The way a VFD facilitates phase conversion differs, compared to phase converters. During installation, the device is wired to the supply line and motor. Compared to a phase converter, controls take place on the VFD interface – not a set of switches or using starters. Common applications include drill presses, large mills and pumps, CNC, cranes, automation and lathes. It’s important to highlight that when sizing a VFD, one must check for the Full-load Amps (FLA) rating on the nameplate, not the horsepower rating.
During operation, the unit converts the single-phase AC supply line (input) into DC, which is converted to three-phase AC on the output side. To start, a bridge rectifier (consisting of 4 or 6 semiconductor diodes) is used to facilitate the initial conversion from AC to DC voltage. At this stage, the DC voltage generated is ‘dirty’ or filled with ripples in its waveform. Next, a capacitator bank acts as a filter to smoothen out the DC voltage. In the next section of the VFD, DC voltage is converted to AC three-phase, via an inverter. Switches in the inverter manages positive and negative voltages that are needed to generate the desired phase and frequency.
The wave produced by the VFD on the output side is rectangular, as the units are unable to generate a sinusoidal wave.
Which One is Better?
Both devices can facilitate phase conversion, from single phase to three phase. Choosing one over the other depends on a handful of factors, such as the number of machines connected to the unit, motor power requirements and costs. All of these variables will be addressed in this section of the article.
For operators who need to simultaneously run more than one three-phase machine from a single-phase supply line, a phase converter is highly recommended. VFDs are more applicable to running one connected machine at a time, as well as for highly accurate control of starting and stopping connected motors, such as an industrial conveyor belt in a manufacturing plant.
As for specific types of phase converters, a static phase converter is suitable for light loads and machines with minimal starting loads. With this in mind, if you’re operating large saws and air compressors at the facility, it would be more advisable to consider a rotary phase converter because the units are more capable of handling large starting loads.
Compared to phase converters, which cannot adjust frequency or motor speed, VFDs provide more control over several aspects of power regulation and distribution. Such factors include motor speed, frequency, direction and soft starting (great for preserving the lifespan of large machines and reducing inrush currents).