JoeGuardian Automation

38. VFD Communication in PLC Systems (38 of 41)

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A VFD, or Variable Frequency Drive, controls motor speed, direction, acceleration, deceleration, torque, and protection.

In older or simpler systems, a PLC may control a VFD with hardwired signals.

Example:

PLC Output → VFD Run Input
PLC Analog Output → VFD Speed Reference
VFD Relay Output → PLC Fault Input

But in modern industrial automation, many VFDs communicate with the PLC through an industrial network.

Example:

PLC ↔ EtherNet/IP ↔ VFD

This allows the PLC and HMI to see much more information than simple hardwiring.

A networked VFD can provide:

Run command
Stop command
Speed reference
Actual frequency
Motor current
Drive ready status
Running status
Faulted status
Fault code
Warning code
Torque feedback
DC bus voltage
Drive temperature
Communication status

A simple way to think about it:

Hardwiring gives basic control. VFD communication gives control plus diagnostics.

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1. What Is VFD Communication?

VFD communication means the PLC exchanges data with the drive through a communication protocol.

Common protocols include:

EtherNet/IP
PROFINET
Modbus TCP
Modbus RTU
Profibus
DeviceNet
ControlNet
EtherCAT

In Allen-Bradley systems, EtherNet/IP is very common.

Examples:

CompactLogix PLC ↔ PowerFlex 525
ControlLogix PLC ↔ PowerFlex 755
PLC ↔ VFD through industrial Ethernet switch

The drive appears as a device in the PLC communication structure, and the PLC exchanges command and status data with it.

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2. Hardwired VFD Control vs Networked VFD Control

Hardwired VFD Control

Typical hardwired signals:

PLC digital output → VFD Start
PLC digital output → VFD Direction
PLC analog output → Speed reference
VFD relay output → Fault feedback
VFD relay output → Running feedback

Advantages:

Simple to understand
Easy to test with a meter
Good for basic control
Less dependent on network configuration

Limitations:

Limited diagnostics
More wiring
More I/O points required
Fault details may not be available
HMI may only show “Drive Fault” with no code

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Networked VFD Control

Typical networked data:

Start command
Stop command
Speed reference
Fault reset
Direction command
Drive ready
Drive running
Drive faulted
Fault code
Actual speed
Output current
Output voltage
Output frequency

Advantages:

More diagnostics
Less hardwiring
Better HMI information
More data for troubleshooting
Easier integration with SCADA
Fault codes available in PLC/HMI

Limitations:

Requires IP address configuration
Requires PLC module setup
Communication fault can stop equipment
Firmware/keying can matter
Technician needs network troubleshooting skills

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3. Basic VFD Communication Path

A typical networked VFD system looks like this:

PLC
 ↓
Industrial Ethernet Switch
 ↓
VFD
 ↓
Motor

And feedback returns:

Motor / Drive Status
 ↓
VFD
 ↓
Industrial Network
 ↓
PLC
 ↓
HMI / SCADA

Example:

PLC sends:
Run command and 45.0 Hz speed reference

VFD returns:
Running status, 45.0 Hz output frequency, 6.2 A motor current, no fault

This gives the technician a much better view of what is happening.

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4. VFD Command Data

The PLC may send command data to the drive.

Common command bits:

Run Forward
Run Reverse
Stop
Fault Reset
Enable
Jog
Speed Select

Common command values:

Speed reference
Torque reference
Acceleration reference
Deceleration reference

Example:

PLC command:
Run = TRUE
Speed_Reference = 35.0 Hz

The VFD receives the command and runs the motor if all drive conditions are satisfied.

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5. VFD Status Data

The drive sends status data back to the PLC.

Common status bits:

Ready
Running
Faulted
Warning
At Speed
Enabled
Local Mode
Remote Mode
Direction
Communication Healthy

Common feedback values:

Actual frequency
Motor current
Motor voltage
Motor power
DC bus voltage
Drive temperature
Fault code
Warning code

This status data is very useful for troubleshooting.

Example HMI display:

Drive Status: Running
Command Frequency: 45.0 Hz
Output Frequency: 45.0 Hz
Motor Current: 7.4 A
Fault Code: None

That is much better than simply showing:

Drive OK

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6. Command vs Feedback

A very important concept:

Command does not prove action.
Feedback proves response.

Example:

PLC Run Command = ON

This only means the PLC is asking the drive to run.

You still need to verify:

Drive Running Status = ON
Output Frequency > 0
Motor Current > 0
Motor physically rotating

A professional PLC program should compare command and feedback.

