Inrush Current 1

[twr2001]

Why is the magnetizing inrush current so high in transformers?

 

[electricpete]

The energization transient creates a decaying dc offset in the excitation current. When the excitation current peaks in the same direction as the dc offset (for instance all positive peaks), the transformer is far into saturation and the exciting current is ineffective... must go higher to match the applied voltage. Maybe easier to explain with equaitons:

Vapplied = N1* d/dt(Phi)
= N1 * dPhi/di * di/dt.
When core enters saturation, we are on the saturated (flat) portion of Phi vs i curve where dPhi/di is very low. di/dt must become very large to compensate.

 

[R22Tailrotor]

I did not know that there was a high inrush current in transformers. Being an iductive device I believe the inrush current is low. NO???

 

[buzzp]

Because it is an inductive device this is why the inrush current is high. To explain it simply, you are essentially applying a voltage across a short. This causes the initial current to be high until energy is stored in the inductor.

 

[electricpete]

Here is a simulation I wrote to model transformer inrush current. The harmonics produced agree fairly well with harmonics seen during actual transformer energization:

http://geocities.com/pschimpf/temporary_stuff/transformer_inrush_simulation.htm

 

[electricpete]

Whoops - if you viewed my link between March 7 and March 8 you say a garbled file... please try again (I fixed it).

 

[Switchable]

How to reduce the Inrush current of the transformer?? If i increase the no of turn in the primary and secondary core, will it reduce the inrush current?? any suggestions to reduce inrush current of transformer?? Thank in advance!!!

 

[1x1x1x]

Swtichable

The good old fashioned way to reduce the inrush current to the transformer, is a resistor in the circuit which is bypassed when the transformer is energized. The inrush current in theory also can be affected by the size of the iron core, the oversized iron core creates less inrush current (this is not a very economical solution either)
The inrush current affected by the following:
the size of the transformer, the resistance of the power system, the type of electric steel is used in the transformer, residual flux of the transformer, and which moment the transformer is energized (what is the residual flux and the system voltage relation at the switching moment).

 

[GusB]

Hold the phone..... if there is no flux in the core, the inductance will limit the inrush current. Edt/L still equals di. Of course, if there is residual flux, then the core could be driven into saturation in which case it would look like a short.

 

[GusB]

Hold the phone..... if there is only residual flux in the core, the inductance will limit the inrush current. Edt/L still equals di.