HOW does voltage lead current in an inductive circuit?

recklessrog

Joined May 23, 2013
985
I thought of myself as being poor at maths, not because I couldn't "do" maths, but not knowing how to relate it to electronics due to my schooling's lack of enough instruction in applied mathematics.
It was only when my science teacher saw some piece of equipment I had built and brought to school, he asked me how it worked and I described the operation of each component, that he asked me how I had calculated the values of the parts I had used. My reply surprised him that I hadn't, and that through studying dozens of various circuits, I had "assumed" them.
This is when he said, "maybe it would be better if you knew how to calculate them" and spent several months teaching me how to find the the maths required, how to use maths "as a tool"
At the age of ten, I built a simple oscilloscope using a VCR197a crt and parts recovered from old tv's, radios and army surplus equipment. The actual design was a mix of two taken from practical wireless magazine and another I can't remember. But, I had calculated the value of the components and changed them where necessary to make it work. This showed me the "value" of maths to enable me to achieve a successful end result.
From then on my enjoyment of electronics grew and grew to the point where I made it a lifetime career. I am now retired, still enjoy electronics and never stop learning. When I'm rusty on a subject, I now know where to look to find the correct information to revise it.
I still don't enjoy maths, but as I said earlier, It is a tool, a very necessary tool if you want to succeed in electronics, it's the process of putting theory into practice, and it is the challenge I enjoy.

BillB3857

Joined Feb 28, 2009
2,519
Having had the great privilege of learning what little I know back in the early '60s, when they taught theory without high level math, this is what we were taught. This is from a US Air Force training manual......

Suppose that current is starting to flow
through a coil. The current causes an expanding
magnetic field which causes a back EMF to be
induced in the coil. The polarity of the back
EMF is opposite to the polarity of the applied
voltage across the coil, and it therefore tends to
oppose an increase in current. The result is that
the rise in current flow is caused to lag behind
the rise in voltage. As current increases through
the coil, energy is stored in the magnetic field.
When the applied voltage starts to decrease from
maximum positive, this energy is returned to
the circuit in the form of current flow. This
current flow is actually greater than the current
flow due to the applied voltage. When the
circuit is purely inductive, maximum current
flows at the instant that the voltage has de-
creased to zero from its positive direction. A
similar action occurs on the negative portion of
the applied voltage, and maximum current flows
in a purely inductive circuit in the reverse
direction at the instant that the voltage has
decreased to zero from its negative direction.

The phase relationships in a resistive circuit
and a purely inductive circuit are shown in the
illustrations on page 92. Note that the cur-
rent is zero when the voltage is zero and the
current is maximum when the voltage is max-
imum in the resistive circuit. In the inductive
circuit, however, the current reaches zero 90°
after the voltage reaches zero. The back EMF
always opposes the applied voltage. In a purely
inductive circuit, it is maximum when the
current is zero since the rate of change of current
and speed of cutting of the magnetic field are
the fastest at this time.

BobaMosfet

Joined Jul 1, 2009
1,012
Hello. Thanks in advance for anyone able to provide clarity to my questions.

I'm desperately trying to making sense of PHASE SHIFT.
I've read statement after statement regarding the description of how voltage leads current in an inductive circuit, and how current leads voltage in an inductive circuit ; however, every statement leaves a lot of open ends through lack of description due to specifics and/or not using practical visuals.

Yes, I can interpret a sine chart showing the 90 degree phase displacements and how at peak X we have zero value Y...that's wonderful and all; however, HOW is that happening in the real world before math is used to describe it?!?!
Does the 90 degree flux line from a coiled conductor cut back on itself, essentially imposing a self-manifested wall on source current but voltage is still able to truck on by unfettered? Wtf?

