Multidrives and their benefits
Multidrive concept is a VFD solution where multiple motors are supplied and controlled from one VFD having several inverters connected to a common dc link. This concept has several advantages and is especially popular in certain applications, for instance rolling mill drives or specific test stands. Our article today is dedicated to multidrives and their benefits. And there are quite many advantages. It is worth reading this post to learn how to benefit the most from this VFD solution.
Concept of multidrives
VFD multidrive is a concept where multiple inverters are connected to one common dc bus. The inverters supply separate motors and have their own control. Each motor can be operated fully independent from other machines. VFD multidrive allows a combination of different types of motors (induction machines, synchronous electrically excited machines, permanent magnet synchronous machines).
Compared to multiple single drive units a multidrive solution offers long list of benefits. We would like to emphasize them in this short series to make sure that you are aware of them and can benefit from this powerful drive concept.
Application example of a multidrive
Applications predestined for using of multidrives are reversing rolling mill applications in metals industry. One such example is depicted in Figure 2 below.
From principle point of view the machines on each side always operate inverted. When one machine works as motor the other one acts as generator and vice versa. Therefore, a multidrive configuration brings considerable advantages.
Modularity: From small to large
VFD multidrives normally incorporate a modular design. The rectifier(s), dc link and inverter(s) can be sized partly independently by following basic dimensioning rules. Such flexibility allows the manufacturer to find the best solution for specific demands. Of course, there are some physical limitations. The multidrive concept of ABB allows to combine up to eight (8) active modules within one lineup. As active module is considered the active rectifier (ARU) or inverter (INU) module. You are free to combine them as needed.
Due to power regeneration through the dc link (will be explained further) the inverter total power capacity is often higher than the power rating on the grid side. Example of a “max configuration” would be for instance multidrive with two active rectifier modules and six inverter modules.
There are several power ratings of the active modules. Most common ratings are e.g. 5 MVA, 7 MVA, 9 MVA or 12 MVA. By combining these modules within a converter lineup you can very well match the required motor power without overdimesioning of the VFD. Such modular concept applies for multidrive as we as single drives. For multidrives, however, the benefits become even more prominent.
Value proposition of multidrives
1. Improved energy efficiency
With a multidrive solution you always save more energy compared to multiple single drives. That is a strong statement, but we will demonstrate to you that this is true. Always.
Having multiple motors in an application where one machine is driving and at the same time another machine is braking it is possible to regenerate the power through the dc link. Therefore, losses in both rectifiers and both input transformers can be saved.
However, also when all machines are driving at the same time you can reach some energy savings. In electric machines (transformers, motors) the efficiency scales up with increasing rated power. Therefore, one larger input transformer will always have slightly better efficiency than two individual smaller transformers.
2. Reduced footprint
Using a VFD multidrive concept you always save some footprint. The savings can range from small (e.g. one module less) to significant. You will see couple of examples in the upcoming articles to convince yourself. For example, a multidrive with two motors might still use the same physical size of water cooling unit as the water cooling unit in each single drive. Hence you save the space of one such unit. We will also show examples where the savings on footprint are far more interesting.
Besides reduced footprint of the multidrive itself compared to several single drives you save substantial space (and cost) on input transformer and switchgear.
3. Reduced rectifier and transformer size
Multidrive concept, as already mentioned, is perfectly suited to use power regenerated from one machine operated at given instant as generator and feed it to another machine operated as a motor. In metals applications both machines usually have the same power rating. Consequently, their power balances nicely and there is very small power consumption from the grid just to cover losses of inverters and motors.
Consequently, the user benefits from smaller and cheaper rectifier and transformer. Savings are also on the interconnecting cables (smaller cross section and/or less cables in parallel).
4. Reduced investment cost
Capital expenditures (CAPEX) of a multidrive are lower than equivalent sum of single drives. The savings can differ from few percent up to substantial portion of the investment. Where do the savings come from?