Example:

Run Command ON
AND Drive Running Status OFF after 3 seconds
= VFD Feedback Fault

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7. Drive Ready Status

Before commanding a VFD to run, the PLC should check that the drive is ready.

A drive may not be ready because:

Drive faulted
Enable input missing
Safety input open
Drive in local mode
Communication fault
Parameter issue
DC bus not ready
Power not applied

A good permissive may include:

VFD_Ready
AND Safety_OK
AND No_Drive_Fault
AND Auto_Mode
AND Motor_Overload_OK

Then:

Start_Request
AND VFD_Start_Permissive_OK
= VFD_Run_Command

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8. Local Mode vs Remote Mode

Many VFDs can be controlled locally or remotely.

Local Mode

The drive is controlled from:

Keypad
HIM module
Local buttons
Drive faceplate

Remote Mode

The drive is controlled from:

PLC
Network command
Remote terminals
HMI through PLC

A common real-plant problem:

PLC sends Run Command
Drive does not start
Drive is in Local mode

The PLC may be correct, but the drive is not accepting network commands.

Technician check:

Is the drive in Remote?
Is the command source set correctly?
Is the speed reference source set correctly?

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9. Speed Reference

The VFD needs a speed reference.

This can come from:

Analog input
Drive keypad
Preset speed
PLC network tag
HMI setpoint through PLC

If the run command is ON but speed reference is zero, the drive may be running at 0 Hz.

Example:

Run Command = ON
Drive Running = ON
Speed Reference = 0.0 Hz
Output Frequency = 0.0 Hz
Motor does not move

This is not always a motor failure.

It may be a missing or incorrect speed reference.

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10. VFD Fault Codes

One of the biggest benefits of networked VFDs is fault code visibility.

Common drive fault categories:

Overcurrent
Overvoltage
Undervoltage
Overload
Ground fault
Motor stall
Communication loss
Heatsink overtemperature
Input phase loss
Output phase loss
Encoder feedback loss

A good HMI should show:

Drive faulted
Fault code
Fault description
Possible cause
Reset status

Bad HMI:

VFD Fault

Better HMI:

Conveyor VFD Fault — Overcurrent
Check motor load, jam condition, acceleration time, motor wiring, or drive parameters.

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11. VFD Communication Fault vs VFD Motor Fault

This distinction is critical.

VFD Communication Fault

The PLC cannot communicate with the drive.

Possible causes:

Bad Ethernet cable
Wrong IP address
Switch port issue
Duplicate IP
Drive adapter fault
PLC I/O tree mismatch
Drive powered off

VFD Motor Fault

The PLC can communicate with the drive, but the drive has a motor/control fault.

Possible causes:

Overcurrent
Motor overload
Short circuit
Overtemperature
Ground fault
Mechanical jam
Parameter issue

Simple difference:

Communication fault = PLC cannot talk to drive.
Drive fault = drive reports a problem.

Do not confuse the two.

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12. PLC I/O Tree and VFD Configuration

In Studio 5000, a networked VFD is commonly configured in the I/O tree.

The PLC needs to know:

Drive type
IP address
Communication adapter
Input/output data size
Electronic keying
Firmware revision behavior
Assembly or datalink settings

If the configuration does not match the real drive, communication may fail.

Common symptoms:

Yellow triangle in I/O tree
Module connection fault
Drive not responding
Wrong data values
Fault code not updating

After replacing a VFD, always verify:

IP address
Drive parameters
Adapter configuration
PLC I/O tree compatibility
Firmware/keying settings
Datalinks
Speed reference source
Start command source

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13. VFD Datalinks

Some drives allow extra parameters to be sent or received using datalinks.

Examples of useful data:

Output current
Output frequency
Fault code
Drive temperature
DC bus voltage
Motor torque
Drive status word
Command word

Datalinks can make troubleshooting much stronger.

Example:

HMI displays:
Motor Current = 9.8 A
Drive Temp = 52°C
DC Bus = 680 VDC
Fault Code = 0

This allows technicians to diagnose problems faster.

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14. Basic VFD Troubleshooting Method

When a VFD-controlled motor does not run, check:

1. Is the PLC run command ON?
2. Is the drive communication healthy?
3. Is the drive ready?
4. Is the drive faulted?
5. Is the drive in Remote mode?
6. Is the enable/safety input satisfied?
7. Is speed reference greater than zero?
8. Is the drive running status ON?
9. Is output frequency increasing?
10. Is motor current present?
11. Is the motor mechanically able to rotate?

This prevents guessing.

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15. Example: PLC Command ON but Motor Does Not Move

Symptom

PLC_Run_Command = ON
Motor does not move

Check

Drive communication healthy?
Drive ready?
Drive faulted?
Drive in remote mode?
Speed reference > 0?
Output frequency > 0?
Motor current > 0?