A textbooks typical description is typically something like "an inductor opposes the change in current...blah-blah-blah" Oh? Is that a current change that's increasing or decreasing at the same time the flux is doing X? Is the back emf of the collapsing mag flux pushing source voltage back FROM the supply or TO the supply? Is the "voltage" that's leading an "EMF" voltage or Potential Difference voltage or Resultant Voltage or Apple-Pie in the Sky voltage?!?!
Like, Jesus Christ, why is this so hard to get a clear answer to? Literally hundreds of sources repeating the same garb.

I'd be forever appreciative if someone can set the record straight and provide some explicitly concise detail and visuals.

Links to suggested sources are appreciated and will be combed over.

Thanks
ELI means this: When voltage (or rather the amount of voltage potential _dropped_ aka _lost_ aka _required to move current_) is placed across an inductor (any conductor), it is at maximum drop-- It's an open circuit. And it remains an open circuit as long as current is pouring into the inductor, creating the electromagnetic field in the inductor. Once that field is created, and can expand no further, the circuit is completed as a short (current flows fully through the inductor)-- and voltage drops to zero, because potential on either side of the inductor is the same.

So if you had a graph with vertical axis being voltage and horizontal axis being time, voltage would start at the top and sink to zero, whereas current would start at the bottom, and rise to the top.

WBahn

Joined Mar 31, 2012
25,297
ELI means this: When voltage (or rather the amount of voltage potential _dropped_ aka _lost_ aka _required to move current_) is placed across an inductor (any conductor), it is at maximum drop-- It's an open circuit. And it remains an open circuit as long as current is pouring into the inductor, creating the electromagnetic field in the inductor. Once that field is created, and can expand no further, the circuit is completed as a short (current flows fully through the inductor)-- and voltage drops to zero, because potential on either side of the inductor is the same.

So if you had a graph with vertical axis being voltage and horizontal axis being time, voltage would start at the top and sink to zero, whereas current would start at the bottom, and rise to the top.
How can current "pour" into something that is an open circuit?

Also, the claim that when voltage is placed across an inductor it is at maximum drop is meaningless and incorrect. Knowing that an inductor has, say, 10 V across it tells you nothing about the current in the inductor except how it happens to be changing at the moment in time. If the inductance is 2 H then the current (with the direction assigned consistent with the passive sign convention) is changing at +5 A/s. But the current could be +100 A, +10 A, 0 A, or -253.4 A -- you have no information from which to conclude that any of those could or could not be the current in the inductor. Similarly, knowing the current in an inductor at any particular moment in time tells you absolutely nothing about the voltage across the conductor.

BR-549

Joined Sep 22, 2013
4,938
The reason that current lags voltage in an inductor by 90 degrees, is, because the counter EMF is at -180 degrees.

Source EMF pressurizes the current forward......+ 180 degrees.

Counter EMF pressurizes the current backward........- 180 degrees.

The resultant current is 90 degrees behind the source or 90 degrees ahead of the counter.

Da da.

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recklessrog

Joined May 23, 2013
985
I have not seen much reference to "reluctance". As the voltage is applied, current flowing becomes the magnetomotive force (m.m.f) This causes the Flux to build up effectively trying to resist the change, by how much it resists the change, is it's reluctance. Magnetmotive force changes directly with current.
Analogous to Ohms law, total flux equals magnetomotive force divided by total reluctance.

BillB3857

Joined Feb 28, 2009
2,519
Me thinks "Elvis has left the building!"

BR-549

Joined Sep 22, 2013
4,938
Well, what do you think papa?

I think post #25 wins. If I do say so.

It's simple, it makes sense and it's easy to remember.

Jony130

Joined Feb 17, 2009
5,145
The resultant current is 90 degrees behind the source or 90 degrees ahead of the counter.
Why I do not see any phase shift in this example ?

BR-549

Joined Sep 22, 2013
4,938
The blue slanted lines.........is phase shift.

Period 2-4 is phase shift and past point 5 is phase shift.

Any time the current is out of time with voltage......phase shift.

At point 2 and point 5, they almost come together.

