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### Author Topic: MH's ideal coil and voltage question  (Read 449245 times)

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #60 on: May 09, 2016, 02:47:45 PM »

Quote
You are just making phony dismissive arguments that take you off track.  The point of the exercise is perfectly clear:  To understand how an inductor works.

No MH. You are making claims you cannot back up,as you do not have access to an ideal inductor.

Quote
A real inductor can be 99.99% identical to an ideal inductor.

99.99% of infinity is an infinite amount away from ideal MH. This may shock you ,but the difference between 99.99% and absolute can be extremely large.
When i asked what the L/R time constant was,you said there isnt one.
With your non ideal coil(being the one that is 99.99% close to ideal)there is an L/R time constant,and so that coil has a finite L/R time constant,where as the ideal coil has either a time constant of 0,or it is infinite. These two values are far from your 99.99% close enough is near enough coil,as it is not even close.

Like i said,you should have thought about your question a little better.

Quote
Ideal voltage sources exist right now within certain limitations.  A good bench power supply is an ideal voltage source.  A car audio amplifier is an ideal voltage source that can output The Star Spangled Banner as an ideal voltage.  I explain that all on the other thread.

They are not ideal at all.
An ideal voltage dose not change--at all,when a load is placed across it-not even by .0000001 of a volt. The ideal power supply would need an ideal transformer,and ideal FWBR,and all other components would have to be ideal--including the power station,the grid supply,and the meter on your house.
All these factors you fail to take into account when you stated an ideal voltage and an ideal inductor.

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The real-world test can easily be done as explained on the other thread.

You cannot do a real world test on fictional components and values.

Quote
The only limitation is that the real-world inductor will behave a tiny smidgen differently from the ideal inductor.

No-the difference is !infinite!--you just dont get this,do you ?.

#### poynt99

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##### Re: MH's ideal coil and voltage question
« Reply #61 on: May 09, 2016, 02:49:13 PM »
If the time constant is infinite for maximum current through the ideal inductor,then that means that no current flows through the inductor--ever,because if it takes an infinite amount of time to reach maximum current flow through the coil,then it also takes an infinite amount of time to reach a 10% value of maximum current flow through the ideal inductor,and it takes an infinite amount of time to reach 1% of the maximum amount of current flow through the ideal inductor--and so on. So it will take an infinite amount of time before current even starts to flow--so there will never be any current flow through the ideal inductor.

My blown universe post was a bit of a ruse. In fact it implied the opposite of what would theoretically happen, that is, "nothing".

#### poynt99

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##### Re: MH's ideal coil and voltage question
« Reply #62 on: May 09, 2016, 02:51:30 PM »
How do you calculate the L/R time constant, when there is no R
L/0=infinite

#### poynt99

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##### Re: MH's ideal coil and voltage question
« Reply #63 on: May 09, 2016, 02:58:52 PM »
A sim of an ideal V source across an ideal inductor will crash the sim.

A tiny bit of resistance must be added as the computational resolution is finite.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #64 on: May 09, 2016, 03:05:10 PM »

<<< No-the difference is !infinite!--you just dont get this,do you ?. >>>

You are just getting belligerent for no reason and you are twisting your logic and not really making sense.

I said that a real-world coil can be 99.99% identical in behaviour to an ideal coil.  That means the difference in their behaviour on the bench will be one part in 10,000.  That's pretty damn similar.

Any good bench power supply with a big capacitor on the output, a huge transformer or huge switching power supply, and a well-designed negative feedback control system to maintain a constant output voltage will be 99.99% identical to an ideal voltage source so the same argument applies.

The point being again that discussing ideal coils or ideal voltage sources is not far fetched at all.

This is the real topic of discussion:  a) What is inductance?  b) Demonstrate your understanding of inductance by solving for the current for an ideal inductor in a simple circuit.

That is what you need to focus on.

MileHigh

#### tinman

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• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #65 on: May 09, 2016, 03:06:23 PM »
You may wish to exercise a few examples to see if your conclusion is correct. Your conclusion being that if the time constant (Tau) is infinite, the load immediately presents itself as a perfect short, meaning the current will be infinite and instantaneous.

Here are a few: (in all cases, L=6H)

1) R=1, Tau=6s
2) R=0.1, Tau=60s (1min)
3) R=1m Ohm, Tau=60ks (16.6 hours)
4) R=1u Ohm, Tau=6M(million)s (1667 hours)
5) etc.

What is happening to Tau as R decreases?
If R could be 0, Tau must be infinite.

Quote
What happens to the inductor current after t=0 when Tau=infinity?

