1. Input of switch command signal
The input signal of the inverter includes a switch-type command signal that controls the operating status such as operation/stop, forward/reverse, section speed, and joging. The inverter usually uses a relay contact or a component (such as a transistor) with relay contact switching characteristics to connect to the PLC to obtain the operating status command.
When using relay contacts, malfunction is often caused by poor contact; When using transistors for connection, it is necessary to consider the voltage and current capacity of the transistors themselves to ensure the reliability of the system.
When designing the input signal circuit of the inverter, it should also be noted that when the input signal circuit is not properly connected, it will also cause the malfunction of the inverter. For example, when an inductive load such as a relay is used in the input signal circuit, the inrush current generated by the opening and closing of the relay may cause damage or failure of the internal components of the inverter, which may lead to malfunction of the inverter, so this situation should be avoided as much as possible.
Crosstalk between the external power supply and the inverter-controlled power supply (DC24V) sometimes occurs when the input switching signal enters the inverter. The correct connection is to connect the collector of the external transistor to the PLC through the diode using the PLC power supply.
2. Input of numerical signals
There are also some numerical (such as frequency, voltage, etc.) command signal input in the inverter, which can be divided into two types: analog input and analog output. The analog input is given externally via a terminal block, usually through a voltage signal of 0-10V/5V or a current signal of 0/4-20mA. Since the interface circuit varies depending on the input signal, the output module of the PLC must be selected according to the input impedance of the inverter.
When the voltage signal range of the inverter and the PLC is different, for example, the input signal of the inverter is 0-10V, and the output voltage signal range of the PLC is 0-5V; Or when the output signal voltage range of one side of the PLC is 0-10V and the input voltage signal range of the inverter is 0-5V, due to the limitation of the allowable voltage, current and other factors of the inverter and the transistor, it is necessary to connect the resistor in parallel and series to limit the current or divide part of the voltage, so as to ensure that the corresponding capacity of the PLC and the inverter is not exceeded when opening and closing. In addition, attention should also be paid to the separation of the control circuit and the main circuit when connecting, and the control circuit should preferably use shielded wire to ensure that the noise on one side of the main circuit is not transmitted to the control circuit.
Note: The size of the input impedance on the PLC side should ensure that the voltage and current in the circuit do not exceed the allowable value of the circuit, so as to ensure the reliability of the system and reduce the error.
In addition, when using PLC for sequence control, because it takes time to process data, and the different sequences and instructions used when programming will lead to a certain time delay in the operation of the system, the above factors should be considered in more accurate control.
Because the inverter will produce strong electromagnetic interference during operation, in order to ensure that the PLC will not fail due to the noise generated by the main circuit breaker and switching devices of the inverter.
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