Inductor has no high inrush current

[gar]

170920-1132 EDT

There is no in rush current to an inductor at time t = 0 when voltage is applied to said inductor. The current at that instant is whatever the current was in the inductor just before voltage was applied.

This thread is simply to provoke discussion and try to get the reader to understand what happens to current in an inductor. There are far too many incorrect statements made about in rush current to an inductor, usually stated as an inductive load.

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[Ingenieur]

i = v/R (1 - e^-( tR/L) )

 

[drktmplr12]

i = v/R (1 - e^-( tR/L) )

for the uninitiated, i(t) = v/R (1 - e^-(tR/L) ) therefore i (t=0) = v/R (1 - e^0) = v/R (1 - 1) = 0

This assumes the initial current was 0.

 

[gar]

1709221-2015 EDT

In most cases it is very difficult to look at an equation and visualize what is happening.

Thus, I created two experiments to show how inductor current varies with time when a voltage is applied.

The circuit tested consisted of a 500 ft roll of #16 wire on a plastic spool. A larger coil with many more turns would have been desirable. Therefore, no ferromagnetic core, just an air core inductor. This is quite a linear device. This coil has internal resistance and that is part of the test circuit. External to the coil was additional resistance used to measure current. This external resistance is also part of the test circuit. Total resistance was about 3.5 ohms.

Thus, we have a simple series L-R circuit with a voltage source and a switch.

Two different voltage sources were used. The first was a regulated 5 V DC source. The second was a transformer fed from a Variac with a maximum output of about 14 V RMS with 120 V input. Test was 60 Hz power.

Multiple trials were performed for the AC plot to randomly have a turn on time with substantial voltage present at turn on.

Estimated L/R time constant is about 3.5 milliseconds.

Just touching two wires together was the switch. The scope was in single shot mode. Triggering was from the voltage channel.

These plots clearly show no inrush current, and because there is no initial inductor current the current at turn on starts at 0.

Blue is voltage, and red is current.


AC Plot:
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There is absolutely no inrush current.



DC Plot:
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There is absolutely no inrush current.



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[Ingenieur]

L/R = time constant
Time x tc / ~pu i
0/0
0.1/0.1
0.2/0.2
0.3/0.25
0.5/0.4
0.7/0.5
1.0/0.63
3.0/0.95
5/>0.99

 

[GoldDigger]

FWIW, the highest inductor current at turn on happens, AFAIK, when the turn on occurs at a voltage zero crossing.
Transformer "inrush" that we see seems to be primarily related to the combination of residual flux and saturation.

 

[gar]

170921-2241 EDT

With 0 initial current in an inductor there is no finite input voltage that will cause the inductor current to be other than 0 just after application of the voltage.

A fundamental characteristic of an inductor is that you can not instantaneously change the current in an inductor.

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[GoldDigger]

170921-2241 EDT

With 0 initial current in an inductor there is no finite input voltage that will cause the inductor current to be other than 0 just after application of the voltage.

A fundamental characteristic of an inductor is that you can not instantaneously change the current in an inductor.

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But in a practical situation, we are comfortable calling it inrush if it happens any time within the first cycle after application of power, rather than immediately at time t=0.

 

[Sahib]

170921-2241 EDT

With 0 initial current in an inductor there is no finite input voltage that will cause the inductor current to be other than 0 just after application of the voltage.

A fundamental characteristic of an inductor is that you can not instantaneously change the current in an inductor.

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True but at 0+ moment there may be large DC decaying component.

 

[Besoeker]

True but at 0+ moment there may be large DC decaying component.

Surely only if there was a large DC component at t(0) ?

 

[FionaZuppa]

well, i think "inrush" is being used in incorrect context here.

the "in" current on a inductor simply lags behind voltage, a voltage follower.
the "in" current on a capacitor simply leads voltage, a voltage leader.
with AC you will see a phase diff of 90° for both.

"inrush" is typically the measured "in" spike of amps during the startup of the ckt, typically a motor, but can be for any ckt. take a simple 12v wall wart, sometimes youll see that spark as you plug it in, thats a nice amps spike but for short period. this spike can be as large as 2x the normal running amps.

hence why many/most clamp-on amp meters have a "inrush" setting, its nothing more than a 1st time max amps recorder that displays to the screen.

 

[Besoeker]

well, i think "inrush" is being used in incorrect context here.

the "in" current on a inductor simply lags behind voltage, a voltage follower.
the "in" current on a capacitor simply leads voltage, a voltage leader.
with AC you will see a phase diff of 90° for both.

I think gar was talking about what happens just after t=0, not the steady state condition you gave above.

 

[winnie]

Agreed, at time t=0 current=0.

IMHO GoldDigger's point is key. What is commonly called 'inrush' happens any time in the first few cycles. For a linear inductive circuit you don't see much if any inrush. But add a core which can saturate and in the cycle just after t=0 you can see much much more than normal steady state current.

-Jon

 

[ggunn]

170921-2241 EDT

With 0 initial current in an inductor there is no finite input voltage that will cause the inductor current to be other than 0 just after application of the voltage.

That depends on what "just after" means. Connected to an ideal current source, V -> infinity as t -> 0

 

[junkhound]

for non-maxwellians :lol:

there be all these electrons sitting there and not drifting anywhere, to get anywhere they gotta run around this big coiled loop of racetrack

close the switch, and the ones at the end push the ones next to it and at near the speed of light that push is felt all the way down the racetrack

now, depending on how many loops there are in the racetrack, the crowd starts drifting toward the other end, buts LOTs slower than the pressure wave, and it is harder to go around the loop than if it were a straight track.

Besides which, the more getting push they 'rubberneck' at the guys on the next loop going the other way and slow down some more, even though the same pressure is still there.

Eventually they go out the other end.

Sometimes, some perverse weeinies start pushing back the other way, and fewer get out over a period of time and then some get pushed back out the other end..

Oh, such fun.....

 

[Besoeker]

That depends on what "just after" means. Connected to an ideal current source, V -> infinity as t -> 0

But not applicable to real life.

 

[ggunn]

But not applicable to real life.

Who said anything about real life?

 

[FionaZuppa]

I think gar was talking about what happens just after t=0, not the steady state condition you gave above.

ok, t=0 ?? meaning that is the time at which voltage is applied? no voltage no amps. kinda weird, the airspeed on wing surface of airplane flying at 500mph is zero.

i did not give steady state view, i said the turn on period, from when voltage is applied to when amps get back to steady run amps.

anyways, still, "inrush" can be any # of cycles, all depends on the ckt. from say when V is applied to the time the amps settle back to normal running state, thats the "inrush". the "inrush" like i said earlier is a short spike during turn on phase.

simple RC or RI ckt will not see "inrush".

 

[junkhound]

simple RC or RI ckt will not see "inrush".

Typo? double typo? no inrush for RC is sure news to lots of folks I'll bet.

Inrush defined as larger than steady state.

 

[Besoeker]

i did not give steady state view,

If that's what you think that's fine.