# Electrical model of a transformer

I would like to know what is the inductance (circled in red) of this part of this model? Is the value infinite? And only the inductance (circled in blue) is the value of the inductance of the transformer. So actually when it is written into a datasheet that the inductance is XX µH, the datasheet is actually talking about the magnetizing inductance (circled in blue on the schematic)? Thank you very much.

• It could be, if it says "magnetizing inductance" or they could be talking about leakage inductance. Transformers have both. Your question is kind of unclear, why would the magnetizing inductance be infinite? What does the datasheet actually say? – John D May 16 at 20:57

Let's start from the very beginning:

• X1 is the leakage reactance related to leakage inductance of the primary winding
• X2 is the leakage reactance related to leakage inductance of the secondary winding
• Xm (blue circle) represents the magnetization reactance which is related to magnetization inductance Lm of the transformer

The Lm is related to mutual inductance of a transformer as: $$L_m = M \cdot a^{2}$$ where M is mutual inductance between pair of coils and a is transformer voltage ratio or turn ratio: $$a = \frac{V_1}{V_2}$$

In red, you've marked a so-called Ideal Transformer. This transformer doesn't contribute any additional reactance or resistance. It simply transforms the voltage depending on given turn ratio of primary and secondary windings. If you'd like to, you can re-evaluate X2, R2 and load (capacitors also) so that everything can be placed on the left side of this ideal transformer like shown below. For example to re-evaluate R2 to the side of primary winding, which is usually marked with apostrophe: $$R_{2}' = a^{2} R_{2}$$ • Thank you for your explanation ! :D – Leo May 18 at 7:20

The blue part of the model, in the above diagram, (also Xm) is the magnetizing inductance. It represents the inductance that would separate the real world transformer from an ideal one. If it is a ferrite core with a saturation or hysteresis, the Xm inductor represents that inductance.

The red portion represents the ideal transformer.

We know how to deal with ideal transformers, they are easy, it transfers voltage and current from the primary to the secondary via a ratio of turns. The problem is real world inductors are not ideal, they have leakage inductance, all of the magnetic field does not transfer from the primary to the secondary.

I would imagine that the red portion is what they are talking about when they specify an inductance (but I'd have to see the datasheet)
If there are two numbers, the leakage inductance should be smaller than the transformer inductance by a large amount since I would not expect the leakage inductance to be more than 5% of the total inductance because most transformers have more than a 95% efficiency rating

• To be clear to the OP: so the reason the red portion exists is to model the desirable behaviour of the transformer - that is, ideal current/voltage transfer to the secondary side according to the turns ratio. – Heath Raftery May 16 at 21:11
• Xm is the magnetizing inductance, not the leakage inductance. – Kevin White May 16 at 21:29
• Misread my CRC book, X1 and 2 are leakage – Voltage Spike May 16 at 21:51