×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Transformer Inrush Currents
4

Transformer Inrush Currents

Transformer Inrush Currents

(OP)
Hello all and happy new year.

I have a problem which is inrush current.

The main problem I have that the transformer is being used as a power supply with mains outlets (you can plug in any mains powered equipment) the unit is 240V and rated at 10A. As you all know when you turn on a power supply / transformer there are very high inrush currents for a few cycles that can go up to 1000A in this case. This would all be ok if the hospital where this is going didn't install C-curve circuit breaker which trips when there are high inrus currents. Have they put curve-D breaker there would be no problem since it has motor start characteristics. And as you have guessed if you power up the power supply it will tripp the circuit breaker due to the high inrush currents.

One way this can be avoided is to put a resistor, capacitor and a relay and create a short timing circuit to stop high inrush currents.

My question is this.... Have any of had a similar problem and how did you go about solving it.

Thank you in advance for your help.

RE: Transformer Inrush Currents

3
The reason for the inrush current is that the transformer core sees more volt-seconds than it was designed for.

As you probably have noticed, the inrush current seems to come randomly. Sometimes there is no inrush at all and sometimes it trips the breaker. The reason is that connection to the 240 V is sometimes made at the beginning of the sinewave, sometimes on top of the sinewave and sometimes somewhere in-between.

It is when connecting at the beginning of the sinewave that you get the most inrush (core sees about twice the design flux), so arranging the switching so that connection is on top of the sinewave will reduce inrush to zero.

There are SSRs with the ability to switch on top of the sinewave - in fact they were designed to eliminate inrush current. There are also NTC thermistors that can be used for the same purpose. Google, google - and google.

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

A 1000A seems a little high for a 10A transformer, you would normally expect it to be around 20A if the secondary was unloaded. The secondary load could be the problem, charging capacitors, which means you need a different solution, there are several easy ways to kill capacitor inrush currents.

RE: Transformer Inrush Currents

An old ham radio trick (used in power supplies) is to have a series resistor on the AC line to limit the in-rush / start-up current surge, and a relay that shorts out that series resistor once one of the conveniently low-voltage secondary DC voltages rises sufficiently to pull-in the relay. So, when you switch on the PS, there is a second click a couple of seconds later as the relay kicks in.

Skogs has provided links towads more-modern solutions...

RE: Transformer Inrush Currents

Long long ago, not far from here I learned that the inrush current in a transformer was due to the core magnetization remaining from when it was last switched off.

If you switch it on and it happens that the incoming volts are crossing zero and will tend to magnetize the core in the same direction then the core saturates.

Once it has saturated, no inductive effect & thus all you are left with is the dc resistance of the winding.

Which will give you a very large current indeed.

This is the same sort of effect that is used in mag amps and saturable reactors and stuff like that, so I understand.

RE: Transformer Inrush Currents

You could just install a larger circuit breaker upstream (along with requisite larger conductors).  This is what is normally done, since the inrush is normal behavior.  

RE: Transformer Inrush Currents

I would just throw that transformer into the nearest trash can.

It is not difficult to design a transformer that has an acceptable inrush current from the unavoidable flux doubling that skogsgurra describes.

The problem is that the magnetic core can go into hard saturation, leaving only the dc resistance of the primary winding to limit the peak inrush current over the first few cycles. The way to reduce the problem is to use a magnetic core material with a much softer saturation characteristic, and design it for a lower operating flux density.

The best way to do it is with E and I laminations, using ordinary cheap transformer iron, and keep the flux density low by using more primary and secondary turns (more turns per volt). The resulting transformer will be larger and heavier but it will be far less aggressive at turn on.

The worst possible way, is to use a high tech grain oriented silicon iron tape wound core, and run the working flux density right up very high. Many transformer manufacturers these days, do it that way to end up with a small hot running transformer at the lowest possible cost to them. It is cheaper, because there will be far less turns and less copper even though the core will be more expensive.

It should be possible to find something commercially available if you know what to look for, or go and talk to a transformer winding company and tell them your problem.