Possible Root Cause

Speed reference was 0.0 Hz.

The PLC was commanding run, but the drive had no speed reference.

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16. Example: VFD Offline After Replacement

Symptom

PLC shows VFD communication fault after drive replacement.

Check

Drive IP address
Subnet mask
Adapter status LEDs
Ethernet link light
PLC I/O tree IP address
Electronic keying
Firmware compatibility
Drive parameters restored

Possible Root Cause

Replacement drive was left at default IP address.

The drive powered up, but the PLC could not connect to it.

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17. Example: Motor Starts Then Trips

Symptom

Motor starts and faults after a few seconds.

Possible causes:

Mechanical jam
Acceleration time too short
Motor overload
Wrong motor parameters
Overcurrent
Bad motor cable
Incorrect VFD tuning
Load too heavy

Check:

Fault code
Motor current
Output frequency
Acceleration time
Motor nameplate parameters
Mechanical load
Motor wiring

The fault code is the clue.

Do not reset repeatedly without investigating.

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18. HMI VFD Faceplate

A good HMI may have a VFD faceplate.

Useful information:

Drive ready
Drive running
Drive faulted
Fault code
Fault description
Command frequency
Output frequency
Motor current
Motor voltage
Drive temperature
Local/remote status
Communication status
Reset button
Manual jog if allowed

This gives operators and technicians a clear diagnostic tool.

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19. Safety and VFDs

Some VFD systems use safety functions such as:

Safe Torque Off
Safety enable
Safety relay input
Safety PLC control

Important:

A VFD may have power and communication but still not run if STO or safety enable is open.

Technician check:

Is the safety circuit healthy?
Is STO satisfied?
Is enable input ON?
Is safety relay reset?

Never bypass safety functions to make a motor run.

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20. Common Mistakes

Mistake 1 — Assuming Run Command means motor should move

Check speed reference, drive ready, enable, feedback, and faults.

Mistake 2 — Confusing communication fault with drive fault

Communication fault means PLC cannot talk to the drive.

Drive fault means the drive reports a problem.

Mistake 3 — Replacing a VFD without restoring parameters

A new drive may not match the original configuration.

Mistake 4 — Ignoring Local/Remote mode

The PLC may command run, but the drive may be in local mode.

Mistake 5 — Resetting faults repeatedly

Repeated resets without root-cause analysis can damage equipment.

Mistake 6 — Ignoring motor and mechanical load

Not every drive fault is electrical. The machine may be jammed or overloaded.

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21. Best Practices

Use these practices:

Document drive IP address.
Backup drive parameters.
Document command source and speed reference source.
Display drive fault code on HMI.
Display actual frequency and motor current.
Monitor communication status.
Use feedback fault timers.
Use proper permissives before starting.
Verify Local/Remote status.
Check STO/safety enable status.
Record drive replacement settings.
Keep firmware and revision notes.

Good tag names:

VFD101_Run_Cmd
VFD101_Stop_Cmd
VFD101_Reset_Cmd
VFD101_Ready
VFD101_Running
VFD101_Faulted
VFD101_FaultCode
VFD101_CmdFreq_Hz
VFD101_ActualFreq_Hz
VFD101_OutputCurrent_A
VFD101_Comm_OK

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22. Technician Checklist

When troubleshooting VFD communication and control, verify:

Drive powered ON
Ethernet link light ON
Drive communication healthy
Correct IP address
Correct PLC I/O tree configuration
Drive not faulted
Drive ready
Drive in Remote mode
Enable/STO satisfied
Run command received
Speed reference greater than zero
Output frequency present
Motor current present
Motor feedback/status present
Fault code reviewed
Parameters restored after replacement
HMI faceplate values correct
Root cause documented

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Final Thoughts

VFD communication is one of the most valuable skills for automation technicians.

A networked drive is not just a motor starter.

It is a smart device that provides control, status, diagnostics, fault codes, current, speed, and performance data.

The troubleshooting path is:

PLC Command → Network Communication → Drive Status → Speed Reference → Motor Output → Feedback → Fault Code

A strong technician does not only ask:

Is the motor running?

They ask:

Is the PLC commanding it?
Is the drive receiving it?
Is the drive ready?
Is the speed reference valid?
Is the drive outputting frequency?
Is the motor drawing current?
Is feedback returning?
Is there a fault code?

A VFD tells a story. The technician’s job is to read the command, status, feedback, and fault data in the correct order.

Understanding VFD communication gives you stronger troubleshooting skills in real industrial systems.