(i) multidrive itself is more cost effective than the sum of single drives for the same total power
(ii) rectifier of the multidrive may, depending on the application, be dimensioned much smaller compared to the sum of power ratings of individual rectifiers
(iii) one input transformer instead of multiple transformers
(iv) one input circuit breaker instead of multiple ones
(v) lower cost for transporation
(vi) lower installation cost
(vii) lower commissioning cost
We will go deeper into the cost savings in a separate article within this multidrive series.
5. Reduced operation cost
Operational expenditure (OPEX) is lower compared to individual drives due to better efficiency, resp. smarter power management. The maintenance cost is lower and the maintenance tasks are less time consuming. Certain components experience less cyclic loading due to dc bus regeneration leading to longer lifetime / slower aging.
One important thing: Many times the user does not just pay for the electricity consumption, but also for the installed (reserved) power. No matter whether you use this power or not – you pay for having it available (!). Therefore, using a multidrive with lower transformer and rectifier rating offloads your energy bill twice:
– reduced power consumption
– reduced contractual installed (reserved) power
6. Network harmonics better under control
With a multidrive the topic of grid harmonics can be handled easier and much more cost effective. How is that possible?
(i) Higher pulse number
– Having just one rectifier it is much more economical to reach higher pulse number and reduce harmonic distortion. You will also need just one multi-winding transformer instead of multiple of them.
(ii) Lower rectifier power demand
As explained, multidrive solutions often benefit from lower rectifier power. IN general, the voltage and current distortion decreases with reduced power.
(iii) Easier implementation of harmonic filter
Analogously, if a grid side harmonic filter is required (rare case though), it is much easier to install it on one multidrive instead of dealing with multiple drive units.
7. Versatility for special applications
For some specific applications a multidrive might be the only solution. It has several goodies that a conventional VFD simply cannot offer.
Example 1: Energy storage
The size (capacity) of a dc link of a standard voltage source inverter is limited. There are several reasons for that. The most important one is the maximum internal fault level that the VFD can handle in case of a short. The energy stored in the dc link is relatively small. When the supply side is interrupted and the inverter would keep delivering full power to the motor, the dc link would be discharged within few milliseconds. Therefore, in ride through mode, the motor torque must be rapidly ramped down to zero (resp. slightly negative value).
The limitation of dc link capacity can be easily solved within a multidrive concept. One more inverter can be added driving a flywheel. The kinetic energy can be set almost freely. A value of 100-times to 1’000-times of the energy of dc link can be reached without any problem.
Example 2: Test stand – Power in the loop (PIL)
Another example is a test stand application using power in the loop principle. Let’s say that we want to test and certify a wind turbine. One inverter (or set of inverters) drive the motors emulating the wind power. The wind turbine generator produces the electric power and feeds it back (via adjusting transformer) through other inverter(s) into the common dc link.
Advantages of multidrives (Wrap-up)
In this introductory part we have emphasized some of the advantages of VFD multidrives. The most significant are:
– Improved energy efficiency by further reducing the power consumption
– Reduced footprint on VFD level and especially on system level
– Reduced investment cost (equipment, transportation, installation)
– Reduced operational cost
– Easier system integration
– Realization of special applications / new concepts
Some of the key advantages will be discussed in more detail in upcoming articles, along with practical examples and illustrations. Stay with us to learn more about VFD multidrives.
References
[1] Medium voltage AC variable frequency drives – Overview, https://new.abb.com/drives/medium-voltage-ac-drives
[2] VFD dimensioning (series), https://mb-drive-services.com/category/vfd_dim/
3 Comments
Davide Andreo · September 30, 2020 at 5:13 am
Great knowledge sharing Martin, kudos! I would add also benefits on maintenance as part of operational costs. Cheers
VFD multidrives: Improved energy efficiency - MB Drive Services · September 28, 2020 at 8:37 am
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VFD multidrive: Reduced footprint - MB Drive Services · October 10, 2020 at 2:39 pm
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