Resistors, relays, NTC thermistors will all work, but may be unreliable or inconvenient. much better to find yourself a decent transformer rather than trying to band-aid rubbish.

RE: Transformer Inrush Currents

scogs,

>It is when connecting at the beginning of the sinewave that you get the most inrush (core sees about twice the design flux), so arranging the switching so that connection is on top of the sinewave will reduce inrush to zero.

Am I being a bit thick here? You say the worst place to switch a transformer on is when the supply voltage is at zero volts. Huh? Surely the worst place to switch it on is at the peak of the sinewave? I accept that an unloaded transformer will have the current and voltage 90 degrees out of phase when the AC transient has decayed, making the worst place to switch it off at zero volts, but I disagree about the switch-on point.

RE: Transformer Inrush Currents

Maximum peak magnetic flux is the point of zero change, and that corresponds to the zero voltage point.

If you switched it off there, the remnant flux remains as residual magnetism in the core.  Next time you turn it on, if you try to drive the flux further in the same direction, the core will saturate.

It is confusing, because transformer magnetising current (and flux) are always exactly ninety degrees out of phase with the voltage.  When one reaches a peak, the other is always at zero.

RE: Transformer Inrush Currents

Its a small enuff transformer..just install a proper breaker ..and call it a day as dpc said. What is the rating of the breaker that trips?

RE: Transformer Inrush Currents

HI logbook, scogs is quite right about the best point to switch on the transformer.

RE: Transformer Inrush Currents

Ok so we all agree that the worst place to switch off the transformer is the highest current point which occurs at the voltage zero crossing point.

Scogs, warpspeed, and Cbarn have the worst switch on point at zero crossing and I am outnumbered three to one in thinking that the if you turn a transformer on at zero crossing there will be the least possible transient. That sort of situation indicates a misunderstanding (which may be mine!) but which is worthy of discussion.

There is a lot of talk of the current and voltage being 90 degrees out of phase, but that does not occur at switch on. Remember that at switch on the volatge is sinusoidal but the current only becomes sinusoidal after ther transient has died out.

Now I could simulate this on SPICE using a linear inductor. Would this result be satisfactory or are we saying that the major problem here is the non-linearity and remnant magnetism in the core? I am not sure if my simulator can handle non-linear inductors.

RE: Transformer Inrush Currents

Hi logbook,

In the absence of other replies, I'd suggest that the transient component causes the problem because of core saturation. The transient is (or can be) a significant DC offset which will shift an ungapped core into saturation. Most transformer cores are ungapped unless they are specifcally designed to handle a DC component.

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: Transformer Inrush Currents

Hi logbook, you seem to have stumbled upon the answer yourself in your 3rd paragraph. You agree that conditions at switch on for the "zero turn on case" are different from normal operation, ie. no voltage and no flux. Now consider the "max voltage on case" and you will see that you have the same conditions at swich on that you would have if the transformer was operating normaly, ie. max voltage no flux.

RE: Transformer Inrush Currents

Sorry that I wasn't awake to catch some of the misunderstandings early enough. But I see that several other workers have done most of the answering for me. Thanks. I still think that a clarification may be needed.

First: Transformer cores are designed to work between a negative maximum flux (Bmin) and a positive maximum flux (Bmax). Outside these limits, the core is saturated and loses its inductivity so that impedance goes to zero - or very close to.

Second: In steady state (i.e. not at switch on), the flux moves between these two limits. And since induced voltage is proportional to flux derivative it follows that the flux is at its maximums when the derivative (or voltage) is at zero.

Third: Since flux is at its maximum at zero volts, there is a flux change equal delta-B = Bmax - Bmin (B used for field intensity) or twice Bmax. This is what saves the transformer's day; it has 2xBmax available to handle the positive half-period's volt-seconds and -2xBmax to handle the negative half-period's volt-seconds.

Fourth: If the transformer is connected at the beginning of the period (flux is zero and not -Bmax) it will have used up the available flux change shortly after 90 degrees (a quarter period) and goes into saturation - which results in inrush current. If the breaker doesn't trip, it will take a few periods before the DC component has died out and current is normal again.

