Understanding Variable Frequency Drives and the appropriate application

A variable frequency drive (VFD) is a controller that drives an electric motor by varying the frequency and voltage supplied to the motor. VFDs can be called many different names, but are most often referred to as variable speed drives (VSDs), inverters, or inverters.

At the most basic level, a VFD rectifies the supplied AC voltage and stores the resulting DC voltage in its capacitor bank. This DC voltage is then rectified to the desired voltage and frequency, causing the motor to rotate at the desired speed and torque required by the process, which may be an air compressor, fan or other device.

How a variable frequency drive works?

The first stage of a variable frequency AC drive is the rectifier, which consists of a set of gating diodes or silicon rectifiers (SCRs). These diodes are usually in groups of 6, and there may be 6 to 24 diodes in total. The diodes allow current to flow in only one direction, similar to a check valve used in a piping system. For example, when the A-phase voltage (a voltage similar to the pressure in a piping system) is more positive than the B- or C-phase voltage, the diodes will open and allow current to flow. When phase B becomes more positive than phase A, the phase B diode will open and the phase A diode will close. The same is true for the 3 diodes on the negative side of the bus. Thus, we get six current "pulses" as each diode opens and closes. For a common 480V RMS power system, the AC ripple will vary between 580V and 680V.

The second stage of the VFD is the capacitor bank, which is used to eliminate this ripple and produce a smooth DC voltage. The capacitor bank acts like a reservoir in a piping system and is designed to minimize ripple to produce a smooth DC voltage.

A diode bridge converts AC to DC, sometimes called a converter. Converters that convert DC back to AC are also converters, but are often referred to as "inverters" to distinguish them from diode converters. In the industry, it has become common to refer to any DC-AC converter as an inverter.

When one of the top switches in the inverter is closed, the motor is connected to a positive DC bus and the voltage on that phase becomes positive. When one of the bottom switches in the converter is closed, the connection is connected to a negative DC bus and becomes negative. Thus, any phase on the motor can become positive or negative, and any frequency can be generated.

The VFD output is in the form of a "rectangular" waveform, rather than the sine output shown in blue in the diagram above. It is important to note that the type of motor used in the VFD application can operate normally with a square wave. However, not every motor is designed this way, so it is important to make sure the motor is suitable for VFD applications. To reduce the motor frequency to 30 Hz, the inverter output transistor can simply be switched more slowly. However, if the frequency is reduced to 30 Hz, then the voltage must also be reduced to 240V to maintain the V/Hz ratio.

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