[Selection and use of relays]: 1. According to the use of environmental selection and use of environmental conditions mainly refers to temperature (maximum and minimum), humidity (generally refers to the maximum relative humidity at 40 degrees Celsius), low pressure (use height below 1000 meters can not be considered), vibration and impact. In addition, there are packaging methods, installation methods, shape size and insulation, etc.
1. Selection according to usage environment
The use of environmental conditions mainly refers to temperature (maximum and minimum), humidity (generally refers to the maximum relative humidity at 40 degrees Celsius), low pressure (use height below 1000 meters can not be considered), vibration and impact. In addition, there are still some requirements, such as packaging method, installation method, shape size and insulation. Because of the different material and structure, the relay bears different environmental mechanics conditions. It may damage the relay when it is used under the environmental mechanics conditions which exceed the product standard. It can be selected according to the environmental mechanics conditions of the whole machine or the high-level conditions.
Around devices sensitive to electromagnetic interference or radio frequency interference, it is better not to choose relays excited by alternating current. When choosing DC relay, the product with coil transient suppression circuit should be selected. Where solid-state devices or circuits are used to provide excitation and are sensitive to peak signals, products with transient suppression circuits should also be selected.
2. Determine the type of relay according to the input signal
It is no problem to select electromagnetic, temperature, time and photoelectric relays according to the input signals of electricity, temperature, time and light. In particular, the selection of voltage and current relays is explained here. If the supply relay coil of the whole machine is constant current, the current relay should be selected, and the voltage relay should be selected for the constant voltage value.
3. Selection of input parameters
The input that is closely related to the user is the working voltage (or current) of the coil, while the suction voltage (or current) is the parameter that the relay manufacturer controls the sensitivity of the relay and judges and evaluates it. For users, it is only a limit parameter value under work. Control safety factor is working voltage (current)/pull-in voltage (current). If the relay is used under the pull-in value, it is unreliable and unsafe. If the ambient temperature rises or under the condition of vibration and shock, the relay will not work reliably. When designing the whole machine, the no-load voltage should not be used as the basis of the working voltage of the relay, but the coil should be connected as the load to calculate the actual voltage, especially when the internal resistance of the power supply is large. When the triode is used as a switching element to control the coil on and off, the triode must be in the switching state. For the relay with working voltage below 6VDC, the saturation voltage drop of the triode should also be deducted. Of course, not the higher the working value, the better. Exceeding the rated working value too high will increase the impact wear of armature, increase the number of contact rebound, shorten the electrical life. Generally, the working value is 1.5 times of the absorptive value, and the error of the working value is generally <10%.
4. Select the types and capacities of relay contacts according to load conditions
Long-term practice at home and abroad has proved that about 70% of the faults occur at contacts, which shows that the correct selection and use of relay contacts is very important.
The combination form and number of contacts should be determined according to the actual situation of the controlled circuit. The common contact combination forms are shown in Table 6. The dynamic contact pair in the dynamic contact group and the switching contact group has higher load capacity and contact reliability than that in the dynamic contact group and the switching contact group because of the fewer times of contact bouncing back and the larger compensation after the contact ablation. The whole machine line can use the movable contact as much as possible by adjusting the position of the contacts appropriately. Point.
It is very important to determine the parameters according to the load capacity and load properties (resistance, induction, capacitance, lamp load and motor load). It is incorrect to think that the contact switching load must be smaller than the switching load. Generally speaking, under the rated voltage, the relay switching load with current greater than 100 mA and less than 75% of the rated current is the best. Current less than 100 mA will increase the carbon deposit on the contacts and decrease the reliability. Therefore, 100mA is called test current, which is the content of professional standards at home and abroad to assess the technological conditions and levels of relay manufacturers. Because general relays do not have low level switching capability, relays for switching loads below 50 mV and 50 mu A should be ordered. Users should note that relay manufacturers should be invited to assist in type selection when necessary.
The rated contact load and life of [$page] relay refers to the number of times that the load is resistive under the rated voltage and current. When the rated voltage exceeds, it can be selected according to the contact load curve. When the load property changes, the contact load capacity will be changed. Users can change the contact load current by referring to the table below.
The relay cover is only marked with resistance rated load. For rated load of other properties, please refer to the detailed technical conditions. The magnitude of surge current is shown in the table.
In the case of polarity conversion and phase conversion load, it is better to choose three-position K-type contacts instead of two-position Z-type contacts, unless the product is specifically specified for three-phase AC load conversion. Otherwise, with the increase of product action times, its arc will also increase, and Z-type contacts may lead to short circuit of power supply.
When switching asynchronous single-phase AC load, there will be phase difference, so the contact rating should be 4 times of the load current and 2 times of the load voltage. Contacts suitable for AC loads are not necessarily suitable for load switching between several power supply phases. Corresponding electrical life tests should be carried out when necessary.