Fifth: This has very little to do with when the transformer was disconnected. The remanence in a modern transformer core is low and the state when disconnected usually doesn't have much influence on the inrush current. It used to be a "popular" explanation several decades ago. It may have been valid then, but it is not now.

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

Ok, after some simulations of the AC transients I see what is going on. An inductor as a load gives no current at all initially at switch on, regardless of the supply voltage phase. If it is switched on at peak supply there seems to be no additional transient, the steady state condition being reached immediately.

If it is switched on at zero volts the transient shifts the whole current waveform up, doubling the peak aiming value of the current/flux. Any normal core would then saturate making the situation worse. Having a higher series resistance damps the AC transient more rapidly. The inductor then simulates an unloaded transformer.

If the transformer is loaded such that the load is several times the magnetising current the "inductor" effects seem to get damped very rapidly. The switch-on current is then pretty much independent of the switch-on phase.

But all of that is with linear magnetics.

We therefore have unanimous agreement smile
Thanks guys.   

RE: Transformer Inrush Currents

Great! Can you also make the transformer core non-linear? Saturating at about 1.5 Tesla?

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

Hi skoggs, nearly right but the core does not have to go into saturation to produce inrush current.

RE: Transformer Inrush Currents

You better give me a very good explanation there before I believe you, cbarn. Do you mean that a linear core (not saturating) could produce a 10 times, or more, inrush current?

Are you not confusing the DC component with inrush?

The DC component exists as soon as you connect anywhere different from 90 (or 270) degrees and it can produce some extra current. It is only when the DC component saturates the core that you get inrush current. So you really need a saturable core to get inrush.

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

No skoggs that isn't what I said. You do not need a saturating core to get inrush it's a basic function of transformer action and amounts to a maximum of 2 times steady state current. If the core does saturate then the current will rise further than that also the inrush properties of the load on the secondary can add even more to it. Logbook has his simulator going so I'm sure he can verify that for you or you can just get a transformer and experiment if your still not convinced.

RE: Transformer Inrush Currents

Sorry cbarn,

What you describe is the decaying transient DC component known from connecting transformers and generators. It does, as you say, amount to twice the steady state current and it does not blow fuses or trip circuit breakers.

The inrush current is, as I say, something else. It has its origin in the DC component, yes, but it needs a saturated core to develop.

As the OP said " The main problem I have that the transformer is being used as a power supply with mains outlets (you can plug in any mains powered equipment) the unit is 240V and rated at 10A. As you all know when you turn on a power supply / transformer there are very high inrush currents for a few cycles that can go up to 1000A in this case."

As you can see, a current being up to 1000 A in a circuit that is expecting (fused for) 10 A is something quite different from the transient DC component.

All this is classical knowledge and you can find good support for the saturation effect in many text-books on basic electricity. I googled "inrush current" and got this as link #1: http://www.allaboutcircuits.com/vol_2/chpt_9/12.html

It clearly shows the saturation and the consequential high current.

I have nothing against a good discussion, but I do not think that it should be carried on in absurdum.

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

skogsgurra is correct.  DC offset is DC offset and inrush is inrush, and while both may happen simultaneously, they are not the same thing and they happen for different reasons.  DC offset is a mere doubling of the peak of the wave and leaves the waveform relatively sinusoidal.  Inrush peaks 10x or more of the normal full load current and is not particularly sinusoidal.  The inrush current will have an extremely high 2nd harmonic content and the peaks are highly distorted.

RE: Transformer Inrush Currents

Well skoggs I just read your link and it says exactly what ive been saying.

RE: Transformer Inrush Currents

One thing is for sure, a suitably designed air core transformer would not have any inrush problem, it is purely due to core saturation and the drastic reduction of inductance that this saturation causes.

An air cored choke will have a current rise proportional to instantaneous applied voltage and inversely proportional to inductance no matter what.  It is the iron core of a mains transformer (and remnant flux) that is the problem.

A mains transformer with a design peak flux swing of less than half the peak saturation value will not have any inrush problem. It would handle the flux doubling phenomena with ease and quickly settle down to normal symmetrical operation over a few cycles.  It would be a wastefully large transformer though.

