A new type of electromagnetic bearing was built and tested. It consists of fixed AC-electromagnets in a star formation surrounding a conducting rotor. The bearing works by repulsion due to eddy-currents induced in the rotor. A single bearing is able to fully support a short rotor. The rotor support is inherently stable in all five degrees of freedom. No feedback control is needed. The bearing is also able to accelerate the rotor up to speed and decelerate the rotor back to standstill. The bearing design and the experimentation to verify its capabilities are described

Nikolajsen, Jorgen L.

English

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EXPER I MENTAL VER I F I CAT I ON OF AN EDDY-CURRENT BEAR I N G ~  
Jorgen L .  Nikolajsen 
Department o f  Mechanical Engineering 
Texas A&M Un ive rs i t y  
College S ta t i on ,  Texas 77843, U.S.A.  
A new type of electromagnetic bearing has been built and tested. It 
consists of fixed AC-electromagnets in a star formation surrounding a 
conducting rotor. The bearing works by repulsion due to eddy-currents 
induced in the rotor. A single bearing is able to fully support a short 
rotor. The rotor support is inherently stable in all five degrees of 
freedom. No feedback control is needed. The bearing is also able to 
accelerate the rotor up to speed and decelerate the rotor back to 
standstill. This paper describes the bearing design and the 
experimentation to verify its capabilities. 
INTRODUCTION 
The Eddy-Current Bearing described here is based on the so-called 
Electromagnetic River suspension for high speed vehicles which was proposed 
and demonstrated by Eastham 6 Laithwaite [ l ]  in 1974. The Magnetic River 
was turned into a journal bearing by bending it into a circular shape (see 
Figure 1). 
the basis of the Magnetic River behavior reported in the literature (seefor 
example references quoted in [ 2 ]  and [3]). No analytical work has yet been 
conducted. The objective was to determine whether the bearing would 
inherit the basic desirable characteristics of the Magnetic River such as 
inherent stability, support capability in five degrees of freedom, motoring 
capability, and emergency shutdown capability. This paper reports on the 
experimental findings to date. Preliminary results were reported in [ 4 ] .  
Additional background can be found in [5]. Work on other types of 
AC-electromagnetic bearings has been reported in [6] through [9]. 
The resulting Eddy-Current Bearing was designed and built on 
THE EXPERIMENTAL APPARATUS 
Four U-shaped electromagnets are spaced 90' apart in a star formation 
The magnet 
to form a 10.16 cm diameter bearing as shown in Figures 1 and 2. 
magnets are mounted in a non-magnetic stainless steel housing. 
cores are made of grain-oriented 0.356 mm laminations with a saturation 
flux density of about 2 Tesla. The coils have 58 turns each. They are 
wound with two parallel flat copper wires with a total cross section of 
The 
1. This work was supported by the TAMU Turbomachinery Research Consortium 
Rig hardware was donated by IMC Corporation and by Magnetic Bearings, Inc. 
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approximately 6.35  mm x 4.32 mm. 
one core was measured to be about 3 . 2  mH. 
The inductance of two coils mounted on 
Each of the four magnets has an electric circuit as shown in Figure 3 .  
Power is supplied from a 115V 60 Hz single phase outlet. The power is 
adjusted by means of the variable transformer. 
used to adjust the power factor (or tune the circuit) such that a large 
current will circulate 
enough current is drawn from the supply to cover the I R loss in the coils 
and in the rotor. The variable capacitor consists of a bank of oil-filled 
capacitors in parallel which can be switched in and out of the circuit 
independently. Each capacitor bank consists of 13 100vF capacitors and 13 
150~F capacitors which are connected such that the capacitance can be 
varied in steps of 100 LIF from zero to 3,2501.1F. 
factor is therefore not possible but the reactive power can be reduced 
sufficiently to permit the experimentation with the available equipment. 
It was generally found that 11 to 13 capacitors needed to be switched in to 
