The universal schematic symbol for a Variable Frequency Drive (VFD) per IEC 60617 and IEEE 315 is a rectangle containing a sine wave transitioning to a square wave (or the letters 'VFD'/'ASD'), while physical terminal symbols follow strict IEC and NEMA alphanumeric codes. Below is the master reference for both schematic drafting and physical terminal block wiring, terminating in a concrete decision path for identifying unmarked drives.
Complete VFD Terminal & Schematic Symbol Reference
When wiring a drive like an Allen-Bradley PowerFlex 525 or a Danfoss VLT FC 302, the physical terminal block symbols dictate your control and power topology. Use this spec sheet to map pins to functions.
| Terminal Symbol | Function | Typical Signal / Voltage | IEC 60204-1 Wire Color | NFPA 79 Wire Color |
|---|---|---|---|---|
| L1, L2, L3 | AC Mains Power Input | 200-240VAC or 380-480VAC 3-Phase | Brown, Black, Grey | Black, Black, Black (or Red, Black, Blue) |
| U, V, W | AC Motor Output (PWM) | 0 to Rated VAC (Chopped PWM) | Black (with UVW tags) | Black (with T1/T2/T3 tags) |
| PE / ⏚ | Protectative Earth (Ground) | 0V (Fault current path) | Green/Yellow Stripe | Green/Yellow Stripe |
| +10V / +24V | Control Power Source | 10VDC (Analog ref) / 24VDC (Logic) | Red (10V) / Blue (24V) | Red (10V) / Blue (24V) |
| DI1 - DI6 | Digital Inputs | 24VDC Logic (Sink/Source) | Blue | Blue |
| AI1, AI2 | Analog Inputs | 0-10VDC or 4-20mA | Blue (Shielded pair) | Blue (Shielded pair) |
| AO1, AO2 | Analog Outputs | 0-10VDC or 4-20mA | Blue (Shielded pair) | Blue (Shielded pair) |
| RO1 (NO/NC/C) | Relay Output (Form C) | Dry Contact (Up to 240VAC/30VDC) | Black (or per load) | Black (or per load) |
| COM / 0V | Control Common | 0VDC Reference | Blue (with white tracer) | Blue (with white tracer) |
Regional Variants: IEC vs. NEMA vs. Legacy Standards
The symbols stamped on the drive chassis and the colors of the wires you pull through the conduit change depending on your region and the governing standard. Assuming a single standard is universal will result in failed inspections or damaged equipment.
- Motor Output Terminals: Modern IEC 60445 and modern NEMA standards both use U, V, W for the motor output terminals. However, legacy NEMA drives (and older US industrial panels) use T1, T2, T3. If you are retrofitting a 1990s US panel, expect T1/T2/T3 on the old drive and U/V/W on the replacement.
- Mains Input Terminals: IEC and modern NEMA use L1, L2, L3. Legacy Asian and older European drives frequently use R, S, T. They are functionally identical, but swapping L1 for R in your documentation prevents confusion during troubleshooting.
- Control Wiring Colors: Under NFPA 79 (US), 24VDC control circuits must be Blue. Under IEC 60204-1 (EU/Global), 24VDC is also Blue, but AC control circuits are Red, and DC interlocks are Blue with a white tracer. Never use Green/Yellow for anything other than PE; doing so is an immediate code violation globally.
The 'Rows People Get Wrong' Notes
Critical Hazard: PE vs. Neutral Bonding
Never bond the Neutral (N) to the Protective Earth (PE) on the load side of a VFD, and never use the VFD's PE terminal as a current-carrying neutral return. The PE terminal is strictly for fault current and high-frequency noise dissipation. Bonding N to PE here will trip upstream GFCI/RCD devices instantly and can destroy the drive's internal common-mode choke.
Never bond the Neutral (N) to the Protective Earth (PE) on the load side of a VFD, and never use the VFD's PE terminal as a current-carrying neutral return. The PE terminal is strictly for fault current and high-frequency noise dissipation. Bonding N to PE here will trip upstream GFCI/RCD devices instantly and can destroy the drive's internal common-mode choke.
