amperage inrush

gar

Senior Member
160226-2119 EST

iwire:

Your statement is probably correct that the Fluke may not be fast enough to measure the very short peak inrush cuirrent that doesn't exist, but also it may not be fast enough to measure the maximum of several cycles of the motor starting currrent.

So I ran an experiment with my Fluke 27 that is at home. With my test motor there is about 6 cycles of startup current at about 123 V. Using this test motor as a signal source, and an LEM Hall sensor the scope read a peak of 5 V ( 5 V = 100 A ). Convert this to RMS and the Fluke, on AC, should have read 3.5 V, but the maximum reading was 0.88 V. This is 25% of what it should have read. Thus, I would estimate the Fluke 27 averaging time is around 0.5 second. Way too short for measuring this startup current signal.

For motor starting current we need an averaging time of 16 to 32 milliseconds. But that is not fast enough for transformer inrush. Here we need something in the fractional millisecond range.

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gar

Senior Member
160226-2239 EST

iceworm:

The statement you quoted from Jraef makes no sense until there is some definition of terms.

From a few cycle transient condition of loading on a transformer and assuming we are not in an unbalanced saturation condition, then the transformer can be considered a linear circuit of resistance and inductance for low frequencies. At high frequencies capacitance has to be added to the circuit.

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mivey

Senior Member
160226-2239 EST

iceworm:

The statement you quoted from Jraef makes no sense until there is some definition of terms.

From a few cycle transient condition of loading on a transformer and assuming we are not in an unbalanced saturation condition, then the transformer can be considered a linear circuit of resistance and inductance for low frequencies. At high frequencies capacitance has to be added to the circuit.

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Jraef was talking about motor starting and transformers so it will not make sense. The energy stored in the transformer fields drops quickly so any voltage sustaining capacity is very very brief and nothing near that needed to start a motor. Also, if the transformer sustained a voltage it would be distorting the sine wave all the time and it does not.

I remember when Jraef brought this up before. It did not make sense then and it still doesn't.
 

gar

Senior Member
160227-0831 EST

In J. G. Tarboux's courses on DC and AC machinery he tried to provide the basic concepts in ways that were as simple as possible to understand the basic concepts.

Searching the Internet I did not quickly find a source that had a presentation that I liked.

For simplicity if you are on the load side of a transformer and look back at the transformer and everything that supplies it, then a good approximate low frequency equivalent circuit is an ideal sine wave constant voltage source with a series internal impedance of a resistor and inductor. Energy storage fields in the transformer have nothing to do with transient response other than as to how a linear series resistor and inductor function. Analysis is by a basic simple differential equation.

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Ingenieur

Senior Member
Location
Earth
Most modern flukes will read this, transformer or motor
they take 400 readings over a 6 cycle peak window and run it thru an algorithm
even the older 'peak' function gives a believable value vs NEC motor starting kva/HP code table
 

mivey

Senior Member
160227-0831 EST

In J. G. Tarboux's courses on DC and AC machinery he tried to provide the basic concepts in ways that were as simple as possible to understand the basic concepts.

Searching the Internet I did not quickly find a source that had a presentation that I liked.

For simplicity if you are on the load side of a transformer and look back at the transformer and everything that supplies it, then a good approximate low frequency equivalent circuit is an ideal sine wave constant voltage source with a series internal impedance of a resistor and inductor. Energy storage fields in the transformer have nothing to do with transient response other than as to how a linear series resistor and inductor function. Analysis is by a basic simple differential equation.

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Other than the capacitive and inductive fields (even though they are too small), do you see any other source Jraef could use to account for the transformer transient capacity he submitted?
 

gar

Senior Member
160227-2115 EST

mivey:

If there is no DC current in either the primary or secondary, and the transformer has recovered from any peak inrush current, then I believe it simply looks like the equivalent of a linear series resistor and inductor. If there is no load on the transformer and a full load is applied, then the initial current in the equivalent series inductor is near zero (meaning the magnitizing current in the primary part of the equivalent circuit can be ignored).

It may be that some people believe that stored energy in the transformer can in some way assist the starting of a motor. This does not make sense. The transformer just looks like a series impedance to the motor. The two different AC induction motors I have tested require at least 6 full 60 Hz cycles to get near full speed. Some small amount of energy from a fractional part of a cycle is not much of a contributor. What can be a contributor is many rotating machines with stored mechanical energy in their inertia loads.

I now have a three phase motor starting current to show. This will be a little later.