This is the very reason that MHs question cannot be answered,as i have stated before.
The conundrum kicks in at T=0.
If R = 0,which id dose,as the inductor is ideal,then no current flows through the ideal inductor.
If Tau is infinite,then 10% of maximum current flow is also an infinite amount of time,and so is 1% of maximum current flow,and so is .1% of maximum current flow-ETC. This means that it will also take an infinite amount of time before current start to flow,as there is no division of infinite that it self is not infinite. This means that there is no inductance,as there is no current flow to create a magnetic field,and there for no CEMF to resist the change in current that isnt flowing anyway

But there is also no resistance in an ideal coil,and so the ideal voltage is now across a dead short.
So now we have an infinite amount of current flow

MH quoted him self--Quote:  there is no time constant--post 14

So,the current either rises instantly,or the current rise time is infinite,which means there is no current flowing through the ideal coil.

#### tinman

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• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #66 on: May 09, 2016, 03:07:04 PM »

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #67 on: May 09, 2016, 03:08:03 PM »
A sim of an ideal V source across an ideal inductor will crash the sim.

A tiny bit of resistance must be added as the computational resolution is finite.

So this means we cannot sim MHs question as it is defined ?

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #68 on: May 09, 2016, 03:11:32 PM »
<<< So,the current either rises instantly,or the current rise time is infinite,which means there is no current flowing through the ideal coil.  >>>

Sorry but I have to lol, when the "answer" is two completely contradictory answers.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #69 on: May 09, 2016, 03:17:14 PM »

That is what you need to focus on.

MileHigh
[/quote]

Quote
You are just getting belligerent for no reason and you are twisting your logic and not really making sense.

No i am not MH. I am working on your question using the values and components you described.

Quote
I said that a real-world coil can be 99.99% identical in behaviour to an ideal coil.  That means the difference in their behaviour on the bench will be one part in 10,000.  That's pretty damn similar.

I think you will find that there is a huge difference between an ideal inductor,and a real world inductor MH. And once this is resolved,then you will find that your near enough to ideal power supply is not even close to being ideal.

Quote
The point being again that discussing ideal coils or ideal voltage sources is not far fetched at all.

If we are going to be accurate and true to our selves in this discussion,then i think you are going to find that there is an infinite gap between real and ideal.

Quote
This is the real topic of discussion:  a) What is inductance?  b) Demonstrate your understanding of inductance by solving for the current for an ideal inductor in a simple circuit.

Your simple circuit requires resistance,and is normally a resistor added in series with an ideal inductor. But your question and comments that follow say there is no resistance,and so, it cannot be resolved,as poynt has just stated in regards to the sim test.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #70 on: May 09, 2016, 03:22:35 PM »
<<< So,the current either rises instantly,or the current rise time is infinite,which means there is no current flowing through the ideal coil.  >>>

Sorry but I have to lol, when the "answer" is two completely contradictory answers.

Exactly MH.
As i said,you did not think about your question to well when you added ideals to it.
It is also the reason i stated that an ideal voltage cannot be placed across an ideal inductor.
You did see the word conundrum attached to that post-along with several other posts--didnt you?.

Your question cannot be answered,as it is a contradiction to it self.
You stated as well that there is no time constant. What dose that mean?--dose the current rise instantly,or is the L/R time constant infinite?.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #71 on: May 09, 2016, 03:31:25 PM »
@ MH

for many years Poynt has told me that a sim can simulate any real world circuit. Poynt also knows how much i hate the use of sim's when dealing with real world circuit's. But time after time ,Poynt has shown me that the sim has been successful 100% of the time in replicating anything i (and others) have built,and has come to more exacting results than i could obtain on the bench with real world devices.

Poynt has just stated that your circuit as defined cannot be simulated,and so your circuit dose not represent a real world device.

But i would like to continue with this discussion ,as it has really made me (and others here)think of what the outcome of your defined circuit would be.

At this point in time,i am sticking to my answers given-both the real world answer-->you cannot place an ideal voltage across an ideal inductor,and also my theoretical answer,being the current would rise instantly,to an infinite value.
The reason for my theoretical answer is because of the resistance value of zero when calculating the L/R time constant.
I have found no math that allows for any value to be divided by zero without the answer being infinite. If the current rise time is infinite,then there is no rise in current as far as the inductor side of it go's. This means that there is no inductor. But as there is an ideal resulting in a zero value of resistance,the ideal voltage is now placed across a dead short--the big conundrum of the whole question using your ideal values.

And so my answer of an instant current rise of an infinite value.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #72 on: May 09, 2016, 03:43:12 PM »

My blown universe post was a bit of a ruse. In fact it implied the opposite of what would theoretically happen, that is, "nothing".

So you are saying that at T=0--the instant the ideal voltage is placed across the ideal inductor,nothing would happen?--no voltage would appear across the ideal inductor?.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #73 on: May 09, 2016, 03:44:57 PM »
<<< Poynt has just stated that your circuit as defined cannot be simulated,and so your circuit dose not represent a real world device. >>>

Of course it can be simulated.  To my surprise in this case he has to add a resistor.  We have seen him add little phantom resistors many times in the past to get the simulator to run so ultimately there is no real surprise.

The place to simulate this setup is in your head, like I explained before.  Then there are no issues.  I am attaching the "short answer that solves everything" again to this posting.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #74 on: May 09, 2016, 03:52:45 PM »