VFD / Variable Frequency Drive Symbol
Definition: The VFD / Variable Frequency Drive symbol represents a power-electronic drive unit used in electrical and industrial schematics to denote a device that converts fixed-frequency AC mains power into variable-frequency, variable-voltage AC output to control the speed and torque of an AC induction or synchronous motor, as recognised under IEC 60617 and NEMA/IEEE conventions; the block designator is VFD or DRIVE.
Also known as: variable frequency drive, VFD, AC drive, inverter drive, adjustable frequency drive, AFD, variable speed drive, VSD, frequency inverter.
What the VFD / Variable Frequency Drive symbol means
The VFD symbol denotes a complete drive block that accepts three-phase AC supply (or single-phase in smaller units) on its input terminals and delivers a three-phase variable-frequency, variable-voltage output to a motor. Inside the block, a rectifier converts AC to DC, a DC bus filters and stores energy, and a PWM inverter reconstructs AC at the commanded frequency, allowing motor speed to be varied smoothly from zero to above synchronous speed.
In a schematic, the VFD symbol is drawn as a rectangular block with three AC input terminals (L1, L2, L3) at the top and three motor output terminals (U, V, W) at the bottom, sometimes with additional signal terminals for control inputs and braking resistor connections. The VFD symbol communicates that speed control, soft-start, and overcurrent protection are all integrated within the drive, replacing the need to draw the individual rectifier, capacitor bank, and inverter stages separately.
How to identify the VFD / Variable Frequency Drive symbol
The VFD symbol is a solid-outlined rectangle labelled 'VFD', 'DRIVE', or 'INVERTER', with three input power pins entering at the top (L1, L2, L3) and three output pins exiting at the bottom (U, V, W) directed to the motor. The label and six-terminal arrangement are the distinguishing features. Some representations add a diagonal arrow or a sine-wave-to-variable-frequency arrow inside the rectangle to indicate the frequency-conversion function. Control terminals (e.g., RUN, REF, COM) may appear on the side. IEC 60617 uses the general block rectangle convention for drive units.
Function in a circuit
The VFD controls AC motor speed by varying the frequency and voltage of its output. Rectifying the incoming AC mains to DC, smoothing it on a capacitor bus, then switching high-power IGBTs via PWM at the desired output frequency, the VFD allows precise speed regulation across the motor's full torque-speed curve. This eliminates mechanical throttling, reduces energy consumption in pump and fan applications by the cube of the speed ratio, and enables controlled acceleration and deceleration to protect mechanical loads.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 represents a VFD using the general functional-block rectangle (IEC 60617-02); the applicable product standard is IEC 61800-2 (adjustable speed electrical power drive systems — general requirements). IEC symbols for the rectifier (bridge symbol) and inverter stages may be shown inside if the schematic is an internal circuit diagram rather than a system block diagram. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 uses a labelled rectangle block for drive units; the more detailed NEMA ICS 7 standard covers VFD installation and ratings. In American industrial schematics, the block is often labelled 'VFD' with input/output terminals explicitly marked. |
| Key difference | Both IEC and ANSI/NEMA use a labelled rectangle for the VFD block at system schematic level. IEC internal drive schematics follow IEC 61800-series conventions; NEMA-based schematics use ICS 7 terminal labelling. The principal difference is terminal designation: IEC uses L1/L2/L3 in and U/V/W out; ANSI/NEMA sometimes labels the output T1/T2/T3. |
Terminals / pins
| Pin | Name |
|---|---|
| l1 | L1 |
| l2 | L2 |
| l3 | L3 |
| u | U |
| v | V |
| w | W |
Typical values
Input voltage: 200–240 V AC (single-phase or three-phase), 380–480 V AC (three-phase), 500–690 V AC (HV drives); output frequency: 0–400 Hz typical (some to 1000 Hz); output voltage: proportional to frequency (V/Hz ratio); power range: 0.2 kW to several megawatts; PWM switching frequency: 2–16 kHz.
