Increasing efficiency by employing multi-motor drive systems (MMDS)

Multi-motor drive systems are one approach to increasing the energy efficiency of drive systems. They consist of an assemblage of two or more motors and - depending on the application - a gear, mutually solving a drive task. Highly efficient system components, a sophisticated control system (control strategy) and a process oriented optimization are the essential part of the system.

Analysis of the mechanical structure of an MMDS

The mechanical structure of an MMDS can vary greatly, depending on different application areas, the number of integrated motors, performance category and mechanical coupling unit. Numerous applications also require a transmission to adjust the rotational speed and torque of the motor to the application. To avoid an additional gearbox, the transmission can be directly integrated into the mechanical coupling unit of the motors. The additional degree of freedom of the engine cutoff can be used to increase the power factor of the drives in an MMDS, as long as the process allows this.

In mechanically coupled multi-motor drive systems a driving drive train becomes the driven drive train at the point of engine cutoff leading to a switch of the adjoining tooth flank of a pinion in the coupling unit. Subsequent to the switch of the tooth flank, a torsional vibration is stimulated/excited which may possibly lead to a reduction of the pinion life span. Instead of compensating the shortening of the life span possibly caused by these vibrations with a larger dimensioning, the research project is aimed at preferably avoiding or possibly dampening the critical vibrations. To this end, different mechanical structures and elements, such as couplings and free-wheels are integrated into the coupling unit and tested for their suitability.

Furthermore, the possibility to increase the efficiency of the overall system by an innovative gear design is explored, focusing on the reduction of friction losses at sealing points and bearings, and also on the reduction of mass inertias when the MMDS is in operation. Another focus of the system design lies on a compact structure to obtain an advantage regarding installation space over conventional transmissions/gears.

Operating strategies

Depending on its design and structure, a technical system has different degrees of freedom, the modification of which influences the system performance during operation. The effects on the system performance can be evaluated looking at the change in the degree of fulfilling the overall task assigned to the system. In this project, an operating strategy is defined as a specific application of the degrees of freedom of the system leading to a desired positive change in the level of fulfillment of the overall task of the system. As far as the overall task can be divided into subtasks, a deterioration of individual degrees of fulfillment of the subtasks is tolerable, if the level of fulfillment of the overall task is improved.

The operating strategies are designed employing heuristic as well as conventional methods of single- and multi-objective optimization. Heuristic methods use previously gained knowledge to qualitatively generate a positive influence on the system performance. Conventional optimization methods quantitatively minimize a deviation from the desired system objectives by target functions that were defined beforehand. The objectives considered may be values or functions that are constant over the operating time as well as functions that are variable over the operating time and system environment. This allows an adjustment of the system procedure and/or the objectives to changing situations.

Within the operating strategies, it is possible to combine heuristic and conventional optimization methods in terms of a self-optimization, while the system automatically decides whether only one or both optimizing methods are pursued in parallel to the operating time.

Application examples

There are test rigs to investigate the operating strategies and the mechanical structure of drive systems; they represent the behavior of real models for research purposes.

Drive system for an internal natural rubber mixer

Drive system for a linear-motor driven rail vehicle

Additional application scenarios

In the future, the advantages of MMDS in combination with intelligent operating strategies and an innovative mechanical structure are transferred to additional applications. Potential applications are:

• Wind energy plants
• Cement mills, rolling mills and crushers
• Stirrers
• Conventionally driven rail vehicles
• High-speed trainsWindkraftanlagen