Why do larger drives achieve better efficiency?
How does the efficiency of a drive system scale with the power? And why do larger drives achieve better efficiency? That is the subject of this blog post.
What is efficiency
Efficiency is the ratio between output power and input power, typically expressed in percentage (%). The higher this ratio the more of the input power is converted into the useful output power. The difference between input and output power represents the losses.
Losses are normally the portion of input power that is not effectively used. Almost all losses are in form of heat. Copper wire has certain resistance R. When the wire conducts a current i the resulting heat losses equal R⋅i². Machine bearing faces certain friction while the shaft rotates. These friction losses also represent thermal power that heats up the bearing or the lubrication oil etc.
Natural aim of the design engineer is to minimize the losses and maximize the efficiency of the device or system. This has both economical and technical reasons. Nowadays we shall mention third important motivation which is the environmental aspect and aim to reduce carbon footprint. Finally there is also a legal reason as certain equipment must fulfill minimum efficiency target valid in the country of installation.
Efficiency scaling with power
The increase of rated power is typically followed by an improvement in efficiency. This “rule” applies practically to all system components. The trend is most prominent for motors and transformers but can be partly seen in VFDs as well.
What are the reasons for improved efficiency at higher power rating?
1. Reduced operational cost
Industrial electric equipment has a design lifetime of 20 years or more. The cost of electric energy accumulated over the lifetime usually largely exceed the initial equipment purchase price. Even a small improvement in the efficiency results in considerable savings. This principle applies to each drive systems regardless whether it is small or large (in terms of power). However, people are naturally more focused on the larger equipment.
Of course, 100 small drives might have larger total consumption than one large drive. Anyway, the large one stands typically more in the spotlight and gets more attention.
2. Larger equipment gives more freedom for efficiency optimization
Equipment in lower power range is mostly produces in larger scale (mass production). The factors such as material cost or compact design are high in the priority list. And customers may be less sensitive regarding efficiency. Thus price is often the main driver. Measures increasing the efficiency beyond certain value may become so costly that the product would not be competitive and the payback would not be guaranteed.
In higher power range the situation is different. The designer has more freedom to optimize the performance of the device or system incl. losses and efficiency. And the customer is more likely willing to pay for a premium efficiency as he understands that it will pay off.
3. Technical necessity due to higher power density
Equipment for higher power rating has higher power density. If the efficiency remains the same it means significantly more losses per unit of volume. That would lead to a major challenge to ensure a proper cooling. One way is to use more efficient cooling medium, other way is to reduce the losses. In practice, both ways are applied.
The power rating approximately scales with the volume (and mass) of the equipment. However, the cooling surface increases as slower pace. We can see it in Figure 1 on an example of a cube. With a length of 1 meter the cube has a volume of 1 cubic metre and surface of 6 square meters. When increasing the length to 1.2 meters the volume increases to 1.73 cubic meters while the surface only reaches 8.64 square meters. Expressing it in percentage the surface increased by 44% while the volume has grown by 73%.
The consequences are essential. If the efficiency of a larger device remains the same as of the smaller device then the losses in % do not change. Device represented by the larger cube would have 73% higher power rating and also 73% higher losses. As the surface only increased by 44%, each square meter of the surface would need to evacuate 20% more losses than in the smaller cube.
This challenge is approached by increasing the cooling efficiency on one hand but also by improving the efficiency on the other hand. Thus, the losses grow slower than the rated power.
4. Legal requirements
There is also a legal frame that impacts efficiency and losses. Some countries have introduced directives and regulations that define minimum efficiency which the equipment must fulfill. And the minimum efficiency targets again scale with rated power (with the background based on arguments above).
Some of the currently used directives regarding energy efficiency:
- EU Directive 2009/125/CE (known as “Ecodesign”) applying for power transformers
- US Department of Energy (DOE) efficiency regulations (e.g. 10 CFR Parts 431 for distribution transformers)
- EU Minimum Energy Performance Standard (MEPS) for electric motors
- EU 2019/1781 Ecodesign for motors and drives
These directives often have several stages. Each new stage increases the minimum efficiency. Aim of these stages is to give the manufacturers some time to prepare for the new regulations.
Fifth point could be motives such as company’s commitment to improve its energy efficiency and reduce the impact of their business on the environment. That is basically complementary to point 4 but more on a volunteer basis rather than being forced by the law.
How large is the efficiency gain?
Now we understand why larger drive achieve better efficiency. But how large is this efficiency gain? On the VFD level itself the improvement in efficiency with increasing power rating is not so significant (and there are other factors, such as e.g. the VFD topology). However, similar or better gains are achieved on the transformer and motor. Thus, the combined efficiency (transformer + VFD + motor) of a high power drive can be approx. 0.5 – 1% higher than his smaller brother.
Summary
Efficiency of main components of a drive system typically improves as the rated power power increases. This post explains why larger drives achieve better efficiency. Such trend has both commercial and technical motivation. While in lower power range there is a strong focus on the purchase price, in high power range the low loss design receives more appreciation. High power equipment offers also more flexibility for the designer to “tune the parameters”. And there are other practical reasons such as the cooling demand. Finally, many countries have already put various directives defining the minimum efficiency which the equipment must meet.
You have read a short blog post version of why larger drives achieve better efficiency. To access the full article purchase one of our premium plans and start discovering the world of variable frequency drives and drive systems.
References
[1] What efficiency can you expect from your drive system? https://mb-drive-services.com/energy-efficiency-part-5/
[2] Ecodesign for motors and drives, ABB, https://new.abb.com/motors-generators/ecodesign-motors-drives
