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.

VFD multidrive is 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.

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.

1. Improved energy efficiency

With a multidrive solution you always save more energy compared to multiple single drives.

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.

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.

(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

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.

– 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.

(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

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

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.

Advantages of multidrives 

– 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