amperage inrush

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ActionDave

ActionDave

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__dan said:
Other than time, not much. But it's three orders of magnitude difference going from 5 milliseconds of time to allow the magnetic field to penetrate the inductor, to 5 seconds for a high inertia load / motor combination to spin up. Very different waveforms.
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Okay. The waveforms are different. That is something you can see in the lab, not something I am going to check in the field.
Also if you Google transformer switching transients, you will see some weird stuff on the captured waveforms, 20kHz ringing.
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Something fun to do on a smoke break, otherwise what am I gaining?
Two very different scenarios when you have to pick one of the two when diagnosing or troubleshooting a problem. Its pretty common for an unloaded transformer located right next to the main gear to trip on the breaker closing. Knowing why that is, is necessary. When it happens in a motor it is not the same effect, that would be the failure of the motor to start turning (or loss of field for a DC motor).
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Nobody is saying motors are the same as transformers
Saying it is inrush tells me nothing, requires more information. When you have more information, you may have more descriptive verbiage to describe the finding.
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The breaker is tripping. If you have narrowed the problem down to transformer inrush, or motor starting current, or just use the word inrush as a way to describe the problem in either case I don't see that as a hindrance to getting down to work solving the problem.
 
iwire

iwire

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ActionDave said:
Okay. The waveforms are different. That is something you can see in the lab, not something I am going to check in the field.
Something fun to do on a smoke break, otherwise what am I gaining?

Nobody is saying motors are the same as transformers

The breaker is tripping. If you have narrowed the problem down to transformer inrush, or motor starting current, or just use the word inrush as a way to describe the problem in either case I don't see that as a hindrance to getting down to work solving the problem.
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:thumbsup:
 
iwire

iwire

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gar said:
160301-1006 EST
The reason for using different names is too distinguish two greatly different effects.
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For an electrician those different effects do not matter.

Either way it is a surge or inrush of current.

I have already referenced a textbook on AC motors that never used the term "inrush" to describe motor starting current, but used the words "motor starting current".
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I work with equipment and people, not text books. Speaking in a language my fellow electricians and customers can understand is much more important than what the text book says.


To troubleshoot problems with starting AC induction motors vs applying power to a transformer is greatly assisted by one having knowledge of the difference in the current waveforms of the two different loads. Having different names helps one to remember that the conditions following application of power are not the same.
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35 years doing this work, always a key employee and the one sent in to fix what others cannot even though I do not use the same terms textbooks use.


Too many electricians make the statement that a large inrush current is the result of an inductive load. This is incorrect. Instantaneously you can not change the current in an inductor, which is a fundamental fact. Instantaneously you can change the current to a capacitor, but not its voltage. Whereas the voltage to an inductor can change instantaneously. These are basic fundamentals to all circuit analysis.
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Yet calling it inrush is what we do and we get by fine.

Our jobs are different.
 
P

Phil Corso

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Ingenieur...

I didn't get the get your point! What I got from Fitzgerald/Kingsley's 2nd edition (in EE-001) was that the transformer transferred electrical-energy from one voltage to another, while a motor transformed it into mechanical-energy!

Regard, Phil Corso
 
I

Ingenieur

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Phil Corso said:
Ingenieur...

I didn't get the get your point! What I got from Fitzgerald/Kingsley's 2nd edition (in EE-001) was that the transformer transferred electrical-energy from one voltage to another, while a motor transformed it into mechanical-energy!

Regard, Phil Corso
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I think we are saying the same thing
energy conversion
one elec-elec power
one elec-mech power
they both store/transmit/convert P + Q to function

they are both 'transfomers' in a sense
one transforms (in most cases the ratio of v/i)
the other elctrical power into mechanical or vice versa
one in mechanically restrained the other not

the basic operating mechanisms are similar, manipulating/controlling electro magnetic fields
that is why in most texts transformers are covered first
the basic theory applies to both and is easier to grasp

I did not have machines til my 3rd year
Needed diff equations and vector calc first
in fact no EE couses til my sec year, then basic circuit theory etc
the first 60 credits or 2 years
21 in math calc 1, 2, 3, linear alg, diff eq, prob/stats
8 in physics
4 in chemistry/bio
12 intro EE stuff plus labs
Balance humanities and electives

the next 2 years 30+ in EE
 
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I

Ingenieur

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ActionDave said:
And you have managed to maintain a senseof humor. Impressive indeed.
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Depends whom you ask :lol:

