VFD Circuit Diagram

VFD Circuit Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable vfd circuit diagram: download the diagram as SVG or open it and print to paper or PDF.

A VFD (variable frequency drive) circuit diagram shows how three-phase AC is rectified to DC, stored on a bus capacitor, then inverted via IGBTs using PWM to produce variable-frequency output for motor speed control.

A variable frequency drive (VFD) — also called a variable speed drive (VSD), adjustable frequency drive (AFD), or inverter — controls AC motor speed by varying both the output frequency and voltage delivered to the motor. The circuit diagram is structured around three functional blocks: the rectifier front end, the DC bus, and the IGBT inverter output stage.

The rectifier section converts the incoming three-phase AC supply (typically 380 V to 415 V at 50 Hz) into DC. In standard drives this is a six-pulse diode bridge rectifier, which produces a DC bus voltage approximately equal to the peak of the AC line voltage — roughly 537 V DC for a 380 V AC input (380 × √2). Higher-performance drives use an active front-end (AFE) rectifier comprising IGBTs that also provide regenerative braking capability (returning braking energy to the mains) and near-unity power factor.

The DC bus consists of a bank of large electrolytic capacitors that smooth the rectified waveform and act as an energy reservoir. A pre-charge resistor and contactor circuit is typically shown in the diagram to limit inrush current during initial energisation — the capacitors are charged through the resistor before the main contactor closes and short-circuits it.

The inverter section uses three half-bridge pairs of IGBTs (six IGBTs total for three-phase output) to produce the variable-frequency, variable-voltage output. Each pair generates one phase of the output. The IGBTs are switched by a PWM (pulse-width modulation) pattern generated by the drive's DSP control board, typically at carrier frequencies of 2 kHz to 16 kHz. The pulse pattern is shaped to approximate a sinusoidal output at the commanded frequency and voltage.

The fundamental control law for standard V/f (Volts per Hertz) control maintains a constant ratio between output voltage and output frequency, preserving motor flux. For example, a 380 V / 50 Hz motor operating at 25 Hz receives 190 V to maintain rated flux and therefore rated torque capability. More sophisticated drives use vector control or direct torque control (DTC) for independent flux and torque regulation.

A braking resistor and brake chopper circuit is often shown in the diagram for loads that overhault the motor during deceleration (cranes, conveyor brakes, centrifuges), dissipating regenerative energy as heat.

How to wire vfd circuit diagram

Identify the three main power circuit sections On the VFD circuit diagram, locate the three-phase AC input terminals (L1, L2, L3), the diode bridge rectifier, the DC bus capacitor bank, and the IGBT inverter bridge with output terminals (U, V, W). These form the main power path through the drive.
Trace the pre-charge circuit Before the DC bus capacitors, find the pre-charge resistor and the main contactor or thyristor bypass. On initial power application, current flows through the resistor, limiting inrush. After a delay (capacitors are charged), the contactor closes to bypass the resistor for normal operation.
Locate the gate driver board and IGBT modules The gate driver circuit receives PWM signals from the control DSP and amplifies them to drive the IGBT gates. Identify the isolation barrier between the low-voltage control circuit and the high-voltage power circuit — this is typically an optocoupler or gate-drive transformer per IGBT.
Identify protection circuits Locate the overcurrent protection (current sensors, typically Hall-effect sensors on the DC bus or output phases), over-temperature protection (NTC thermistors on the IGBT heatsink), and DC bus overvoltage circuit that triggers the brake chopper or trips the drive on regenerative overvoltage.
Trace the control power supply Identify the SMPS (switch-mode power supply) that derives low-voltage control power (typically ±15 V, 5 V, and 24 V DC) from the DC bus or from the AC input. This supply powers the DSP, gate drivers, display, and communication circuits.
Identify the brake chopper and resistor terminals If shown, locate the brake chopper IGBT in parallel with the DC bus between the positive bus and the braking resistor terminals (usually labelled BR+ and BR- or R+ and R-). The brake resistor is an external component connected to these terminals by the installer.
Review the control terminal block and I/O connections The control terminal block (low-voltage, typically 0–10 V, 4–20 mA, and digital inputs/outputs) is shown on a separate section of the diagram. Identify the speed reference input, start/stop and direction contacts, fault relay output, and communication port connections.

Specifications

Typical input voltage (industrial)	380 V to 415 V AC, three-phase, 50 Hz
DC bus voltage (six-pulse rectifier, 380 V AC)	Approximately 537 V DC (380 × √2)
Inverter IGBT carrier frequency range	2 kHz to 16 kHz (drive and load dependent)
V/f control voltage-to-frequency ratio example	380 V / 50 Hz = 7.6 V per Hz (maintained across frequency range)
DC bus capacitor minimum discharge time before access	5 to 15 minutes after AC isolation (manufacturer specific — always measure before access)
Maximum motor cable length (no output filter, typical)	30 m to 100 m (varies by drive rating and carrier frequency)
Input line reactor impedance (recommended)	3 % at rated input current
Operating temperature range (standard)	0 °C to 40 °C ambient (derate above 40 °C per manufacturer curve)