RE: Transformer Inrush Currents

cbarn24050, the link from skogsurra's post DOES NOT say what you've been saying.  The material at the link is talking about the saturation driven inrush of a iron cored inductor; what you were talking about is the DC offset of any inductor, air core or iron core.  It takes the saturation of the iron core to get an inrush current 10x full load current.  The DC offset you are talking about can only get to 2x.

RE: Transformer Inrush Currents

(OP)
Hi All

Thanks for helping me out on this topic.

Yes the normal transformer that you can buy (not an ideal transformer which would cost an arm and a leg) has a nominal inrush current at 10x which occurs mostly when a transformer that has a load is turned on. The highest inrush current will occur when it is turned on anywhere on the positive part on the cycle.

The easiest way to overcome this would be to change the circuit breaker that can withstand the inrus ie motor start circuit breakers with curve D. Only thing is that this is for a Hospital and you would have to change thousands of circuit breakers and that is not an option.

At the moment I am looking into making a timing circuit or using a current limiting diode to lose few cycles and prevent circuit breaker from tripping unless any of you have any better solution.

Thank you all again for your inputs

Regards Vlad

RE: Transformer Inrush Currents

Easy now!

How would you apply the current limiting diode?

If you are going to do what I think you are going to do, then don't do it! It will DC saturate your core very effectively.

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

Vlad, is this just one transformer, or a whole series of transformers feeding for example, a class A electrical area ?

If you absolutely MUST use that transformer, try fitting a negative temperature coefficient thermistor. These large thermistors are commonly used to limit the high inrush into the cold filaments of large projection lamps. See if you can get one to try.

Cold they have a fair bit of series resistance, but they quickly self heat, and the resistance then falls quite low.

A diode is only going to make things much worse, don't do it !

RE: Transformer Inrush Currents

NTC thermistors are great BUT if the transformer has been on for a while and then gets switched off and back on almost immediately the thermistor will still be hot and therefore will give no surge protection.

RE: Transformer Inrush Currents

I have no idea what a C or D curve is, sound like an IEC thing, but come on; transformer inrush is not a surprise.  Ideal transformer would cost more than an arm and a leg, they just don't exist.  You could throw enough iron at the problem and make a transformer that didn't exhibit inrush characteristics, but that would cost more than an arm and a leg for most application and would not be a better transformer in the steady state condition.  Somebody goofed and the situation needs to be corrected, and the only practical way of correcting it is to use a breaker that coordinates with the transformer inrush; you won't find a practical means of eliminating or limiting the inrush, although the more load on the other side of the transformer, the faster the inrush will damp out.

RE: Transformer Inrush Currents

Yes that is very true, but it is simple to do, and would improve the situation significantly.

O/k then. How about combining an NTC thermistor and a suitably rated "polyswitch" in series ?

It may take a bit of experimentation to get it all working satisfactorily, but a polyswitch would certainly open the circuit fairly smartly if the NTC was ever badly caught out with a hot restart.

I am sure the thermal time constants of NTC and polyswitch could be arranged so it did not repeatedly cycle, but automatically recovered after a suitable cool down delay for the NTC.

RE: Transformer Inrush Currents

"On power transformers, the industry standard is to assume the RMS inrush current is 25 times RMS full load current for .01 seconds and 12 times RMS full load current for .1 seconds. Control transformers can have peak inrush currents that are 100 times the full load RMS current for 1/2 cycle."

Quote by Vince Saporita, Bussman Fuses

RE: Transformer Inrush Currents

I had an interesting mail only a few days ago. It is from a German guy named Michael (probably a forum member, but he didn't say). Anyhow, he pointed out that a large part of the inrush current in a toroidal core is dependent on remanence. I obviously had said somewhere that remanence was a minor problem - although I cannot find where I said that.

I have now done some experimentation. I used a toroid core, a fast recorder and current and voltage transducers. I also used an SSR to switch on/off in a controlled way.

The result is that remanence seems to be contributing to inrush current in about the same degree as the phase angle where you switch on. I had thought that remanence had less influence than that.

