Development of Magnetic Coupling with Variable
Thrust Structure for Flywheel Energy Storage System
in Long Lifetime UPS*Daisuke Sato, *Jun-ichi Itoh, *Tomoki Watanabe, *Kazuma Kawagoe, *Noboru Yamada, and **Koji Kato*Nagaoka University of Technology
Nagaoka, Niigata, Japandsato@stn.nagaokaut.ac.jp, itoh@vos.nagaokaut.ac.jp
Abstract— This paper introduces the flywheel energy storage
system (FESS) in a long lifetime uninterruptible power supply.
The first prototype FESS (3.0-MJ) uses low cost ball bearingsand general purpose induction motor in terms of cost reduction.
From the experimental results, it is confirmed that the charge
and discharge efficiency of the FESS is 60.7% (charge 80.7%,
discharge 75.1%). In order to improve the efficiency and lifetime,a permanent magnet synchronous motor, a spherical spiralgroove bearing and a magnetic coupling with variable thrust
structure are applied to the second prototype. The estimatedcharging efficiency is achieved to 86.9% and the estimateddischarging efficiency is achieved to 82.8% at the second
prototype. Then, the thrust of the proposed magnetic coupling isanalyzed by the finite element method (FEM). As a result, thethrust can be changed by 11.1% with the electromagnet. In
addition, the thrust analysis results are evaluated by using theprototype electromagnet.
Keywords—Energy storage; Flywheel; Magnet coupling;
Thrust control

I. INTRODUCTION
An uninterruptible power supply (UPS) is widely used inorder to prevent instantaneous power failure in data centers orfactories. The UPS needs the energy buffer such as electric
double layer capacitors (EDLCs), batteries, or flywheels
because the UPS has to supply the power to load until anemergency generator starts.
Table I shows the characteristics of each energy storage
devices. The battery can achieve a high energy density at low
cost. However, one of the problems in the battery energy
storage is the short life time. In particular, the lifetime depends
on the ambient and the number of charge and discharge time.
In addition, the battery cannot cope with rapid charge anddischarge due to the large internal resistance in the battery. On
the other hand, an EDLC has a high charge and discharge
efficiency. Moreover, the rapid charge and discharge arepossible because the internal resistance is very small. However,similar to the battery, the lifetime is decreased rapidly due tothe influence of the ambient temperature [1]. On the other hand,
flywheel energy storage systems (FESS) has following
advantages compared to chemical batteries: (i) environmental
friendly, (ii) low maintenance cost, (iii) long lifetime due to no**Sanken Electric Co., Ltd.
Kawagoe, Saitama, Japank.kato@sanken-ele.co.jp

TABLE I.

Energy storage
Charge &Discharge ofshort period
Temperature
characteristic
Energy density

CHARACTERISTICS OF EACH STORAGE DEVICES.
Flywheel
Kinetic
energyFast
Excellent
Good

