What is a VFD (variable frequency drive)?
A VFD, also called an inverter or AC drive, lets you run a standard 3-phase induction motor at any speed instead of only its nameplate speed. It also starts the motor smoothly and can save a lot of energy on pumps and fans.
Why speed control needs a drive
The speed of an induction motor is set by the supply frequency: a 4-pole motor runs at about 1450 rpm on 50 Hz. To run it at 700 rpm you must feed it 25 Hz. A VFD does exactly that: it takes the fixed mains (for example 400 V, 50 Hz) and creates a new 3-phase supply with an adjustable frequency and voltage. See types of electric motors for how the rotating field works.
Inside a VFD
- Rectifier: diodes turn the AC input into DC.
- DC bus: large capacitors smooth it. On a 400 V supply the bus sits at about 540-565 V DC.
- Inverter: six IGBT transistors switch the DC bus on and off thousands of times per second (the carrier frequency, typically 2-16 kHz). By varying the width of these pulses (PWM, pulse width modulation) the drive makes the motor current follow a sine wave of the frequency you ask for.
V/f and vector control
A motor needs its voltage to rise with frequency, so the magnetic flux stays constant: full voltage at 50 Hz, half the voltage at 25 Hz. This simple method is called V/f control and is fine for pumps, fans and conveyors. Sensorless vector control calculates the motor's flux and torque from the currents and gives strong torque even at low speed; closed-loop vector adds an encoder for precise speed holding.
Why factories use drives
- Speed control: one conveyor, many product speeds.
- Soft start and stop: the acceleration and deceleration ramps stop jerks, spilled products and water hammer, and avoid the 6-8× starting current of a direct-on-line start.
- Energy saving: for fans and centrifugal pumps the power falls with the cube of the speed. Running a fan at 80 % speed needs only about half the power (0.8³ ≈ 0.51) compared with throttling it with a damper.
- Protection: current limiting and motor overload protection are built in.
Key parameters to set
| Parameter | What it does |
|---|---|
| Motor nameplate data | voltage, current, power, frequency, speed: the drive protects and controls the motor with these |
| Min / max frequency | limits the speed range (e.g. 10-50 Hz) |
| Acceleration / deceleration time | how fast it ramps (e.g. 0 → 50 Hz in 5 s) |
| Command source | where RUN comes from: keypad, terminals (PLC outputs) or fieldbus |
| Speed reference source | keypad, analog input 0-10 V / 4-20 mA, preset speeds or fieldbus |
| Stop mode | ramp to stop or coast to stop |
Braking
When the drive decelerates a heavy load, the motor works as a generator and pushes energy back into the DC bus. If the bus voltage rises too high, the drive trips on overvoltage. Fixes: longer deceleration time, a braking resistor (burns the energy as heat), or a regenerative drive that feeds it back to the mains.
Wiring rules
- Mains to L1, L2, L3; motor to U, V, W. Never connect the mains to U, V, W.
- Use shielded motor cable, with the shield earthed at both ends, and keep it away from signal cables.
- Do not switch a contactor between the drive and the motor while it is running.
- Emergency stops use the drive's STO (Safe Torque Off) input through a safety relay.
- The DC bus stays charged for minutes after power-off: wait and measure before touching the terminals.
Common faults
| Fault | Typical cause |
|---|---|
| Overcurrent | acceleration too fast, jammed load, motor cable short |
| Overvoltage | deceleration too fast, no braking resistor |
| Undervoltage | supply dip, missing phase |
| Overload / motor thermal | motor overloaded for too long, wrong nameplate current |
| Overtemperature | blocked cooling fan, hot cabinet |
Next: controlling a VFD from a PLC. The PLC Sim Lab Playground has a VFD you can wire and run.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →