Increasing motor efficiency with a copper rotor - Copper India

Increasing motor efficiency with a copper rotor
A. Bárdos 1* , S. Bauer 1 , W. Braun 2 , P. Orbán 1 , K. Schleede 1 , P. Smith 1 , Cs. Walczer 1
1 Kienle-Spiess GmbH, www.kienle-spiess.com
2 Dr.-Ing. Ernst Braun GmbH, www.braun-inst.de
* Corresponding author: andras.bardos@kienle-spiess.com
New efficiency classes were defined for the electro motors. This new ranging will be valid in 3 steps from 2011 till 2017. To meet with
that demand, electro motors should be developed. 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 used aluminium. This
replacement could result in ca. 15-20 % overall motor loss reduction. 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 extremely quick die
deterioration and high porosity in the end rings. The key of copper usage, as a squirrel cage material in the rotors, is based on the
solution of these challenges.
keywords: copper rotor, induction motors, new efficiency classes, new IEC standard, high pressure die-casting
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1 Introduction
Electric motor applications in industry consume between
30 % and 40 % of the generated electrical energy worldwide.
The total energy saving potential of an optimised system is
assumed to be around 30 % to 60 % [1]. For this reason the
IEC60034-30 standard was redefined and was accepted to
classify the induction motors according to Table 1 and Fig. 1.
Table 1 The electrical motors with different power have to reach
the new IE-classes [1]
Deadline Motor power
(kW)
Efficiency
class
16/06/2011 0.75 – 375 IE2
01/01/2015 7.5 – 375 IE3
01/01/2017 0.75 – 375 IE3
Fig. 1 The new efficiency classes for motors
with 2, 4 and 6 poles [1]
Today one has already accepted that the main reason for the
greenhouse effect is the increasing CO2 emission. It is
reducible with the usage of machines with high efficiency
since a great percentage of the produced electrical energy
comes from the burning of fossil materials. There are more
ways of decreasing the losses of an induction (electric) motor
but one of the most successful possibilities is the copper
where the motor losses sink significantly [2].
2. Methods of increasing the efficiency of an induction
motor
An electrical motor transforms electrical energy into a
mechanical one. During this conversion there are different
losses that can be divided into 4 groups (Fig. 2). For a motor
with higher efficiency, one of these losses must be decreased.
The stator resistance losses are reducible by using other slot
designs, or by increasing the copper amount in the stator
slots, or by changing the winging energy. For decreasing the
iron losses one can also offer different ways. They are: using
thinner lamination, better quality of the lamination electrical
steel or the optimisation of the slots. The rotor resistance
losses strongly depend on the size of the conductor bars, the
slot and end-rings design and the electrical resistance of the
squirrel cage material. The improved fan design and the
bearings with sealing and low friction cause a decrease in the
friction losses.
Fig. 2 The four main reasons for the motor losses [3]
3. Advantages of copper usage
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 %
which would result in a cooler run motor (Fig. 3). The cooler
motor improves the reliability and gives longer motor life
[4].
Increasing motor efficiency with a copper rotor, Journal of Machine Manufacturing, Vol. XLIX (2009) Issue E3-E5, 83

Fig. 3 Overall losses of aluminium and copper motors
with similar power [4]
The lower overall motor loss will result in higher motor
efficiency, which definitely contributes to the CO2 emission
reduction. A further positive effect is that one can reach the
same motor power with a shorter copper rotor than with
aluminium. For this reason the complete motor built with
copper will be lighter and cheaper as well. A very important
benefit is that only with the substitution of Al rotor to Cu, the
“old” motor is already in the higher efficiency class
(according to IEC 60034-30 standard) without having to
change the housing (Fig. 4).
Fig. 4 The same core length with copper results in higher efficiency
4. Challenges during the production of copper rotors
Nowadays the aluminium squirrels cages are high pressure
die cast (HPDC) because this is the most economical and
fastest process [5]. For these reasons it would also be
obvious to produce copper rotors with this technology. There
is an important difference between the melting point of
copper (1089°C) and of aluminium (660°C). Their viscosity
is also significantly different at ca. 150° overheated
temperature (Cu: 3.9 mNs/m 2 , Al: 2.5 mNs/m 2 ). For these
reasons the cast ability of copper is much worse than that of
aluminium. Therefore copper is ready for high pressure die
casting, but it is enormously difficult. The same gating
system as used with aluminium is not convenient and the die
has to be able to endure extremely high heat stress during
copper die casting. An alternative production method for
copper rotors is its assembly from previously produced
copper bars and rings, fixed by hard soldering. But this is a
slow and costly technology.
On the basis of different tests [6] the expected porosity of
copper products in the end rings is around 4 %. This value of
aluminium rotors is less than 1 %. The copper density
(8.9 g/cm 3 ) is 3 times higher than that of aluminium
(2.7 g/cm 3 ). On the basis of the physical properties of these
two elements, ca. 2 times more heat is necessary for the
melting of the same volume of copper [7]. During the HPDC
more the 2/3 heat of the cast metal goes through the die. This
is why the die must be flat; not having wings or balancing
bars cavity. Since the copper density is high, the balancing of
a copper rotor with significant porosity is quite difficult. For
this reason one of the main precondition for the serial
production of copper rotors is to cast them with low porosity.
The other main challenge is the die life. Because of the
extreme high casting temperature, the tool life is very
limited. Normal tool steel (1.2344) cannot survive more than
50 to 100 shots with copper. This tool material is suitable for
ca. 50 000 shots with aluminium [7]. For an economical
production process it is indispensable to have a low-priced
technology and/or material which are ready to extend the die
life with copper die casting.
5. Summary
Since the electro motor efficiency must be increased and has
to reach the higher efficiency classes, the R & D teams
should work out new technologies which are ready to satisfy
the mentioned requirements. With the substitution of
aluminium with copper as a squirrel cage material, the motor
has already penetrated into the higher efficiency class,
without having to change the motor housing. Since copper is
difficult to cast, the high pressure die casting technology has
to be developed according to the copper requirements. End
rings of copper rotors are flat and their porosity must be low
because of the balancing problems coming from the high
density.
Acknowledgements
This work was supported by the Hungarian National
Development Agency and was co-supported by the European
Regional Development Found through Grant No GOP-1.1.1-
08/1-2008-0086.
[1] IEC 60034-30 Ed. 1: Rotating electrical machines –
Part 30: Efficiency classes of single-speed, three-phase,
cage-induction motors (IE Code)
[2] F. Cerra, SIRF – Electrical Maintenance & Safety
National Forum, Australia, September 2007
[3] http://users.telenet.be/b0y/content/gen_techin
/Induction.Motor.cutaway.jpg
[4] 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] http://www.diecasting.org/faq/ (09/06/2006)
[6] 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
[7] A. Bárdos, „Kupfer als Käfigläufermaterial für den
höheren Wirkungsgrad eines Elektromotors“ 17.
Kleinmaschinenkolloquium, Ilmenau, 19-20 März 2009
Increasing motor efficiency with a copper rotor, Journal of Machine Manufacturing, Vol. XLIX (2009) Issue E3-E5, 83