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iwire

Moderator
Staff member
Location
Massachusetts
160227-2239 EST

See post #27 at http://forums.mikeholt.com/showthread.php?t=174880&p=1718198#post1718198 for several 3 phase motor starting current waveforms. There is no inrush current.

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Gar, most electricians they are going to call the large surge of current that happens ( and is shown on the waveforms) when the motor is energized and reduces as the motor comes up to speed 'inrush'.

I have no idea why you feel it is helpful to confuse this thread with your personal choice of terminology.
 

mivey

Senior Member
160227-2115 EST

It may be that some people believe that stored energy in the transformer can in some way assist the starting of a motor.
Yeah, but Jraef is smarter than the average bear so it would be nice if he would explain.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I recall this coming up in another thread. I never got a clear understanding. I did a search on IEEE papers, IEEE 100, and even google - nothing except for some heat capacity related issues.

Do you have a reference for this?

If not, do you have some physics explanation?

How about personal test data - test setup, what measured, how measured?

I'm absolutely open to some education this morning.

ice
I keep making this same mistake. You keep catching it... :slaphead: I do a lot of consulting on generator capacity for large motor starting, sometimes I get my head lost in the fog of one project when responding here on another. It's a bad habit I have of responding without thinking it through enough.

GENERATORS have a transient overload capability, not transformers. In general, transformers are often over sized to ALLOW for minimal voltage drop when starting a large motor, but that's not the same. Mea culpa...
 

mivey

Senior Member
GENERATORS have a transient overload capability, not transformers. In general, transformers are often over sized to ALLOW for minimal voltage drop when starting a large motor, but that's not the same.
Well that clears up that. Exercised the brain cells anyway.

Spec'n a low-impedance transformer is also used as well as oversizing the transformer to reduce drop. Hopefully the transformer manufacturer actually comes through.
 

gar

Senior Member
160301-1006 EST

iwire:

From your post:
Gar, most electricians they are going to call the large surge of current that happens ( and is shown on the waveforms) when the motor is energized and reduces as the motor comes up to speed 'inrush'.

I have no idea why you feel it is helpful to confuse this thread with your personal choice of terminology.
The reason for using different names is too distinguish two greatly different effects.

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".

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.

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.

Instantaneously you can change the voltage and current to a resistor.

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topgone

Senior Member
160301-1006 EST

iwire:

From your post:

The reason for using different names is too distinguish two greatly different effects.

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".

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.

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.

Instantaneously you can change the voltage and current to a resistor.

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Nice one, @gar!
 

Ingenieur

Senior Member
Location
Earth
V = L di/dt
i = C dv/dt
v = ir

in L v must change/integrate over time for i to occur, not instantaneous
opposite for C
R no time constraints or integration, ie, instantaneous for both

most genset alternator specs giveva kva motor starting current
 

__dan

Senior Member
Nice one, @gar!
My thought exactly as well when I read it.

Motor starting current is called "locked rotor" which is descriptive as well. Once the rotor starts turning, a back EMF is generated in the winding, lowering voltage and current through the motor winding.

Inrush I got a good understanding of from reading this board, the transformer magnetizing current when there is no residual magnetization to reduce the inrush, essentially a short, but a very fast transient short as it does not need to get a physical mass moving. It just needs enough time to get the magnetic field to penetrate its normal range, probably less than three cycles.

Inrush on the job, I have mostly only seen used by professionals who had no clue, they were using it as BS term to explain or blame something that they should have honestly said "I don't know". I recall once they blamed something (large) on inrush, and when it got to me I told them immediately no, that could not be inrush. They persisted and I looked up their engineering licenses on the DCP website and found they had none (while billing the customer as "Engineers"). It was obvious and I explained, at which point one said I could "shove the code" while holding the book.

Yes, there was an inrush, but only for the customer's money.
 

gar

Senior Member
160201-1942 EST

ActionDave:

The big deal is that transformer inrush current has a totally different characteristic shape, is probably larger, and is shorter in duration than is a motor starting current curve. These characteristic curves derive from different causes.

Additionally a motor starting current curve is, for the most part, the same every time the motor starts. Transformer inrush current is very random in magnitude from one turn on occurance to another.

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__dan

Senior Member
So you have a big slug of current magnetizing some wires and inducing on a piece of iron. The iron rotor on the motor turns, the iron core on the transformer doesn't. What's the big deal about calling it inrush in both cases?

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.

Also if you Google transformer switching transients, you will see some weird stuff on the captured waveforms, 20kHz ringing.

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).

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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