Good to know. Thanks Michael!

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

Hi skogsgurra;
The e-mail may have been refering to your 14 Jan 06 5:27 post. I was left with the understanding that remnance was unimportant. Thank you for the correction and clarification.
Respectfully

RE: Transformer Inrush Currents

Thanks Bill,

I was searching for "toroid" - and couldn't find it. So, I didn't say anything about toroids, actually. Now, do I have to measure EI cores as well? Sigh...

I will probably be back when I done that. But there is a holiday going on. And Sweden is playing Trinidad-Tobago right now. Have to help our guys there.  winky smile

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

Strip would grain oriented silicon iron toroids are absolutely the very worst thing conceivable for creating an inrush problem.  They combine very sudden saturation with almost no air gap. Wound and cut U cores are nearly as bad. E and I laminations of soft iron are always much more friendly at turn on.

Remnance is the real demon here.  With negligible air gap and very high permeability, the iron maintains a high remnant flux after the primary is de-energised. If you try to drive the flux higher in the same direction at turn on, and then hit hard saturation, the resulting current spike will be very spectacular indeed.  

RE: Transformer Inrush Currents

OK, Sweden didn't win that match. I think it is because I was doing some inrush current experiments instead of supporting them.

This is what an ordinary transformer looks like. I was running a slight overvoltage (250 V instead of 220 V). The periodic switching was done with an SSR and it was controlled in such a way that an integral, but odd numbers of half-periods were applied. The upper trace in picture below shows transformer primary voltage. Primary current is shown in bottom trace. Note that the last half-wave is negative. The first half-wave the transformer sees in the next burst is also negative. Remanence makes the inrush current very large as evidenced by the negative current spike. The inrush current decays to normal values in about two periods.




To verify that it is really the remanent flux that causes the current spike, I arranged a simple "automatic degaussing" by connecting a 2 microfarad/250 V capacitor across the primary. The SSR was still controlled by the same bursts with odd number of half-periods.

The influence of the capacitor can be seen at the end of the burst. In the first picture, there was a typical snap-off high voltage transient when the SSR stopped conducting. The voltage spike actually goes off scale. With the capacitor, things are a lot different. Instead of getting an overvoltage, we get a decaying oscillation which goes to zero in about 30 milliseconds. The decaying oscillation effectively degausses the core, so when we next time apply the burst (same polarity) we do not get any inrush current at all. Beautiful, I would say.



Actually, I think that this could be the remedy for the OP. Adding a capacitor of suitable size does bring remanent flux to zero and thus eliminates the inrush current.

I must admit that I was wrong in my first assumption. I have really learned from this - and I hope you did, too.

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

That is lovely picture Gunnar.  

What happens if there is an appreciable secondary load ?   Is there enough Q left to get that same beautiful damped oscillatory rundown ?

RE: Transformer Inrush Currents

Well you certainly are having fun with your new logger..roll2

Wide pics I actually had to spread it across two monitors!

Nice demo.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Transformer Inrush Currents

You guessed! Hmm.. How do I reduce the picture size?

Gunnar Englund
www.gke.org

RE: Transformer Inrush Currents

Reduce the picture size.. hmmmm.  Well you need a jpeg viewer that can "RESIZE"  jpegs.

I use Firehand Ember.
http://www.firehand.com/Ember/index.html

I use an old paid version that appears to have only the free  features now listed.

You open a picture anywhere on your system by clicking it.  It opens to its native dimensions.

Pull down edit.

Select resize.

Make sure "constrain proportions" is checked so the picture doesn't distort.

Then enter 400 into the width.  (The height will change accordingly.)

The image will jump to the new size.

Close the image.

It will ask, "do you want to keep the changes?"

Answer "yes".

Click the picture just to see if you like things.

Proceed with the standard posting procedure.

Or use any other graphics program you have that can resize.


Too much info?   BUWHAaahahah

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Transformer Inrush Currents

Thanks!

No, not too much.

At least not too WIDE!

Gunnar Englund
www.gke.org

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources