Tips for Applying PLC to Variable Frequency Drive

When VFD is used to form an automatic control system in current production conditions, the VFD shall cooperate with PLC in many cases, such as gear cleaning, packing paper printing, PCB plate production. PLC can provide on-off signals of various control signals and instructions through output points or by communication. A PLC system consists of three parts, namely the central processing unit, input/output module and programming part. Next this paper will introduce the matters needing attention when the VFD and PLC cooperate.

Input of switch order signal
Input signals of VFD include switch order signals that control the operation states of running/stopping, forward/reverse, segment speed and point motion. VFD is usually connected to PLC by relay contacts or components (such as transistors) with a characteristic of relay contacts switch, so as to get the operation state orders.

When using the relay contacts, malfunctions are often caused by poor contact. When connecting with the transistor, the voltage and current capacity shall be considered to ensure the reliability of the system.

When designing the input signal circuit of variable frequency drive, it shall also be noted that VFD may also make malfunctions if the input signal circuit is not properly connected. When the input switch signal enters the VFD, crosstalk occurs occasionally between external power supply and VFD control voltage (DC24V). The correct connection is to connect the collector of external transistor to the PLC through the diode with the PLC power supply.

Input of digital signals
There are also some numerical input signals (such as frequency, voltage) in the VFD, which can be divided into two types of analog input and analog output. Analog input is given externally through the connecting terminal, it’s generally input through 0～10V/5V voltage signal or 0/4～20mA current signal. As the interface circuit varies from input signal to input signal, PLC output module shall be selected according to the input impedance of VFD.

When the voltage signal ranges of VFD and PLC are different, for example the input signal of VFD is 0～10V while the output voltage signal of PLC is 0～5V, or voltage range of output signal at one side of PLC is 0～10V while input signal voltage range of VFD is 0～5V, the resistance shall be connected in series or parallel due to the limitation of allowable voltage and current of transistors and VFD, so as to limit the current or bear partial voltage, ensure not to exceed corresponding capacity of PLC and VFD when opening and closing. Moreover, control circuit and main circuit shall also be separated in wire connection, and it’s best to choose shielding wire for control circuit, so as to ensure that the noise on one side of main circuit is not transmitted to the control circuit.

VFD also outputs corresponding monitoring analog signals to the outside through the connecting terminal, such as voltage and rotation speed, DC voltage signal with a signal range of 0～10V. According to the user’s needs, a voltmeter or tachometer can be connected to display the output voltage or rotation speed of VFD in operation. But in all cases, it shall be noted that the input impedance at the PLC side should ensure that the voltage and current in the circuit should not exceed the allowable value, so as to ensure the system reliability and reduce errors.

In addition, when using PLC for sequential control, there are some time delays when the system is running due to time cost required by data processing, different arrangement order and different applications of orders in programming, above factors should be taken into account in more precise control.

VFD will produce strong electromagnetic interference in operation. In order to ensure that the PLC doesn’t break due to the noise produced by the main circuit breaker and switching device, following attentions shall be noticed when connecting VFD with PLC:

1. PLC shall be grounded by the specified wiring standards and grounding conditions, don’t use the same ground wires with VFD. When grounding, VFD and PLC should be separated as far as possible.
2. When the power supply conditions are not good, noise filter, reactors and noise-reducing devices should be connected to the power supply lines of PLC power supply module and input/output module. In addition, corresponding measures should also be taken on the input side of VFD if necessary. 
3. When VFD and PLC are installed in the same cabinet, related wires shall be separated as far as possible. 
4. Shielding wire and twisted-pair can be used to improve the level of noise interference.