Two frequently development ways of the induction ... - Kienle + Spiess

Two frequently development ways of induction motors:
High Efficiency and High-Speed Drives
Dr. András Bárdos
Kienle + Spiess GmbH
Bahnhofstraße 23
74343 Sachsenheim
Germany
Dr. Walter Braun
Dr. –Ing. Ernst Braun GmbH
Martin-Luther-Strasse 1
88400 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
fan blade
rotor body made of
laminated steel sheet
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 60034-30. This new ranging will be
introduced in 3 steps from 2011 till 2017.
4 th International Conference on Magnetism and Metallurgy, WMM’10, June 9 th to 11 th 2010 p.387

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. 15-20 % 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 10000 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.
4 th International Conference on Magnetism and Metallurgy, WMM’10, June 9 th to 11 th 2010 p.387

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
c) Stress level with smooth end ring
at 10000 rpm; 100 m/s
b) 10000 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 th International Conference on Magnetism and Metallurgy, WMM’10, June 9 th to 11 th 2010 p.387

eliability 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
4 th International Conference on Magnetism and Metallurgy, WMM’10, June 9 th to 11 th 2010 p.387

[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,
T03-046
Presentation Company Kienle + Spiess
One of the leading manufacturer of punched lamination and die-cast rotors
Revenue 2008: 219 Mio. EUR
Employees: 1’150
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: 45’000 sqm
Steel consumption: 100’000 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
4 th International Conference on Magnetism and Metallurgy, WMM’10, June 9 th to 11 th 2010 p.387