Intelligent drive selection
– Answers to the central question

Mains-connected drives have been increasingly giving way to more efficient and flexible alternatives – combinations of geared motors with frequency inverters are gaining ground. These power electronics used to be situated at a distance from the machine in a control cabinet. Now, however, inverters mounted near the motor and integrated units combining drive mechanics and electronics open up new possibilities. This trend towards distributed control installations, while it must be applauded, also has its limits that users should be familiar with when designing drive concepts.

Distributed drive solutions are especially well suited for large conveyor architectures

Drive control units installed centrally or in the field offer their respective advantages and disadvantages, all depending on the application conditions. Consequently, machine and plant engineers keep using different tools for different tasks. Preferably, these will come from a drive supplier who manufactures conventional cabinet inverters as well as innovative distributed systems, continuously improving both lines. Ideally, all models will enable the same comfortable handling, no longer requiring users to learn different operating procedures for mixed or changing solutions.

New approach for large-scale facilities

Drive electronics moving from the control cabinet to the machine has economical reasons as well as technological ones. The systems save costs over their total lifecycle, they take up less space, and allow for an increased flexibility in facility design. Many former restrictions no longer apply. Facilities with many drives used to require centrally installed frequency inverters in control cabinets. The drives, of course, were installed near the machine or application. And since the control cabinet with the drive electronics was the starting point for cabling all motors or geared motors in the field, star wiring was the automatic result. Materials handling facilities in particular, such as loading zones in high-bay warehouses and belt or roller conveyors, benefit from the fact that integrated drive solutions do not require the usual long motor cables and large numbers of control cabinets. Instead, highly flexible setups become possible in many cases, with minimal wiring and installation effort.

Thermal problems limit distributed solutions to the lower output range – for control cabinet inverters, on the other hand, efficient additional cooling can be well ensured even when they are packed densely

Easier handling decreases costs

Integrated drive units can be completely wired and tested before installation. In addition, designing the drive concept and commissioning takes up much less time. The remaining control cabinets are much smaller and cooler and easier to standardise. The solutions support current communication buses, and line wiring is possible. Shielded motor cables are either not required at all (with direct motor mounting), or very short (with wall mounting near the motor). Integrated drive units in the field are easily accessible for service personnel. Diagnostics can be performed directly at the site. Due to pluggable connections, many units can even be exchanged without an electrician if required. Via sensor and actuator interfaces, data can comfortably be transmitted to higher-level controllers, and integrated status LEDs facilitate on-site diagnosis. Applications with distributed drive technology can reap considerable economic advantages over centrally installed power electronics. The components have a 20 to 30% higher purchase price, but that is outweighed by up to 40% lower engineering costs, up to 60% savings during installation and commissioning, and maintenance costs reduced by up to 50%. Over the product lifecycle, the savings amount to about 30%, depending on the size and type of application.

Have cabinet-installed inverters done their duty?

It seems that the advantages of distributed electronics units suggest that conventional installations with control cabinet inverters are outdated – particularly when compared with the latter’s constraints. Connecting remote geared motors with a centrally situated control cabinet, the maximum motor cable lengths must not be exceeded. Furthermore, depending on possible interferences, shielded cables might be obligatory. Alongside the power lines, a number of other wiring lines including signal cables for motor brakes, temperature sensors, incremental encoders, and other components must be laid – the drive solution is complicated by the fact that all these diverse components need to be made compatible. Furthermore, different floor plans with drives in different positions require different control cabinet configurations, e.g. when long motor cables necessitate motor chokes. Consequently, it remains difficult to standardise control cabinets.

Central option for difficult cases

Despite the obvious drawbacks over the new, integrated drive electronics, there is often no way around centrally installed control cabinet inverters, even today. After all, the requirements of widespread, distributed facilities are quite different from those for shredders, chippers, and similar machines. In applications such as these, the extreme environmental conditions alone are reason enough to put the electronic drive technology somewhere safe and enclose it in a robust housing. Furthermore, in some applications, drives installed in too close proximity of each other or for high power outputs – generally everything above 22 kW – make distributed inverter solutions difficult since the temperature can quickly rise above their limit. These applications should therefore follow the centralised route. In such cases, even large drive components such as motor chokes or mains filters can be integrated in control cabinets without problems. In distributed systems, on the other hand, space for all kinds of optional equipment (I/Os, circuit breakers, etc.) is considerably limited. Also, certain applications appreciate the fact that cabinet inverters can be accessed and operated along with the controller. Another typical application, series machines that are shipped in one piece, will keep on profiting from centralised drive technology as the most cost-efficient solution.

Fully integrated distributed systems are very low-maintenance and provide maximum flexibility (pictured: a geared motor with an SK 200E inverter from NORD)

Conclusion

In recent years, distributed drive solutions have opened up new avenues for mechanical engineering and plant construction and resulted in decreased handling effort and costs. However, this development is not a one-way street. Besides fully integrated units and inverters mounted near the motor, cabinet-installed devices maintain their position. The optimal concept for electric drive control depends on the specific application requirements and needs to be determined individually. For many users, it will sometimes be the one and sometimes the other solution. In most cases it will therefore be beneficial to cooperate with a drive manufacturer such as NORD DRIVESYSTEMS who can supply compatible inverter lines for both options – and who continues to invest in the development both technologies. Moreover, the best drive partner will be able to provide a wide range of solutions with a performance-graded functional spectrum including positioning tasks.