VFD Operation

Although VFDs offer many application benefits such as energy efficiency, reduced stress on motors and equipment, diagnostic capabilities, and process control integration—the primary reason to use a VFD is to control motor speed. The bonus is that lowering motor speed usually increases energy efficiency.

For example, using VFDs to lower speed or flow by just 20% can potentially reduce energy use by 50%. Although a VFD is typically about 95% to 97% efficient—its ability to directly vary motor speed instead of using dampers, flow controls and blocking valves usually results in an overall increase in system efficiency along with lower maintenance costs.

A VFD rectifies the AC line voltage into DC voltage, which is applied to a DC bus. The DC bus voltage is “inverted” into pulsed DC, the RMS value of which simulates a sine-coded AC voltage. The result is a pulse-width-modulated DC output gated through insulated-gate bipolar transistors.

The output frequency of a VFD typically varies from 0 to the AC input line frequency. However, on some applications, the frequency can exceed the line frequency to beneficial effect. Like RVSSs, VFDs use CTs to measure motor current. Sensing current allows VFDs to implement motor overload monitoring, stall prevention, and torque and current limiting.