1. To correctly select
To make good use of relays, correct selection of models is very important. First, we must have a thorough understanding of the nature, characteristics, and use requirements of the controlled objects and consider them carefully. The principles, applications, technical parameters, structural features, and specifications of selected relays must be mastered and analyzed. On this basis, the relay should be correctly selected according to the actual conditions and specific conditions of the project.
2. The understanding of the joint
When the relay coil is not energized, the static and dynamic contacts in the disconnected state are called “normally open contactsâ€; otherwise, they are called “normally closed contactsâ€. A moving contact is normally closed with one static contact and is normally open with another static contact, and they are called "changeover contacts". In the same relay, there can be one or more pairs of normally open contacts or normally closed contacts (both can also be present at the same time), and there can also be one set or array changeover contacts.
3. Elimination of contact sparks
Because the current of the relay contact is small, there will be no arc between the contacts, but there will be “spark dischargeâ€. This is due to the presence of inductance in the contact circuit, and the overvoltage will appear on the inductor when it is disconnected. It is connected to the power supply voltage. They are added together on the contact gap, and the contact gap just separated by a certain distance is broken down and discharged. Due to the limited energy, only spark discharge will occur. The alternating capacitance between the capacitor and the energy in the inductor will make the spark discharge appear hidden and become a high-frequency signal, and the spark discharge joint will also cause damage. , And will reduce the service life, so we must try to eliminate, there are two kinds of practical anti-sparking circuit, as shown in Figure 1. 1. Its basic principle of action is to make the energy in the inductor pass through the rc without passing through the contact; 2. it is consumed on the load rl through the diode v when it is disconnected. Choose one of the applications. However, it should be noted that the rc parameter should be selected appropriately, and the parameter is mainly determined by experiments. Usually, the capacitor c can be selected according to the load current 1a/1 micro method. When using a diode, its positive and negative polarities should be connected correctly.
4. Increase the contact load method
In use, if the load capacity of the contact can not meet the requirements of the use, several pairs of contacts can be used to solve in parallel. However, adjustments should be made before use so that the synchronization of the contacts can meet the requirements. Otherwise, it will be counterproductive. The best method is to use intermediate relays or contactors to increase the load capacity of the contacts.
5. When the return coefficient is not satisfactory
The so-called return coefficient kf is a parameter that reflects the degree of cooperation between the suction force characteristics and the reaction force characteristics, that is, the difference between the relay action value and the release value. Relays of different uses often require different return coefficients. When the return coefficient of the relay cannot meet the requirements for use, the circuit shown in Figure 2 can be used to improve the return coefficient. Figure 2a is a circuit that reduces kf, while Figure 2b is a circuit that increases kf. The value of r in Fig. 2 should be appropriate, that is to say that the voltage applied to the relay coil must still be greater than the operating voltage (Fig. 2a) or the holding voltage (Fig. 2b) after the series r.
6. Pull release time does not meet the required improvement
When the relay's pull-in and release time can not meet the use requirements, the time constant of the relay coil loop can be changed to solve it. 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 series-connected in the relay coil loop, t2 (t2=l/r+rf) is smaller than t1 (t1=l/r).
When it is necessary to accelerate the pull-in, a resistance rf is connected in series in the coil circuit of the relay, and the power supply voltage is appropriately increased to ensure that the pull-in current of the coil remains unchanged, and the purpose of acceleration and pull-in can be achieved. If a capacitor c is connected in parallel across r, the pull-in time is shorter as shown in Figure 3a.
A diode is connected in parallel across the relay coil to achieve the purpose of delayed pull-in, see Figure 3b. Its principle is that after the relay coil is de-energized, it is equal to adding a short-circuit coil to the iron core, so that the release time is extended.
7. Correct selection of the relay's alarm action status
In general, alarms and interlocking instruments, dcs, and frequency converters are indispensable for the use of relays, that is, alarms and interlocks are mostly connected through relay contacts and alarms and interlocking circuits. When the alarm is to make the relay coil "charged" or in the "de-energized" state? We start from the reliability to analyze the "charged" and "lost power" state of the advantages and disadvantages.
The relay coil is "live" and operates to make the circuit alarm. This is the most easily understood design, but there is a hidden danger, that is, when the relevant wiring is not connected and an open circuit occurs, or if the relay coil power supply has a problem, the accident needs to occur. When the alarm occurs, the relay coil should be "live" but it will fail because of the above reasons. This result is very serious.
If the alarm is changed to “de-energizedâ€, the meter wire will not be connected or open, the relay coil power supply problem, or the instrument failure. The reason is that when there is no alarm, the relay coil is in the "live" state. Once the above abnormal phenomenon occurs, the relay coil will return to the state of "de-energized". The operator and maintenance personnel will find out the cause of the alarm because of "alarm". , When the signal is found to be normal and alarm, it will go to find other reasons, and eliminate the fault, so that the alarm circuit back to normal, so as to avoid the occurrence of the phenomenon of missing, obviously "lost power" alarm is more reliable than "live" alarm.
