VFD multidrives: Additional energy savings
In our introduction [1] to VFD multidrives we have listed the key benefits of multidrive concept. One of them is improved energy efficiency. Now it is time to look into it closer and demonstrate that it is true. This article is about improved energy efficiency with VFD multidrives.
VFDs help to reduce the power consumption of industrial electric motors [2, 3]. The principle is well known and already used for several decades. With the multidrive solution you can further improve the energy efficiency and reduce the energy consumption. The concrete savings always depend case by case. However, we look at basic principles that reduce the energy bill. Some of them work always, others are applicable for specific applications.
What we personally like on improved energy efficiency is the fact that it helps to be competitive and reduces environmental impact at the same time.
1. Reduced transformer losses (better efficiency of one larger transformer)
The first principle benefits from having one larger input transformer rather than several smaller transformers. As you probably know, the efficiency of electric equipment increases with power rating. Therefore, larger transformer has lower losses (in %) than a smaller unit and the absolute losses of one bigger transformer are lower than sum of the losses of smaller transformers.
Note that this is a general rule that you can benefit from no matter if your application is regenerative or not. It is based on same motor power consumption for single drives and for multidrive. The saving seems rather small, but the cumulative savings over the lifetime of the equipment may still be quite interesting. Especially since you don’t pay anything extra for this efficiency improvement (vice versa, the capital investment into a multidrive is smaller than buying several single drives).
Let’s illustrate this consideration on a real example:
We have an application with three industrial asynchronous motors 2 x 4.0 MW and 1 x 2.5 MW. In one case each motor has its own drive, in other case a multidrive for all three motors is used.
2.5 MW motor requires approx. 2.9 MVA input transformer (considering power losses, input power factor 0.95 and grid undervoltage factor 0.95), 4 MW motors require approx. 4.7 MVA transformers and the multidrive has approx. 12.2 MVA input transformer.
Transformer no-load and load losses are based on real cases (projects). We will consider 100%, 75% and 50% loading in below calculations to illustrate the difference in losses. The no-load (core) losses of the transformer are kept constant while the load losses are scaled with square of the load. This is a typical calculation of transformer losses with reasonable accuracy. The transformer losses of three single drives are summed up and compared with losses of larger input transformer of a multidrive.
Remark:
We do two simplifications to make this model case easier to follow. Despite of these “shortcuts” the case is still accurate. Reasons for that are:
1. It is assumed that with 100% motor load the transformer sees 100% load as well. This is not fully true. Transformer is designed for certain grid undervoltage and the dimensioning power is not the same as the load. Therefore, 100% motor shaft power will lead to 90-95% loading of the transformer nameplate power. Therefore, the losses drop by 10-20% (0.9² = 0.81, 0.95² = 0.9).
2. At the same time the load losses are declared for sinusoidal supply and do not contain any harmonics. In real operation some harmonics are of course present and real losses will be slightly higher – typically 10-20% higher.
As you can see, both effects neutralize each other very well and therefore our “simplified” calculation is still rather accurate and at the same time easy to follow.
The comparison of three individual VFDs with one multidrive is summarized in Table 1.
Table 1: Transformer loss calculation for 100% / 75% / 50% loading – VFD single drives versus VFD multidrive
Such reduction of transformer losses in multidrive case is not huge. However, even small improvement of losses might turn into interesting savings. Let’s consider average loading of 75% resulting into 12.4 kW less transformer losses of a multidrive solution compared to three individual single drives (as per table above). With typical annual operating time of 5500 hours and electricity price of 8 cent per kWh (average industrial electricity prices in Europe, see [4]) it yields 5’456 EUR annual savings or 109’120 EUR savings over the 20 years of lifetime.
– Average 75% loading
– Transformer loss reduction 12.4 kW (54.1 kW instead of 66.5 kW)
– 5’500 hours per year
– Electricity price 0.08 EUR/kWh [4]
– Annual additional savings 12.4 kW * 5’500 h * 0.08 EUR/kWh = 5’456 EUR
– Accumulated savings over 20 years: 109’120 EUR
The calculation is oversimplified considering e.g. constant electricity prices in the future. In reality, the prices are expected to grow due to increase in consumption, shift to more renewable sources etc. All these aspects make the savings even more attractive.
