VFD multidrives: When footprint matters
Reduced footprint as additional benefit of VFD multidrives
VFD multidrive solution can, among other benefits (see [1]), substantially reduce the total space required for the installation. In this article we will explore the ways how multidrives reduce footprint providing the users with another competitive edge.
Space required for the installation of VFD really matters. According to numerous customer surveys the footprint of a VFD is one of the key decision criteria. The reasons are:
— Larger footprint increases direct and indirect investment cost (size and cost of a building or container)
— In specific installations, such as offshore, space and weight is key item
— Brownfield projects have serious space restriction – it is not only question of cost, but overall feasibility
Space savings with VFD multidrives
Multi-motor VFD has reduced space requirements compared to an installation of multiple single motor drives. The reduction of footprint can be anything between 10% to 50%, in specific cases even higher, depending on the configuration, type of application and other requirements.
Besides the VFD itself, multidrive solution helps to reduce the overall footprint on the drive system level. Such savings might be even more interesting. They involve system components such as input transformers, switchgears, external re-coolers etc.
1. Reduced footprint on VFD level
First, we will look how a VFD multidrive compares with a conventional solution based on single motor drives. To be objective and fair, we shall look into two scenarios A and B.
In scenario A we compare a multi-motor VFD with single drives in application that is not regenerative. Therefore, the advantage of of regeneration through dc link cannot be used here. Despite of that, a multidrive still helps to reduce the overall VFD footprint. How is it possible?
Multidrive saves space by better utilization of the essential modules (cooling unit, control unit, rectifier unit, various options etc).
Capacitor bank in dc link
– VFDs with modular design have the dc link capacitors as one specific module. Such module is typically 800 mm to 1’600 mm wide. For lower power ratings the dc link module might not be entirely filled with capacitors (i.e. some slots are free), but the outer dimensions of the module remain the same.
Water cooling unit
– Each liquid cooled VFD single drive has a water cooling unit containing the heat exchanger, pumps, valves, deionizer, expansion vessel, instrumentation and piping. Such cabinet is typically 600 mm to 1’400 mm wide (depending on manufacturer, VFD model, cooling capacity etc). When we switch to a multidrive that supplies three motors we don’t need three such cooling units. Instead, the water cooling unit gets a bit wider or we might need two of them in larger multidrives. Therefore, we save at least one complete module, maybe even two of them.
Control cubicle
– On the motor side there is no space saving. The motors are controlled fully independently and each one has its own control. However, on the grid side there will be just one controller of the active rectifier (in case of diode rectifier this is of course not applicable).
Braking chopper (optional)
– In some cases a braking chopper is required. Since this is an option and multidrives have modular design, the chopper comes with a separate cabinet. Such cabinet may be between 600 mm and 1200 mm wide depending on the capability (peak shaving versus full braking). In individual VFDs each of them need such cabinet. However, in a multidrive you just need one cabinet. It does not necessarily get larger when the braking power increases.
Examples of reduced footprint
Now let’s illustrate above considerations on specific examples.
Case A: 2 x 7MW asynchronous motors
In our first case we compare two individual VFDs, each powering a 7 MW induction machine, with a multidrive having two independent inverters, one for each motor.
For this case we consider both the individual drives as well as the multidrive to have a 24-pulse diode rectifier. The VFD are all liquid cooled and have a modular design.
Each single drive has an input terminal field, diode rectifier, capacitor bank (dc link), inverter module, control unit and cooling unit. All modules together create a VFD lineup of approx. 6,350 mm of length.
The multidrive solution consists of identical or similar modules. The capacitor bank unit has 1’000 mm compared to 800 mm of single drives due to larger amount of capacitors. Also the input terminal field is 400 mm wider (in some cases the terminal fields of single- and multidrive could be of same size making the multidrive even more compact). There are two inverter modules and each one has its own control unit as the motors are operated fully independently. The water cooling unit remains of same physical dimensions (slightly larger cooling pump, however fitting into the same cabinet size). Total length of a multidrive is approx. 8,850 mm.
