Variable Frequency Drives (VFDs) can look complicated at first. A typical drive may have hundreds of parameters, multiple control terminals, communication interfaces, protection functions, and different operating modes.
But the basic control logic behind most VFDs is surprisingly simple.
Whether you are working with WOLONG, ABB, Siemens, Schneider, Danfoss, Delta, or another major VFD manufacturer, the fundamental principles are largely the same. Parameter numbers and menu structures may differ, but the way a VFD receives commands, generates a speed reference, and controls a motor follows the same basic logic.
Once you understand this foundation, commissioning a different VFD becomes much easier.
At its core, a VFD is an electronic device used to control an AC motor.
From a practical point of view, most VFD applications involve four basic tasks:
Start and stop the motor
Control the direction of rotation
Control motor speed
Control motor torque
These four functions cover a huge range of industrial applications, including pumps, fans, conveyors, compressors, machine tools, mixers, extruders, and many other types of machinery.
The advanced functions may vary considerably from one drive to another, but these basic requirements remain the same.
When commissioning a VFD, two settings are especially important:
Where does the run command come from?
and
Where does the speed reference come from?
Understanding these two concepts makes VFD commissioning much easier.
The run command tells the VFD whether the motor should run and, depending on the control configuration, in which direction.
Typical run command sources include:
The operator starts and stops the motor directly from the VFD keypad.
This is commonly used during commissioning, testing, and standalone applications.
External switches, push buttons, relays, or PLC outputs are connected to the VFD digital inputs.
For example:
DI1 = Run/Stop
DI2 = Forward/Reverse
DI3 = Fault Reset
The exact terminal assignments depend on the VFD model and configuration.
A PLC, HMI, SCADA system, or industrial controller sends commands through a communication network.
Common industrial protocols include:
Modbus RTU
Modbus TCP
PROFINET
EtherNet/IP
EtherCAT
CANopen
The specific protocols available depend on the VFD.
The speed reference tells the VFD how fast the motor should run.
Depending on the control mode, the reference may represent frequency, speed, torque, or another control variable.
Common speed reference sources include:
The operator enters a frequency or speed value directly through the VFD keypad.
For example:
50 Hz → normal motor speed
30 Hz → reduced speed
An external analog signal provides a continuously variable reference.
The most common industrial signals are:
0–10 V
4–20 mA
For example, a pressure transmitter may provide a 4–20 mA signal to control the speed of a pump.
A PLC or automation controller can send a digital speed reference through a fieldbus or industrial Ethernet network.
This approach is common in automated production lines and process control systems.
This is one of the most useful concepts for anyone working with VFDs.
The run command determines whether the motor should run.
The speed reference determines the required operating speed.
They can come from different sources.
For example:
Run command: external push button
Speed reference: 0–10 V potentiometer
Or:
Run command: PLC communication
Speed reference: PLC communication
Or:
Run command: keypad
Speed reference: keypad
Once this relationship is understood, many VFD control configurations become much easier to understand.
Another fundamental concept is the difference between digital and analog signals.
Digital inputs represent discrete states.
Typical devices include:
Push buttons
Selector switches
Relays
Proximity sensors
PLC digital outputs
Typical VFD functions include:
Run/Stop
Forward/Reverse
Fault Reset
Jog
Multi-speed selection
External interlock
A digital input normally has two basic states: active or inactive.
The exact wiring logic can be either sourcing or sinking, depending on the VFD design. Always follow the manufacturer's wiring diagram rather than assuming that every drive uses the same terminal logic.
Analog inputs provide a continuously changing signal.
Common examples include:
0–10 V
4–20 mA
Typical applications include:
Speed control
Pressure control
Flow control
Temperature-related control
Process feedback
For example, a 4–20 mA pressure transmitter can be scaled so that:
4 mA = minimum pressure reference
20 mA = maximum pressure reference
The VFD then converts this signal into the corresponding speed or frequency reference according to its configured scaling.
The internal power conversion process of a conventional VFD can be simplified into three stages.
The incoming AC power is converted into DC power by the rectifier.
For example:
AC input → Rectifier → DC
The DC-link circuit uses capacitors, and in some designs inductors or reactors, to smooth and stabilize the DC voltage.
The inverter section converts the DC voltage back into AC voltage with adjustable frequency and voltage.
The simplified process is:
AC → DC → AC
This is why a VFD is often described as an AC-DC-AC converter.
A common mistake when learning VFD technology is to think that the drive only changes frequency.
