Inrush current in a transformer

Discussion in 'Electrical Engineering' started by Cubus, Dec 31, 2003.

1. CubusGuest

Here is a photo of the data-label of a transformer:

How do you estimate/calculate the inrush current of this
transformer when power is switched on?

Does it make any difference, concerning inrush current, if

Why does it state two kVA-ratings? (2,5kVA/15,6kVA).

2. RowbotthGuest

OK, but it was n ot a bad question. In thepast (like maybe up to about
5-10 yearrs ago), youcould go to the IEEE and get a typical value for
Inrust current. Now, in this era of high efficiency" transformers, I;'m
told theat some of them have an inrush of 10 to 14 times nominal
nameplate amps. (Same thing for the newer motors.)

So what do people do when they are trying to perform studies that will
give them an idea of how their system is supposed to respond to the new
loads - so they know how much capacity they need to add to the system?
Waiting for the actuals from the mfr. seems to be the wrong way to go -
and you need to know WELL in advance of receiving the transformer or
motor, as the lead time on system components is long, too.

So what rules of thumb are we supposed to use now?

Thanks, folkjs.

HR.

3. CubusGuest

What im actually looking for, is some rules of thumb, not the exact
calculated value.

Purpose: to be able to estimate what size of mini-circuit breaker
would be nessesary on the primary side without tripping when the
transformer is powered on.

A smaller transformer was replaced with this one and a 10 A type C
mini-cicuit breaker tripped almost everytime power was turned on.

The maximum inrush current would be determined by the inductance of the coil
with an air core.
The inrush current depends upon a number of things and so can not be
computed.
1 Shape of the Hysterieses loop.
2 Point on the hysterieses loop when the transformer was last deenergized.
3 Available margin of spare volt seconds designed into the core over the
required.
4 The applied voltage.
5 The series impedance of the coil and the source.

So you see, it is basically impossible to compute what the inrush current
will be at turn on.
It depends on to many things that have happened in the past and to measure
those things will change them.

The inrush current can be vastly different each time you energize the
transformer and never ever repeat itself.

6. Eur Ing John RyeGuest

Hello All

The inrush current can be vastly different each time you energize the
transformer and never ever repeat itself.[/QUOTE]

Not quite correct. I have set up a circuit in the laboratory, where one can
get the same inrush current time after time. However you do need :-

(a) Constant Supply voltage

(b) A load on the transformer significantly greater than the magnetising
current

(c) A circuit-breaker that breaks accurately at current zero.

(d) Point on wave control of the closing circuit breaker.

With a set up like this you can plot the distribution of the magnetising
inrush peak with closing angle.

Items (b) & (c) together will guarantee that you have the same remanent flux
each time, although for a single phase transformer it could be in either of
two directions.

John

7. Spehro PefhanyGuest

You can get an upper limit by measuring the DC resistance of the
primary winding. (Divide the peak input voltage by that value). This
works because that's all that's left when the core saturates. Toroidal
power transformers seem to be particularly bad.

Best regards,
Spehro Pefhany

Yes with the setup below, you could measure the inrush current and get

But you need a bit more than you outline.
Your lab setup must always turn the power off at the same phase zero
crossing and must also turn it on at the same phase zero voltage crossing.

Turning the power off at a Zero current crossing does not insure repeatable
power measurements.
This only helps if you have a large gap in the core.

If you have a square loop material then 0 current crossing occurs at or
near max flux density.
If the power is turned on with the voltage trying to drive the flux further
in this direction there will be saturation and a large current flow.
If you turn the power off at 0 flux you will still get a large surge if the
power is turned on at 0 voltage in either direction as the core will have to
go from 0 flux to max flux (with the core saturated) as it will have to hold
twice the volt seconds that a well designed transformer is designed to
withstand.

To avoid a surge, you would have to turn to power off at max flux and then
apply a voltage that drives the flux in opposite direction. There would then
be no saturation as the core could absorb the full VS's going in the opposet
direction.

In any case load current plays on part in surge currents.

Your correct Spehro but you must also include the line impedance.
Torriodal cores are generally worse because they are, in general are nade
from square loop materials or at least squarer than the material used in E-I
cores like Selectron.
Also E-I cores in general have some effective gap which brings the flux down
from saturation closer to 0 flux so when the power is turned on there is at
least half a BH loop to climb.

http://www.speff.com

10. RowbotthGuest

The inrush current can be vastly different each time you energize the
transformer and never ever repeat itself.
[/QUOTE]

Yes, we know this. But one of the basics of engineering is to provide a
conservative (i.e., worst case) estimate and base all calcs on this
value.

You see, that way your system will usually be in better shape than you
calculated, and your system will never fail due to a higher value of
inrust current! (Relays won't operate unexpectedly; etc.)

It would be more helpful if you would try to supply information of
assistance.

HR.

