The BH curve - For the H, what does the 'ampere' in ampere-turns per meter refer to? 2

[Soloten]

In the BH curve - H (the magnetic field strength) is in ampere turns per meter. If I keep the magnetic path length constant, H can be varied by varying the current. However, I want to make sure I understand this correctly - is current on the X axis of the BH curve referring only to the magnetizing branch current or is it the total current. If it is the total current, then an increase in transformer load current increases the H on the X axis as well... which would mean the total flux density on the transformer would increase even if the transformer primary and secondary voltage remain vastly unchanged...which doesn't seem right...this further makes me think that the H on the X axis is referring to the magnetizing branch current...What do you think?

 

[dpc]

The only way current can increase beyond magnetizing current is if current can flow on both windings. Current in the secondary counteracts the flux created by the primary current, so equilibrium is maintained and the net flux remains the same.

 

[Soloten]

Hi dpc, thanks for your reply; my understanding is also the same regarding the net flux on the core. So would you also say that the X axis (H component) of a typical BH characteristic curve is referring to the magnetizing current component only (assuming length and turns are constant), i.e. going from origin to the right shows current on the magnetizing branch increasing -- and not the total current on the primary or secondary circuit which can increase based on loading? Because if the H referred to total current on the circuit, then the B on the core shouldn't increase as shown in a typical BH curve. All this leads me to think that only increasing the primary voltage on the transformer increases the core flux and magnetizing current --and the BH curve supports this. Kindly let me know if I made an error in this assessment.

The reason I'm asking is because I'm reading a bit on transformers and saturation; most of these start with explaining the BH curve, and then go on to show various BH curve characteristics for various materials, gapped vs. ungapped etc. So I need a good understanding of what the X axis represents.

 

[waross]

B is the magnetic field strength in the material under test.
H is the Amp turns per weighted meter.
If your magnetic circuit has an air gap, remember that a meter of air gap is equivalent to about 50,000 meters of transformer iron.
For an open faced magnet, acceptable results may often be obtained by ignoring the iron and just using the air gap.
Even if the iron saturates, the percentage difference on the flux density and on the strength of the magnet may be negligible.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

BH curve refers to peak values of B & magnetizing component of no-load current of transformers. Practically this will not change with loading. But in case of very large transformers, there will be substantial leakage flux from the gap between HV and LV windings, the magnitude varying with load current. This flux will augment the flux density in yoke and reduce in limb part of the core.

 

[fouadf]

This flux will augment the flux density in yoke and reduce in limb part of the core.

Hi prc,for very large transformer with high magnetizing flux density, could this additional leakage flux make the core saturate?

 

[prc]

Yes it can and transformer designers take care of it. There was a CIGRE paper on this subject during 1980s from SMIT engineers Netherlands.