1. VFD Main Circuit Wiring (R/S/T input → U/V/W output)
VFD main circuit wiring3 in, 3 out, plus braking resistor and DC reactor:
| Terminal | Connect | conductor size | What it should show |
|---|---|---|---|
| R/S/T | three-phase supply incoming line | selected per VFD power rating | wired in through breaker + contactor |
| U/V/W | VFD output → motor | selected per motor power rating | Do not install a contactorbetween the VFD and the motor |
| PD1/P+ | DC reactor | standard | remove factory shorting bar, then connect reactor |
| P+/PB | braking resistor | per VFD specification | connect PB-PA when a braking unit is fitted |
2. VFD Control Terminal Wiring (digital + analog)
VFD control terminals are divided intoDigital inputs (DI), digital outputs (DO), analog inputs (AI), analog outputs (AO), communication5 types, using the Siemens G120 as an example:
- DI: DI0 (start/stop), DI1 (forward/reverse), DI2 (fault reset), DI3 (multi-speed 1), DI4 (multi-speed 2). Common terminal COM connects to 0V (PNP wiring).
- DO: relay output (fault alarm, NO/NC selectable), collector output (running signal).
- AI: AI0 (frequency reference, 4-20mA or 0-10V selectable), AI1 (feedback signal, e.g. pressure sensor).
- AO: AO0 (output frequency indication, 0-10V), AO1 (output current indication, 0-10V).
- Communication: PROFINET (RJ45 port) or MODBUS RTU (RS-485 A+/B-).
3. Analog Wiring Rules (4-20mA vs 0-10V)
Analog signals are the most interference-prone part of VFD panel wiring,The two signal typesrequire different wiring rules:
| Signal type | Advantages | Drawbacks | Wiring rules |
|---|---|---|---|
| 4-20mA | Strong noise immunity, broken wire detectable (0mA = fault) | Requires a 250Ω precision resistor to convert to voltage | Two-wire, shield grounded at one end |
| 0-10V | Simple wiring | Susceptible to interference; at 0V a broken wire cannot be distinguished | Three-wire, shielded twisted pair |
4. Typical Wiring Diagram Examples for a VFD Panel (22kW constant pressure water supply VFD panel)
Using a 22kW constant pressure water supply VFD panel as an example, the wiring scheme:
- Main circuit: circuit breaker NSX100F (80A) → contactor LC1D40 (for bypass) → VFD G120 22kW → 22kW motor.
- Control circuit: start button → DI0, stop button → DI1 (or controlled via PLC), pressure sensor 4-20mA → AI0 for PID feedback.
- Frequency reference: PLC analog output (4-20mA) → VFD AI0, or the VFD's built-in PID computes automatically (sensor wired directly to AI0).
- Fault output: VFD relay output → intermediate relay → fault indicator light + buzzer.
VFD output terminals U/V/Wmust never be connected to a contactor— if the output-side contactor opens while the VFD is running, high voltage is generated inside the VFD and damages the power module. If bypass switching is required, it must be done only after the VFD hascome to a complete stop. Cable length between the VFD and the motor≤50m(shielded cable ≤100m); if longer, add an output reactor.
5. Common Wiring Mistakes in VFD Panels (5 red lines)
① VFD output terminals U/V/Wwired through a contactor→ Opening the contactor while the drive is running damages the VFD; ② Analog signal cables and power cablesrun in the same wireway→ Interference causes frequency fluctuation; ③ Braking resistor wire gauge too small → resistor overheats and catches fire; ④ VFD not connected to PE → leakage current has no return path, enclosure becomes live; ⑤ Communication cable without a 120Ω terminating resistor → MODBUS drops nodes.
FAQ FAQ
Can a contactor be connected to the VFD output terminals?
Prohibited. Opening the output-side contactor while the VFD is running produces high-voltage spikes that damage the VFD's IGBTs. If bypass switching is required, stop the VFD first, then operate the contactor, and add mechanical interlocking.
For the VFD analog signal, which is better: 4-20mA or 0-10V?
4-20mA is recommended. Strong noise immunity (a current signal is unaffected by line voltage drop), and a broken wire reads 0mA so the fault can be detected. 0-10V suits short-distance (≤10m) simple applications.
Is there a limit on cable length from the VFD to the motor?
Yes. Cable length≤50mstandard cable is fine; 50-100m use shielded cable; >100m must add anoutput reactor(dv/dt filter), otherwise the motor insulation can be punctured by high-frequency pulse voltages.
How do you select a VFD braking resistor?
Select the braking resistor resistance per the VFD manual (e.g., 400Ω/2kW for 22kW), and calculate power based on braking frequency and inertia. For frequent braking applications (such as cranes), select 1-2 power steps higher, and the resistor must be mounted in a location with heat dissipation.
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