How to Wire a VFD (Variable Frequency Drive): Complete Guide

A variable frequency drive controls motor speed and torque by converting incoming fixed-frequency AC power into a variable voltage and frequency output. Wiring one is not the same job as wiring a contactor or a simple motor starter -- a VFD has separate power and control circuits, strict terminal polarity between line and load sides, and safety inputs older across-the-line starters never had. Get the terminal assignments wrong and you can destroy the drive's output transistors in the first half-second of power-up.

This guide covers line-side and load-side power wiring, motor winding configuration (delta vs star) as it relates to drive output voltage, control terminal wiring including speed reference and Safe Torque Off, shielded cable practices for EMC, commissioning parameters, and troubleshooting. It is written for electricians, controls technicians, and engineers who already understand three-phase motor circuits and need the VFD-specific details, not a general electrical primer.

Line-Side vs Load-Side Power Wiring

Every VFD has two separate three-phase power sections, and confusing them is the single most expensive mistake you can make on a drive install.

Line side (input) -- terminals labeled R/S/T or L1/L2/L3 -- connect to the incoming utility or mains supply through appropriate branch circuit protection (fuses or a breaker sized per the drive's rating) and, on larger drives, a line reactor or contactor. Smaller drives (typically under 1-2 HP) may accept single-phase input on R/S or L1/L2 and still produce three-phase output; check the nameplate before assuming that.

Load side (output) -- terminals labeled U/V/W or T1/T2/T3 -- connect only to the motor. The output of a VFD is not a passive pass-through of line power; it is a synthesized waveform built by the drive's internal power transistors (IGBTs), switched at high frequency to approximate a sine wave at the commanded voltage and frequency.

Never connect incoming line power to the U/V/W (or T1/T2/T3) output terminals. This is the most common and most expensive VFD wiring mistake in the field. The output stage has no equivalent of a diode bridge to absorb reverse-applied line power the way the input rectifier does -- line voltage backfed into the output terminals drives current straight through the IGBTs and typically destroys them instantly. This happens most often with a mis-wired bypass contactor, or a drive reinstalled after troubleshooting without re-checking which terminal set is which. Always confirm R/S/T versus U/V/W from the label silkscreened on the drive itself, not from memory or wire color on a previous job. If a bypass contactor is used, interlock it so line power and drive output can never reach the motor at the same time.

Motor Delta vs Star Configuration and VFD Output Voltage

Dual-voltage three-phase motors have six winding leads brought out to a terminal box, and links inside that box set whether the windings are connected in delta (mesh) or star (wye). A common example is a motor nameplated 230V delta / 400V star -- the windings are identical either way, but the link configuration changes how line voltage divides across each winding.

The critical point for VFD applications: the motor must be linked for the voltage the drive will actually output, not the raw voltage entering the building. A VFD's maximum output voltage is bounded by its own input/DC bus voltage -- a drive fed from a 400V three-phase supply cannot output more than roughly 400V line-to-line at its rated (base) frequency, regardless of the motor nameplate. If that drive is set for a lower base frequency and correspondingly lower voltage, the terminal links must match that intended voltage, not whatever configuration was set for direct-on-line operation.

Before landing a single motor lead: read the nameplate and terminal box wiring diagram (usually riveted inside the box lid) to identify which links produce which voltage; confirm the VFD's output voltage and V/Hz curve, which most drives compute from the nameplate data entered during commissioning; and link the terminal box to match the drive's intended output voltage at base speed, not the incoming supply voltage. If reusing a motor previously wired direct-on-line at a different voltage, re-link it rather than assume the existing links are correct.

Getting this backward -- leaving a motor linked for 400V star on a drive that only outputs 230V at base frequency, for instance -- will not damage anything immediately, but the motor will be underpowered and unable to develop rated torque, running effectively under-voltage for its winding design.

Control Terminal Wiring

The control terminal strip is low-voltage (typically 24V DC or less) and physically separate from the power terminals, usually behind its own cover or on a separate row at the bottom of the drive. Typical control terminals include:

• Run/Start and Stop inputs -- often two separate digital inputs for forward and reverse run. Some drives instead use a single run input plus a direction input -- check the specific drive's control wiring diagram.
• Speed reference input -- commonly 0-10V analog voltage or 4-20mA analog current loop. A voltage reference is often supplied from a wired potentiometer: the drive's +10V reference terminal feeds one end of the pot, the wiper goes to the analog input terminal, and the other end goes to the analog common. A 4-20mA loop resists voltage drop and noise better over long runs.
• Analog/digital common -- the reference ground for control circuit signals, not the same as protective earth; do not bond it except at the single point the manufacturer specifies.
• Additional digital inputs -- preset speed selection, external fault/interlock inputs, jog commands, or drive enable.
• Relay or transistor output -- signals run/fault/"at speed" status to a PLC or lamp. Do not exceed the rated contact voltage/current.