To make good use of relays, correct selection of models is very important. First, we must have a thorough understanding of the nature, characteristics, and use requirements of the controlled objects and consider them carefully. The principles, applications, technical parameters, structural features, and specifications of selected relays must be mastered and analyzed. On this basis, the relay should be correctly selected according to the actual conditions and specific conditions of the project.
2. The understanding of the joint
When the relay coil is not energized, the static and dynamic contacts in the disconnected state are called “normally open contactsâ€; otherwise, they are called “normally closed contactsâ€. A moving contact is normally closed with one static contact and is normally open with another static contact, and they are called "changeover contacts". In the same relay, there can be one or more pairs of normally open contacts or normally closed contacts (both can also be present at the same time), and there can also be one set or array changeover contacts.
3. Elimination of contact sparks
Because the current of the relay contact is small, there will be no arc between the contacts, but there will be “spark dischargeâ€. This is due to the presence of inductance in the contact circuit, and the overvoltage will appear on the inductor when it is disconnected. It is connected to the power supply voltage. They are added together on the contact gap, and the contact gap just separated by a certain distance is broken down and discharged. Due to the limited energy, only spark discharge will occur. The alternating capacitance between the capacitor and the energy in the inductor will make the spark discharge appear hidden and become a high-frequency signal, and the spark discharge joint will also cause damage. , And will reduce the service life, so we must try to eliminate, there are two kinds of practical anti-sparking circuit, as shown in Figure 1. 1. Its basic principle of action is to make the energy in the inductor pass through the rc without passing through the contact; 2. it is consumed on the load rl through the diode v when it is disconnected. Choose one of the applications. However, it should be noted that the rc parameter should be selected appropriately, and the parameter is mainly determined by experiments. Usually, the capacitor c can be selected according to the load current 1a/1 micro method. When using a diode, its positive and negative polarities should be connected correctly.
4. Increase the contact load method
In use, if the load capacity of the contact can not meet the requirements of the use, several pairs of contacts can be used to solve in parallel. However, adjustments should be made before use so that the synchronization of the contacts can meet the requirements. Otherwise, it will be counterproductive. The best method is to use intermediate relays or contactors to increase the load capacity of the contacts.
5. When the return coefficient is not satisfactory
The so-called return coefficient kf is a parameter that reflects the degree of cooperation between the suction force characteristics and the reaction force characteristics, that is, the difference between the relay action value and the release value. Relays of different uses often require different return coefficients. When the return coefficient of the relay cannot meet the requirements for use, the circuit shown in Figure 2 can be used to improve the return coefficient. Figure 2a is a circuit that reduces kf, while Figure 2b is a circuit that increases kf. The value of r in Fig. 2 should be appropriate, that is to say that the voltage applied to the relay coil must still be greater than the operating voltage (Fig. 2a) or the holding voltage (Fig. 2b) after the series r.
6. Pull release time does not meet the required improvement
When the relay's pull-in and release time can not meet the use requirements, the time constant of the relay coil loop can be changed to solve it. 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 series-connected in the relay coil loop, t2 (t2=l/r+rf) is smaller than t1 (t1=l/r).
When it is necessary to accelerate the pull-in, a resistance rf is connected in series in the coil circuit of the relay, and the power supply voltage is appropriately increased to ensure that the pull-in current of the coil remains unchanged, and the purpose of acceleration and pull-in can be achieved. If a capacitor c is connected in parallel across r, the pull-in time is shorter as shown in Figure 3a.
A diode is connected in parallel across the relay coil to achieve the purpose of delayed pull-in, see Figure 3b. Its principle is that after the relay coil is de-energized, it is equal to adding a short-circuit coil to the iron core, so that the release time is extended.
7. Correct selection of the relay's alarm action status
In general, alarms and interlocking instruments, dcs, and frequency converters are indispensable for the use of relays, that is, alarms and interlocks are mostly connected through relay contacts and alarms and interlocking circuits. When the alarm is to make the relay coil "charged" or in the "de-energized" state? We start from the reliability to analyze the "charged" and "lost power" state of the advantages and disadvantages.
The relay coil is "live" and operates to make the circuit alarm. This is the most easily understood design, but there is a hidden danger, that is, when the relevant wiring is not connected and an open circuit occurs, or if the relay coil power supply has a problem, the accident needs to occur. When the alarm occurs, the relay coil should be "live" but it will fail because of the above reasons. This result is very serious.
If the alarm is changed to “de-energizedâ€, the meter wire will not be connected or open, the relay coil power supply problem, or the instrument failure. The reason is that when there is no alarm, the relay coil is in the "live" state. Once the above abnormal phenomenon occurs, the relay coil will return to the state of "de-energized". The operator and maintenance personnel will find out the cause of the alarm because of "alarm". , When the signal is found to be normal and alarm, it will go to find other reasons, and eliminate the fault, so that the alarm circuit back to normal, so as to avoid the occurrence of the phenomenon of missing, obviously "lost power" alarm is more reliable than "live" alarm.
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