What Signal Outputs Does the VFD Have?

Published: November 25, 2025

VFDs have become one of the most common control devices in industrial automation. Beyond the core "speed control" function, their output terminals play a critical role—transmitting the VFD’s operating status, fault signals, or speed information to external systems, or directly controlling external devices.
 

1. What Are VFD Output Terminals?

Simply put, output terminals let the VFD tell the outside world what it is doing, as opposed to input terminals, which tell the VFD what to do.

 

They are generally divided into two main types:    

  1. Switching Outputs (Relay / Open Collector)
  2. Analog Outputs (0–10V / 0–20mA)

2. Switching Output Terminals


1.Relay Output


Function

Relay outputs use a mechanical contact to open or close based on VFD conditions such as RUN, AT FREQUENCY, or FAULT.


Advantages

  • Can drive higher-current loads such as relays or indicators.
  • Provides strong electrical isolation and noise immunity.
  • Highly compatible with industrial circuits.


Disadvantages

  • Mechanical contacts wear out over time.
  • Slower response compared to electronic outputs.
  • Not ideal for high-frequency switching.


2. Open Collector Output


Function

Open Collector outputs use an NPN transistor to pull the signal low to indicate an ON condition.


Advantages

  • No mechanical wear; extremely long life.
  • Very fast switching speed.
  • Compatible with multiple voltage levels (5V / 12V / 24V).
  • Easily interfaces with PLCs and microcontrollers.


Disadvantages

  • Requires an external power source.
  • Cannot directly drive high-current loads.
  • Requires proper grounding to avoid noise issues.

3. Analog Output Terminals

VFDs provide analog output signals such as 0–10V or 0–20mA for feedback of actual frequency, current, torque, or PID values.


Advantages

  • Provides continuous data suitable for monitoring systems.
  • Enables closed-loop control.
  • Works with PLC analog inputs, meters, and SCADA.


Disadvantages

  • Susceptible to noise; requires shielded cable.
  • Voltage-type signals may attenuate over long distances.
  • The receiving device must match the correct signal type.

4. Common Assignable Output Functions

 Switching Output Terminals (Relay / Open Collector)

  • Inverter Running
  • Fault Output
  • Frequency Reached
  • Zero-speed Running
  • Motor Overload Pre-warning
  • Reverse Running
    ……

Analog Output Terminals (0–10V / 0–20mA)

  • Running Frequency
  • Set Frequency
  • Analog Input
  • Output Power
  • Motor Rotational Speed
  • Pulse Input 
    ……

5. Typical Applications

1.Fans & Pumps

  • Relay RUN → Indicator Light
    Used to display the fan/pump running status.

 

  • OC FAULT → PLC Input
    Sends fault signals to the PLC or monitoring system.

 

  • AO Actual Frequency → Building Management System (BMS)
    Provides real-time frequency feedback for HVAC/BMS monitoring.
     

2.Machinery

  • Relay → Buzzer Alarm
    Outputs alarm signals during overload, fault, or abnormal process status.

 

  • OC At-Frequency → Trigger Next Step
    Used for step-by-step industrial sequencing, such as feeding, cutting, or positioning.

 

  • AO → Speed Feedback for Monitoring System
    Used in closed-loop or real-time monitoring systems for production quality control.
     

3.Central Air Conditioning

  • Relay → Fan–Valve Interlock
    Ensures cooling/heating valves only open when the fan is running.

 

  • OC → Alarm Output to Control Room
    Sends fault or warning signals to the central monitoring panel.

 

  • AO → Airflow Feedback (Frequency-Based Calculation)
    Sends airflow or fan speed information to HVAC controllers.
     

6.Precautions

  1. Confirm the terminal type (relay / OC / analog).
  2. OC outputs require an external power supply.
  3. Match PLC input specifications.
  4. Do not exceed relay contact ratings.
  5. Use shielded cable for analog signals.

7.Conclusion

VFD output terminals are essential for monitoring, interlocking, alarming, and data acquisition.

 

Understanding these terminals transforms a VFD from a simple speed controller into the intelligent core of your automation system.

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