Two frequently development ways of induction motors: High Efficiency and High-Speed Drives

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1 Two frequently development ways of induction motors: High Efficiency and High-Speed Drives Dr. Walter Braun Dr. Ing. Ernst Braun GmbH Martin-Luther-Strasse Biberach / Riss Germany Introduction Nowadays there are two development ways of the induction motors. One is focusing on the high rotating speed and the other on the high efficiency applications. Beyond the different production challenges, there is a common problem i.e. porosity, which can cause undesirable effect at the end users. High speed application During the normal applications of asynchronous motors the maximum reachable rotation speed with a 2 poles product is 3000 or 3600 depending on the applied frequency. For this application field the end ring shape and the form with the well known fan blades and balancing lugs was developed (Figure 1). end ring rotor body made of laminated steel sheet fan blade Figure 1. An aluminium die-cast rotor segment with a fan blade (outer diameter: 190 mm, which means a motor frame size of 200) Mechanical strength analyses in consideration of different speeds and end ring geometries for such a rotor are described in this to show what is important for the mechanical strength during dimensioning such drives. High efficiency New efficiency classes were classified by the European Motor Manufacturers for the electro motors and were summarised in the IEC This new ranging will be introduced in 3 steps from 2011 till 2017.

2 There are more methods to reduce the motor losses which will give higher efficiency. One opportunity is the usage of copper as squirrel cage material substituting the nowadays applied aluminium. This replacement could result in approx % overall motor loss reduction. Figure 2. The new efficiency classes for motors with 2, 4 and 6 poles Copper is difficult to cast because it has high density and a high melting point compared to aluminium. During the high pressure die-casting of copper, one would face high porosity in the end rings and extremely quick die deterioration. The key of copper usage, as a squirrel cage material in the rotors, is based on the solution of these challenges. Performance at Different Speeds In the last few years the applied or reachable motor frequency is already higher than 1000 Hz, which results in higher motor rotation speed too. The not alloyed aluminium has a yield point of 20 MPa and a tensile strength of 64 MPa at 200 C. Simulation shows (Figure 3. a) that the stress level in the area of the blades and the end ring connection is not more than 9 MPa which definitely does not cause any problem in case the motor rotation is not more than 3600 rpm. However the mentioned stress level will be higher if the rotor is operating at higher speed. According to a calculation presented in Figure 3 b), the mechanical stress in the blades area of the rotor with an other diameter of 190 mm is significantly higher i.e. 30 MPa if the applied speed was rpm which results a surface speed of 100 m/s. This elevated stress at this surface speed would certainly lead to breaking of the blades and therefore destruction of the stator winding. With a smooth end ring it is possible to reduce the mechanical stress under the admissible limits (Figure 3. c). However, it should be considered that this requires a properly cast rotor with ideal end ring without porosities or shrink holes. Such defects result from the casting process due to shrinking of aluminium and it is difficult to avoid (Figure 4). The shrink holes have an absolutely subordinate influence on the electrical performance of the motor and cannot even be measured (e.g. deterioration of efficiency). The shrink holes in Figure 4 do not cause strength problems as long as the surface speeds do not exceed 100 m/s and the rotor temperatures are under 200 C. Nevertheless, such defects are often not accepted by the final customers for unfounded fear of motor failures or poorer motor performance.

3 For this reason it is necessary not to use end rings with blades and balancing lugs in case of such high-speed rotors and to turn the end rings. Kienle + Spiess offers a large variety of suited casting dies for such end rings and it is always possible to offer customer-tailored casting dies at favourable conditions. a) Stress distribution of a 2 poles rotor at 60 Hz in the end ring and the bars at a speed of 3600 rpm b) rpm; 100 m/s c) Stress level with smooth end ring at rpm; 100 m/s Figure 3. The mechanical stress in the blades area. Rotor other diameter is 190 mm Figure 4. Shrinking porosity in the end ring, open up by turning Copper Rotors superior to Aluminium Rotors Premium Efficiency Since the electrical resistance of copper (16.78 nω m) is almost 40 % lower than that of aluminium (26.5 nω m), one would expect lower I 2 R losses in the rotor if aluminium were substituted by copper as the squirrel cage material. Motor modelling has shown that motors with copper-containing rotors would have an overall loss reduction of 15 to 20 % (Figure 5.) which would result in lower temperatures. The cooler motor improves the

4 reliability and gives longer motor life [1]. Although copper is ready for high pressure die casting, but it is enormously difficult to cast since its density, viscosity and high melting point. Figure 5. Overall losses of aluminium and copper motors with similar power [1] Figure 6. The expected maximal porosity of the copper end ring [2] On the basis of different tests [2] the expected porosity of copper products in the end rings is around 4 % (Figure 6.). This value of aluminium rotors is less than 1 %. Since casting difficulties, copper rotors have a smooth end ring, therefore there are no balancing lugs or fans. Because of the high copper density, the end ring porosity occurs difficulties by the balancing. Due to the Kienle-Spiess innovation, it was possible to decrease radically the end ring porosity at the copper product as well. Figure 7. The maximal porosity of the copper end ring cast by Kienle + Spiess is around or less than 1 % [1] C. Stark, J. G. Cowie, D. T. Peters, E. F. Brush, Jr., Copper in the Rotor for Lighter, Longer Lasting Motors, ASNE San Diego Section Fleet Maintenance Symposium 2005, 30 Aug.-1 Sept. 2005

5 [2] E. F. Brush Jr, S. P. Midson, W. G. Walkington, D. T. Peters, J. G. Cowie, Porosity Control in Copper Rotor Die Castings, NADCA Indianapolis Convention Center, Indianapolis, IN September 15-18, 2003, T Presentation Company Kienle + Spiess One of the leading manufacturer of punched lamination and die-cast rotors Revenue 2008: 219 Mio. EUR Employees: Manufacturing sides: Kienle + Spiess GmbH, Sachsenheim, Germany Kienle + Spiess GmbH, Vaihingen/ Enz, Germany Kienle + Spiess Kft, Tokod, Hungary Kienle + Spiess UK, Bilston, Great Britain Production Area: sqm Steel consumption: t Our products LAMINATIONS PACKS ACCESSORIES Main areas of expertise: Laser cutting/ prototypes/ Sample pressing Stamping: Blank & Notch/ Segment Dies/ Progressive Dies Assembling: Interlocking/ / Riveting/ Welding/ Bonding/ Cleating Die-casting: Aluminium/ Aloys/ Copper