Why does IE3 classification not make sense for submersible motor pumps?

The worldwide increase in the demand for energy and the burning of fossil fuels are playing a really significant role in driving climate change. In order to halt this trend as far as possible, the EU has set itself ambitious targets for reducing CO2 emissions. One important approach to solving this problem is to reduce power consumption by making use of high-efficiency technologies.

Most centrifugal pumps in commercial applications are driven by three-phase motors. The efficiency levels of both the motors and the pumps need to be improved in order to optimise the pumping process in water applications. With its Ecodesign Directive 2009/125/EC, the EU has set a framework for steering practical implementation of these targets in various product groups (lots). The USA and China are also pursuing similar aims.

A submersible motor pump is generally understood to be a close-coupled unit comprising a pump and a motor. The following special technical features apply:

Submersible motor pumps differ significantly from pumps for deep wells that have a standardised, water-filled special motor. No separate, binding standards have been published for submersible motors. All manufacturers apply the regulations stipulated by IEC 60034 / EN 60034 for three-phase standardised motors where appropriate. NEMA MG1 is the reference standard for the US market. Commission Regulation (EU) 2019/1781 stipulates requirements for the ecologically compatible design (ecodesign) of electric motors. The efficiency limit values are based on standard EN 60034, Part 30. The Regulation explicitly excludes motors that are designed to operate completely immersed in a liquid (Article 1 (2)). In a study on Lot 30 in which all previous exceptions were reassessed, the fact that the highly intermittent operation of submersible motors makes them a special case was confirmed.

Since submersible motor pumps are only in operation for brief periods, the scope for reducing the CO2 emissions associated with submersible motors is very limited by comparison with other pump applications. The technical control requirements are out of all proportion to the potential savings. The EU has therefore excluded submersible motors of the kind used in the KSB type series Amarex N, Amarex KRT and Amacan from an obligatory IE3 efficiency classification. Nonetheless, many national and international manufacturers are still using the IE3 efficiency classification as a quality benchmark for their submersible motors – especially for marketing reasons. The result: Operators and consultants are often uncertain as to how submersible motors should be classified in terms of energy efficiency.

Unlike clean water pumps and heating circulators, waste water pumps are not currently subject to any binding rules with respect to minimum efficiency. Given the absence of a regulation and the drive to minimise investment costs, the efficiency of many submersible motor pumps on the market could be improved considerably. We should not, however, neglect the importance of operating reliability in our attempts to save energy. Considering the CO2 emissions and the costs incurred for service jobs, we will obviously not get any closer to our goal by designing pump sets that are highly efficient but also highly susceptible to clogging.

The current Commission Regulation (EU) 2019/1781 and the standard EN 60034 Part 30 explicitly exclude submersible motors. EN 60034-2-1 ‒ the standard governing methods of determining efficiencies from tests ‒ does not stipulate any requirement concerning submersible motors either. As a consequence, IE3-classified submersible motors do not exist in the sense of CE conformity. Pump manufacturers widely agree that significantly greater savings can be achieved by selecting a suitable hydraulic system than by optimising drives. Various impeller types optimised for specific applications, trimming the impeller to meet customer requirements, and matching flow rates to the actual waste water volume offer plenty of scope for improving efficiency.

In view of the demands for energy-efficient submersible motor pumps and the absence of applicable standards, some individual manufacturers are publicising various motor concepts. They promote their products by emphasising, for example, that they have used stator and rotor components from IE3 standardised motors manufactured on a large scale by sub-suppliers. But these kinds of statements only indicate that they have used high-quality materials. They do not guarantee any specific, binding minimum efficiency level. Despite the use of such components, the actual efficiency of a submersible motor can be significantly lower than efficiency level IE3.

From an engineering viewpoint, this can be explained by a number of factors. Unlike the rolling element bearings used in standardised motors, those of submersible motors must be larger in size because they need to absorb hydraulic forces as well. But larger bearings also mean higher friction losses. Mechanical seals and shaft seal rings apply additional friction, which brakes the rotor. To be able to compensate for the shaft deflection caused by hydraulic forces and the shaft overhang, submersible motors often need a larger air gap between the stator and rotor. This in turn requires a higher magnetising current and causes electrical losses. If the pump needs an integral coolant circulation system due to the installation conditions (when it is dry-installed or pumping in semi-submersed state, for example), this will also have a negative impact on the efficiency balance.

The constant, load-independent losses are determined in a separate no-load test. During the actual load test, only the electrical power input and the speed are measured. On the basis of these values, the engineers finally calculate all separate losses and add them up to arrive at an efficiency figure. EN 60034-2-1 deems this method to be unsuitable for the purpose of an IE3 verification owing to the large number of influencing factors. For many decades, this procedure was nonetheless the recognised state of the art for assuring quality in motor production.

The direct measurement procedure appears simple at first glance because it only requires two measuring instruments in order to record electrical power input and mechanical power output. In reality it involves a fairly large amount of time and costs. Extremely precise measuring instruments are needed because two measured values of almost identical magnitude have to be compared.

The motor torque can only be determined by means of expensive, high-precision torque measurement shafts. These are extremely sensitive to the shock loads that occur when motors are started direct on line. For this reason, a so-called soft starter is needed, a device with electrotechnical features that require substantial investment depending on the motor size. A further complicating factor is that submersible motors do not have standardised feet, flanges or shaft ends.

The process of fixing them mechanically to a test facility and coupling them to a load brake is a complex one. A water cooling system to dissipate the heat generated by the motor is often also required. Water cooling is achieved by providing for a sealed shaft passage between motors and brake in a water basin. Alternatively, the motor housing can also be sprinkled with water by a shower fitting. Were this test method to be applied generally, the large number of available sizes of waste water pumps would require substantial investment and incur high operating costs. Furthermore, there would be virtually no independent testing institutes that could provide market monitoring, at least not for pump sets with higher ratings. As a result, this method too is only suitable for submersible motors to a limited extent.

To make comparisons between pumps for waste water applications easier, studies are currently being conducted in Europe into standardised tests designed to verify the suitability of pumps for use with waste water. Pump manufacturers are also working on proposals that will mark a departure from the separate declaration of efficiency levels for pumps and motors and provide alternatively for an overall efficiency specification under the term "wire-to-water efficiency" as a comparison criterion.

Points of reference with respect to the overall efficiency of pumps are already given in chapter "Performance acceptance test for centrifugal pumps" of ISO 9906 (2012), or in "Performance acceptance test for submersible centrifugal pumps" in the ANSI/HI standard dated 11 June 2012. These methods merely require electrical power, flow rate and head as measurands. They can be implemented without large investment because they only need a pump test facility.

From the customer's viewpoint, an energy efficiency index of pumps based on their typical annual load profile would be desirable. The introduction of energy efficiency labelling would enable end users and consultants to compare different products and so select the best pump for a specific application. Customers would then have an objective criterion for determining the energy requirements of a pumping task. And it would provide transparency, allowing the efficiency of individual products to be compared with each other.