Keep control wiring in its own gland or conduit, physically separated from the motor power cable, for the EMC reasons covered below.

Safe Torque Off (STO) Wiring

Safe Torque Off is a hardwired safety function built into most modern drives, satisfying machinery safety standards such as IEC 61800-5-2. STO removes the drive's ability to generate torque at the power semiconductor level, rather than issuing a software stop command, so the motor cannot rotate while STO is open even with a run command present.

• STO is implemented through one or two dedicated terminals, sometimes labeled STO1/STO2, separate from the general-purpose digital inputs.
• The circuit must be closed (bridged or supplied with the specified safety voltage) for the drive to run normally. When open, the drive cannot produce torque regardless of any run command present.
• Many drives ship with a factory jumper across the STO terminals. If you are not implementing a safety circuit through STO, leave it in place -- removing it without wiring a replacement simply prevents the drive from running.
• If STO is used in a machine safety system (e-stop, guard interlock, light curtain), remove the jumper and wire the terminals into the safety circuit per the drive's safety manual and the machine's risk assessment -- this has specific requirements for routing, response time, and diagnostic coverage beyond general control wiring.
• STO removes torque but does not isolate the motor electrically or discharge stored energy. It prevents unexpected rotation; it is not a substitute for lockout/tagout.

Shielded Motor Cable and EMC Practices

The VFD output stage switches the DC bus at high frequency to synthesize the output waveform, producing fast voltage transitions (high dv/dt) on the motor leads. This creates radiated and conducted noise that can couple into nearby signal wiring if the installation is done poorly.

• Use shielded (screened) cable between the VFD output and the motor. Unshielded output cable is one of the most common causes of nuisance noise complaints, from erratic PLC analog readings to interference on communication cables.
• Ground the shield at both ends. A 360-degree bond at the cable gland -- full-circumference shield contact with a conductive gland -- is far more effective at high frequencies than a pigtail (a drain wire twisted to a ground lug), whose inductance makes it a poor high-frequency ground path.
• Physically separate motor power cable from control and signal wiring -- at least 12 inches (300mm) of separation for parallel runs, crossing at 90 degrees if a crossing is unavoidable.
• Keep motor cable runs as short as practical. Very long runs (100+ feet, varies by drive and cable) can cause voltage reflection at the motor terminals even with correct shielding -- check the manufacturer's cable length guidance and consider an output reactor or dV/dt filter.
• Bond the drive chassis and motor frame to the facility grounding system.

Step-by-Step Wiring and Commissioning Procedure

1. De-energize and verify. Lock out the upstream disconnect per your facility's LOTO procedure and verify zero voltage at the drive's line-side terminals before touching any wiring.
2. Confirm the DC bus is discharged. The bus capacitors store energy after power is removed -- check the manual's rated discharge time and verify with a meter before opening the enclosure. Do not rely on a charge LED alone.
3. Confirm drive sizing -- voltage rating matches the supply, and current rating meets or exceeds the motor's full-load amps.
4. Land the line-side conductors to R/S/T (or L1/L2/L3) through branch protection and any specified line reactor, at the drive's rated terminal torque.
5. Land the load-side conductors -- shielded motor cable to U/V/W (or T1/T2/T3). Double-check against the drive's silkscreened labeling before energizing.
6. Ground the drive, motor frame, and cable shield, and confirm the motor terminal box links match the voltage the drive will output at base frequency, per the motor's nameplate diagram.
7. Wire the control terminals -- run/stop, direction, any preset-speed or interlock inputs, and the speed reference (potentiometer or 4-20mA loop), kept separate from power conductors.
8. Wire or confirm STO -- leave the factory jumper in place if unused, or wire it into the machine safety circuit if it is.
9. Wire the status output -- relay or transistor run/fault output to any PLC input or indicator lamp.
10. Close the enclosure, remove lockout devices, and restore power.
11. Enter motor nameplate data -- rated voltage, current, frequency, power, and base speed (RPM) -- so the drive can build its internal motor model.
12. Set ramp times and frequency limits, sized to the load's inertia, and select control mode -- basic V/Hz for variable-torque loads like fans and pumps, or vector control for higher starting torque on constant-torque loads.
13. Run a no-load or light-load test, confirming smooth acceleration, correct direction, and target speed before connecting the driven load.
14. Verify direction and reconnect the load. If the motor runs backward, swap any two U/V/W leads at the motor terminal box (power locked out), or use the drive's reverse-rotation parameter if allowed.

Set carrier (PWM switching) frequency deliberately: higher carrier frequency quiets the motor's audible whine but increases drive heat and cable losses; lower carrier frequency reduces heating but makes the motor noisier. Long cable runs sometimes need a lower carrier frequency to stay within the drive's derating limits.