Where the VFD / Variable Frequency Drive symbol is used
- Pump and fan speed control in HVAC systems, saving energy by reducing flow rather than throttling
- Conveyor and material-handling systems requiring smooth, controllable belt or roller speed
- Compressor drives in industrial refrigeration and compressed-air systems for demand-based control
- CNC machine tool spindle drives requiring precise speed and torque control over wide ranges
- Elevator and hoist drives for controlled acceleration, deceleration, and regenerative braking
- Water and wastewater treatment pumps where variable flow reduces pipe stress and energy use
- Wind turbine generators using back-to-back VFDs for grid-tied variable-speed operation
Example
In a water-pump station schematic, the VFD symbol appears between the 400 V three-phase supply (L1, L2, L3 input terminals) and a 7.5 kW induction motor (U, V, W output terminals); a pressure transducer signal feeds the VFD's analogue input, allowing the drive to modulate pump speed from 20 Hz to 50 Hz, maintaining constant outlet pressure while consuming only the energy the load demands.
Key facts
- The VFD symbol is drawn as a labelled rectangle with three AC input terminals (L1, L2, L3) and three variable-frequency output terminals (U, V, W), following IEC 61800-2 and IEC 60617-02 conventions.
- VFD stands for Variable Frequency Drive; the device is also called an AC drive, inverter drive, adjustable frequency drive (AFD), or variable speed drive (VSD).
- The VFD designator in schematics is VFD or DRIVE; it is treated as a functional block, not assigned a single-letter designator like resistors or capacitors.
- Inside a VFD, three power-conversion stages operate: an AC–DC rectifier, a smoothing DC bus capacitor bank, and a PWM IGBT inverter; only the block boundary is shown at system level.
- Motor speed in an induction motor is proportional to output frequency: at 25 Hz the motor runs at approximately half the rated synchronous speed, and power consumption drops by roughly 87.5% (cube law) in centrifugal-load applications.
- IEC 61800-2 is the governing product standard for adjustable-speed AC drive systems, defining ratings, test methods, and EMC requirements.
- VFD output terminals are labelled U, V, W (IEC) or T1, T2, T3 (NEMA/ANSI); mixing these labels without annotation is a common wiring error.
- A braking resistor terminal (BR+ / BR-) is often shown as an additional output on the VFD symbol to indicate the connection point for dynamic braking during deceleration.
Diagrams that use this symbol
Frequently asked questions
What does the VFD symbol mean in a circuit diagram?
The VFD symbol represents a Variable Frequency Drive — a power-electronic block that converts fixed-frequency AC mains into variable-frequency, variable-voltage AC to control motor speed. The symbol shows three AC input terminals (L1, L2, L3) and three output terminals (U, V, W) connected to the motor.
What does the VFD symbol look like on a schematic?
The VFD symbol is a labelled rectangle marked 'VFD', 'DRIVE', or 'INVERTER'. Three input power lines enter at the top (L1, L2, L3) and three output lines exit at the bottom (U, V, W). Some versions include a small sine-to-variable-frequency arrow inside the box to indicate the frequency-conversion function.
What is the IEC standard for a VFD?
IEC 61800-2 (Adjustable speed electrical power drive systems) is the primary product standard for VFDs. For schematic representation, IEC 60617-02 covers the general functional-block rectangle used to depict the drive at system level.
What is the difference between IEC and ANSI terminal labelling on a VFD?
Under IEC conventions, VFD output terminals are labelled U, V, W; under ANSI/NEMA conventions they are labelled T1, T2, T3. Both sets refer to the same three motor phases; incorrect mixing of label conventions is a common wiring error.
What is the designator for a VFD in a schematic?
The VFD is designated 'VFD' or 'DRIVE' in schematics. It is treated as a functional equipment block rather than a standard passive component, so it does not receive a single-letter designator like R (resistor) or C (capacitor).
How does a VFD save energy compared to a fixed-speed motor?
For centrifugal loads such as pumps and fans, power consumption follows the cube of speed: reducing motor speed to 80% of rated drops power to approximately 51% (0.8³ = 0.512). A VFD achieves this electrically without mechanical throttling, making it far more efficient than using a damper or valve to reduce flow.
What are the input and output voltage levels of a typical VFD?
Most industrial VFDs accept three-phase 380–480 V AC at 50/60 Hz on the input. Output voltage is variable from near 0 V to the input voltage, with output frequency variable from 0 Hz to typically 400 Hz, proportional to the speed command signal.
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