Transformer Saturation Curve - magnetizing current

[tiftif]

I got the transformer saturation curve from the manufacturer. It shows the excitation current vs the voltage.

Is this current pure magnetizing current or it includes copper & eddy losses? Is this the line current during the test or it is the magnetizing only current?

 

[prc]

Depends- if you got actual measured excitation current vs voltage, it is a combination of lagging magnetization component + in line component to meet eddy, hysteresis plus resistive loss + leading capacitive current from charging of capacitances between windings and winding to earth.

 

[HamburgerHelper]

If your secondary is open ended, your line current will be the excitation current plus the core losses. When you are saturating, your excitation current will be much larger than your core losses.

 

[waross]

"If your secondary is open ended, your line current will be the excitation current plus the core losses."
plus the resistive or copper losses
"When you are saturating, your excitation current will be much larger than your core losses."
When saturated, there is very little further increase in the magnetizing component of the excitation current, however the increase in resistive or copper losses becomes close to linear with respect to voltage increase.
Note the magnetizing current is one component of the excitation current.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Let's back up a bit. The excitation test is performed on the CT secondary with the primary open. Perhaps the subscripts should be switched in the diagram above.

And if the magnetizing current doesn't increase and the losses are linear, to what do you attribute the non-linear current shown in the excitation curve above the knee?

 

[HamburgerHelper]

Waross,

The inductor Xm is non-linear. I should maybe have copied a model diagram that had an arrow crossed through it to show this. When the core saturates it moves into another region that is linear. You can have nice looking waveforms if you super saturate a transformer. They won't match the turns ratio but they won't have the distortion we are used to seeing due to saturation.

 

[prc]

In the normal working range of induction (<1.7T),the capacitive charging current part plays a major role in the total excitation current of HV transformers. Sixty years back, power transformer no-load power factor was 0.05-0.25.(Transformer Engineering-L.F.Blume,1951)But today it is 0.7-0.8. This improvement came by two routes-capacitive current became prominent as excitation and loss component came down,thanks to the dramatic improvements in transformer steel and core manufacturing methods. Earlier exciting kVA was 5-20 times the loss kW, but today it is hardly two times the loss kW.