Table 1: Footprint comparison – 7 MW single drives versus 2 x 7 MW multidrive (VFD level)
Looking at the numbers in above table we clearly see that a 2-motor multidrive can save roughly 1/3 of the footprint. That is quite an attractive space saving, isn’t it?
When adding some options to the drives, such as e.g. a braking chopper unit, the difference between individual VFDs and multidrive VFD becomes even more prone.
Case B: 3 x 7 MW asynchronous motors
In our second example we look at a 3-motor multidrive where the space savings are even more significant.
The motors are same as in previous example: 7 MW common industrial motors. The drives this time are regenerative with active front end. Smaller individual drives use 6-pulse active rectifier in combination with an input filter module that is integrated into the VFD lineup. The multidrive has a 12-pulse active rectifier and comes without an input filter. The application supports partial regeneration through common dc link. It allows reduction of rectifier total power as well as the size of dc link. Due to a dynamic nature of such application a small chopper (called voltage limiting unit) is included to shave off potential voltage peaks in dc link.
Table 2: Footprint comparison – 7 MW single drives versus 3 x 7 MW multidrive (VFD level)
This time the footprint reduction of a multidrive reaches almost 50% or the individual VFDs require in total almost twice as much footprint, respectively.
We could continue this way and prove that the more motors are supplied from a multidrive the larger is the space saving. With 4 motors supplied from a multidrive the required space is less than half of that of individual VFDs. When adding some optional modules the difference is typically further growing.
One might argue that it is easier to arrange several smaller VFD lineups inside a building rather than one very long multidrive. Well, the modularity of VFD supports high flexibility in mechanical layout. User can benefit from configurations such as back-to-back, L-shape, U-shape etc as alternative to standard straight lineup.
2. Reduced footprint on system level
After discovering the space savings on the VFD level we move to the system level and explore the saving potential here.
Obviously, lot of footprint can be saved on system components such as input transformers and switchgears. We take again our case A with two 7 MW motors and look at the footprint comparison from a system perspective.
The switchgear of a multidrive has one incomer and one feeder whereas the solution with single drives requires two individual feeders.
The dimensions of input transformer depend no just on power rating, but also grid voltage, cooling method, ambient conditions etc. To compare apples with apples we have assumed ONAN cooling, grid voltage in the range 11 – 24 kV and standard ambient with max. temperature 40°C.
Table 3: Footprint comparison – 7 MW single drives versus 2 x 7 MW multidrive (system level)
As you can see, the switchgear footprint is reduced by 33%, transformer total footprint by approx. 26% and the VFD total footprint by 30%. This is just an example of a 2-motor multidrive. here again the trend is the same: the more motors within the multidrive the larger the overall reduction of footprint.
Note that above plot compares the footprint of the equipment itself. The overall space requirements of single drive solution will further increase since:
– oil filled transformers need sufficient clearance between each other otherwise a firewall is necessary
– escape routes inside the buildings must be provided
Summary
VFD multidrive solution allows significant savings on footprint of the VFD itself as well as overall space demand. As illustrated in our case models, the savings depend on the system configuration and character of the application. Space reduction on VFD level can be 25-30% on a two motor multidrive with diode rectifier. In case of three motors the savings can already be close to 50%.
Similar space saving figures are observed on the system level as well. In addition, you may save on infrastructure (e.g. fire wall between transformers).
In order to benefit the most from reduced footprint that multidrives can offer to you, contact the VFD manufacturer or drop us an email. This series will continue with other benefits of VFD multidrives. Stay tuned and learn more useful hints.
References
[1] VFD multidrives and their benefits, https://mb-drive-services.com/vfd-dimensioning-multidrives-and-their-benefits/
[2] VFD multidrives: Improved energy efficiency, https://mb-drive-services.com/vfd-multidrives-improved-energy-efficiency/
[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
