Fixed-speed drives

Energy Efficiency

2 Translated reprint from No. S4/2011 •

A wide-spread argumentation is as follows: A fan operated with constant speed, for example switched via a contactor, works against a throttle valve controlling the partial load demand with full power. Recom-mended counter-measure: Remov-al of the throttle valve and appli-cation of a fre-quency convert-er. This opens up high saving po-tentials. As a re-sult, the frequen-cy converter is often regarded as the best general solution for in-creased energy efficiency.

Undoubtedly, the frequency

converter offers decisive advan-tages thanks to its flexible load adjustment options, particularly with applications subject to varia-ble load requirements, frequent op-erations and energy recovery. In many cases however, there are more simple and more efficient so-lutions.

Accurate application analysis First of all, the application has to

be accurately analyzed before de-ciding on the drive system to be employed. The optimum solution can only be determined with precise

knowledge of the processes’ load behavior. In particular, partial load operation of the motor below 40 % of the nominal load has to be avoid-ed. From a load range of 50 % to 60 %, optimum efficiency is attained with direct mains operation.

If, for example in a container fill-ing application, no special require-ments are placed upon the hysteresis,

Fixed-Speed Drives – Dying Breed or Efficiency Lever?

Fixed-speed drives – i.e. drive solutions in which three-phase motors are operated with mains voltage at mains frequency – are frequently considered potential replacement candidates when it comes to energy efficiency measures. At first sight, this stance is justified in some ways. However, often only the symptoms instead of the cause are tackled in this context. Nevertheless, the total cost view represents the ultimate factor to be considered.

Andreas Thomas

Andreas Thomas is project manager for energy efficiency at Siemens AG

in Fuerth.

E-mail: andreas.thomas@siemens.com

Fig. 1. With water supply systems, a cascade consisting of a speed-controlled pump and one or several downstream fixed-speed drives represents an efficient solution.

Energy Efficiency

3 • Translated reprint from No. S4/2011

the task can be easily implemented using a two-point controller with fixed-speed drives in a technically simple, yet very efficient manner.

If a fault-tolerant solution is re-quired with varying demand in a water supply system, a cascade with one frequency converter and one or several downstream fixed-speed drives, for example with soft start-ers, should be preferred over a large speed-controlled pump (Fig. 1).

While the frequency converter serves the variable, controlled de-mand, the soft starters take care of the base load demand (Fig. 2). In many cases, the frequency convert-er’s intelligence already covers the function of the control unit, as is the case with a Sinamics G120P [1].

Considerable drive overdimen-sioning (as a cumulative effect of the safety margins associated with hasty and broad-brush configura-tion) represents a common phenom-enon. As a consequence, not only the motor, but also power units, cables and infeeds are dimensioned significantly larger than actually required. Even though this superflu-ous capacity can be throttled with a frequency converter on the basis of special control algorithms, it would be better to dimension the capacity

in accordance with the actual de-mand already during the planning phase. This not only saves invest-ment costs, but also minimizes ma-terial consumption – and thus fa-cilitates double efficiency.

Good reasons for fixed-speed technology

What are the advantages of fixed-speed technology when permitted by the application?

Applications which do not require any speed adjustments due to proc-ess reasons can be easily assembled in a highly energy-efficient manner with a motor switching device and (if required) a gear unit. With opti-mum dimensioning, operation at nominal speed facilitates the entire application’s optimum efficiency.

In comparison, not only the con-verter’s intrinsic losses (ranging from approx. 3 % to 10 %) have to be considered with variable-speed drives, but also additional losses in the motor (approx. 1 %) and in the filters (approx. 1 %) required in most cases. When employing fixed-speed technology, low-voltage switching devices with low intrinsic power loss (<0.2 % of the motor power are used, thanks to their elec-tromechanical contacts). Even with

electronic soft starters, the thyris-tors are only actively switched dur-ing the start-up and run-down phase. During operation with nomi-nal speed, electromechanical bypass contacts ensure the current feed and thus facilitate minimized losses.

Due to the switching devices’ cor-respondingly reduced energy losses, the temperature inside the control cabinet does not rise to the same extent as with permanently con-nected power electronics of the var-iable-speed drive technology. Addi-tional air-conditioning costs can be reduced – from an energetic point of view, this exceeds the converter losses by up to a factor of two. Fre-quently, this allows for a fan-free and thus low-maintenance heat dis-sipation of the control cabinet.

Easy configurationType-tested direct-on-line and re-

versing starters, star-delta and soft starters represent standard circuits which can be easily integrated via CAE macros. No parameters (or only few) have to be considered during commissioning. Familiarization with additional product-specific software is not required.

In the range up to 32 A, the switching device assemblies can be

Fig. 2. With the cascade circuit, a frequency converter serves the variable demand (orange); the base load is switched via soft starters (green).

Energy Efficiency

rapidly interconnected, without tools, by means of the spring-loaded connection system and can then be snapped onto the DIN. Infeed sys-tems supply complete groups of feeders without the normally re-quired wiring overhead. Wiring is implemented via unshielded stand-ard cables.

When comparing the space re-quirements per kW, switching devices stand out with their com-pact design. Furthermore, they fea-ture a comparatively low weight. The devices are available both with IP20 protection for application in-side the control cabinet and with IP65 protection for direct installa-tion on the machine.

The purchase costs of fixed-speed drives are lower than those of vari-able-speed drives, not only due to the level of functionality, but also as a result of the lower costs in terms of ancillary equipment.

Low-voltage switching devices represent a technology which is known and available worldwide. In most cases, devices by various man-ufacturers can be effortlessly ex-changed or combined if required, for example in remote areas. Main-tenance technicians are able to rec-tify any problems on their own, with just a basic knowledge of elec-trical engineering.

The high reliability of the tech-nology, which has been tried-and-tested for decades, coupled with long lifetime characteristics facili-

tate long-term and fault-free opera-tion.

Low-voltage switching devices by Siemens are completely recyclable and thus further contribute to sustainability at the end of their lengthy application period.

The assurance of future-proof so-lutions is paramount: As of 2015, the European Union Ecodesign Di-rective ErP Lot 11 requires the ap-plication of class IE3 efficiency motors in connection with switch-ing devices. Such motors are al-ready available and should particu-larly be considered for applications subject to continuous operation (more than 1,000 h per year). Nor-mally, existing configuration solu-tions and, in most cases, even the available switching devices can be used further without any problems.

This keeps operators on the safe side, both with regard to new in-stallations as well as motor replace-ments.

The right procedureWhen the responsible configura-

tion engineer takes all essential cri-teria for the optimum drive system into account, lots of money can be saved both in terms of system de-sign and runtime. Besides economi-cal factors, this also accounts for the aspects of energy efficiency and ecobalance – in short: Sometimes, less is more.

References[1] Siemens AG, Nuremberg:

www.siemens.com/sirius/energysaving

• Operation of the application with no-minal speed with optimum efficiency

• Application of switching devices with negligible energy losses

• Low temperature rise inside the con-trol cabinet

• Easy configuration of the drive train • Low installation costs • Minimum space requirements inside the

control cabinet coupled with low weight • Favorable cost/performance ratio of

switching devices

• Simplified maintenance thanks to ge-nerally known technology – available worldwide

• High reliability and availability thanks to tried-and-tested technology and easy maintenance and repair

• Low-voltage switching devices by Siemens are easily recyclable as well as

• future-proof: Switching devices har-monize well with class IE3 efficiency motors and partially with IE4 motors

Eleven reasons in favor of fixed-speed drives: