Variable Frequency Drives, Tesla and Energy Efficiency

Variable Frequency Drives (VFDs) are known by many names: Variable speed drives, Adjustable speed Drives, Adjustable Frequency Drives, Variators and Frequency Converters. Contrary to popular belief, VFDs are not limited to industrial settings but are in fact quite ubiquitous. Their applications range from smaller appliances to larger motors in building, agricultural and industrial settings.

AC Induction Motors: The Geek’s Gift

To learn about VFDs we must talk one of the greatest inventors of the 20^th century. Nikola Tesla’s contribution to the design of modern AC electricity supply systems is particularly well known.

We also know about the 3-phase AC Induction motor which he patented in 1888. The AC induction motor invented by Tesla was more efficient and reliable than (his supposed arch-nemesis) Edison's direct current (DC) motor. However, one significant weakness of the invention was that speed control in AC motors unlike DC motors was an extremely difficult task.

For DC motors, speed control could be achieved by installing a simple rheostat. However, for AC motors it requires varying the magnetic flux or changing the number of poles on the motor. Even decades after AC motors became widespread, varying the frequency for speed control in AC motors remained a concept per se.

It was only in the 1980s, almost 100 years after their introduction by Tesla, that varying AC motor speeds entered the mainstream of industry.

Variable Frequency Drives: A century’s worth of effort

The innovation that allowed for changing AC motor speed was Variable Frequency Drive technology. A Variable Frequency Drive (VFD) is a device that controls the voltage and frequency that is being supplied to a motor and therefore controls the speed of the motor and the system it is driving.

There are three types of VFDs: Current source inversion (CSI), Voltage source inversion (VSI) and Pulse-width modulation (PWM) VFDs.

Of these the first two are not widely used due to issues with motor cogging, i.e. motor refusing to start even when full voltage is applied. PWM VFDs are widely used in industry because of excellent input power factor due to fixed DC bus voltage, absence of motor cogging, higher efficiencies, and lower costs.

VFDs: How do they save energy?

A motor does not need to operate at the same load throughout the day. Depending on demand, the load required varies throughout the day. With VFDs you can control the speed of the motor according to the load. This reduces excess energy from being wasted.

This can result in tremendous savings especially for machines such as variable-torque centrifugal fans and pump applications. The load's torque and power for these two set of machines varies with the square and cube, respectively, of the speed. A relatively minor reduction in speed can thus lead to a large power reduction. For example, at 63% speed a motor load consumes only 25% of the power it would have consumed at 100% speed. That’s some serious energy saving!

VFD and Energy Savings: We are talking about some big numbers here

Electric motors are the single biggest consumer of electricity worldwide. According to IEA estimates, they consume about 45 per cent of global power produced. Lighting is a distant second with 19 per cent.

Although, VFD as the go-to option for reducing energy consumption in motive loads has been around for a while, it had seen limited adoption earlier due to high risk perception. The global market penetration of VFDs for all applications is still relatively small even though the VFD market is estimated to be nearly USD 19 billion in 2016, according to a report by MarketsandMarkets.

The same report projects the VFD market to grow by 6.9% to touch USD 27 billion by 2021.

VFDs: They do more than just energy savings

In addition to reducing energy costs, VFDs also save costs in other ways by:

• Increasing system reliability
• Reducing downtime
• Reducing equipment setup time
• Lowering maintenance cost
• Lessening wear and tear of machine

VFDs in India: Energy Efficiency, PAT and Climate Change

Experts believe that there is a huge untapped potential for VFDs in the Indian industry. Sectors including Iron and Steel, Cement, Aluminum and Fertilizer in particular have a sizable potential for this application in India.

The significance of VFDs has risen in the context of the Perform, Achieve and Trade (PAT) scheme of the government of India launched in 2008. The PAT scheme imposes mandatory specific energy consumption targets on certain designated industries with less energy efficient facilities having higher reduction targets than the more energy efficient ones.

The scheme has acquired renewed significance in the context of India’s Intended Nationally Determined Contributions (INDCs) which have pledged reduction in emissions intensity of its GDP by 33 to 35 per cent by 2030 from the 2005 level.

Simply put, India has to become more energy efficient and what better way to do so than by deploying VFDs for industrial motor applications; which as we discussed earlier are the biggest consumer of electricity globally.

Thus, VFDs would have an important role to play if India wishes to achieve its emission targets. Sustainability Outlook in its PAT Pulse 2 Report estimates a potential market size of more than USD 1 billion (INR 7 billion) for Variable Frequency Drives across sectors in India.

VFDs primarily have two main market segments, namely High Tension drives (HT) and Low Tension drives (LT), with the former having a relatively higher potential in the Indian industry and being more efficient in terms of savings per unit investment. Both the segments have seen varying penetration rates in the Indian Industry with the uptake of LT drives being relatively higher than that of the HT drives. This is because, while on the one hand there the upfront capex for LT drives Rs.6000-7000/kW and simple payback is <2 years the capex for HT drives is Rs.15000-20000/kW and the associated payback period is 3-4 years. Furthermore, the costs of LT drives have fallen in the past decade but the costs of HT drives haven’t seen a similar drop.

Click here if you want to know more about how VFDs, WHRs (waste heat recovery systems) and other technologies can contribute to realization of India’s energy efficiency targets.

In conclusion, VFDs hold enormous potential for energy efficiency across the globe. It is time industry leaders and policy-makers realize its importance and make the necessary efforts to attain our shared global goal of sustainable energy.