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Mixer efficiency gets a boost with IE4 permanent magnet motors

10 October 2015

Production processes that run 24/7 need reliable and energy efficient drive systems to maintain a cost effective process. For one pharmaceutical manufacturer the drive for efficiency led it to replace some existing equipment with the latest IE4 electric motor technology.

Fresenius Kabi Austria is a subsidiary of the international healthcare group Fresenius Kabi and its plant in Linz manufactures active pharmaceutical ingredients (API), such as lactulose, hydroxyethyl starch and sinistrin. The production of lactulose API involves a 24 hours a day, seven days a week operation with an order-related and just-in-time production system. A significant part of the production process involves the product being held in large vats that are continuously stirred by agitators, driven by electric motors.

It was this process, among others, that was under scrutiny in terms of energy and process efficiency, so Fresenius Kabi approached Bauer with a request for support in investigating a more efficient drive system, and one that was also flexible enough to cope with process changes. The initial step was to carry out an assessment of the existing equipment which comprised an IE2 (2-pole) direct-on-line (DOL) started asynchronous geared motor (ASM) rated at 18.5kW with a stated efficiency of 91.2 percent. This was connected to the agitator paddles that turned at about 105rpm with an approximate load of 1,632Nm.

Initial measurements showed that this drive arrangement was working at 10.5kW and therefore had a reduction in energy efficiency due to the partial load conditions. At this point the production engineers explained that they wanted to change the geometry of the agitator and also reduce its speed from 105rpm to just 53rpm. The effects of these modifications were verified prior to selecting the new geared motors.

Initially, Fresenius Kabi was interested in installing an IE4 asynchronous motor (motor size 132) that had a stated efficiency of 92.6 percent but after consultation with Bauer, decided to use an IE4 permanent magnet synchronous motor (PMSM) (motor size 112) to achieve even greater efficiency gains.

Bauer's consultation started with a comparison of running costs of the original equipment with those of the latest motor technology. This was accompanied by calculations for total cost of ownership that provided a clear evaluation of the differences in energy consumption between the ASM and PMSM designs, as well as detailed technical calculations for bearing lifetime. The test and evaluation work was completed with help from Bauer's Gear Centre, ESKO in Fohnsdorf, Austria.

As part of the project, Bauer proposed the BF60 and BK60 models of geared motors that would provide a 7.5kW IE4 PMSM with parallel or bevel gearboxes respectively. The most significant advantage when compared with an IE4 ASM is seen when operating at lower speeds or under partial load.

Although both the ASM and PMSM motors were classed as IE4, the stated efficiencies were 92.6 percent and 92.1 percent respectively. However, the ASM efficiency was stated for DOL duty and after this was corrected for inverter duty it became 90.2 percent. There was also a considerable difference in the physical mass of the motors, with the ASM weighing in at 84kg and the PMSM at only 31kg.

Figure 1 shows the calculated differences at 50Hz with differing loads on the motor. The ASM in partial load has a considerably higher decrease in efficiency than the PMSM for the same load conditions. At loads under 50 percent the differences are extremely high. A further factor to be considered is, when the speed is also variable, the efficiency also reduces.

During the consultation period Bauer was able to reference data from a physical comparison test that was carried out on an IE3 ASM and IE3 PMSM. In this example the stated efficiency of the ASM was 89.6 percent while the PMSM motor was rated at 90.8 percent. As with the IE4 examples above, there was a considerable difference in the physical mass of the motors, with the ASM weighing in at 56kg (size 132) and the PMSM only weighing 31kg (size 112).

The efficiency charts (Figures 2 and 3) demonstrate the advantages of the PMSM when the application requires a lower speed or less than full load from the motor. These charts show that the efficiency of the ASM is significantly lower than that of the PMSM at lower speeds (6Hz) when operating under partial load. This data is transferable to the IE4 counterparts considered for the application.

While motor nameplates offer the rated efficiency of the motor, it is also important to consider the efficiency of the geared section which can be affected by friction in the oil, bearings and seals. Bauer was able to improve the overall drive efficiency by approximately 35 percent while maintaining the required quality of mixing.

However, these gains in efficiency cannot be at the expense of reliability which is heavily influenced by the running temperature of the drive train. With this in mind, thermal imaging equipment was used to provide accurate information on the normal running temperature of both the motor and the gearbox.

The attention to detail provided by Bauer’s engineers in providing a comprehensive demonstration of the energy saving potential of IE4 PMSMs and the depth of knowledge relating to the application resulted in the Bauer geared motors being selected by Fresenius Kabi. Furthermore, as the initial units performed to the expected levels, additional units were ordered to complete a comprehensive overhaul of the original process equipment.

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