Does a VFD pay off?
In turbulent times companies are cautious of making investments. However, exact these periods are times when specific companies set apart from the others by making smart decisions. VFD can reduce the energy consumption, make the production more flexible and improve the quality of the products – just to mention the largest benefits. Still, you may ask yourself the question: Does a VFD pay off for my company/plant/process/service?
In most cases VFD definitely pays off. There will be special cases where it maybe does not. Whether the answer is yes or no depends partly on the process, partly on system design and partly on the end user himself.
Energy efficiency and energy savings
We have already described in multiple articles what is the background of energy savings and how to calculate them. There are many success stories where VFD significantly reduce the energy bill. If you design a new plant or a part of it and the VFD concept is included in the considerations then it results in most cases in a short payback time. If it is an existing plant you shall have lot of valuable operational data to decide whether VFD does pay off or not. In both cases the energy savings can be calculated. The precision of the calculation is as good as the quality of input data.
Pay as you save
New business concepts are emerging and one of them is ‘pay-as-you-save’ principle (PAYS). The idea is sort of VFD leasing where the user pays the VFD from the energy savings. In applications with short payback time this is an interesting approach how more users can benefit from VFDs even with limited financial resources. Again, a good energy efficiency tool helps to predict the saving potential and commercial viability of a specific case.
An energy saving assessment [2] is a starting point to find out whether your application is eligible for PAYS.
Reduced reactive power
The energy saving approach usually considers active power only (kW). However, VFD helps to reduce reactive power as well. That is an extra benefit and you surely like such ‘extras’, especially if they come free of charge. Diode front end (DFE) rectifier typically has input power factor above 0.95. This is better than any standard industrial asynchronous motor can reach. At partial load the VFD power factor further increases (towards 1.0) while for motors the power factor generally drops. This saved reactive power may reduce your energy bill or may reduce the demand for external reactive power compensation. What it for sure does is to reduce the power losses.
Motor start
VFD allows a smooth motor start. This is something that the utility will appreciate (no more large starting current). It is good for the motor itself (thermal impact of starting current) as well as for the load (large oscillating torque during direct on-line start). Finally you get more flexibility, as DOL motor is usually designed for 2-3 consecutive starts while with VFD there is normally no such limitation so you can start 10-time per hour if required.
More information on motor start with VFD is published in [4].
Extended lifetime of mechanical equipment
As mentioned, VFD enables a soft start that is not only gentle towards the grid and motor, but also towards the load. Reduced mechanical stress extends the maintenance intervals ($$ savings) as well as overall lifetime of mechanical parts. Motor air gap torque during direct on-line motor start is depicted in figure 5. In contrast, motor air gap torque during VFD motor start is shown in figure 6. When comparing the figures please note different scaling of y-axis.
While the transient motor torque during direct on-line (across the line) start reaches approx. 5 pu at low speed (example of typical 7 MW, 4-pole asynchronous machine), the VFD start-up features smooth ramping up of the torque and much lower torque ripple.
Utilize the digital potential
VFD offers vast potential to automate the process, collect and visualize various data, assess the health condition of the equipment etc. Most VFD products now come with excellent connectivity possibilities supporting almost any type of communication protocol.
The digital potential unlocks better possibilities for predictive maintenance. it also allows better overview of the process performance. Remote connectivity allows faster and more efficient troubleshooting by service experts.
Use VFD as smart sensor
VFD contains several measurements (usually quite precise) plus mathematical models to estimate other quantities from the measured signals. This is necessary for the VFD control and protection, but can be further utilized for condition monitoring.
Just imagine the potential on one example: VFDs are often speed-sensorless. It means that the rotor speed is not directly measured by speed encoder, but calculated by motor model from electric signals. This is state of the art. Now imagine that the VFD calculates the speed, detects an unbalance and notifies the user. He can then make a closer check that reveals e.g. problem with foundations or misalignment on coupling. You know what I mean? The VFD does not only controls and protects itself and the motor. It can even detect issues somewhere else in the drive system. This is a potential that is still mostly untapped.
That was just one example, but there is actually much more of such “low hanging fruit” that could be realized with almost no extra cost and very little effort.
Summary
There is lot of advantages and benefits that modern VFDs can provide. Diving deeper into that and estimating the value that it has for your business you could be surprised how attractive the VFD might be for your organization.
Many people would only look at energy savings. And this argument alone is in most cases strong enough to justify the investment into VFD technology. But even if the electricity payback does not look that great in your case (e.g. your process allows only very narrow range of speed variation) that are still many more benefits that you get in addition.
Commercial benefits when installing VFD:
– reduced energy consumption (less kW)
– reduced reactive power consumption (less kVAR) in most cases
– extended lifetime of mechanical parts due to reduced stress
– less maintenance on mechanical parts
– central collection of information about process performance
– predictive maintenance and extended diagnostics
References
[1] How to calculate VFD energy savings, https://mb-drive-services.com/how-to-calc-energy-and-cost-savings/
[2] Energy saving assessment, https://mb-drive-services.com/energy-saving-assessment/
[3] VFD power factor and reactive power consumption, https://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
[4] Motor start with VFD, https://mb-drive-services.com/vfd-motor-start/
[5] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
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