Best VFD topology - One size fit all?

Best VFD topology: One size fit all?

If you deal with variable frequency drives (VFD) you have certainly asked yourself the question: What is the best VFD topology? Why do the big brands have several different products in their portfolio? And is there anything like one size fit all?

Does this topic sound interesting? Then keep reading our article.

Search for the best VFD topology

Humans like to think in simple terms (and please don’t get offended). Above all, most popular videos on YouTube are “the best…”, “the most…”, “top 10” etc. Logically, many engineers would like to know what is the best VFD topology on the market right now. Of course, you want to buy the product with best performance (and affordable price). So let’s elaborate on this thought.

One size fit all

Imagine that you have your own workshop (maybe you actually have one). What tool is more important for you? A screw driver or a hammer? Or maybe a saw? A planer? A drilling machine?

Most likely your answer is that you need all of them. The choice depends on the job or task you are about to do. Right?

Exact same thing applies for the variable frequency drives. Nowadays, VFDs are used in large variety of applications. Naturally, the requirements of those applications are not all the same. Consequently, what is a good topology for particular application is not necessarily the best solution for another application.

Full palette of VFD models

Renowned VFD manufacturers have a full palette of VFD models. There must be a good reason for that. Otherwise they would streamline the portfolio to one single product which would obviously reduce their development and product maintenance cost. However, that is not the case and they maintain a variety of VFD topologies. It must be very costly! Each VFD topology has slightly different (or very different) software. The protection scheme may be somewhat different. There is larger variety of sub-components in smaller quantities. And still they do it…And the reason is simply because there is nothing like the “best VFD topology” fitting for all possible requirements.

Common VFD models and topologies include:

— 3-Level Neutral Point Clamped Voltage Source Inverters (3-L NPC VSI)

— 5-Level Neutral Point Clamped Voltage Source Inverter (5-L NPC VSI)

— Flying capacitor based Voltage Source Inverters

— Multicell (Cascaded H-bridge) Voltage Source Inverter

— Modular Multilevel Converter (MMC based VSI)

— Load-Commutated Inverter (LCI)

— Cycloconverter

Cycloconverters exist on the market for over 50 years. LCI can look back at similar track record. Major development of high power VSI happened in 1990s. Some of the topologies, such as MMC, were born relatively recently. Certainly, the technology evolves. But the different VFD topologies do not necessarily compete. Instead, they often complement one another.

Pros and cons of popular VFD topologies

Life is tough. You will not get all the goodies without any drawbacks. Let’s use few examples for illustration.

– Multicell topology is quite cheap and scalable. However, you cannot separate the complex multi-winding input transformer. Consequently, transformer losses are dissipated into the electric room. In addition, the drive requires larger footprint inside the building and the integrated transformer has substantial limitation in terms of input voltage. Moreover, protection of this topology is not very elegant, either.

– 3-L NPC without an output sine filter has excellent performance, particularly dynamic response. Therefore, it is the preferred topology for dynamic applications such as rolling mills. On the other hand, such drives require specifically designed inverter duty motors.

– Drives with an output sine filter are extremely motor friendly. So it does not surprise that they are popular in combination with “standard” motors or in motor retrofit cases. One drawback of this solution is reduced dynamics. Another one is limitation in terms of maximum power (bulky sine filter).

– MMC based inverters provide motor friendly output waveform and scalability in output voltage. But you pay for larger footprint and may need a redundancy due to larger quantity of active components. Also, constant torque applications are not the best fit for this topology.

– LCI has unbeatable power density and excellent reliability. Also, N+1 redundancy is very easy to realize (if needed). Yet, it requires a harmonic filter and only works together with a synchronous machine.

Link to the entire series LCI versus VSI

– Cycloconverter is still being used for low speed high power applications. It is simply a perfect fit there. But obviously it cannot be used for typical industrial motors.

Let’s face the truth

You cannot have ideal motor friendliness, smallest footprint, excellent dynamics, superior efficiency, best scalability, far reaching versatility, full modularity, highest reliability and fit for any thinkable motor. Well, actually you can! But not all of them at the same time. is it an issue? Most likely not. Depending on the application one criteria is essential, other is less important and another one may be irrelevant.

Finally, it is important to have a portfolio to select from and a partner to guide you through this selection.

Best VFD topology – Full article

This article was substantially shortened. Full version is available for our premium members.

Summary

VFDs are being further developed and improved. Yet, there is no “best VFD topology” that would cover all possible application needs. For instance, you can still find older VFD technology, such as LCI or Cycloconverter, being offered by leading drive manufacturers. Also within the family of Voltage Source Inverters there are several popular models, but none of them is able to cover all the needs.

However, that is no reason to be concerned. There is a broad portfolio to select from and you will find a suitable solution for your particular needs.

References

[1] Selection of a variable frequency drive, https://mb-drive-services.com/selection-of-a-variable-frequency-drive/

[2] LCI versus VSI – series comparing load-commutated inverter and voltage source inverter, https://mb-drive-services.com/category/lci-versus-vsi/

[3] Current source and voltage source inverter, https://mb-drive-services.com/current-source-and-voltage-source-inverter/

[4] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives

Best VFD topology – One size fit all?

Published by admin on February 26, 2021February 26, 2021