PlantFlow 3D › Guides › Variable frequency drives (VFD) explained
Variable frequency drives (VFD) explained
How a VFD controls motor speed, why it starts without inrush current, how its 24 V control inputs work and what to check when it trips.
What a VFD does
A variable frequency drive feeds an induction motor from a supply whose frequency and voltage it can change. The motor speed follows the frequency: at 25 Hz a 4-pole motor turns at roughly half its 50 Hz speed. The drive first rectifies the three-phase supply to DC, smooths it in a DC link, then switches that DC with transistors to build a new three-phase output.
Because the drive raises the frequency gradually, from a few hertz up to the set point, the motor accelerates smoothly. The current stays close to the motor rating instead of the large inrush of a direct-on-line start.
Voltage follows frequency
To keep the magnetic flux in the motor roughly constant, a basic drive keeps the voltage in proportion to the frequency (V/f control). At half frequency the output voltage is about half, which is why a multimeter on the drive output reads less than 400 V while the motor runs slowly.
Control wiring
Most drives have their own 24 V DC supply for digital inputs. A typical three-wire scheme uses a normally-closed stop button on one input (the drive runs only while it is closed), a start-forward button and a start-reverse button. Never connect 230 V control voltage to these inputs: it destroys them.
The acceleration and deceleration times, the speed set point and the motor data are parameters in the drive. Reversing does not need a second contactor: the drive simply changes the order of its output phases.
Things that must never happen
Mains voltage must never reach the drive output terminals U, V and W, for example through a wrongly wired bypass contactor. Contactors between the drive and the motor should only switch when the drive is stopped. If a supply phase is missing, the drive stops and reports an undervoltage or phase-loss fault.
Try it yourself
Choose “VFD drive” in the simulator, start forward, change the speed set point to 25 Hz and press reverse. Watch the frequency, current and output voltage.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.