PLC and VFD Control Explained for Beginners: Start, Stop and Speed Control

Easy Electra September 26, 2026

PLC and VFD Control Explained for Beginners: Start, Stop and Speed Control



Introduction

Variable Frequency Drives, commonly known as VFDs, are widely used in industrial automation to control the speed and operation of AC motors.

A PLC can be used to control a VFD by sending commands such as Start, Stop, Forward, Reverse, and Speed Reference.

The PLC can also monitor VFD information such as Running status, Fault status, Frequency, and other available parameters.

When a PLC, VFD, motor, and HMI are combined, they provide a flexible control system for many industrial applications.

In this article, we will learn how PLCs control VFDs, different control methods, speed control, fault monitoring, HMI integration, and a practical conveyor example.

What Is a VFD?

VFD stands for Variable Frequency Drive.

A VFD is an electronic device used to control the speed and operation of an AC motor by controlling the electrical supply provided to the motor.

In many applications, the VFD changes the frequency and voltage supplied to the motor according to the required operating condition.

A simple system is:

PLC → VFD → AC Motor

The PLC provides the required control commands, while the VFD controls the motor according to its configuration.

Why Is a VFD Used?

VFDs are commonly used when a motor needs variable-speed operation.

Typical applications include:

  • Conveyors
  • Pumps
  • Fans
  • Compressors
  • Mixers
  • Packaging machines
  • Material handling systems
  • Process equipment

For example, a conveyor may need to run at different speeds depending on the production requirement.

A VFD allows the motor speed to be adjusted instead of operating only at a fixed speed.

What Is PLC and VFD Control?

PLC and VFD control means using a PLC to send operating commands and, depending on the system, speed references or communication data to a VFD.

The PLC can receive feedback from sensors and execute the machine control logic.

The VFD then controls the motor based on the commands and its configuration.

A basic system can be represented as:

Sensor → PLC → VFD → Motor

The PLC may also receive VFD status information:

Motor → VFD → PLC → HMI

Common VFD Control Methods

There are several ways a PLC can communicate with and control a VFD.

Common methods include:

  • Digital inputs and outputs
  • Analog speed reference
  • Industrial Ethernet communication
  • Modbus communication
  • Other manufacturer-specific communication methods

The correct method depends on the VFD, PLC, available communication hardware, application requirements, and control-system design.

VFD Control Using Digital Signals

Digital signals can be used for basic VFD commands.

For example, a PLC output can provide a Run command to the VFD.

Other digital commands may include:

  • Run
  • Stop
  • Forward
  • Reverse
  • Reset

A simple sequence is:

PLC Output ON → VFD Run Command → Motor Runs

When the PLC removes the Run command, the VFD can stop the motor according to its configured stopping method.

VFD Speed Control Using Analog Signal

A PLC can use an analog output to provide a speed reference to a VFD.

A common signal is 0–10 V, although other signal types and configurations are also possible.

For example:

0 V → Minimum Speed Reference

5 V → Approximately 50% Reference

10 V → Maximum Speed Reference

The actual motor frequency and speed relationship depends on the VFD configuration and motor/application parameters.

The PLC can calculate the required reference and send the corresponding analog output.

VFD Control Using Industrial Communication

Modern automation systems can also control VFDs through industrial communication networks.

Depending on the equipment, examples may include:

  • PROFINET
  • EtherNet/IP
  • Modbus TCP
  • Modbus RTU

Communication can allow the PLC to exchange more information with the VFD than simple Start and Stop signals.

For example, the PLC may be able to access:

  • Run command
  • Frequency reference
  • Actual frequency
  • Motor current
  • Drive status
  • Fault information
  • Alarm information

The exact data available depends on the VFD and communication configuration.

PLC Start and Stop Control

A basic PLC-VFD application may require Start and Stop control.

The PLC receives a Start command from a push button or HMI.

The PLC then checks the required permissive and interlock conditions.

If the conditions are satisfied, the PLC sends a Run command to the VFD.

The sequence is:

Start Button → PLC Logic → VFD Run Command → Motor Running

For Stop:

Stop Button → PLC Logic → Run Command OFF → VFD Stops Motor

The VFD stopping behavior depends on its configured stopping mode.

PLC Speed Control

PLC speed control allows the machine to change motor speed according to the process requirement.

For example, an operator may enter a speed setpoint through an HMI.

The PLC reads the setpoint and generates the appropriate VFD speed reference.

