Relationship between transformer and inductor

bigbat

Relationship between transformer and inductor [Copy link]

 

For different ideal transformers, under the same power supply (ideal AC power supply! Same frequency, same voltage, and consistent changing waveform), the larger the inductance, the greater the power of the transformer. It has nothing to do with the transformer ratio. The larger the power frequency transformer, the greater the power than the smaller one, because the larger the capacity of the transformer, the greater the inductance than the smaller one. Here, there is a doubt: if the transformer has a large inductance and the voltage is the same, then the current change rate of the large inductance will be small, that is, the current cannot be very large. Without considering the problem of transformer saturation, just consider the situation of the voltage output change of the power supply. Even if the voltage changes faster, the product of current and time will not be very large due to the influence of inductance, that is, the flux change is small, and the output power of the transformer is also small. That is, the larger the inductance, the greater the inductive reactance, and the larger the inductive reactance, the smaller the current. This is why the frequency of the switching power supply cannot be very high. It is also not possible if the frequency is low, then the current will not change much and the power cannot be output.

cruelfox

Ideal transformer? That would require infinite power.

cruelfox

The magnetic flux caused by the current in the primary and secondary coils of the transformer cancels each other out. When the secondary is open, the transformer only looks at the primary, which is equivalent to an inductor; but when a component such as a resistor is connected to the secondary, the secondary has an induced electromotive force and a current output. If the current in the primary coil remains unchanged, the magnetic flux in the transformer will increase, and the magnetic field will increase: this is of course impossible. In fact, the primary current also increases, canceling out the magnetic flux caused by the secondary current. Looking from the primary of the transformer, it is now equivalent to an inductor connected in parallel with a resistor.

maychang

Here we have a question: if the inductance of the transformer is large and the voltage is the same, then the current change rate will be small with large inductance, that is, the current cannot be very large.

That's right. If the primary inductance of the transformer is large, the no-load current must be relatively small. Note: it is the no-load current.

maychang

Due to the influence of inductance, the product of current and time will not be very large, that is, the flux change is small, so the output power of the transformer will also be small.

When the secondary of the transformer is connected to a load, the change in magnetic flux generated by the secondary current will produce an induced electromotive force in the primary. This induced electromotive force is opposite to the direction of the voltage across the primary, causing the primary current to increase.

The transformer output power is the secondary output.

maychang

Because the transformer with large capacity has a larger inductance than the transformer with small capacity

This requires a prerequisite, which is that the primary rated voltage of the transformer is the same.

maychang

The primary inductance of the transformer is only important in single-ended flyback switching power supplies. For other forms of switching power supplies and power frequency transformers, the inductance mainly depends on the primary voltage and the maximum magnetic flux density of the core. It is a derived parameter, not a parameter that needs to be met during design.

PowerAnts

maychang published on 2022-4-22 18:39 The primary inductance of the transformer is only important in the single-ended flyback switching power supply. For other forms of switching power supplies and power frequency transformers, the inductance mainly depends on the primary...

In active clamp forward, asymmetric half bridge and magnetic integrated LLC, the primary inductance is also important and needs to be calculated and determined.

maychang

PowerAnts posted on 2022-4-22 21:19 In active clamp forward, asymmetric half bridge and magnetic integrated LLC, the primary inductance is also very important and needs to be determined after calculation

The OP has not even figured out how the power frequency transformer works, and does not know where the no-load current comes from, let alone asymmetric half-bridge and LLC. Otherwise, the OP will be even more confused.

maychang

PowerAnts posted on 2022-4-22 21:19 In active clamp forward, asymmetric half bridge and magnetic integrated LLC, the primary inductance is also very important and needs to be determined after calculation

Just like the case where three capacitors are connected in series and parallel and a DC voltage is applied, what is the voltage across the three capacitors? If you can't even come up with an algebraic equation, don't even mention differential equations, let alone partial differential equations.

bigbat

maychang posted on 2022-4-22 22:14 Just like the one who connected three capacitors in series and parallel and applied a DC voltage, he asked, "What is the voltage on the three capacitors?" If you can't even write an algebraic equation, don't even mention differential equations...

I can still understand simple differential equations, but I really don't understand partial differentials except for examples. Thank you. The purpose of asking these questions is to understand what is the most important first design factor of the transformer. Just like wires, other parameters are very important, but resistivity is the first to consider. Other parameters can be calculated from resistivity. Other characteristics are excellent, but it depends on whether it can conduct electricity, just like why wires are not made with ropes. It is economical and environmentally friendly.

I have been thinking these days: Is the inductance the first factor of a transformer? Once the output voltage and power are determined, can the inductance be determined? Then what is the material, volume, number of turns...

maychang

bigbat posted on 2022-4-23 11:01 I can still understand simple differential equations, but I really don’t understand partial differentials except for examples. Thank you. The purpose of asking these questions is to think...

I have been thinking these days: Is the inductance the first factor of a transformer? Once the output voltage and power are determined, can the inductance be determined? Then what is the material, volume, number of turns...

Let's talk about the power frequency transformer first

The first step is to select the material. Power frequency transformers usually use silicon steel core and copper wire, while switching power transformers usually use ferrite core and copper wire. There are other materials to choose from, such as iron powder core. This mainly depends on the requirements for volume, weight and cost.

The second step is to select the core size according to the power.

