What is the difference between VFD and VSD?

What is the difference between VFD and VDS?

This article aims to answer one relatively simple question: What is the difference between VFD and VSD? As the question was raised again recently, we would like to deliver an understandable explanation.

Let’s start with the abbreviations first to ensure that we are on the same page:

VFD = Variable Frequency Drive

VSD = Variable Speed Drive

Variable speed drive (VSD) is any technology that drives the load and can adjust its speed. The solutions are based on electrical, mechanical or hydraulic principle.

Steam turbine is a mechanical driver. In conventional thermal power plants the turbine runs at constant speed as the frequency of the generator shall match the frequency of the power grid. However, steam turbine can be used for variable speed control as well. Certain industrial processes (e.g. refineries) produce steam as a by-product. Therefore, it might make sense to use such steam in a turbine to drive e.g. compressors.

Gas turbine is a mechanical driver, too. In fact, gas turbine is the main competitor of VFD in Oil & Gas industry to drive fix speed and variable speed compressors.

Hydrodynamic coupling, also called fluid coupling, is a hydraulic drive. It is used in combination with electric motor to adjust its speed in a limited range. Hydrodynamic coupling can only reduce the speed of the motor.

DC motor drive is also a variable speed drive when used with a controllable DC voltage source. In fact, DC drive was a benchmark for variable speed technologies due to its high performance and straight forward control implementation (at constant excitation speed is proportional to the induced voltage and torque to the armature current) .

Variable frequency drive (VFD) is another solution for variable speed control. The term VFD is used specifically for power electronic based frequency converters that supply AC electric motors. VFD can be realized as current-source inverter (CSI) or voltage-source inverter (VSI).

Therefore, VFD is a subset of a larger VSD family. That family includes other technologies as well (various types of turbines, fluid couplings, diesel engines, DC motor drives etc).

Take away

VSD operate on mechanical, electromechanical, hydraulic or electronic principle. However, VFD is a term exclusively used for (power) electronic solution.

Few years back I have read a quote that summarizes above explanation very well:

“Every VFD is a VSD but not every VSD is a VFD.”

It is basically the same like saying that strawberry is a fruit but not every fruit is necessarily a strawberry. Latest now I am sure that you got the point.

If you remeber the above quote then the blog post fulfilled its purpose.

Leading VFD manufacturers, such as e.g. ABB, call their products mostly VSD. As we have learned this is of course correct as VFD belongs to the VSD group. However, VFD as a term is more precise as it directly points out to a power electronic frequency converter. The F standing here for electric frequency is reserved for frequency converters.

Note that there are cases where different types of VSDs can be used to drive a load. One such example is an LNG compressor train. Such system may consist of a combination of a gas turbine and an electric drive (i.e. VFD plus motor). While the gas turbine provides most of the power, it does not have the starting capability and also its response time is rather slow. The VFD fed motor is then added to solve those two issues. It is therefore referred to as a “starter-helper”. This is actually a great example for our today’s topic. The main driver is mechanical (gas turbine) while there is an electronic driver (VFD fed motor) on the same shaft string.

Don’t get confused about other abbreviations and terms frequently used as synonyms for VFDs. In North America the term Adjustable Speed Drive (ASD) is very common and basically has the same meaning as VFD. Although strictly speaking it shall rather be Adjustable Frequency Drive (AFD), right? Another abbreviation is Static Frequency Converter (SFC). This term is also used for power electronic converters. However, it is not exclusive for motor drives. In fact, it is mostly used for power conversion of one fix frequency to other fix frequency (such as 50/60 Hz SFCs as grid interties or railway interties 50 Hz, 3-ph/16.7 Hz, 1-ph).

When the technology is clear from the context, then short expressions such as “converter” or “drive” are being used.

Table 1: Most common VSD technologies

Table 2: Terms frequently used as synonyms for VFD

The terms alternative used instead of VFD are shown in Table 2. Note that some of them are rather vague (frequency converter, converter, drive) but can be well used if the meaning is clear from the context. SFC is rather used for frequency conversion with fix input and output frequency.

Summary

These guys are VFDs:

But you can also call them VSDs (as ABB does, in fact).

VFD is a subset of VSD. The VSD family includes besides VFDs also other technologies for variable speed such as gas or steam turbines, fluid couplings or DC motor drives.

The different variable speed solutions were introduced in [1]. Some frequent terms and definitions have been summarized in [2].

If you want to learn more about particular advantages of electric drives against mechanical or hydraulic counterparts, refer to [3], [4] and [5].

And finally if you sincerely want to learn more about VFDs and drive systems and dive deeper into interesting subjects, hurry up and purchase our premium subscription. An investment that clearly pays off.