Safety warnings

The DC bus of a VFD remains energised at 537 V DC or above (for 380 V AC input) for several minutes after AC supply disconnection, due to the stored charge in bus capacitors. Always measure DC bus voltage before opening any cover or touching internal components. Minimum wait times are stated by the manufacturer — typically 5 to 15 minutes.
Never connect or disconnect output motor cables while the VFD is energised. Switching the motor on the output side of a running VFD can cause immediate IGBT failure from the transient current surge.
Always install an isolator between the AC supply and the VFD — do not use the VFD's start/stop control as the means of isolation. Disconnect and lock off the upstream isolator before any access to the drive or motor wiring.
The VFD output waveform contains high-frequency components. Standard CAT III or CAT IV rated multimeters may display incorrect voltage on the VFD output — use a true-RMS meter with adequate bandwidth, and be aware that test probes on VFD output phases carry potentially lethal voltage.
VFD installation must comply with applicable electrical codes (IEC 61800-5-1, IEC 60364, BS 7671, NEC Article 430) and the manufacturer's installation manual, including cable segregation, earthing, and EMC requirements. Qualified personnel only.

Tools needed

True-RMS multimeter with CAT III/IV rating (>= 1 000 V)
Clamp meter (AC and DC current)
Oscilloscope with high-voltage differential probes (for waveform analysis)
Insulation resistance tester (500 V or 1 000 V Megger) — for motor insulation, not on VFD output
Laptop with drive parameter software (manufacturer specific)
Torque screwdriver for terminal block connections
Personal protective equipment: insulated gloves, arc-flash rated as required

Common mistakes

Performing a megger (insulation resistance) test on the motor with VFD output cables still connected — the high test voltage destroys the IGBT output stage. Always disconnect and short the VFD output terminals before any insulation resistance testing.
Installing a variable frequency drive on a power factor correction (PFC) capacitor bank — the non-sinusoidal rectifier current causes capacitor overheating and resonance. VFDs must always be installed on the load side of any PFC capacitors, or the PFC bank must be removed.
Undersizing the supply cable based on motor rated current only, ignoring the drive's input current which includes harmonic content — the RMS input current of a six-pulse rectifier can be 20–30 % higher than the output current.
Connecting motor cables without observing the manufacturer's maximum cable length for the chosen carrier frequency — exceeding this causes reflected wave overvoltage at the motor terminals, accelerating insulation failure.
Setting acceleration and deceleration ramps too aggressively for the load inertia, causing DC bus overvoltage trips on deceleration (without a brake resistor) or overcurrent trips on acceleration.

Troubleshooting

Drive trips on DC bus overvoltage during deceleration: Cause: Motor is regenerating energy into the DC bus faster than it can be dissipated; brake chopper resistor absent, undersized, or not connected Fix: Increase the deceleration ramp time to reduce regenerative power. If fast deceleration is required, install or verify a correctly sized braking resistor on the BR terminals. Confirm the brake chopper is enabled in drive parameters.
Drive trips on overcurrent at start-up or during acceleration: Cause: Acceleration ramp too fast for the connected inertia; motor cable too long causing parasitic capacitive current; motor has an internal winding fault Fix: Increase the acceleration ramp time. Check actual output current with a clamp meter during the trip condition. Measure motor winding resistance and insulation resistance to rule out a motor fault. Reduce carrier frequency to lower capacitive cable current.
Excessive motor bearing noise or premature bearing failure: Cause: High-frequency common-mode currents circulating through motor bearings to earth, caused by VFD PWM switching and unbalanced stray capacitance Fix: Install a common-mode choke on the output cables, use shielded motor cable with the shield correctly terminated at both ends, and fit insulated bearings or a shaft grounding brush on the drive end of the motor as recommended by the motor manufacturer.

Frequently asked questions

What is the purpose of the DC bus capacitors in a VFD?

The DC bus capacitors serve two functions: they smooth the ripple from the diode rectifier to produce a stable DC supply for the inverter, and they act as an energy reservoir that can supply the inverter during brief AC input disturbances and absorb regenerative current during motor deceleration. Their health directly affects VFD reliability.

What does the carrier frequency (switching frequency) affect in a VFD?

The carrier frequency is the rate at which the IGBT inverter switches. Higher carrier frequencies (e.g. 16 kHz) produce a smoother output waveform with less acoustic motor noise but increase IGBT switching losses and drive heat output. Lower frequencies (2–4 kHz) reduce losses and extend IGBT life but produce more audible motor noise. Drive manufacturers specify the derate in output current at elevated carrier frequencies.

What is V/f (Volts-per-Hertz) control and when is it used?

V/f control maintains a constant voltage-to-frequency ratio across the output frequency range, preserving magnetic flux in the motor and thus its torque-producing capability. It is simple, robust, and suitable for variable-torque loads like fans and pumps. It is not optimal for constant-torque applications at very low speeds, where vector control (field-oriented control) provides better performance.

What is a brake chopper in a VFD circuit?

A brake chopper is a switching circuit (IGBT and diode) that connects a braking resistor across the DC bus when bus voltage rises above a threshold during motor deceleration. As the motor acts as a generator during braking, energy flows back into the DC bus. The brake chopper dissipates this energy as heat in the resistor to prevent bus overvoltage and protect the capacitors.

Why does a VFD require output filters for long cable runs?

VFD output is a PWM voltage waveform with fast-rising edges (high dV/dt). Long cables between the drive and motor act as transmission lines — these fast edges reflect at the motor terminals, causing voltage peaks that can reach twice the DC bus voltage. This accelerates motor insulation breakdown. Sine-wave filters or dV/dt filters are installed at the drive output for cable runs typically exceeding 30–50 metres.