By Isaiah David Updated Mar 24, 2022
Electric power comes in two flavors: alternating current (AC) and direct current (DC). DC flows in one direction, while AC reverses polarity many times per second. AC motors draw power directly from AC. The motor’s speed is proportional to the frequency of the alternating voltage; a faster frequency means a faster spin. An AC controller therefore changes the current’s frequency to adjust the motor’s speed.
Motor controllers are normally supplied with mains AC. The first step inside the controller is to rectify that AC into DC. A rectifier, built from diodes, acts like one‑way valves. During the negative half‑cycle a diode connected to the negative rail conducts while the opposite diode blocks the positive rail; the roles reverse during the positive half‑cycle. The result is that all negative current is combined into one wire and all positive current into another, yielding a stable DC supply for the rest of the circuit.
The final stage is to recreate an AC waveform at the desired frequency. High‑speed electronic switches (typically MOSFETs or IGBTs) turn on and off thousands of times per second. Each pulse nudges the voltage up or down in tiny increments, producing a staircase‑like waveform that closely mimics a true sine wave. This pseudo‑sine wave is then fed to the motor, giving it the precise voltage and frequency needed for smooth, variable speed operation.
