Understanding Variable Frequency Drives

A Variable Frequency Drive (VFD) is a type of controller that drives an electric motor by varying the frequency and voltage supplied to that electric motor. A VFD can have many different names but is most commonly referred to as a variable speed drive (VSD), frequency converter or an inverter.

At the most basic level, the 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 to spin the motor at the desired speed and torque required by the process, which could be an air compressor, fan or other equipment.

How Variable Frequency Drives Work

The first stage of a Variable Frequency AC Drive is the rectifier, which is made up of a group of gated diodes or silicon rectifiers (SCRs). These diodes are typically in a group of six and may vary from six to 24 diodes total. Diodes only allow current to flow in one direction, similar to check valves used in plumbing systems. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing systems) is more positive than B- or C-phase voltages, the diode will open and allow current to flow. When B-phase becomes more positive than A-phase, the B-phase diode will open and the A-phase 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 the common 480V RMS power system, the AC ripple will vary between 580V and 680V.

The second stage of a VFD is the capacitor bank which acts to remove this ripple and create a smooth DC voltage. The capacitor bank acts similarly to a reservoir in a plumbing system and is designed to minimize fluctations, creating a smooth DC voltage.

The diode bridge converts AC-to-DC and is sometimes referred to as a converter. The converter that converts the DC back to AC is also a converter, but to distinguish it from the diode converter, it is usually referred to as an “inverter.” It has become common in the industry to refer to any DC-to-AC converter as an inverter.

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

A VFD outputs the “rectangular” waves form, not the sinusoidal outputs shown in blue in the figure above. It is important to note that the types of motors used in VFD applications can run properly with the square waveform. However, not every motor is designed this way, so it is important to ensure that the motor is suited for a VFD application. To reduce the motor frequency to 30 Hz, one can simply switch the inverter output transistors more slowly. But, if the frequency is reduced to 30 Hz, then the voltage must also be reduced to 240V in order to maintain the V/Hz ratio.