Transformer Inrush current

Dear Sir,

You might have read in theory like this wheras this is a momentry current when the CB is closed. If it prolong means then the O/L realky acts. Differential Relay will not act since it is biased for this in rush current.

Bet Regards,

N.Suresh

of late most of the electronic relays address this particular situation. Relays sense the inrush current and will disable tripping for some time. Example of that would be GE Multilin Relay SR745. Please go through this link http://www.gedigitalenergy.com/products/brochures/745.pdf

Sorry, you misunderstand. Transformers do not have inrush currents unless the secondary windings are loaded with motors (6X inrush) or incandescent lighting (10X inrush). The inductance of a transformer actually delays current because a magnetic field must be established before current flows from the secondary windings.

If you mean inrush to an electromagnetic motor, the inrush caused by a stalled rotor at start up is easily controlled by the proper fuses, breakers and/or switch gear suggested by many respondents. Unless you know more, the safest path is to have the power supplied by a distribution designed and constructed by a licensed electrician.

_{"Sorry, you misunderstand. Transformers do not have inrush currents unless the secondary windings are loaded with motors (6X inrush) or incandescent lighting (10X inrush). The inductance of a transformer actually delays current because a magnetic field must be established before current flows from the secondary windings.}

_{If you mean inrush to an electromagnetic motor, the inrush caused by a stalled rotor at start up is easily controlled by the proper fuses, breakers and/or switch gear suggested by many respondents. Unless you know more, the safest path is to have the power supplied by a distribution designed and constructed by a licensed electrician."}

No, you are incorrect about this. All inductive winding devices, both motors AND transformers, have "inrush" current. This is the amount of high current that flows through the windings when the winding wire itself is the only resistance to the flow of power in the circuit as the magnetic flux builds in the core. This can be very very high, as much as 20X FLA, but only lasts for less than a cycle; fractions of a second. Inrush then is in fact specifically referring to what happens BEFORE inductance can take place, so your statement about inductance "delaying" inrush is wrong.

What you are thinking of is technically not "inrush" current but it is commonly misconstrued as such. What you are referring to with regards to AC motors is officially called "starting" current and takes place AFTER the initial inrush current has taken place.

For the OP;

Capblanc's explanaion below is best, although the implementation of a limitng resistor is not something I would suggest that a casual user should try. All transformers have inrush current and it can indeed cause nuisance tripping of protective devices. That is what protective device coordination is all about. If you do not understand this issue, you need the help of a professional Electrical Engineer.

Try it. Switch a voltage pulse across the primary of a transformer with the secondary open or connected to a high impedance. The primary will act like an inductor, the current will start at zero and rise linearly with Vl = L * di/dt or I = V*t/L. The additional current in the primary will be a multiple of the load current. The secondary impedance is reflected to the primary as Rp = Rs/(n^2).

Transformers do not have inrush current.

If the inductance of the transformer is actively carrying current (it has not been reset) and you switch into it again with the same polarity it will immediately step to that current level. This is not a normal way to operate a transformer as it leads toward saturation. A Flyback converter in CCM operation which is not common but sometimes done does Maintain current in the windings (energy in the core) of the transformer (coupled inductor) at all times.

Sorry Mr Guest,

your maths do not explain the FACT that a 63A circuit breaker will trip on connection to a 14kVA transformer even though the output is open circuit? These are not faulty transformers as they draw in the region of 4A to 6A once the field is established.

regards

Chas

Sometimes there is no inrush, sometimes there is a severe inrush. When the input voltage is switched off after a full half cycle of the mains, there is a magnetising flux remaining in the transformer in a given direction. When the mains is switched back on again, if it's at the start of a half cycle which is in the SAME DIRECTION as the one which occurred at switch-off, the transformer will saturate because it tries to add the normal flux on top of the existing flux. Tests will confirm this behaviour. The result is a large pulse of inrush current SOMETIMES, which can trip input protection devices.

Wow, this is nasty and unfortunate that you do not know what inrush is. You really should not be posting.

The inrush current of a transformer is way in excess of the full load current even with the output disconnected. I would like someone with a better grounding in power electrics to confirm this, but I believe that the current that will flow at the instant of switch on is simply V/R where V is the instantaneous voltage and R is the winding resistance. The duration of this current is by its nature less than one half cycle.

These currents do trip the circuit breakers upstream. The solution we use is to connect limiting resistors in series with the windings and a contactor to bridge these out after a second. Care must be taken that the output is unloaded while this takes place.

If you need it I can send a proven diagram

Kind Regards

Chas

Hi, capblanc. Please can I see this diagram. If it can't be posted pls send to apdor@yahoo.com. Thanks a million.

Hi Dorsey,

This is for single phase obviously, but three phase is the same in each live leg. You will note the slow blow 4A fuse seems too low for the current in the limiting resistor. This is deliberate and is set to blow before the resistor burns out if the contactor fails to close properly. The output of the transformer needs to be contactor controlled to make sure that it cannot be loaded at start-up. The resistors do not need to be powerful enough to carry the calculated power but they must accept the current pulse at inrush. You can use standard high power resistors but we prefer to use mains heater elements.

The timer we have set at 1 sec to allow for a "shaky" switch on, i.e. when a shore plug is inserted. You could have this much shorter. You probably will not need the voltmeter for your application.

As JRaef says it would be better to select your input protection breaker to the conditions. We don't have this option as the input breakers in our case are in locked boxes on the dockside and cannot be changed.

Regards

Chas

I think Mr Guest is actually correct. What is confusing here is talk between inrush and magnetizing current. Magnetizing current i believe is around 1% of flc. That is what you have when a transformer is switched into circuit when there is no load on the secondary.