How to Wire a VFD with Motor, PLC, Switches & External Devices

Controlling a 3-Phase Motor using VFD for Speed Control and Changing the Direction of Rotation

Wiring a Variable Frequency Drive (VFD) is a fundamental skill in industrial automation and control systems. As a modern component, it facilitates precise regulation of an electric motor’s speed, torque, and direction by adjusting the frequency and voltage of the power supplied to the motor. In the following article, we will demonstrate how to wire and control a three-phase motor using VFD, external switches, and additional devices such as PLCs and PCs, with the help of power and control wiring diagrams.

What is a VFD?

A Variable Frequency Drive (VFD), also known as an Adjustable Frequency Drive (AFD), is an electronic device used to control the speed and torque of an electric motor by varying the frequency and voltage of the power supplied to the motor. VFDs are a key component in modern industrial automation and control systems, allowing precise regulation of motor speed and performance.

VFDs convert incoming AC power into DC voltage using rectification, and then convert it back into AC power at the desired frequency using an inverter. By adjusting the frequency of the AC power, the VFD can control the motor’s rotational speed. This speed control capability provides several advantages, including energy savings, improved process control, reduced wear and tear on equipment, and enhanced system efficiency.

VFDs not only enhance energy efficiency but also offer smoother starts, stops, and speed transitions, reducing mechanical stress on the motor and connected machinery. This technology finds widespread use across industries, from manufacturing to HVAC systems, enabling optimized performance, increased automation, and significant energy savings.

Components Required

1. 3-P MCCB
2. Three-Phase Motor
3. Variable Frequency Drive (VFD)
4. Three Phase Supply
5. 24V DC SMPS (For NPN Mode)
6. 3 Nos. of ON/OFF switches for manual ON/OFF and REV/FWD Operation
7. Wires & Cables
8. Programmable logic controller (PLC) for controlling the VFD’s operation (Optional)

Wiring a Three-Phase Motor and VFD

The following power and control circuit diagram shows how to wire a VFD for speed control and changing the direction of rotation of three phase motor.

Power Wiring:

Connect the 415V AC three-phase power supply from the 3-P MCCB to the VFD’s L1, L2, and L3 terminals. Then, wire the three-phase motor terminals (U1, V1 and W1) to the VFD’s output terminals T1, T2, and T3. Ensure that the correct phases are properly matched and aligned in sequence. If you’re unsure, refer to the manufacturer’s guidelines for accurate wire sizing and connections.

Control Wiring:

Connect control devices to the VFD’s control inputs such as Start/Stop, REV and FWD etc. to the digital inputs on the VFD. You may also additional devices such as PLC, PC/Laptops etc. To do so, follow the following steps.

Click image to enlarge

For NPN Mode:

External Switches

1. Connect the 24V DC Switched Mode Power Supply (SMPS) from the 2-P MCB via Phase and Neutral wires.
2. Connect the Negative (-) from 24VDC to the D1 terminal (COM) of VFD.
3. Connect the Positive (+) from 24VDC SMPS to the first terminals of all three external switches (Start/Stop, Forward and Reverse).
4. Connect the second terminal of external Start/Stop switch to the DI1 terminal of VFD.
5. Connect the second terminal of external Forward switch to the DI2 terminal of VFD.
6. Connect the second terminal of external Reverse switch to the DI3 terminal of VFD.

PLC & Other External Devices.

• The PLC can be connected to the VFD via RS485 or RJ45 serial communication. To do so, Connect the PLC to the A1 and AI1 terminal of VFD.

This feature is used to provide analog input to run the VFD with varying frequency or speed using PLC or other external devices.

To check the running status of VFD, you may connect external devices such as PLC to the A0 and A01 terminals of VFD.

The LAN port on the VFD can be used to connect an external device such as PLC, Computer or HMI to be interfaced with VFD using RS485 or RJ45 connectors.

Good to Know:

We have used a 24V DC SMPS to control and operate the VFD in the above wiring and control circuit because the circuit is based on NPN Mode.

In more recent VFD models, the options of both PNP and NPN modes are available for operation. If a VFD supports NPN Mode (as per bulletin features), there is no need to provide 24V DC supply because, the VFD internally convert and supply the DC power to the circuit.

In this scenario (PNP Mode), you can use a 100-230V AC supply for wiring the circuit, and the VFD will take charge of the remainder by converting the AC supply into DC to facilitate circuit operation.

Note, To wire a VFD in the PNP Mode, you may check the previous post for FWD-REV Control of motor using a VFD.

VFD control can be used without using optional terminals and devices. VFD can also be used with the built-in switches and display without control wring connections.

Programming the VFD

You can utilize the relevant programming software for your VFD, such as VFDSoft for Delta VFDs, ATV312 for Schneider, or Drive Composer for ABB. If connecting a PLC is necessary, ensure that the software is compatible with WPLSoft or the programming software of other PLC brands. To do so,

• Simply access the program dashboard.
• Configure the basic motor parameters (such as voltage, rated current, frequency, etc.).
• Assign the digital inputs to the corresponding switches (FWD, REV, ON/OFF, etc.).
• Program the Reverse, Forward, and Start/Stop functions to activate when the REV, FWD, and ON/OFF switches are pressed, respectively.
• Set the acceleration and deceleration for motor speed control according to your requirements.
• Test the program, and if everything is functioning correctly, proceed to real-time operation.

VFD Control Terminals

We have used the following terminal to wire a VFD for motor control.

• DI1 = S/S = Start and Stop
• DI2 = FWD = Forward
• DI3 = REW = Reverse
• AI1 = Analog input for varying speed and frequency
• A01 = Analog output for VFD frequency or speed status

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