If there was no phase shift.............the bottom graph would mirror the top graph.

Replace the coil with a resistor and re-plot.

That is what "in phase" looks like.

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ErnieM

Joined Apr 24, 2011
8,019
Why I do not see any phase shift in this example ?
View attachment 105408
Because you are working a problem in the time domain whilst complaining techniques suitable for the frequency domain do not work.

crutschow

Joined Mar 14, 2008
24,404
Why I do not see any phase shift in this example ?
View attachment 105408
The phase-shift applies to a periodic excitation of the inductor with zero DC average value.
If you use a square-wave with equal plus and minus excursions, you will see the phase-shift.

BR-549

Joined Sep 22, 2013
4,938
Counter EMF is really neat stuff.

Most forget that the most flux, and therefore the most counter EMF is at 0 CURRENT crossover.

Flux comes from the CHANGE IN CURRENT.

When a current stops, or when a current starts.......IS..... the greatest change in current.
This is when the maximum number of flux lines is cutting the conductor.

It's not the amount of current, it's the change in current.

Also the least counter EMF (0 volts) occurs at maximum current.

At maximum current.....the change in current is 0.

Pretty wild, ain't it.

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MrSoftware

Joined Oct 29, 2013
1,676
Here's how I think of it in simplified layman's terms, I hope this helps.

Picture a very heavy metal block on wheels. Movement of the block represents current in an inductor, the force you apply to the block to make it move represents voltage. If you want the block to move fast, you apply a whole lot of force. Initially the force (voltage) is very high but the block isn't rolling yet (current is zero) thanks to inertia. As the block starts to move (current starts to flow) and approach maximum velocity (max current), the force required to keep it moving (voltage) goes down. Now if you try to stop the block instantly (stop the current flow), the amount of force required (voltage) is extremely high. You have to push hard before anything moves, i.e. pressure (voltage) leads the velocity (current).

For a capacitor, picture filling a bucket from the bottom. The water flow into the bucket is the current, the water pressure required to make the flow happen is the voltage. As you begin to fill the bucket, there is lots of water flow (current flow) but almost zero pressure (voltage) since there's no water yet in the bucket to push back. As the bucket fills, the pressure required to insert more water from the bottom (voltage) increases and the flow of water (current) decreases. Here you have water flow (current) before pressure (voltage), i.e. the current leads the voltage.

I'm not sure if this is what you were looking for, but I hope it helps.

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recklessrog

Joined May 23, 2013
985
Whilst this thread is running, I would like to ask a question addressed primarily to American members.
I was always taught that the unit of magnetic flux is the Weber, (Wb) pronounced Vayber because Wilhelm (Vilhelm) Weber was a German Physicist, but a scientist friend insists that it is now universally pronounced Weber as in spider web. Can anyone settle the argument for me?

BR-549

Joined Sep 22, 2013
4,938
This is the first I heard Vayber.

Papabravo

Joined Feb 24, 2006
12,976
Well, what do you think papa?

I think post #25 wins. If I do say so.

It's simple, it makes sense and it's easy to remember.
It doesn't do anything for me, but I'm not the one with the problem. I'm actually OK with using the time derivative of the current to calculate the voltage across the inductor. It even gives the correct sign. The TS/OP is really the only authoritative voice that can settle this argument, and as @BillB3857 has observed, he may have left the building.

recklessrog

Joined May 23, 2013
985
This is the first I heard Vayber.
The German pronunciation (and Polish) of W is V, as in Wolkswagen is correctly pronounced Volksvagen. So should not the Weber be correctly pronounced? I guess it's like Mainwaring, pronounced Mannering.

Aha, I'm wrong with VW as Kubeek pointed out...............
The company name si Volkswagen, which si why the logo is VW, and is pronounced folksvahgn. Initial V is allways pronounced F, just like initial S is pronounced Z, so Siemens is pronounced Zeemens in Germany.

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