Relay use common sense

Relay use common sense

1. To choose the right type

To use the relay well, it is very important to choose the correct type. First, you must have a thorough understanding of the nature, characteristics and application requirements of the controlled object, and carefully consider it. The principle, use, technical parameters, structural features, specifications and models of the selected relays should be mastered and analyzed. On this basis, the relay should be correctly selected according to the actual conditions of the project and the specific conditions.

2. Understanding of the junction

The static and dynamic contacts that are in the off state when the relay coil is not energized are called "normally open contacts", otherwise, they are called "normally closed contacts". A moving contact is normally closed with one static contact and normally open with another, and they are called "switching contacts". In the same relay, there may be one or several pairs of normally open contacts or normally closed contacts (both may also have), or a set or array of conversion contacts.

3. Method of eliminating contact sparks

Since the current of the relay contact is small, there is no arc between the contacts, but there will be a "spark discharge". This is because there is an inductance in the contact circuit, and an overvoltage occurs in the inductor when it is disconnected. They are added together on the contact gap, so that the contact gap just separated by a certain distance is broken down and discharged. Due to the limitation of energy, only spark discharge is generated, and the alternating energy between the capacitor and the inductor in the contact makes the spark discharge appear in time, and becomes a high-frequency signal, and the spark discharge is also damaged. , but will reduce the service life, so must be managed to eliminate, there are two practical spark-eliminating circuits, as shown in Figure 1. 1. The basic principle of action is to make the energy in the inductor pass through rc without passing through the contact; 2. When it is disconnected, it is consumed by the diode v on the load rl. Just choose one of the apps. However, it should be noted that the rc parameter should be selected appropriately, and the parameters are mainly determined by experiments. Usually, the capacitance c can be selected by the load current 1a/1 micro method. When using a diode, its positive and negative polarity should be connected correctly.

4. Method of increasing contact load

In use, if the load capacity of the contact can not meet the requirements of use, it can be solved by adopting several pairs of contacts in parallel. However, adjustment should be made before use to make the synchronization of the contacts meet the requirements, otherwise it is counterproductive. The best method is to use an intermediate relay or contactor to increase the load capacity of the contacts.

5. Solution when the return factor is not satisfactory

The return coefficient kf is a parameter reflecting the degree of cooperation between the suction characteristic and the reaction force characteristic, that is, the difference between the action value and the release value of the relay is characterized. Relays for different purposes often require different return factors. When the return factor of the relay does not meet the requirements of use, the circuit shown in Figure 2 can be used to improve the return factor. Figure 2a shows the circuit for reducing kf, while Figure 2b shows the circuit for increasing kf. The value of r in Figure 2 should be appropriate, that is, after stringing into r, the voltage applied to the relay coil must be greater than its operating voltage (Figure 2a) or holding voltage (Figure 2b).

6. Suction release time does not meet the required improvement

When the pull-in and release time of the relay cannot meet the requirements of use, the time constant of the relay coil circuit can be changed to solve the problem. We know that the time constant t of the relay coil is equal to the ratio of the coil inductance l to the resistance r. If a resistor rf is inserted in the relay coil circuit, t2 (t2 = l / r + rf) is less than t1 (t1 = l / r).

When accelerating the pull-in, a resistor rf is inserted in the relay coil circuit, and the power supply voltage is appropriately increased to ensure that the pull-in current of the coil remains unchanged, so that the purpose of accelerating the suction can be achieved. If a capacitor c is connected in parallel across r, the pull-in time is shorter as shown in Figure 3a.

The reverse parallel connection of a diode at both ends of the relay coil can achieve the purpose of delaying the pull-in, see Figure 3b. Its principle is that after the relay coil is de-energized, it is equivalent to adding a short-circuit coil to the iron core, which prolongs the release time.

7. Correctly select the alarm action status of the relay

Generally, instruments, dcs, and inverters with alarm and interlock functions are indispensable to use relays, that is, alarms and interlocks are mostly connected through relay contacts and alarms and interlock circuits. Is it good to make the relay coil "charged" or "de-energized" when the alarm is issued? We analyze the advantages and disadvantages of the "charged" and "power loss" states from the reliability.

The relay coil is "charged" and the action causes the circuit to alarm. This is the most easily understood design, but there is a hidden danger, that is, when the relevant wiring is not connected and there is an open circuit, or when the relay coil power supply has a problem, an accident is required. When the alarm occurs, the relay coil should be "charged" and act, but the above-mentioned reasons do not operate and make mistakes, the consequences are very serious.

If you change to the "power loss" alarm, once the instrument wiring is not connected or open, the relay coil power supply problem, or the instrument failure, there will be no possibility of misreporting. The reason is that when there is no alarm, the relay coil is in the "charged" state. Once the above abnormal phenomenon occurs, the relay coil will return to the "power loss" state, and the operation and maintenance personnel will find the cause of the alarm because of the "alarm". When the signal is found to be normal and the alarm is issued, it will find other causes and eliminate the fault, so that the alarm circuit returns to normal, thus avoiding the occurrence of false alarms. Obviously, the "power loss" alarm is more reliable than the "live" alarm.