can form a circuit. Resistors are energy absorbing components that generate heat. Capacitors and inductors by contrast, are energy storage components, that store energy temporarily and release it later. The concept of resistance can be generalized as what we call impedance. A ratio between voltage and current across/through a component. Unlike resistors that have exclusively real resistance as their impedance, capacitors and inductors have imaginary numbers as their impedance. The "imaginaryness" indicates energy is time delayed instead of absorbed.

The reason they are "linear" is that the response current is proportional to the applied voltage, for a simple (i.e. sinusoidal) AC signal. In resistors, it is a direct proportion with both voltage and current being synchronized. In inductors and capacitors, it is still a direct proportion, except there is a quarter cycle time delay between current and voltage. Voltage leads current in an inductor. Current leads voltage in a capacitor.

A common application of capacitors, is filter circuits, which can block or pass certain frequencies. Capacitors block low frequency currents like an open circuit (in the extreme limit), and pass high frequencies like a short circuit (in the extreme limit). Inductors do exactly the opposite. A basic low pass filter has a resistor from the source to the load, and a capacitor in parallel across the load. This lets voltages through at low frequencies, and diminishes them at high frequencies. A basic high pass filter is built with the components in opposite positions.

 

[Besoeker]

In my real world, all calculations are done and what is on the drawings is what is to be installed. Very seldom do I do service work. Some of the guys in my class may be service electricians, in which case this information will be useful but as we know with time things are forgotten or like in your 10th grade algebra class, "when will I use this information".

Had a similar conversation about algebra with my nephew and he made an almost identical remark "will I use algebra tomorrow?" He's in the army, the US army. So maybe he has a point but you probably use mathematics much more than you realise in everyday life.
The electrical field more than many others.

My background is power electronics, mainly in variable speed drives for industrial applications. Capacitors were used for a lot of different things but for alternating currents, and I assume you mean mains frequency, the most common use I saw was power factor correction (PFC), particularly for ac induction motors. The motor power factor was commonly about 0.8. It's a measure of how much the current lags behind the sine wave voltage. This has two unwanted consequences. The current in the supply to the motor is higher so the conductors have to be larger to accommodate that. The other thing is that some electricity authorities penalise for low power factor.

So, to mitigate this, PFC capacitors get fitted. For those, the current leads the sine wave voltage so some of the lagging current is cancelled. In my experience the usual target is a PF of 0.95 lagging. You need to do the calculations or draw out the vectors with kVA, kW and kVAr to visualise what's happening. And that needs a bit of trigonometry. And a bit of Pythagoras if you want to draw it out.

Anyway, glad you found that the posts were of help.

 

[Ingenieur]

Just as you need to read to understand subjects like history and philosophy you need to 'read' equations for physics and engineering

'a picture (with equations) is worth a 1000 words' holds no greater meaning than in engineering and such

 

[Sahib]

Some of the guys in my class may be service electricians

You may also stress precaution to be taken while handling of capacitor after disconnection from power. Its terminals to be shorted together for some time before touching. Some have the habit of shorting the terminals only momentarily, say with a screw driver. It is risky because the voltage can reappear across the capacitor terminals. For this reason, standard power capacitors have resistors connected across the terminals permanently.

 

[Besoeker]

You may also stress precaution to be taken while handling of capacitor after disconnection from power. Its terminals to be shorted together for some time before touching. Some have the habit of shorting the terminals only momentarily, say with a screw driver. It is risky because the voltage can reappear across the capacitor terminals. For this reason, standard power capacitors have resistors connected across the terminals permanently.

Yes. I don't agree with the screwdriver shorting method either.
We made slip recovery drives where the DC link had up to 360 2,200uF electrolytic caps is a series/parallel arrangement. With up to 1,140V at the link there was no choice but for that kind of arrangement when you can't get electrolytics much above 450Vdc as a permanent rating. Sticking a screwdriver across that lot would not have been a pleasant experience either for you or the screwdriver.

We fitted a 22kohm 15W resistor across each capacitor and a very prominent safety notice on the outside of the panel about waiting ten minutes then checking voltages before handling them.

 

[kwired]

Probably me with finger trouble then........

And thank for responding!

Your posts won't be deleted if you stop using the middle finger in them

 

[ggunn]

A friend who served in the military in electronics told me of a prank they used to play on each other. A guy would charge up a sizeable capacitor and toss it at/to a buddy and laugh at the reaction.

 

[ActionDave]

A friend who served in the military in electronics told me of a prank they used to play on each other. A guy would charge up a sizeable capacitor and toss it at/to a buddy and laugh at the reaction.

High school electronics class, we would charge them up, go to the next class and short them out on the underside of somebody sitting in a metal fold up chair.

 

[gar]

171022-0945 EDT

Fishn sparky:

In your first post you made the following statement:

In regards to volt amps reactive in a capacitor, capacitors are a source of VARS. They don't use energy but provide energy through the electrostatic field. While inductors are a consumer, capacitors are a source. Inductors consume VARS due to the concept that they use energy to create a magnetic field which stores energy. Capacitors do not use energy but create a field via laws of physics and attraction and repulsion of electrons.