The third step is to calculate the number of turns based on the core material size, allowable magnetic flux density and primary voltage. The principle is that the magnetic flux density in the core is still within the allowable range under this number of turns.

The fourth step is to determine the cross-sectional area of the primary and secondary wires based on the transmission power and secondary voltage (of course, the copper wire diameter is also determined at the same time).

Until now, the inductance has not been involved. In fact, until this power frequency transformer design is completed, the primary inductance is not considered.

maychang

bigbat posted on 2022-4-23 11:01 I can still understand simple differential equations, but I really don’t understand partial differentials except for examples. Thank you. The purpose of asking these questions is to think...

If you want to know how to design a transformer, you can refer to the "Electronic Transformer Design Manual" compiled by Wang Ruihua et al. However, it is best to understand its working principle before designing. The working principle of the transformer is discussed in books such as "Electrical Engineering".

PowerAnts

maychang posted on 2022-4-22 22:12 The OP has not even figured out how the power frequency transformer works, and does not know where the no-load current comes from, let alone asymmetric half-bridge and LLC. No...

I'm afraid the OP doesn't know much about the flyback transformer.

maychang

PowerAnts posted on 2022-4-23 12:57 I'm afraid the OP doesn't know much about the flyback transformer either

Yes. I am afraid that the basic knowledge of magnetism is not very clear, and I always rely on the circuit. That is why we need to determine the inductance first and then consider the material and number of turns...

bigbat

maychang posted on 2022-4-23 11:46 I have been thinking these days: Is the first factor of the transformer the inductance? If the output voltage and power are determined, can the inductance be determined? Then...

Inductance is a bit like a "flywheel" in a mechanical structure. The speed of a flywheel is very similar to the current in a circuit, and the voltage in a circuit is a bit like the force acting on the flywheel. Even if the force is large, the speed of the flywheel cannot change suddenly, but the acceleration of the flywheel is large. The mass of a flywheel is a bit like the inductance. People with a little common sense know that the flywheel on a tractor cannot be used on a watch. People who design watches may go to the market to find a suitable flywheel for watches, but I think they will definitely not go to a tractor factory to choose a flywheel. Of course, if you insist on doing this and like to wear a tractor flywheel on your hand, it seems that there is no law that can prohibit you. Vice versa, tractors will not use flywheels on watches, although watch flywheels and tractor flywheels look similar in shape and have the same function.

Why? Because the design principle in this example is based on the laws of physics. Objects with greater mass have greater kinetic energy at the same speed.

Back to the transformer, is it true that the capacity of the transformer/(converter) is determined by the inductance of the transformer when other conditions are ignored? Is the boss's method based on engineering practice, first selecting the material, because the material and its volume can determine the capacity or power of the transformer? Is the principle: the maximum inductance of the transformer can be determined at this time, although this is not rigorous, because factors such as the number of turns of the coil are also necessary. However, the transformer in the charger should not be able to produce the power supply transformer used in the community.

Boss, please don't be angry. Please read my description first before you start scolding me.

The method you recommended is the one I despise the most. Although this solution is feasible, it is very confusing for me to understand it in depth. When it comes to transformer design, it is all about empirical formulas. What experience is there? It's like a primary school student asking an idiot teacher a question, "Just remember it...blah blah blah." I have a low level, but I have the courage to improve! I ask these questions not to actually design a transformer, but to understand the deeper principles in this area. Otherwise, you can just copy it.

maychang

bigbat posted on 2022-4-23 14:36 Inductance is a bit like a "flywheel" in a mechanical structure. The speed of the flywheel is very similar to the current in the circuit, and the voltage in the circuit is somewhat like the effect on the flywheel...

Inductance is a bit like the "flywheel" in mechanical structure. The speed of the flywheel is very similar to the current in the circuit. The voltage in the circuit is somewhat like the force acting on the flywheel. Even if the force is very large, the speed of the flywheel cannot change suddenly. It's just that the acceleration of the flywheel is large.

It seems that the problem is not the lack of knowledge of magnetism, but the lack of knowledge of mechanics.

It is possible to compare the voltage applied to the inductor to force and the current in the inductor to speed. However, the speed cannot change suddenly, which is not the case with "the acceleration of the flywheel is large".

maychang

bigbat posted on 2022-4-23 14:36 Inductance is a bit like a "flywheel" in a mechanical structure. The speed of the flywheel is very similar to the current in the circuit, and the voltage in the circuit is somewhat like the effect on the flywheel...

"It looks like the watch flywheel and the tractor flywheel are similar in shape and function."

The shapes are indeed similar, but the functions are definitely different.

maychang

bigbat posted on 2022-4-23 14:36 Inductance is a bit like a "flywheel" in a mechanical structure. The speed of the flywheel is very similar to the current in the circuit, and the voltage in the circuit is somewhat like the effect on the flywheel...

"Is it true that, ignoring other conditions, the capacity of a transformer/(converter) is determined by the inductance of the transformer?"

No. Transformer capacity has nothing to do with inductance.

maychang

bigbat posted on 2022-4-23 14:36 Inductance is a bit like a "flywheel" in a mechanical structure. The speed of the flywheel is very similar to the current in the circuit, and the voltage in the circuit is somewhat like the effect on the flywheel...

"Isn't the principle: the maximum inductance of the transformer can be determined at this time?"

No. The transformer primary inductance does not need to be determined, because the transformer capacity has nothing to do with the primary inductance. The larger the primary inductance, the larger the transformer capacity.