industrial//industrial automation//VFD

2026-09-06

A VFD (variable frequency drive) is a motor controller that sets the speed and torque of an AC motor by changing the frequency and voltage it feeds. In a machine the chain is sensors, then the PLC, then the VFD, then the motor: the PLC decides what should happen, the drive solves the electrical control of the motor. It is the industrial actuator that turns a numeric setpoint into shaft speed, the box between the controller and the motor.

A PLC output rarely powers a motor directly. It commands the drive with a setpoint (speed, torque, start, stop), over a fieldbus or industrial Ethernet such as PROFINET, and the drive closes its own fast current and speed loops inside.

Many drives run without a position sensor on the motor. The microcontroller that generates the PWM at tens of kilohertz also runs an observer, a copy of the motor model corrected by the measured currents, that estimates the rotor position and speed; the rule of thumb is to make it several times faster than the speed loop it feeds (Luenberger observer, PWM). Removing the encoder removes a cable and a failure point, at the price of poor estimates near standstill.

Drives can be told which speeds to avoid. A skip-frequency setting makes the drive pass quickly through the speeds where the motor would excite a resonance of the pump, fan or frame it drives.

Drives are sold by the big automation vendors (ABB, Siemens, Rockwell, Schneider) and are one of the components a system integrator has to make talk to a PLC of another brand.

Beyond speed, a modern drive is a data source: current, torque, temperature, energy, fault codes, all readable as tags for condition monitoring (telemetry).