By Andy Pasquesi
Updated Mar 24, 2022
A variable frequency drive (VFD), often referred to as a variable‑speed drive, adjusts the rotational speed of single‑phase or three‑phase AC induction motors by modulating the input frequency. Since the mid‑twentieth century, VFDs have become indispensable in industrial automation, yet many process‑level schematics, such as Piping and Instrumentation Diagrams (P&ID), omit a dedicated symbol. The conventional practice is to represent a VFD with a small rectangular box labeled “VFD” (or “VS” for variable speed) adjacent to the motor it controls.
Extend a short line—vertical or horizontal—from the motor symbol. Keep the line length no greater than three times the motor’s symbol. Note that the motor may appear as a generic “M” box or as the specific equipment (fan, pump, conveyor) it powers.
Place a rectangle on the opposite end of the line. Ensure the rectangle’s area does not exceed the motor’s symbol area.
Enter the letters “VFD” inside the rectangle. If you prefer to denote the device as a variable‑speed drive, use “VS” instead.
Connect the rectangle to the motor control center (MCC), programmable logic controller (PLC), or other control device that governs the VFD.
Including a clear VFD symbol enhances readability and ensures that all stakeholders accurately understand motor speed control within the system.