Note that above calculation does not represent energy savings due to usage of VFD. It only shows additional savings of a VFD multidrive compared to three individual VFD single drives. Total energy savings due to VFD technology are much larger and not scope of this article. This portion is only an extra bonus coming from multidrive concept and synergies. Everyone can profit from it. It is not linked to a specific application. As soon as you have multiple motors that shall be speed controlled and you choose to supply them from a multidrive you will benefit from this effect.
After all you receive this financial benefit practically for free because the multidrive solution does not require higher initial investment than the solution with single drives. In fact, it is rather the opposite way – you save on CAPEX as well (more about it in a separate article within this series).
2. Reduced rectifier power due to regeneration through the common dc link
This principle results in substantial reduction of power consumption and savings as well. The reason why we mention it as number 2 is because it is not applicable for every application. However, multi-motor applications with machines mechanically interconnected such that one motor is driving while another one is braking heavily benefit from this principle. Let’s have a look.
It is important to understand the power flow in the corresponding VFD drive setup in order to understand the energy savings. We take again the example as in model case 1, but look at the 2 x 4 MW machines only. This time the application supports regenerative operation. When one 4 MW machine operates as motor, the other one runs in regenerative braking mode and vice versa. In rolling mills such mode is very common. The principle can be used in other applications as well. Particularly when the machines are mechanically connected through the load and work in push-pull kind of tandem.
For better visualization we will re-draw the figure 3 and leave the third machine out. Our focus is on the power flow and the associated losses. In both cases the power regeneration is used. However, in case of two single drives the power is exchanged through the grid side. The rectifiers and input transformers are substantially loaded and certain amount of losses is being produced.
In a multidrive the power is exchanged through the common dc bus. The rectifier and input transformer only transfer the system losses and therefore operate at very light load. Consequently, the power losses in those elements are much smaller compared to the solution with single drives.
The above case can be generalized a bit using typical efficiency values of VFD components and input transformers. The motor losses are the same regardless of single or multidrive.
Table 2: System power losses in a regenerative mode – single drives versus multidrive
Table 2 presents system losses in a regenerative mode as a percentage of rated motor power. We can see that a multidrive solution has roughly 3% less power consumption compared to single drives. In our model case it makes 3.1% savings which is 124 kW as absolute figure based on 4’000 kW machines. Interestingly, it is 10-times larger than the power savings in case 1 without regenerative operation that were based on reduced transformer losses due to better efficiency.
Using 124 kW difference and multiplying it with amount of hours operated in this mode per year and using your electricity price level (e.g. 0.08 EUR/kWh) you can directly calculate the extra energy savings.
Note again that the single drive solution already saves large amount of energy, especially in such regenerative setup. The calculated 124 kW in above example are additional savings on top when utilizing a multidrive.
Another obvious advantage of a multidrive in such case is that the rectifier and input transformer can be dimensioned much smaller. This benefit goes into the cost of equipment and capital expenditures. We will dig into the system cost in a separate article within the multidrive series.
Summary:
“1 for all, 2 for some”
Let’s quickly summarize the conclusions of this article. VFD multidrives enable to further boost the energy efficiency of VFD technology. Compared to conventional single drives a multidrive reaches additional energy savings.
First effect of one larger and more efficient input transformer rather than several smaller transformers is something that every user with multiple variable speed motors benefits from. Therefore we say “1 for all”.
The second effect counts for applications where some electric motors are in driving mode while other(s) are in regenerative braking mode. By exchanging and balancing the power through a common dc link substantial savings can be achieved (-> elimination of rectifier and transformer losses). Since not every application works in this way we call it “2 for some”.
References
[1] VFD dimensioning: Multidrives and their benefits, https://mb-drive-services.com/vfd-dimensioning-multidrives-and-their-benefits/
[2] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[3] Energy efficiency in variable speed motor drives, https://new.abb.com/drives/energy-efficiency
[4] Industrial electricity prices in Europe as per 2018, https://www.statista.com/statistics/267068/industrial-electricity-prices-in-europe/