I am back in school now for a grad cert in power engineering
just had my midterm
4 guys in my class (started with 8)
the other 3's ages almost add up to mine lol
at least the prof is older

spent yesterday in a mine
500' down 2 miles in
40" seam
crawling on my hands a knees in 6" of mud to do some testing on a safety system
getting too old for this crap lol
I better have a sense of humor :)
 
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iwire

iwire

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Ingenieur said:
Depends whom you ask :lol:

I am back in school now for a grad cert in power engineering
just had my midterm
4 guys in my class (started with 8)
the other 3's ages almost add up to mine lol
at least the prof is older

spent yesterday in a mine
500' down 2 miles in
40" seam
crawling on my hands a knees in 6" of mud to do some testing on a safety system
getting too old for this crap lol
I better have a sense of humor :)
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Seems like poor management to waste your knowledge doing things a normal human could.
 
I

Ingenieur

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Earth
iwire said:
Seems like poor management to waste your knowledge doing things a normal human could.
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I AM a normal human? Except to my wife at times lol
my knowledge is exactly why I WAS there

it is a very complex issue
the testing needed made up as we went along depending results of the previous steps
believe me, when I feel I can in good conscience I send a subordinate
but this might have been involved a recent fatality
I do not consider myself of higher value
I did have Inspectors with me
 
Sahib

Sahib

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India
gar said:
The possible peak inrush current in a transformer does not violate my statement that current can not change instantaneously in an inductor
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Correct. But to explain inrush, phase angle of voltage at instant of switching also required.
 
G

gar

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Ann Arbor, Michigan
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EE
160304-2018 EST

If you look at a pure L-R (meaning an air core inductor) series circuit with AC sine wave excitation, and no initial current at -t, then for t positive and starting at t = 0, with theta being the phase angle of the AC voltage at turn on the instantaneous current in the circuit is:

i = i (transient) + i (steady state)

and expanded

i = k1 * e^(-t*R/L) + Vm * sin (wt - theta) / (R^2 + w^2 * L^2)^0.5

where
L-R impedance = (R^2 + w^2 * L^2)^0.5
k1 = Vm * wL / (R^2 + w^2 * L^2)
Vm = peak AC voltage
R = series resistance in ohms
L = series inductance in Henrys
t = instantaneous time in seconds
theta = phase angle of voltage at t = 0 (turn on)
w = 2*Pi*f
f = frequency in Hz

As R gets very large compared to L, then the circuit looks very much like a resistance.

As L gets very large compared to R, then the circuit looks very much like a pure inductance.

So long as there is some inductance the current has to start at zero.

The small peaking that occurs is about where inductive reactance Xl = 2*PI*L = R.

The above equations are from "Analysis of A-C Circuits", by M. B. Stout, 1952.

When a ferromagnetic material is introduced as the core material, then magnetic saturation has to be taken into account. Peak transformer inrush current is a funtion of the last magnetic state of the core, the applied voltage, and the point in the AC cycle when voltage is applied to the transformer. In the reql world this inrush current is very random in magnitude up to a maximum. Its major impact lasts only about 1/2 cycle. Maximum possible value, assuming no initial current, is v/R.

A typical induction motor has a consistent starting current that probably lasts for at least 6 cycles in most applications, and very much longer on some high inertia loads.

.
 
Sahib

Sahib

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India
gar said:
When a ferromagnetic material is introduced as the core material, then magnetic saturation has to be taken into account. Peak transformer inrush current is a funtion of the last magnetic state of the core, the applied voltage, and the point in the AC cycle when voltage is applied to the transformer. In the reql world this inrush current is very random in magnitude up to a maximum.
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Even when there is no ferromagnetic material, if the voltage phase at the time of switching is 90 degree, it would cause inrush due to DC offset.
 
mivey

mivey

Senior Member
Sahib said:
Very nice pdf. Do you have Vincent Del Toro's Electric machines and power systems in pdf, which is also good?
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Del Toro is good but I find Fitzgerald to be more technically appealing. I would prefer Fitzgerald over Del Toro for EE courses. Both are great references.

FWIW, the pdfs like that are copyright violations and I am suprised the link posted here survived this long.
 
I

Ingenieur

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mivey said:
Del Toro is good but I find Fitzgerald to be more technically appealing. I would prefer Fitzgerald over Del Toro for EE courses. Both are great references.

FWIW, the pdfs like that are copyright violations and I am suprised the link posted here survived this long.
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Pretty sure that is a legal open source doc
it's from a University website
???
 
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