Wire and Terminal Reference Table

Troubleshooting

Common Mistakes

• Connecting line power to U/V/W -- destroys the output IGBTs and is the costliest wiring error on a VFD job.
• Leaving the motor terminal box linked for the wrong voltage relative to the drive's actual output voltage at base frequency.
• Running unshielded cable to the motor, or pigtailing the shield instead of a proper 360-degree gland termination.
• Routing motor power cable alongside control and signal wiring without adequate separation, coupling noise into analog references and communication lines.
• Removing the STO jumper without wiring a replacement safety circuit, leaving the drive unable to run.
• Entering incorrect or incomplete motor nameplate data, which degrades the motor model and shows up later as poor speed control or nuisance trips.
• Opening the enclosure before confirming the DC bus has discharged.

Safety Warnings

• Lockout/tagout is mandatory before opening a VFD enclosure or disconnecting/reconnecting power or motor wiring -- follow your facility's documented LOTO procedure, not just the local disconnect switch.
• DC bus capacitors remain charged and dangerous after power is removed. Check the manual's rated discharge time before opening the enclosure, and verify with a meter rated for the voltage present -- do not rely on elapsed time or a charge LED alone.
• This work is for qualified personnel only -- familiar with three-phase power, arc-flash risk, and the drive's documentation. If you cannot confidently identify line-side versus load-side terminals, bring in a qualified electrician first.
• Never bypass STO with a jumper on a machine where STO is required for personnel safety.
• Verify zero energy state on both line and load sides, and confirm the motor cannot be back-fed from another source before working on any conductor.

Key Takeaways

• Line-side terminals (R/S/T or L1/L2/L3) connect to the incoming mains supply; load-side terminals (U/V/W or T1/T2/T3) connect only to the motor -- never cross the two, or you will destroy the drive's output IGBTs.
• The motor's delta/star terminal box links must match the voltage the drive will actually output at base frequency, not the raw incoming supply voltage.
• Control wiring includes run/stop and direction inputs, a speed reference (0-10V or 4-20mA), an analog/digital common, preset-speed or interlock inputs, and a status relay output.
• Safe Torque Off (STO) must be bridged or supplied for the drive to produce torque -- leave the factory jumper in place if unused, or wire it into a proper safety circuit if it is used.
• Use shielded motor cable, ground the shield at both ends with a 360-degree gland termination, and separate motor power cable from control wiring to control EMC noise.
• Enter accurate motor nameplate data, set realistic ramp times and frequency limits, and choose V/Hz or vector control mode based on the load before commissioning.
• DC bus capacitors stay charged and dangerous after power-off -- check the manual's discharge time and verify with a meter before opening the enclosure, and let only qualified personnel perform this work.

Frequently asked questions

What happens if you wire line power to the VFD output terminals?

Connecting incoming line power to the U/V/W (or T1/T2/T3) output terminals typically destroys the drive's output IGBTs instantly, because the output stage has no diode bridge to absorb reverse-applied power the way the input rectifier does. This is the most common and most expensive VFD wiring mistake and usually requires replacing the drive.

Should a motor be wired in delta or star for a VFD?

The motor's terminal box links should match the voltage the VFD will actually output at base frequency, not the raw incoming supply voltage. Check the motor nameplate and terminal box diagram, confirm the drive's intended output voltage, and link accordingly -- a mismatch underpowers the motor rather than damaging it.

What is Safe Torque Off (STO) on a VFD?

STO is a hardwired safety input that removes the drive's ability to produce torque at the power semiconductor level. When the STO circuit is open, the motor cannot rotate even with a run command present. Many drives ship with a factory jumper across STO terminals that must stay in place unless a safety circuit is wired in.

Why does a VFD need shielded motor cable?

The drive's output stage switches at high frequency, creating fast voltage transitions (high dv/dt) that radiate and conduct electrical noise. Shielded cable, grounded at both ends with a 360-degree gland termination, contains this noise and prevents it from coupling into nearby signal wiring, PLC inputs, and communication cables.

How long do VFD capacitors stay charged after power is removed?

DC bus capacitors can remain charged and dangerous for several minutes or longer after input power is removed, depending on the drive's size and design. Always check the specific drive manual's rated discharge time and verify with a meter before opening the enclosure -- never rely on elapsed time or a charge indicator LED alone.

Why does a motor run backward after connecting a VFD?

Reversed rotation almost always means two of the three U/V/W phase leads are swapped relative to the motor's expected wiring. Lock out power and swap any two motor leads at the motor terminal box, or use the drive's reverse-rotation parameter if the application allows changing direction in software instead.

Interactive diagrams for this guide

• Vfd Wiring Diagram
• Vfd Connection Diagram
• Vfd Control Wiring Diagram
• Vfd Circuit Diagram
• Vfd Diagram
• Star Delta Motor Connection Diagram
• 3 Phase Star Delta Motor Connection Diagram
• 3 Phase Motor Wiring Diagram
• 3 Phase Motor Connection Diagram
• Motor Starter Wiring Diagram

Related guides

• Motor Control Circuit Diagrams: Star-Delta, DOL, and VFD