Motor drives · intermediate
Variable Frequency Drive
An electronic motor controller that turns fixed mains power into adjustable-frequency power so an AC motor can run at different speeds.
Why it exists
Control speed and torque smoothly, reduce starting current, and expose motor diagnostics to the PLC.
Quick answer
What is a variable frequency drive?
A variable frequency drive controls an AC motor by converting incoming AC to DC, smoothing it on a DC bus, and switching that DC through an inverter to create a variable-frequency, variable-voltage three-phase output. The PLC can send run, direction and speed commands while reading ready, running, current and fault status.
Inside the control system
How the PLC relates to it
A simple installation uses digital inputs for run and direction plus an analog or pulse reference for speed. A networked drive can exchange commands, reference values, status words, actual speed and fault codes over an industrial protocol. Whichever interface is used, define where control comes from, scale engineering units explicitly, separate normal stop from safety functions, and decide what the sequence should do if communication or feedback is lost. The motor leads belong on U, V and W; switching contactors on the drive output while it is running can damage equipment.
Cause and effect
How it works, step by step
- 01
A rectifier converts incoming AC to DC.
- 02
Capacitors smooth and store energy on the DC bus.
- 03
Fast power transistors rebuild a variable-frequency three-phase waveform.
- 04
Motor speed follows commanded frequency while protection monitors current and voltage.
01 / Open the case
Watch the mechanism do the work
Follow one highlighted causal link at a time, then operate the component and deliberately create the fault.
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Know the terminals
- L1 L2 L3 — supply
- U V W — motor
- +24 V / DI / COM — control
- Network port — PLC comms
Recognise the faults
- Motor data not commissioned
- Ramp too short causes overcurrent
- Incorrect control source
- Unsafe isolation of charged DC bus
Read the field guide
Go deeper on sizing, wiring conventions, test procedure, and the mistakes that damage equipment.
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Selection checks
- Supply voltage, phase and available fault current.
- Motor current, overload duty and required speed range.
- Load type: variable torque, constant torque or regenerative.
- Braking resistor, line reactor, EMC filtering and cable-length requirements.
- Control I/O, safety functions and communication protocol.
Commissioning sequence
- 1Record the motor nameplate voltage, current, frequency, speed and power before entering motor data.
- 2Confirm supply, motor, protective-earth and control wiring against the drive manual.
- 3Select the intended command source and speed-reference source; do not leave local keypad control active by accident.
- 4Test direction at low frequency with the machine uncoupled or otherwise made safe.
- 5Prove acceleration, deceleration, current limit, stop behaviour and representative fault recovery under load.
Troubleshooting answers
Frequently asked questions
Does a VFD reduce motor voltage as well as frequency?
Yes. The drive coordinates output voltage and frequency to maintain appropriate motor flux, then applies its control method and current limits across the operating range.
Can a VFD be used as an emergency stop?
A normal run command is not a safety function. Safety designs may use certified Safe Torque Off or external isolation as required by the risk assessment and drive documentation.
Why does a VFD trip during deceleration?
A fast-ramping load can regenerate energy into the DC bus and cause overvoltage. A longer ramp, suitable braking method or regenerative drive may be required.
Now use it in a machine
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