Relays and AC contactors look very similar. They both have coils and electromagnetic systems, and both have contacts or contacts. But the relay does not have an arc extinguishing system. What does this mean?
The relationship between relays and contactors is quite deep, and they belong to the category of electromagnetic relays.
We know that relays have voltage relays and current relays, and voltage relays are divided into over-voltage relays and under-voltage relays. The contactor is similar to a voltage relay.
So what is the essential difference between the two? It is the scope and occasion of application.
The contactor is used in the switching and control of electric energy, and it works in the main circuit of the distribution network. The so-called main loop refers to the loop for power transmission and control.
Since the current in the main circuit is very large, the contactor generally needs to be matched with an arc extinguishing device.
The relay is used for the amplification, transmission, control, etc. of the control signal, and it works in the auxiliary loop. The so-called auxiliary loop refers to the control loop.
It can be seen that the applications of relays and AC contactors are completely different, and the working characteristics of the two are of course different.
It can be seen that the contactors and relays are completely different in terms of their operating characteristics and applications.
Below, I use the length of a few articles to analyze the similarities and differences between contactors and relays.
One of the differences: contacts and contacts
The contact names of relays and contactors are different. For relays, we still use the term contact, but contactors must use the term contact.
In fact, for all main circuit components working in the main circuit, their contact points are contacts. For example, circuit breaker contacts, isolation switch contacts, contactor contacts, and so on.
Correspondingly, we will see various relay contacts in the auxiliary circuit, such as intermediate relay contacts, time relay contacts, voltage relay contacts, and so on. Sometimes, the auxiliary contacts of the circuit breaker and the auxiliary contacts of the contactor are also seen.
So what is the essential difference between contacts and contacts?
1. Different touch point characteristics
Contacts, as the name suggests, are point contacts.
The characteristic of point contact is that the contact pressure is relatively large, but the load capacity is relatively weak, and large current cannot flow.
In addition to the point contact form, the contact has a line contact form and a surface contact form.
Among the three contact forms, the line contact is self-cleaning, because in the contact process, the line contact has a sliding process, which will wear off the oxide layer on the contact, so the line contact has a better load capacity .
Surface contact has a large current, but its contact pressure is low, so it is necessary to clean the contact surface frequently.
Why does contact pressure have such a big influence on electrical contact? This is related to the repulsive force characteristics of electrical contacts.
2. Repulsion characteristics
The repulsion characteristics are related to a famous engineer, who is the electrical engineer of Siemens Home.
Holm, he is not even found on Baidu Encyclopedia. However, his contribution in the field of electrical contact is very impressive, and he is the originator of electrical contact theory. Although Holm is no longer there, every year the Holm Electric Contact Annual Conference is held in the world and hundreds of latest developments are released. Our country has also participated in the Hom Electrical Contact Organization.
In Figure 3, the left picture is the electrical contact, the bottom is the static contact, and the top is the moving contact. We see that since there is only one contact point, the current lines are distributed inside the contact as shown in the figure.
Pay attention to the current line Ijr on the right side of the static contact, and use the right-hand screw rule to determine the direction of the magnetic field lines it produces, as shown in the right figure. We can see that the magnetic field line on the left side leaves the paper, and the right side enters the paper.
Look at the current line Idr on the right side of the moving contact, and use the left hand rule to judge the direction of its electric power F, as shown in the right picture. We decompose F into the vertical Y direction and the horizontal X direction. We find that the horizontal electric power Fx will passively offset the horizontal electric power generated by the current line Idl on the left side of the contact, but the vertical electric power will be Overlay.
In this way, we can see that the moving contact receives an upward repulsive force Fhy. In the same way, the static contact is also subjected to a downward electric repulsion Fhy'. Fhy and Fhy' are collectively called the Holm repulsion of the contacts. The role of Holm's repulsion is to repel the contacts, and the greater the current, the greater the repulsion.
The apparent area of the contact here refers to the area where the bottom of the contact meets the conductive rod, and the contact area is the area where the dynamic and static contacts contact.
Generally, no matter how big the contact is, the number of their contacts will not exceed 3, and the radius of the contact point is less than 0.5mm, generally about 0.2mm. It can be seen how severe the heating of the contacts at the contact points can be.
Let's take a look at it through an example.
Suppose, the rated current of a certain AC contactor is 100A, its contact area is 5mmX8mm, and the contact radius is 0.5mm. Knowing that its dynamic and static contact pressure is 35N, try to find its home repulsion.
We can substitute the data into the formula:
We see that this value is so small that it is not worth mentioning for the contact pressure.
But now there is a short circuit at the load of the contactor, we assume that the short circuit current is 10kA, what will happen? Let's substitute in the data to calculate:
We see that the Holm repulsion of the contact is now greater than the contact pressure.
But what impact will this have?
The impact is huge: because the contacts are repelled, an arc will appear at the contact gap and ablate the contacts. At the same time, once the contacts are repelled, only the arc current flows between the contacts, and its value is small, so the contacts will reclose under the pressure of the contacts and then repel. After such a few times, the contact material will be welded.
This phenomenon is called the dynamic stability of the electrical contact, and it is related to the Holm repulsion of the contact.
Due to the occurrence of a short circuit, the circuit breaker will perform the operation of breaking the circuit. However, it takes time for the circuit breaker to open, so during this period of time, there is a mutual cooperation relationship between the contactor and the circuit breaker.
This coordination relationship is defined as a short-circuit coordination relationship in the national standard GB14048.4-2008 "Low-Voltage Switching Appliances and Control Appliances Part 4: Contactors and Motor Protectors...". Among them, type 1 (type1) contactor contacts will be welded, and type 2 (type2) contactor contacts will not be welded. Therefore, be sure to pay attention to the short-circuit coordination relationship between the contactor and the circuit breaker when selecting the type.