The sequence is:

HMI Speed Setpoint → PLC → Speed Reference → VFD → Motor

This method is commonly used in conveyors, pumps, fans, and other variable-speed applications.

Frequency and Motor Speed

For an AC induction motor, motor speed is related to the supply frequency and the motor's pole configuration.

The synchronous speed relationship can be expressed as:

Ns = 120 × f ÷ P

Where:

Ns = Synchronous speed in RPM

f = Frequency in Hz

P = Number of motor poles

For example, increasing the frequency generally increases the motor's synchronous speed, while reducing the frequency generally reduces it.

Actual motor speed is affected by motor slip and the operating conditions.

VFD Fault Monitoring

A PLC can monitor VFD fault and status information.

Common conditions that may be monitored include:

  • VFD running
  • VFD stopped
  • VFD fault
  • Overload
  • Overcurrent
  • Drive ready
  • Drive warning

If a fault occurs, the PLC can stop the related machine sequence and display an appropriate message on the HMI.

The actual fault information depends on the VFD model and available communication or I/O signals.

PLC, VFD and HMI System

A complete automation system can combine a PLC, VFD, HMI, motor, and sensors.

A typical architecture is:

HMI ↔ PLC ↔ VFD → Motor

Sensors → PLC

VFD Status → PLC → HMI

The HMI can provide controls such as:

  • Start
  • Stop
  • Speed Setpoint
  • Forward
  • Reverse
  • Auto/Manual selection
  • Fault Reset
  • Frequency Display
  • Motor Status

Practical Conveyor Example

Consider an industrial conveyor controlled by a PLC and VFD.

The operator selects Auto mode from the HMI.

A sensor detects that the machine is ready.

The PLC checks all required permissives and interlocks.

The PLC sends a Run command to the VFD.

The VFD starts the conveyor motor.

The PLC provides the required speed reference.

The VFD adjusts the motor operation according to the configured reference.

The PLC monitors the VFD status and can display the information on the HMI.

The complete sequence is:

HMI → PLC → VFD → Motor

Motor/VFD Status → PLC → HMI

How to Troubleshoot PLC-VFD Control

If a motor does not run through the PLC and VFD, troubleshoot the system step by step.

  • Check the VFD power supply.
  • Check whether the VFD shows a fault.
  • Check the PLC Run command.
  • Check the PLC output status.
  • Check the VFD digital input configuration.
  • Check the analog speed reference if used.
  • Check the communication status if a network is used.
  • Check the VFD frequency reference.
  • Check the motor wiring.
  • Check the motor protection and overload conditions.
  • Check the VFD parameter configuration.

Online monitoring of the PLC program and VFD diagnostic information can make troubleshooting easier.

Common Beginner Mistakes

Beginners often make several mistakes when setting up PLC-VFD control.

One common mistake is forgetting that the VFD itself must be configured for the selected command source and speed reference source.

Another mistake is sending a Run command from the PLC while the VFD is configured to receive commands from its keypad.

Incorrect analog scaling can also cause the motor to run at an unexpected speed.

Communication addressing and network configuration must also be checked when using a communication protocol.

How to Practice PLC-VFD Control

You can practice PLC-VFD control using small automation projects.

  • PLC Start and Stop control
  • Forward and Reverse control
  • 0–10 V speed reference
  • HMI speed setpoint
  • VFD fault monitoring
  • Auto and Manual mode
  • Conveyor speed control
  • Motor interlocking
  • PLC-VFD communication

Start with basic digital Start and Stop control, then move to analog speed control and finally industrial communication.

Important Safety Considerations

VFDs are connected to electrical power systems and can control rotating machinery.

Always follow the VFD manufacturer's documentation, electrical drawings, isolation procedures, motor specifications, and applicable safety requirements.

Do not bypass protective devices or safety circuits to make a motor run.

Emergency stops and other safety functions should be implemented using appropriate safety-rated components and architecture where required by the application.

Conclusion

PLC and VFD control is an important part of industrial automation.

A PLC can control VFD Start and Stop commands, speed references, operating modes, and fault monitoring.

VFDs can be controlled using digital signals, analog signals, or industrial communication protocols depending on the application.

Understanding PLC-VFD integration will help you work with conveyors, pumps, fans, compressors, and many other industrial machines.

In the next article, we can learn about PLC sequence control and how to design an automatic machine sequence step by step.

Meta Description

Learn PLC and VFD control for beginners. Understand Start and Stop control, speed reference, 0–10 V signals, industrial communication, VFD faults, HMI integration, troubleshooting, and conveyor applications.

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