I really don't know what you were trying to say. But possibly your statement was because some loads are energy consuming, such as a motor or magnetic ballast fluorescent, and look more inductive than capacitive that an inductive load is energy consuming. A true ideal pure inductor or capacitor in steady state on average neither consumes or generates energy. On an instantaneous basis either an ideal inductor or ideal capacitor can receive or export energy. Averaged over a fully cycle no energy is dissipated.

With ordinary materials at room temperature there are some capacitors that are close to ideal, not so with inductors.

A polypropylene capacitor will hold a charge for a long time because it is close to ideal, very low leakage current.

The very best of inductors have enough series resistance that when charged with a current flow, initial current, and then shorted with that initial current will not maintain that current long because of dissipation of that energy in the inductor's series resistance.

More later.

.

 

[ggunn]

In regards to volt amps reactive in a capacitor, capacitors are a source of VARS. They don't use energy but provide energy through the electrostatic field. While inductors are a consumer, capacitors are a source.

Think about it. Capacitors cannot be a net source of energy; if they were we wouldn't need power plants that burn fossil fuels or use nuclear, gravitic hydro, geothermal, wind, or solar energy sources. All we'd need is a bunch of big caps to power the world.

Capacitors store energy during one part of an AC cycle and give it back later; there is no net energy gain but a small amount of loss as heat. The Laws of Thermodynamics have not been repealed.

Inductors do the same thing, in a way. Capacitors store voltage and inductors "store" current.

 

[gar]

171022-1923 EDT

Fishn sparky:

By experimental observation it was learned that one could mathematically model a capacitor by v = q/C where v is the instantaneous voltage across the capacitor and q is the instantaneous charge on the fixed capacitor of capacitance C.

Quite obviously you can not instantaneously change v without an instantaneous change in q. To instantaneously change q would require an instantaneous infinite current. Not possible in the real world. Note: when you remove current flow to a capacitor, then its voltage remains constant.

A similar condition exists for an inductor. But here you can not instantaneously change current because that would require an infinite voltage across the inductor. If you attempt to change the current thru an inductor, then the inductor will generate whatever voltage is required to maintain that current flow.

Think about this for a while.

.

 

[gar]

171022-2002

Fishn sparky:

This post will mean very little to you, it is useful information, but possibly not to your students.

If you consider a series circuit of a capacitor, inductor, and resistance, then the differential equation for this is
L d(dq)/dt + R dq/dt + q/C = 0
that is the sum of the voltages around a closed loop is 0.

If R is small, then the result is a damped sine wave assuming an initial charge of energy at time t = 0.

If R = 0, then there is no damping and a continuous sine wave exists.

The frequency of this oscillation is called the resonant frequency.

What is occurring here is an exchange of stored energy back and forth between the inductor and capacitor.

Suppose you change the circuit to a resistor, capacitor, and inductor in parallel, and apply an excitation sine wave at the resonant frequency, then the power source only sees the resistance as a load because the capacitor and inductor are exchanging energy back and forth between each other.

Next suppose the L and C are not at resonance, then some of the energy associated with the L and C is being exchanged with the excitation voltage source.

More later.

.

 

[gar]

171023-0834 EDT

If you create a series RLC circuit, place an initial charge on the capacitor, and close the circuit, then you get some form of damped oscillation. Two illustrations follow. Here 18 V was applied to a 30 ufd capacitor for an initial charge, and he voltage displayed is the capacitor voltage after circuit closure. All resistance in the loop was that of the inductor internal resistance.

First waveform is using a small transformer primary with no secondary load.

.


.

Second waveform is using a DC power supply filter inductor. Higher Q, longer decay time.

.


.

Can't load pictures, no space.

.

 

[ggunn]

171022-1923 EDT

A similar condition exists for an inductor. But here you can not instantaneously change current because that would require an infinite voltage across the inductor. If you attempt to change the current thru an inductor, then the inductor will generate whatever voltage is required to maintain that current flow.

Something that is of interest to some of us is that this is why you don't want to send signal into a tube amplifier that is powered up with no speaker connected to the output terminals of the output transformer.

 

[gar]

171023-1218 EDT

ggunn:

For a triode output amplifier is should be possible to run the transformer unloaded because the triode is not a constant current source. See 6AS7 curves at http://wooaudio.com/docs/tube_data/6AS7GA(6080).pdf

A pentode is a constant current device and in this case the amplifier must not be run unloaded. See 6L6 curves for pentode mode. Also shown are 6L6 curves for triode mode. http://www.audiomatica.com/tubes/6l6.htm
I was unaware that this tube was ever connected as a triode.

.

 

[winnie]

In regards to volt amps reactive in a capacitor, capacitors are a source of VARS. They don't use energy but provide energy through the electrostatic field. While inductors are a consumer, capacitors are a source. Inductors consume VARS due to the concept that they use energy to create a magnetic field which stores energy. Capacitors do not use energy but create a field via laws of physics and attraction and repulsion of electrons.

I would state it as follows:

Both capacitors and inductors store energy during one part of the AC cycle and return it during another part of the AC cycle. It is simply sign convention that we call one sort of energy storage a 'source' of VARs and the other sort a 'consumer'; they are as similar as left and right hands.

For maximum efficiency, inductive and capacitive loading must be balanced.

Because numerous common loads (such as motors) are inductive and must be balanced out by a suitable capacitive supply, we consider the inductive loads to be consumers. But if some strange factory had a 'leading power factor', the power company would charge them to provide a suitable inductive balance.

-Jon