How does the power outlet transmit electricity? Simple problems that have plagued my little friend for so many years

How the power outlet transmits electricity ？ A simple problem that has plagued my little friend for so many years

Let's start with the answer ： Electric energy transmission between plugs and sockets depends on electrical contact .

Let me give the subject some knowledge about electrical contact .

1. Let's start with science popularization

Let's look at the picture below ：

chart 1： Inside of plugs and sockets

When the plug is inserted into the socket , The insert of the plug is in contact with the elastic contact copper strip of the socket , And friction , Then remove the oxide layer on the insert , Achieve electrical contact . so , Electric energy is transmitted between the plug and socket through electrical contact .

We know from high school chemistry , Electrical contact metal materials are exposed to air , Their surfaces are oxidizable , Once oxidized , The contact resistance will increase . Besides , Dust cover can also produce a dust film , There are also inorganic and organic membranes . These films are collectively called surface facial mask .

From the conductivity of the film , There are mainly two types of surface facial mask with completely different properties . One is insulating film , The resistivity of this kind of film is very large , about . The other is conductive film , The resistivity of such films is in the order of magnitude , The thickness is in the order of magnitude , Electronic borrowing “ Tunnel effect ” Conduct electricity through a thin film , This type of membrane is composed of “ The adsorption ” The effect produces , So it is also called adsorption membrane .

Common plugs and sockets use copper as conductive material . The resistivity of copper oxide is very high , For example, the resistivity of oxide film can reach , And as the temperature rises , The thickness of the oxide film increases rapidly , The contact resistance is increased thousands of times . So for copper - Electrical contact of copper , The contact pressure must be increased F, Crush the oxide film , To reduce the contact resistance .

Let's look at the picture below ：

chart 2： The three elements of electrical contact

Let's look at the picture carefully 2, When the plug piece or rod of the plug is pushed into the socket , There is a friction process between the inserting piece or the inserting rod and the elastic electric contact elastic piece of the socket . meanwhile , Elastic electrical contact with the shrapnel exerts a force F Acting on the inserting piece or rod , Use friction to grind the oxide layer . There are three elements in this process , One is the material coefficient of electrical contact K, Different materials have different K value ; The second is contact pressure F, The third is the form of contact . In a general way , If the electrical contact is point contact ,m=0.5; If the electrical contact is wire contact ,m=0.5 To 0.8, The general values m=0.7; If the electrical contact is face contact , be m=1.

We look at the material K value , as follows ：

chart 3： Of materials K value

chart 3 in , We see copper - Copper contact K=100, silver - silver （ Or nickel plated copper - Nickel plated copper ）K=60, Tinned copper - Tinned copper K=100, The biggest is brass , brass - Brass K=670, Than copper - Copper K It's worth it 6.7 times ！

Let's look at the empirical formula for calculating contact resistance , as follows ：

, type 1

Attention style 1 The unit of , yes μΩ, That is, micro Europe ！

We set the contact pressure between the elastic contact copper strip of the socket and the plug insert as 20N, The pressure of almost two bottles of mineral water . Because it is wire contact , take m=0.7. Because it's copper - Copper electrical contact , So the material K The value of the coefficient is 100. We substitute these values into the equation 1 in , Look at the contact resistance ：

Notice the graph 2 There are two sides of the contact surface of the middle inserting piece or inserting rod illustrated spring piece , The contact resistance is in parallel , The actual contact resistance is only 60.70 Half of micro Europe . We set the rated current of the plug socket combination to be 10A, Thus, the contact voltage generated by the electrical contact is ：

almost 0.3 Millivolts .

Knowing the use of contact voltage ？ We can calculate the operating temperature rise or operating temperature of the plug socket combination . The calculation method is as follows ：

We see first 10A Plug size of , See the picture below ：

chart 4： Size of plug insert

We set the comprehensive heat dissipation coefficient of this plug , The ambient temperature is 25℃, The current flowing is full load 10A. The section perimeter of the plug insert , Sectional area of insert , Then the temperature rise produced by a single plug τ by ：

Temperature rise between plug and socket electrical contact spring τj The formula of is ：, there Uj Contact voltage ,L Is the Lorentz coefficient , Its value is ;T Is the absolute temperature of the insert , Its value is ：

.

Substitute into τj In the calculation formula of ：

We get the result ：

The temperature of the plug insert is ：

The temperature of the elastic electrical contact spring of the socket is ：

We see , The temperature of a single plug in the plug and socket at full load is only higher than the ambient temperature 1.4℃ and 1.6℃. in consideration of 2 The root inserts work at the same time , Its temperature rise will not exceed 3℃.

2. Discuss the answers to the questions

Power outlets rely on electrical contact to transmit electrical energy . The transmission of electric energy will inevitably bring about the temperature rise of the plug and socket combination , If the temperature rise value under full load condition conforms to the maximum value specified in the technical parameters , The plug and socket assembly works stably when transmitting electric energy .

We can see from this , It must be ensured that the contact resistance of the plug socket combination is small . So , For long-term working plug socket combination , For example, the socket of the refrigerator , Need regular （ About three months to six months ） Unplug and plug the refrigerator plug several times after shutdown , To remove the oxide layer on the plug insert and socket spring , Only in this way can the reliability of the plug socket combination be ensured .

in addition , The socket has been used for a long time , The elastic pressure will drop , The operating temperature of the socket will rise , In this case, it is necessary to replace the socket .

Come again , Under no circumstances should we use brass as the material for plugs and sockets .

Once I went to a hardware store to buy a plug , The merchant told me that the plug was made of brass , I looked at the real thing , It's just a copper coating , But I'm not sure . So , I asked the merchant to change another plug for me . Businessmen don't understand , I told her the worst thing was the brass socket , Contact resistance N Big , Only then did she realize .

Electrical contact theory is one of the five theories of electrical technology . These five theories are ： Heating theory of electrical appliances , Electrical contact theory of electrical appliances , Electrodynamic theory of electrical apparatus , Electric arc theory of electric apparatus and electromagnetic system theory of electric apparatus . There are also books to read N many , For example, hexianyan's 《 Electrics 》 The third edition , Although it is a college textbook , But relatively simple , It can be used as a reading material for electrical workers . in addition ,《 Technical Manual of high and low voltage electrical apparatus 》 This book is more detailed , But it is obviously not suitable for ordinary readers .

I wrote this document for nearly an hour , Writing speed is OK ？！

I'm going to do morning exercises , I'll just leave it there .

I heard my mobile phone ringing during morning exercise and running , It turned out that a group friend read this post , She asked me ： The temperature of my socket is very high , What should I do ？

My answer is ： If this socket has been used for a long time , It indicates that the internal contact pressure has been reduced , Cause contact resistance to increase and heat . Recommend replacing .

The group friend replied , This socket has been used for more than ten years , The decoration has never been changed . She said , Next to the other socket , Last year there was a fire . It seems , Ask an electrician to replace the sockets in the house in batches .

Another group friend said that his company's GGD The switch cabinet uses aluminum busbar , The lower branch bus is copper bar , The temperature of the lap joint is very high , Ask what to do ？

I told him , Look at this post 3 Data in , copper - Aluminium K The value is 980, Than copper - The copper is big 10 times . therefore , It is necessary to replace the branch bus with aluminum bar .

From the interaction of group friends, we can see , The contact pressure of the electrical contact inside the socket is very important , The surface oxidation condition of electrical contact materials is also very important , If the contact pressure drops or the electrical contact material is seriously oxidized , The temperature rise of the socket is inevitable .