In many basic motor control applications, voltage must also be adjusted as frequency changes.
For a conventional induction motor operating in the constant-flux region, the VFD commonly maintains an approximately constant V/Hz ratio.
For example, if a motor is rated at:
400 V / 50 Hz
the approximate ratio is:
8 V/Hz
The actual control strategy depends on the motor and VFD. Modern drives may use sensorless vector control, closed-loop vector control, or other advanced algorithms rather than simple V/Hz control.
The important principle is that motor voltage and frequency must be controlled together to maintain appropriate magnetic flux and motor performance.
The inverter section of a VFD uses high-speed semiconductor switching devices, commonly IGBTs or other modern power semiconductor technologies.
The drive rapidly switches the DC voltage on and off according to a control algorithm.
This process is commonly called Pulse Width Modulation (PWM).
The resulting voltage waveform consists of high-frequency pulses whose average effect produces the desired fundamental motor voltage and frequency.
The motor's inductance naturally smooths much of the high-frequency switching current, allowing the motor to produce a rotating magnetic field.
This is one of the fundamental technologies that allows a VFD to provide variable-speed AC motor control.
One of the most common VFD control arrangements is:
Digital input for Run/Stop + Analog input for speed reference
For example:
A push button or PLC output is connected to:
DI1 → Run command
Another digital input can be assigned to:
DI2 → Forward/Reverse
A potentiometer or analog signal can be connected to:
AI1 → 0–10 V speed reference
Depending on the VFD design, a potentiometer may use:
+10 V → Potentiometer → AI1 → Analog common
The motor is connected to the VFD output terminals:
U / V / W → Motor
The actual terminal names and wiring requirements vary by manufacturer, so the drive's installation manual must always be followed.
If you switch from one major VFD manufacturer to another, the parameter numbers may look completely different.
For example, one manufacturer may use a parameter such as P700 for a command source, while another may use a completely different parameter structure.
The menu names can also vary:
Run command source
Start command
Operation command
Frequency reference
Speed reference
Main reference
Reference source
However, the underlying questions remain the same:
Where does the run command come from?
Where does the speed reference come from?
What do the digital inputs do?
What does the analog input represent?
How does the drive control the motor?
Once these questions are answered, the brand-specific parameters become much easier to understand.
When commissioning an unfamiliar VFD, do not start by trying to understand every parameter.
A more practical approach is to work through the system step by step.
Check the motor nameplate and record:
Rated power
Rated voltage
Rated current
Rated frequency
Rated speed
Motor connection
Enter the correct motor data into the VFD.
Depending on the application, select an appropriate control strategy such as:
V/Hz control
Sensorless vector control
Closed-loop vector control
Decide whether the motor will be controlled by:
Keypad
Digital terminals
PLC
Communication network
Select the required reference source:
Keypad
0–10 V
4–20 mA
Communication
Preset speeds
Before connecting the machine to the full mechanical load, verify that the motor rotates in the correct direction.
Start with a low frequency or speed reference and check:
Motor rotation
Current
Vibration
Noise
Load response
After confirming stable operation, gradually increase the speed and verify that the motor and machine operate normally.
This method is much safer and more efficient than changing parameters randomly.
For technicians and electricians working with VFDs, three simple rules are particularly useful.
A VFD fundamentally controls:
Run/Stop + Direction + Speed + Torque
Always identify:
Run Command Source + Speed Reference Source
Remember:
Digital = discrete commands
Analog = continuously variable reference
Once these three principles become familiar, most VFD control circuits become much easier to troubleshoot.
A VFD may contain hundreds of parameters, but you rarely need to understand every parameter before you can commission or troubleshoot it.
Start with the basic control architecture:
Where does the run command come from?
Where does the speed reference come from?
What are the digital inputs assigned to?
What does the analog input represent?
What motor data has been configured?
How is the motor being controlled?
The parameter numbers, terminal names, and software interfaces may change from one manufacturer to another, but these fundamental concepts remain largely universal.
For electricians, commissioning engineers, and automation technicians, understanding the underlying control logic is far more valuable than memorizing parameter numbers.
Once the logic is clear, learning a new VFD brand becomes a matter of learning its interface and parameter structure—not learning motor control from scratch.
WOLONG develops variable frequency drive solutions for industrial motor control applications, providing reliable drive technology for pumps, fans, conveyors, compressors, machinery, and other industrial systems.
For VFD technical information and application support, please contact:
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