Analyses problems - PLC and frequency converter connected applications

1. PLC switch command signal input

inverter input signal includes the run/stop, forward/reverse, running state of operation such as micro switch type command signal. Frequency converter is usually the use of relay contacts or components (relay contact switch characteristics Such as a transistor) With PLC) Connected to the operation instruction.

using relay contacts, often poor contact and bring the false action; Should be considered when using the transistor are connected, the transistor itself factors such as voltage, current capacity, guarantee the system reliability.

design should also pay attention to the inverter input signal circuit, when the input signal circuit connection was not also causes inverter misoperation. For example, when the input signal circuit adopts the inductive load such as relay, relay open closed surge currents bring noise may cause the inverter misoperation, should avoid as far as possible.

when the input switch signal into frequency converter, an external power supply and frequency converter control power supply (happens DC24V时) Crosstalk between. Is properly connected power supply based on PLC, the external transistor collector diode to PLC.

2。 Numerical signal input converter

inverter also save some numeric ( Such as frequency, voltage, etc. ) Command signal input, can be divided into two kinds of, digital input and analog input. Digital input more adopt inverter panel keyboard and serial interface to a given; The analog input is given by an external terminals, usually 0 ~ 10 v / 5 v voltage signal or 0/4 ~ 20 ma current signal input. Owing to the different input signal interface circuit, must choose the input impedance of PLC output module.

when the inverter and PLC voltage signal range is not at the same time, such as the input signal is 0 ~ 10 v, and the PLC output voltage signal range of 0 ~ 5 v; Or on one side of the output signal of PLC and frequency converter input voltage range of 0 ~ 10 v voltage signal range of 0 ~ 5 v, frequency converter and transistors allow other factors, such as voltage, current limits, need series connected to the current limiting resistor and the partial pressure to open and close in order to make sure no more than the PLC controller and frequency converter capacity accordingly. In addition, the attachment also should pay attention to separate wiring, guarantee the main circuit on one side of the noise is not to control circuit.

usually inverter also terminals to the external output corresponding monitoring analog signals. Electrical signal is usually range is 0 ~ 10 v / 5 v and 0/4 ~ 20 ma current signal. Cases, all should be paid attention to: PLC controller side size to ensure that the input impedance circuit circuit voltage and current shall not exceed the allowable values, to ensure the system reliability and reduce error. In addition, the monitoring system of each other is not the same, there is no clear party shall consult the manufacturer.

in addition, the use of PLC in sequence control, the CPU for data processing need time, time delay, so the more precise control should be considered. Strong electromagnetic interference will be produced in the operation of the inverter, PLC in order to ensure inverter main circuit breaker and switch devices and other noise and fails, will the inverter connected to the PLC should pay attention to the following:

1) Itself should be in accordance with the provisions of PLC controller wiring standards and conditions, we should pay attention to avoid using common wiring and inverter, and makes it both to separate as soon as possible.

( 2) When the power supply condition is not very good, should be the PLC power supply module and input/output module power line access noise filter and reduce the transformer noise, etc. In addition, if necessary, the inverter side should also take corresponding measures.

( 3) When the frequency converter and PLC is installed on the same operation ark, should as far as possible make related to inverter power lines and separated from PLC on wires.

( 4) Use the shielded wire and twisted pair to raise the level of noise interference.

3。 Conclusion

PLC and frequency converter connected applications, both involved with high voltage and low voltage control should pay attention to the connection appear interference, avoid the interference caused by frequency converter misoperation, improper connection lead to damage of PLC and frequency converter.

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