Lesson 19 · Automation

Variable Frequency Drives (VFDs)

A VFD controls motor speed by controlling frequency. It soft-starts, saves serious energy, and lets a PLC run a motor at any speed.

What a VFD does

A VFD — Variable Frequency Drive — controls the speed of an AC motor. A motor across the line runs at one fixed speed set by the power frequency (60 Hz) and the number of poles. A VFD changes that frequency on the fly, so it can run the same motor at any speed from a crawl to full — and ramp smoothly up and down.

How it works (the idea)

An AC motor’s speed follows the frequency of its power. The VFD does three things in sequence:

Rectify: convert the incoming AC into DC.
Filter: smooth that DC on a capacitor bank (the "DC bus").
Invert: electronically chop the DC back into AC at whatever frequency it wants, using fast switching transistors (IGBTs).

Lower the output frequency and the motor slows; raise it and the motor speeds up. The drive carefully keeps the voltage-to-frequency ratio right so the motor makes proper torque at every speed.

Why use one

Energy savings. On pumps and fans, slowing the motor a little saves a lot of energy — running at 80% speed can cut power draw dramatically. This alone pays for many VFDs.
Soft start. The VFD ramps the motor up gently, eliminating the huge across-the-line inrush — it’s a reduced-voltage starter and a speed control in one.
Process control. Match motor speed to what the process needs — conveyor rate, flow, pressure — instead of running flat-out and throttling.
Less mechanical stress. Smooth ramps mean no belt-snapping torque shocks.

How it’s controlled

A VFD takes a start/stop command and a speed reference. The start/stop can come from its own terminals (a run contact) or over a network. The speed reference is usually an analog signal — often a 4-20mA or 0-10V signal from a PLC analog output, or a potentiometer. So a PLC can tell a VFD both "run" and "run at this speed," which is exactly how modern process control works.

Where it fits with what you know: a VFD replaces the contactor-and-overload starter for many motors — it does the starting, the protection, and adds speed control. You’ll still see contactors feeding VFDs, and control circuits enabling them, but the drive itself handles the motor.

A caution

VFDs introduce their own considerations — electrical noise, special cable, the DC bus stays charged after power-off (wait before servicing), and motors need to be VFD-rated for hard duty. Respect the drive’s manual and discharge times.

What to take away

A VFD varies an AC motor’s speed by varying its frequency — rectify to DC, then invert back to AC at the desired frequency. It saves energy, soft-starts, and enables true process control, taking a start/stop command and a speed reference (often from a PLC). It’s the modern way to run a motor that doesn’t need to be full-speed all the time.

Concept lesson: this one is about understanding equipment and ideas rather than wiring a circuit, so there’s no Sandbox build for it. From here, the remaining lessons cover practical field knowledge every electrician needs.