Cost benefits of AC drives

By ABB.

In addition to their technical advantages, AC drives also provide many cost benefits. In this post, these benefits are reviewed, with the costs divided into investment, installation and operational costs.

AC Motors with and without AC Drives
AC Motors with and without AC Drives

At the moment there are still plenty of motors sold without variable speed AC drives. This pie chart shows how many motors below 2.2 kW are sold with frequency converters, and how many without. Only 3% of motors in this power range are sold each year with a frequency converter; 97% are sold without an AC drive.

This is astonishing considering what we have seen so far in this site. Even more so after closer study of the costs of an AC drive compared to conventional control methods. But first let us review AC drive technology compared to other control methods.

Technical differences between other systems and AC drives
Technical differences between other systems and AC drives

AC drive technology is completely different from other, simpler control methods. It can be compared, for example, to the difference between a zeppelin and a modern airplane.

We could also compare AC drive technology to the development from a floppy disk to a CD-ROM. Although it is a simpler information storage method, a floppy disk can only handle a small fraction of the information that a CD-ROM can.

The benefits of both these innovations are generally well known. Similarly, AC drive technology is based on a totally different technology to earlier control methods. In this site, we have presented the benefits of the AC drive compared to simpler control methods.

No mechanical control parts needed
No mechanical control parts needed

To make a proper cost comparison, we need to study the configurations of different control methods. Here we have used pumping as an example. In traditional methods, there is always a mechanical part and an electrical part.

In throttling you need fuses, contactors and reactors on the electrical side and valves on the mechanical side. In On/Off control, the same electrical components are needed, as well as a pressure tank on the mechanical side. The AC drive provides
a new solution. No mechanics are needed, because all control is already on the electrical side.

Another benefit, when thinking about cost, is that with an AC drive we can use a regular 3-phase motor, which is much cheaper than the single phase motors used in other control methods. We can still use 220 V single phase supply, when speaking of
power below 2.2 kW.

Factors affecting cost
Factors affecting cost

This list compares the features of conventional control methods with those of the AC drive, as well as their effect on costs. In conventional methods there are both electrical and mechanical components, which usually have to be purchased separately. The costs are usually higher than if everything could be purchased at once.

Furthermore, mechanical parts wear out quickly. This directly affects maintenance costs and in the long run, maintenance is a very important cost item. In conventional methods there are also many electrical components. The installation cost is at least
doubled when there are several different types of components rather than only one.

And last but not least, mechanical control is very energy consuming, while AC drives practically save energy. This not only helps reduce costs, but also helps minimize environmental impact by reducing emissions from power plants.

Price comparison for pumps
Price comparison for pumps

Investment costs: mechanical and electrical components

In this graph, the investment structure as well as the total price of each pump control method is presented. Only the pump itself is not added to the costs because its price is the same regardless of whether it’s used with an AC drive or valves. In throttling, there are two possibilities depending on whether the pump is used in industrial or domestic use. In an industrial environment there are stricter requirements for valves and this increases costs.

The motor

As can be seen, the motor is much more expensive for traditional control methods than for the AC drive. This is due to the 3-phase motor used with the AC drive and the single phase motor used in other control methods.

The AC drive

The AC drive does not need any mechanical parts, which reduces costs dramatically. Mechanical parts themselves are almost always less costly than a frequency converter, but electrical parts also need to be added to the total investment cost.

After taking all costs into account, an AC drive is almost always the most economical investment, when compared to different control methods. Only throttling in domestic use is as low cost as the AC drive. These are not the total costs, however. Together with investment costs we need to look at installation and operational costs.

Installation costs: throttling compared to AC drive
Installation costs: throttling compared to AC drive

Because throttling is the second lowest investment after the AC drive, we will compare its installation and operating costs to the cost of the AC drive. As mentioned earlier, in throttling there are both electrical and mechanical components. This means twice the amount of installation material is needed.

Installation work is also at least doubled in throttling compared to the AC drive. To install a mechanical valve into a pipe is not that simple and this increases installation time. To have a mechanical valve ready for use usually requires five hours compared to one hour for the AC drive. Multiply this by the hourly rate charged by a skilled installer to get the total installation cost.

The commissioning of a throttling-based system does not usually require more time than commissioning an AC drive based system. One hour is usually the time required in both cases. So now we can summarize the total installation costs. As you can
see, the AC drive saves up to USD 270 per installation. So even if the throttling investment costs were lower than the price of a single phase motor (approximately USD 200), the AC drive would pay for itself before it has even worked a second.

Operational costs: maintenance and drive energy
Operational costs: maintenance and drive energy

In many surveys and experiments it has been proved that a 50% energy saving is easily achieved with an AC drive. This means that where power requirements with throttling would be 0.75 kW, with the AC drive it would be 0.37 kW. If a pump is used
4000 hours per year, throttling would need 3000 kWh and the AC drive 1500 kWh of energy per year.

To calculate the savings, we need to multiply the energy consumption by the energy price, which varies depending on the country. Here USD 0.1 per kWh has been used.

As mentioned earlier, mechanical parts wear a lot and this is why they need regular maintenance. It has been estimated that whereas throttling requires USD 40 per year for service, maintenance costs for an AC drive would be USD 5. In many cases however, there is no maintenance required for a frequency converter.

Therefore, the total savings in operating costs would be USD 185, which is approximately half of the frequency converter’s price for this power range. This means that the payback time of the frequency converter is two years. So it is worth considering that instead of yearly service for an old valve it might be more
profitable to change the whole system to an AC drive based control. To retrofit an existing throttling system the pay-back time is two years.

Total cost comparison
Total cost comparison

In the above figure, all the costs have been summarized. The usual time for an operational cost calculation for this kind of investment is 10 years. Here the operational costs are rated to the present value with a 10% interest rate.

In the long run, the conventional method will be more than twice as expensive as a frequency converter. Most of the savings with the AC drive come from the operational costs, and especially from the energy savings. It is in the installation that the highest individual savings can be achieved, and these savings are realized as soon as the drive is installed.

Taking the total cost figure into account, it is very difficult to understand why only 3% of motors sold have a frequency converter. In this post we have tried to present the benefits of the AC drive and why we at ABB think that it is absolutely the best possible way to control your process.

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