A variable frequency drive does this by taking the AC supply, converting it to DC, and then converting it back again to AC. The VFD consists of three main sections. The rectifier is made up of six (or four for single phase supplies) diodes that conduct one way only. In the arrangement shown in figure 1, a DC voltage is built up on the capacitor (which stores energy a little like a battery) and supplies the load resistor.
Figure 2 shows the rectifier and capacitor connected to an inverter. The inverter uses six IGBTs to convert the DC back to AC. The IGBTs are fast, electronic switches; by switching them on and off very rapidly, and adjusting the time that they are on and off, the current on the motor can be controlled as needed. Varying the on and off times is known as Pulse width Modulation (PWM). Because the motor current changes relatively slowly, the pulses of current are averaged out, and a nice sine wave current of any practical frequency can be built up, as shown in figure 4. The average voltage can also be controlled, but the motor voltage will still consist of a series of square waves.
The diodes in the inverter allow current to continue to flow in the motor as the IGBTs switch on and off.
The next article will look at the practical design of a variable frequency drive.