All about variable frequency drives

21 June 2024

Today, variable frequency drives (VFDs) are ubiquitous components across many industries. Control Engineering Europe spoke to Kes Beech, Technical Manager at Invertek Drives, find out more about their history and how they are able to ensure greater efficiency in electric motor applications.

Q: How does a variable frequency drive work?
A VFD operates by transforming fixed frequency AC power into variable frequency and voltage. This control occurs in three stages – Firstly, AC input power is converted into DC power via a rectifier. Second, this DC power is smoothed and stored using capacitors within the DC link. Finally, the inverter utilises the DC power to synthesise a new AC waveform with the desired frequency and voltage to power the motor. This controlled supply enables the VFD to seamlessly adjust motor speed for specific application demands.

Q: Can you talk us through the history of VFDs and why they were originally created?
The ability of VFDs to precisely manipulate motor speed and torque significantly benefits energy efficiency, process control, and equipment longevity. 

Despite their widespread use in electric motor-controlled applications, many in the industry are still unaware of the core principles of VFDs, their evolution, energy-saving potential, additional advantages, potential drawbacks, and considerations for optimal VFD selection.

While primitive VFD concepts emerged in the early 20th century, it wasn't until advances in power electronics in the 1960s and 70s that VFDs became commercially viable. 

Early applications centred mostly around heavy industries which required precise speed control, such as textile manufacturing, paper mills, and mining operations. When their benefits were realised more widely, VFDs became a key component in most electric motor applications.

Q: Today VFDs have an important role to play in helping reduce energy consumption. Could you discuss  how this is achieved and what sort of energy savings are possible in a few typical application areas?
VFDs offer substantial energy savings by tailoring motor power consumption to match the exact requirements. Traditional motor control methods often rely on throttling or bypass mechanisms, resulting in energy waste.

In HVAC systems, energy savings of up to 50% can be achieved by matching fan or pump speeds to demand. Conveyor systems can realise 30-40% savings by adjusting speed based on load, while centrifugal pumps can see reductions of up to 60% through reduced-speed operation where possible.

Q: In addition to energy savings, what other advantages can VFDs provide?
In addition to energy savings, VFDs deliver several other performance advantages. Precise speed control enhances product quality and reduces process variation within many applications. 

Additionally, smooth starts will reduce wear and tear on motors, belts, and other driven components. Modern VFDs, such as Invertek’s Optidrive range, can also help improve an entire facility's power factor.

Q: Are there any potential drawbacks to using VFDs and if so how can these drawbacks be overcome?
Like any technology, VFDs are not without their potential shortcomings. But these can be easily overcome and, many of today’s drives have been designed to alleviate such issues. 

Some VFDs, for example can introduce harmonics into the power system. By choosing models with active front ends or harmonic filters this issue can be effectively addressed. 

The high-frequency switching of VFDs can, in some cases, accelerate motor winding and bearing wear – selecting VFDs with appropriate output filters and motor-rated cables helps mitigate this risk.

Invertek's design philosophy addresses potential VFD challenges. Our Optidrive drives incorporate features that minimise harmonics and protect motor integrity.

Invertek Drives Ltd has been designing and manufacturing its Optidrive range of VFDs for 26 years in the UK . Today it is able to offer a wide range of VFDs which are able to meet virtually all  motor control applications and have addressed any traditional drive issues through design.

Q: Today there appears to be a VFD designed for most applications – what advice can you offer to help ensure that engineers specify the right VFD for their application? What are the most important considerations?
While a VFD's technical specifications – horsepower, voltage, current rating – are essential, they are merely the foundation of any decision. A deeper understanding of the application is paramount. Start by meticulously analysing the load profile: is it constant torque, variable torque, or something in between?  Different loads demand different VFD capabilities.

Consider the operational environment. Will the VFD be exposed to harsh conditions like dust, moisture, or extreme temperatures? IP ratings and enclosure types become crucial here, such as IP66 rated enclosures for outdoors or harsh environments. 

Additionally, never overlook the control requirements. Does the application require simple speed control or a more sophisticated setup with complex feedback loops and communication protocols?

Beyond the technical specifications, consider the manufacturer's reputation and support. A reliable VFD is an investment in your operation's efficiency and longevity. Look for manufacturers with a proven track record of delivering high-quality products backed by comprehensive technical support.

Don't hesitate to engage with VFD specialists directly. There are many companies involved in the sales and support of VFDs.  Their expertise can be invaluable in translating your application needs into the right technical specifications. A collaborative approach can lead to a VFD solution that not only meets but exceeds your expectations.

As VFD technology continues to advance, we anticipate even more specialised solutions will become available, tailored to meet the specific needs of niche applications. However, the core principles of VFD selection will remain the same: understand your application, scrutinise the technical specifications, and prioritise quality and support.”

Variable frequency drives continue to be a cornerstone of industrial automation. Their ability to conserve energy, enhance control, and extend equipment life makes them invaluable. They are also crucial in the work to reduce emissions globally through the use of more efficient systems that consume less energy.

By understanding their working principles, application benefits, potential drawbacks, and selection criteria, engineers can harness the full power of VFDs across a vast range of industrial processes.

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