minimize the supply current. 
The variable capacitor is 
between the capacitor and the $oil while only 
Fine-tuning of the power 
Five rotors made of construction aluminum were available for 
levitation. Three of these were 15.2 cm long with an inner diameter of 
7.62 cm and outer diameters of 9.50 cm, 9 .73  cm, and 9.91  cm respectively. 
They were used to study the effect of bearing clearance. A 15 .2  cm long 
solid iron cylinder with 7.62 cm diameter was made to fit snugly inside the 
aluminum sleeves to provide extra weight. Two additional aluminum sleeves 
with lengths 12.7 cm and 17.8 cm were used to study the effect of sleeve 
length. Also, a pure copper sleeve with length 15.2  cm and inner and outer 
diameters of 7.37 cm and 9.50 cm was made to study the effect of improved 
conductivity. The masses of the three 15.2  cm aluminum sleeves were 1.038 
kg, 1.176 kg, and 1.288 kg. The iron core mass was 4 . 8 8 3  kg and the copper 
sleeve mass was 3.859 kg. 
RESULTS 
All three 15.2  cm aluminum sleeves were successfully levitated as 
shown in Fig. 2 confirming the inherent stability of the bearing and the 
five degree-of-freedom support capability. 
slightly larger currents in all four magnets were required to levitate the 
large diameter sleeve than the medium and small diameter sleeves ( 7 5 A  [82V] 
versus 60A [65VJ in the bottom magnet). The larger current was not 
required to lift the sleeve but to provide sufficient support stiffness to 
prevent excitation of radial vibrations of the sleeve by the 60 Hz magnetic 
flux pulsations. Such excitation otherwise led to rattling of the sleeve 
against the pole faces. 
Contrary t o  expectations, 
With the iron cylinder inserted in the aluminum sleeves, there was 
insufficient power available to energize all four magnets for full 
levitation. With the bottom magnet excited only, the 6kg sleeve/core 
combination could be lifted free of the bottom with 130A [128V] and 140A 
(133VJ for the large and medium diameter sleeves respectively while the 
390 
small diameter sleeve was unable to fully lift off at 160A [142V] which was 
the maximum current available. For comparison, 40A [44V] to the bottom 
magnet was required to lift each of the three sleeves without the iron 
core. 
Although the copper sleeve geometry is almost identical to the small 
diameter aluminum sleeve, it suffered axial instability and tried to exit 
the bearing when lifted. The long (17.8 cm) aluminum sleeve behaved 
similarly suggesting that the copper sleeve could possibly be stabilized by 
reducing its length. The short (12.7 cm) aluminum sleeve had the largest 
thrust capability of all the sleeves but inferior radial support 
capability. This suggests, as expected, that it probably will not be 
possible to optimize the bearing with respect to all its capabilities 
simultaneously. 
The motoring capability was also confirmed. With single phase current 
supply, the three 15.2 cm levitated sleeves would rotate in the bearing 
when the magnet currents were adjusted to position the sleeve eccentrically 
within the bearing clearance. 
to the concentric position and would rotate in the opposite direction when 
brought to a diametrically opposite eccentric position. The larger the 
eccentricity, the higher the speed. The maximum speed recorded with single 
phase current was about 50 rpm. With 3-phase current, the rotational speed 
could be increased to about 750 rpm with the sleeves supported 
mechanically. 
3-phase current, apparently because the strong torque at zero speed would 
rotate the sleeves before metal contact could be broken, thus initiating a 
backward whirl instability. 
They would stop rotating when brought back 
It was not possible to fully levitate the sleeves with 
After a few minutes of operation, the aluminum sleeves got too hot to 
be hand held whereas the magnets remained cool with only a slight 
temperature increase to be felt. 
hot as the aluminum sleeves, attesting to its greater conductivity. An 
increase in supply current to one circuit would result in a similar current 
in the other circuits, indicating strong mutual inductance between the 