- RO (Relay Output) vs. DO (Digital Output): Technicians frequently wire 24VDC into an RO terminal, frying the internal trace. RO is a dry contact (mechanical relay). It provides no voltage; you must supply your own voltage to the COM terminal of the relay. DO is a solid-state transistor that sinks or sources the drive's internal 24VDC.
- AI (Analog Input) 0-10V vs. 4-20mA: The symbol 'AI1' does not tell you the signal type. On drives like the Danfoss FC 302, you must physically flip a DIP switch (e.g., SW1/SW2) on the main PCB to change the shunt resistor for current mode. Leaving it in voltage mode while feeding 4-20mA will result in a maxed-out, unreadable 10V signal.
- Sinking vs. Sourcing DI: The 'DI' symbol requires a return path. If the drive's internal 24V COM is tied to 0V (Source), your external switch must connect DI to 0V (Sink). Check the drive's specific PNP/NPN jumper settings before applying power.
Decision Path: Identifying Faded or Missing VFD Markings
When dealing with a surplus, legacy, or sun-faded VFD where the terminal silkscreen is illegible, use this decision tree to safely map the control block before applying mains power. Always use a bench power supply for control testing.
| Target Pin to Find | Measurement / Test Procedure | Concrete Action / Result |
|---|---|---|
| PE (Ground) | Set DMM to continuity. Probe chassis metal and terminal screws. | The screw with < 0.5Ω continuity to the bare metal chassis is PE. Mark it with green/yellow tape. |
| Power In/Out | Visual inspection of bus bars and screw gauge. | The largest screws connected to heavy copper bus bars are L1/L2/L3 and U/V/W. The set with diode/IGBT module traces is U/V/W. |
| Control COM (0V) | Set DMM to continuity. Probe control block pins against the PE terminal. | In most modern drives, Control COM is isolated, but some legacy drives tie COM to PE. If isolated, look for the thickest trace on the control PCB leading to the DC-DC converter ground. |
| Digital Inputs (DI) | Apply external 24VDC to COM and DI candidates via a 1kΩ current-limiting resistor. | If the drive's status LED or display registers an input state change, you have found a DI. Do not test with raw 120VAC. |
| Analog Inputs (AI) | Inspect the PCB directly behind the terminal block. | Look for op-amps (e.g., LM358) and precision shunt resistors (e.g., 250Ω). The terminals routing to these components are your AI pins. |
Default Recommendation: If a legacy drive's markings are entirely unreadable and the manual is lost, do not risk a miswire that could feed 480VAC into a 24VDC logic board. Scrap it and default to a modern, IEC-compliant drive with clearly labeled, removable terminal blocks, such as the Yaskawa GA800 or Allen-Bradley PowerFlex 525.
Schematic Representation in Single-Line Diagrams
When drafting or reading electrical schematics, the vfd symbol changes based on the standard your engineering firm adheres to. Understanding these prevents misinterpretation of P&IDs and single-line diagrams.
- IEC 60617-11 (Global/EU): The standard symbol is a rectangle. Inside the rectangle, a sine wave transitions into a square wave, indicating frequency conversion. Alternatively, the letters 'f' and 'Hz' with a variable arrow are used. The input lines are labeled L1, L2, L3 and output lines U, V, W.
- IEEE 315 / NEMA (US): The symbol is typically a rectangle containing the letters 'VFD' or 'ASD' (Adjustable Speed Drive). The input and output lines often feature a specific 'variable frequency' symbol (a sine wave with a diagonal arrow striking through it). Legacy US drawings may still use the diamond-shaped motor-generator set symbol with a rheostat, though this is obsolete for solid-state drives.
- Control Wiring Schematics: In ladder logic or IEC 81346 schematics, the VFD's internal relays (RO) are drawn as standard relay coils and contacts, but enclosed in a dashed box labeled with the VFD's tag (e.g., '-M1' or 'VFD-101') to indicate they are internal to the drive chassis, not discrete panel components.
