Design principle of power frequency transformer

The principle of power frequency transformer is very simple, and the related formulas are not complicated in theory, so everyone has formed a view: it is too simple, and there is nothing to study in those three or four formulas. As long as the design is based on those simple formulas, it will be successful immediately.

I think the above understanding has both merits and merits. The advantages are: according to the calculation formula, the result can be calculated quickly and the problem can be solved; What is worth studying is: Do you know the performance of the product you designed? Is the design reasonable? Is the design optimized? What is economy?

For example, selecting iron core specifications according to power is a very complicated problem because there are many factors involved. Some books recommend using the following semi-empirical formula to choose:

S = K Sqrt(P) ( 1)

After s is determined, other calculations are performed. This is indeed a practical method, but it is also a simplified design method, which is wasteful in most cases. This design method does not need to be discussed for amateurs (only occasionally designing a transformer for their own use), but it is worth discussing for enterprises. When this design is widely used in products, it reflects the reduction of economic benefits.

So, what can they do in professional occasions, such as transformer manufacturers?

In principle, the core specification is selected according to the occupancy rate of conductors in the window, but this calculation is complicated and the relationship is not simple. For example, the relevant calculation formula is:

P = 0.0222 f B J Sc Sm (2)

When the current density is determined by the voltage regulation rate, the calculation formula is:

p = 0.0555(f f)(B B)(Sc Sc)SmδU/(Z Lm)(3)

For such a complicated relationship, it is a headache to come up with the design scheme manually. Therefore, experts compile these relationships into a series of tables according to the actual situation, and design engineers can get a set of practical data by looking up the corresponding tables according to different design indicators, such as power and other indicators to get the core specifications.

The convenience brought by this method is so great that we can come up with some schemes quickly, and then make appropriate judgments and make optimization according to the accumulation of experience. Even so, this method has two major disadvantages. One is that it is impossible for experts to make all the series tables, and the other is that engineers have to come up with relatively complete designed product parameters and spend a lot of time calculating them one by one. Engineers must have a big head to come up with several schemes a day. Fortunately, with the popularization of computer application and the appearance of special software, computers can do boring calculation work, and engineers only pay attention to the design and optimization of the scheme.

For another example, for the selection of current density, it is generally recommended to be 2.5a ~ 3a/(mm mm) in simple design, but according to formula (3), when the low power adjustment rate is large, the current density can be very large, reaching 8 ~ 9a/(mm mm) [for the chat of low-frequency EI transformer, see this column, and I have provided the landlord with six schemes]. However, when the high power regulation rate is limited, the current density can be less than 2A/(mm mm), otherwise the temperature rise of the transformer may be too high and burn the transformer. It should be emphasized that the current density can not be increased casually, but calculated according to the design index, which will often lead to the deterioration of performance or even burn the transformer.

Some people may think: Why haggle over every ounce? I don't think so. Everyone is talking about how foreign economic growth can save energy and raw materials. Isn't that dug up bit by bit? Where does the efficiency of switching power supply come from? Isn't it also developed bit by bit? In this case, if you are really engaged in the production and design of power frequency transformers, you must conscientiously do your own job and find out the relevant contents of your work. Imagine that an enterprise needs all kinds of transformers for its products and wants to recruit an engineer to design them. Give a few examples of on-site design. The design results of engineers A and B meet the requirements, but the average cost of A is lower than that of B 10%. Who do you think the enterprise will hire?

From the perspective of saving energy and raw materials, I would like to share with you here:

1, reducing the amount of copper can be achieved in two ways. One is to reduce the wire diameter, which means that copper resistance will increase and copper loss will increase. The other is to reduce the number of turns, which will increase the no-load current and the no-load loss. If the transformer is in standby state for a long time, the waste of power resources is very great. Billions of dollars of electricity are wasted every year due to the long-term standby state of household appliances in China.

2. When designing the transformer, the copper loss and iron loss should be equal, so that the transformer loss is the lowest and the operation is the most stable. If a transformer is designed, because the wire diameter is very small, in order to save copper wire, the method of reducing the number of turns is adopted, and there is still a lot of room in the iron core window, which means that the size of the iron core is too large, resulting in the waste of the iron core. Due to the large size of wire core, the average circumference of winding grows, and the amount of copper wire will also increase. At the current price,

Therefore, when designing, try to choose a small iron core while ensuring the performance to meet the customer's requirements. If 4 1 can be used, then never use 48. Regarding no-load current, it is best to save standby loss as much as possible.