11. RowbotthGuest

The maximum inrush current would be determined by the inductance of the coil
with an air core.
The inrush current depends upon a number of things and so can not be
computed.
1 Shape of the Hysterieses loop.
2 Point on the hysterieses loop when the transformer was last deenergized.
3 Available margin of spare volt seconds designed into the core over the
required.
4 The applied voltage.
5 The series impedance of the coil and the source.

So you see, it is basically impossible to compute what the inrush current
will be at turn on.
It depends on to many things that have happened in the past and to measure
those things will change them.
[/QUOTE]

OK, I guess I should have said "If you cannot help, you don't have to
kill any electrons."

A collection of motherhood statements which appear to sound impressive
but mean nothing is of no help to either the original poster, or to me.

HR.

A highly tecnical question was asked.
There is no simple way of telling whqt the inrush current will be.
It is not my fault if you can not understand the answere.
Perhaps I should have answered you The grass is green because.
Those mother hood statements are not impressive.
They are the fact.
Perhaps I should have started you with basic electronics 101.

13. Don KellyGuest

----------------
This is a power transformer- it will definitely not have a square loop BH
curve as the material used should have as small a loop area as possible.
Core gaps in such a transformer are not deliberate (and unwanted) and the
effective gap in the EI core will have little effect. Some transformers use
a toroid made of a continuous strip of transformer steel to eliminat such
gaps- I do not know whether any significant improvement was achieved by this
construction.
- however speculation on the core construction is simply that- speculation.

The case that Spehro gave is a good upper limit estimate.

The reason for the dual kVA rating is not apparent. Air cooled vs. oil
cooled, possible autotransformer operation - insufficient information.

As I said power transformers for 60 hertz are generally made of 17 mil
selectron or some such material
I realize that EI, DU and all the other types the desire is to keep the gap
as small as possible but it is always there and it does tend to drop the
residual flux down to a very low value.

Some transformers use
I have never heard of a power transformer made with a torroid.
Power transformers in general have heavier wire sizes and they are hard to
wind on a torroid.
I used to work for Arnold who made torroid winders.

When I designed I generally used some sort of nickle core which are square
looped.
Some times for converters you want softer curves so the saturation currents
are not so high.

But I had developed teqniques to use very square loop and was able to
prevent the flux from creeping up the loop on one side of the other.

In fact on one mag amp controlled converter for one satelite we went all
through tests and passed every test and then I knowticed that the wave did
not have dead zones on both sides pulses.

We checked and found that there was an open connection and half the
converter was not working.
The flyback on the current drive was providing the drive to the open side.
Because the transformer had base emmiter and external return diodes, both
sides of the wave shape were symetrical.

Mag amps work out nice in converters as they are Volt-Second devices and you
can get open loop response to the input changes which cancel out input
variations. We used a lower gain amplifier with 12db role off and had an
inherrently stable circuit.

Been in orbit and still working now after about 35 years.

15. Eur Ing John RyeGuest

Hello All

[/QUOTE]
SNIP on Magnetic Amplifiers

As part of a study relating magnetising inrush currents to system protection
I measured the remanent flux on a selection of British Distribution
transformer. These included single phase transformers ranging from 5 to 50
kVA, and three phase transformers from 50 to 500 kVA.

The single phase transformers clearly fell into three groups according to the
core construction :-

Strip wound (Torroidal) - 0.8 to 0.9 pu

Stacked core - 0.5 to 0.6 pu

"C" core - 0.05 pu

The three phase ones were all stacked core, and the figure on the highest
limb ranged from 0.4 to 0.6 pu

John

16. CubusGuest

In the meantime I have found som datasheet over the
transformer. It categorizes the two kVA ratings as
nominel power and shorttime power.

By the way, the datasheet doesn't say anything about
inrush current but proposes a specific Siemens circuit
breaker on the primary side. This circuit breaker can
handle inrush currents over 208 A with the primary
side given 415 V.

17. CubusGuest

The proposed circuit breaker is adjustable from 7-10 A.
Should be adjusted to 9,2 A.

Don't know if these adjustable types of cicuit breakers
have a special name in English?

18. CubusGuest

Well, input was 415 V and the problem about tripping was
due to inrush current, not the load.

19. daestromGuest

No, I think you misunderstood. When Don Kelly (or I if I had responded
first) says 'power transformers' he most likely means *POWER* transformers.
Ratings from a few kVA to several MVA in our business. You know, things the
size of a desk and upwards ;-)

Transformer 'in-rush' with large *power* transformers can exceed the trip
ratings of the differential protective relaying. Time delays, or 'harmonic
restraint' relaying is used to avoid nuisance tripping when first energizing
the primary.

daestrom

They don't really make transformers that size, do they ;-)
The core would be to long for the flux to go all the way around and you
could never get it on aboard a space shuttle.
So what would the need for one be.