Sometimes referred to as "variable speed," a variable frequency drive (VFD) controls the rotational speed of a single-phase or three-phase AC inductor motor by altering the frequency of the electrical input energy. Since their introduction in the mid-twentieth century, VFDs have become ubiquitous in the fields of industrial engineering and machine design. So ubiquitous, in fact, that engineers may not even include a separate symbol for them in process-level schematics such as Piping and Instrumentation Diagrams (P&ID). While a particular client may require that the symbol for a VFD in his P&ID include certain technical specifications, the standard way to signify a VFD is to draw a small, rectangular box with the letters "VFD" or "VS" written inside of it.
Draw a short line (either vertical or horizontal) extending away from the motor controlled by the VFD. Try to keep the line no more than three times the length of the motor's symbol. Note: in some schematics, the motor will be signified separately as a rectangle or circle with an "M" inside of it. In other cases, the motor will be symbolized by the valve, fan, pump, conveyor belt or other device to which it supplies torque.
Draw a rectangle on the opposite end of the line from step 1. Make sure that the rectangle's area is no greater than that of the motor's symbol.
Write the letters "VFD" inside the rectangle. If you prefer to refer to the device as a "variable speed drive" instead of a "variable frequency drive," write "VS" inside the rectangle instead.
Draw a line connecting the rectangle to the motor control center (MCC), programmable logic controller (PLC) or other device responsible from controlling the VFD.