magnet coils. 
Also, the copper sleeve did not get as 
The magnetic support stiffnesses, with the three 15.2 cm sleeves 
levitated, were estimated by impacting the sleeves and counting vibration 
periods with a stopwatch. The average radial, axial and angular 
stiffnesses were found to be in the neighborhood of 80 N/m, 60 N/m, and 0.3 
Nmlrad. The damping was also very low, as evidenced by the long time taken 
for any vibrations to die out. 
Finally, the effect of power supply frequency was studied using a 60 
Hz magnet and a 400 Hz magnet. 
bearing magnets except that the pole faces were flat. The 400 Hz magnet 
had identical geometry but was made of 0.1 mm laminations, and the number 
of turns was 15 per coil to achieve the same magnetic flux density as with 
the 60 Hz magnet but using a 150V, 400 Hz power supply. According to the 
analysis of [ 8 ] ,  a frequency increase should result in a significant 
improvement of the lift capacity. This did not occur. The main effect was 
The 60 Hz magnet was identical to the 
391 
that thinner aluminum plates could be lifted with 400 Hz power supply, 
presumably because the eddy-current penetration depth decreases with 
increasing frequency. 
CONCLUSIONS 
The experiments have confirmed that the Eddy-Current Bearing retains 
the basic advantages and disadvantages of the Magnetic River suspension: 
1. 
2. 
3. 
4. 
5. 
1. 
2. 
3 .  
4 .  
A single eddy-current bearing is sufficient to fully support a 
short rotor in all five degrees of freedom simultaneously. 
The bearing provides inherently stable rotor support. No 
feedback control is needed. 
The bearing will act as a motor and as a support simultaneously. 
The lift capacity and the stiffness and damping achieved so far 
are low. Additional damping can be supplied by a simple 
eddy-current damper and design optimizations may improve the 
stiffness and the lift capacity but it is unlikely that they will 
ever exceed those of DC-type magnetic bearings. 
The efficiency of the bearing is low due to a high I R loss in 
the rotor. This is considered to be the most serious problem 
which must be overcome before the bearing finds practical 
application. The current thinking is to move the design closer 
to a conventional induction motor, thereby further reducing the 
load carrying capacity but gaining motoring efficiency. 
bearing could then be used in space-based applications such as 
flywheel energy storage systems where it could support the 
flywheel and also act as the motor/generator. 
2 
The 
REFERENCES 
Eastham, J.F. and Laithwaite, E.R. "Linear Induction Motors as 
Electromagnetic Rivers", Proc. IEE, Vol. 121, No. 10, pp. 
1099-1108, 1974. 
Jayawant, B.V. "Electromagnetic Levitation and Suspension 
Techniques," Edward Arnold, London, 1981. 
Jayawant, B.V. "Electromagnetic suspension and Levitation," IEE 
Proc., Vol. 129, Pt. A ,  No. 8, pp. 549-581, 1982. 
Nikolajsen, J.L. "An AC-Electromagnetic Bearing for Flywheel 
Energy Storage in Space," To be published in NASA CP, Magnetic 
392 
Suspension Workshop, NASA Langley, Feb. 1988. 
5. Nikolajsen, J.L. "A Magnetic Bearing Based on Eddy-Current 
Repulsion," NASA Conference Publication 2443, Rotordyn. Instab. 
Prob. in Hi-Perform. Turbomach. pp. 460-465, 1986. 
6. Bolton, H. "An Electromagnetic Bearing," IEE CP 120, Linear 
Electr. Machines, London, pp. 45-50, 1974. 
7. Iskierka, S. "Influence of Irregularities of Exciting Current 
Density Distribution on Induction Bearing Work," Acta Technica 
Csav, No. 5, pp. 572-582, 1984. 
8. Iskierka, S. "Analysis of an Induction Bearing by the Finite 
Element Method," Archiv fur Electrotechnik, Vol. 67, pp. 375-380, 
1984. 
9. Connor, K.A. and Tichy, J.A. "Analysis of an Eddy-Current Journal 
Bearing," ASME Pap. 87-Trib-10, 7pp., 1987. 
w I I ,\k\'\R w w m co 
LENGTHS I N  mm 
F i g u r e  1 The Eddy-Current Bear ing  (Schematic)  
393 
Figure 2 The Eddy-Current Bearing Prototype 
I 
115V 
60Hz 
3 
D 
0 z 
m 
--.I 
0 
0 
r 
- 
Figure 3 Elec t r ic  Ci rcu i t  f o r  Each Magnet 
ORIGINAL PAGE 
BLACK AND WHITE FHOTOGR4pH 
394 


Experimental verification of an eddy-current bearing

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