EDLC
Ion transfer

Battery
Chemical
reaction

Fast

Slow

Limitedbytemperature
Good

Limitedbytemperature
Excellentchemical structure, and (iv) the charge and discharge
characteristic of high cycle are excellent. For these reason, theflywheel has attracted attention as an energy storage system[2-8].
In recent years, an ultra-high speed rotation of the flywheel
has been studied [2-4]. The kinetic energy which is stored in
the flywheel is proportional to the square of the rotational
speed. Therefore, the magnetic bearings have been studied inorder to achieve high energy density. However, theconventional five-axis magnetic bearing system requires anadditional control system [7]. In addition, the flexibility of thissystem is low because this system is necessary to design inaccordance with the capacity of the power source to becompensated. On the other hand, in order to increase storage
energy, it is necessary to increase the mass of the flywheel inthe low speed rotation FESS because the kinetic energy is
proportional to the inertia. However, when the typical ballbearing is applied, the bearing loss is increased by increasing
the mass of the flywheel. Therefore, it is effective to apply
spherical spiral groove bearing (SSGB) as the bearing loss
reduction method for the flywheel because the SSGB can float.
Furthermore, the SSGB has longer lifetime than that of the ballbearing [9].
In addition, a motor/generator for driving the flywheel is
put inside the vacuum case (MG-inside type) or outside it(MG-outside type). MG-outside type has advantage in terms ofheat dissipation. In addition, torque is transmitted to theflywheel by a magnetic coupling [10-11]. However, it isdifficult to apply the SSGB to MG-outside type because theshaft of the SSGB side must be float, whereas the shaft of theMG side is fixed. Therefore, the thrust of the flywheel iscontrolled by the proposed magnetic coupling. The proposedmagnetic coupling can not only transmit torque but also control
the thrust of the SSGB as the magnetic bearing. The
conventional magnetic bearing has five electromagnets (twofor x-axis, two for y-axis and one for z-axis). By contrast, theproposed magnetic bearing has only one electromagnet for zaxis.
In this paper, the first prototype FESS made from thegeneral products is developed. Next, the charging anddischarging efficiency are measured. Then, in order to improvethe efficiency and lifetime, the PMSM, the SSGB and the
proposed magnetic coupling are employed in the second
prototype FESS. This paper considers the proposed magnetic
coupling with variable thrust structure as the first step in order
to apply the SSGB. Therefore, the thrust and the torque of the
proposed magnetic coupling are analyzed by the finite element
method (FEM). Besides, the prototype electromagnet isdeveloped, and the thrust analysis results by the FEM arecompared with the experimental results.
II. CONSTRUCTION OF FLYWHEEL ENERGY STORAGE SYSTEM
Fig. 1 shows the configuration of the first prototype FESS
that employs a general induction motor and ball bearings. In
this system, it is possible to store the kinetic energy of 3.0 MJ
at 2900 r/min. The typical ball bearing and the general purpose
motor can be applied in such low rotation speed region. In
addition, the flywheel vacuum case and the motor are separated
by the magnetic coupling. As a result, the windage loss can begreatly reduced because it is possible to reduce the pressure ina vacuum case by the vacuum pump. Moreover, the vacuum inthe vacuum case does not affect the heat dissipation of the
motor because the motor and the vacuum case are separated.
Therefore, the general purpose motor can be applied to drive
the flywheel without adding special cooling mechanism.
Fig. 2 shows the block diagram of the flywheel system
including the measurement system and auxiliary devices. In the
flywheel system, the induction motor is operated as a generator
during deceleration; the kinetic energy is converted into theelectrical energy. On the other hand, during acceleration, theelectrical energy is stored as the kinetic energy. Therefore, thissystem uses a regenerative converter. Furthermore, an overtemperature of the bearing and the motor can be prevented by
the oil cooler.
Table II shows the specification of the flywheel. It can beseen that the shape of the flywheel is disk-shaped. The
flywheel is capable of storing sufficient energy for low
rotational speed due to the large inertia weight. In addition, thewindage loss and the bearing losses are also reduced by the
reduction of the rotational speed. The pressure in the vacuum
case is reduced by using the vacuum pump in order to reduce
the windage loss. In addition, the magnetic coupling is adopted
to maintain the vacuum in the vacuum case.
III. EVALUATION OF CHARGE AND DISCHARGE EFFICIENCY
In this section, the energy storage efficiency of the
prototype system (Fig. 1) is analyzed. This property is divided
into charging efficiency and discharging efficiency. The
charging efficiency is the ratio of the energy consumed forInduction motor

Magnet coupling
Ball bearing
Vacuum case
FlywheelFig. 1. Configuration of a flywheel system that employs an induction
motor and ball bearings. This system can store the kinetic energyof the 3.0 MJ at 2900 r/min.

3Φ200V
3
Power
meter

GeneralPurposeInverterPower
3 meter

Regenerative
converter
Regeneration
3
3

Induction
motor
3

Magnetic
coupling

Flywheel

Bearing

Oil Pump
VacuumPump

Vacuumcase

Fig. 2. Block diagram of the flywheel system including themeasurement system and the auxiliary devices.

TABLE II.

SPECIFICATIONS OF THE FLYWHEEL WHICH IS
CALCULATED FROM THE LOW LOSS DESIGN.

Flywheel

Motor/Generator
Inverter

SCM440500 mm
65 mm
392 kg
3.0 MJ at
Stored Energy
2900 r/min
MLC1115C (Fuji Electric)
FRN 37G11S-2 (Fuji Electric)
Material
RadiusThickness
Weight of the Flywheel

storing the desired energy. On the other hand, the discharging
efficiency is the ratio of the actual energy can be released withrespect to the stored energy.
Fig. 3 shows the experimental results of (a) the energy
storage and (b) the energy discharge. In the experiment ofenergy storage, the flywheel is accelerated from 2300 r/min to2900 r/min. In the discharge test, the flywheel is decelerated
from 2900 r/min to 2300 r/min. The stored energy is
calculated using (1) from the rotational angular velocity andthe moment of inertia of the flywheel J.

IV. EFFICIENCY IMPROVEMENT OF FESS

6

Power[kW]Electrical Loss

MechanicalLoss

5

The low efficiency of the first prototype FESS is caused
mainly by the additional loss when the induction motor is used
to drive the flywheel. Besides, the cause of the mechanical loss
is the bearing loss. Thus, it is necessary to apply a PMSM anda SSGB to achieve high efficiency for the entire flywheel
system. In addition, SSGB is longer lifetime than the ballbearing. However, the shaft of the SSGB side must be float,
whereas the shaft of the motor side is fixed. In other words, theshaft of the SSGB needs a degree of freedom. In order to solve
this problem, a magnetic coupling with a variable thrust
structure is proposed in this paper. The conventional magnetic
coupling is effective method to transmit torque in the isolated
system such as the first prototype FESS. The proposedmagnetic coupling can not only transmit torque from the motorin the atmosphere to the flywheel in vacuum but also control
the thrust of the SSGB. The proposed magnetic coupling can
give a degree of freedom indirectly to the SSGB.

4
3
2
Stored Energy

1
0
2300

2400

2500
2600
2700
Rotation Speed[r/min](a)6Power[kW]

5

2800

2900

Charging efficiency

Electrical Loss

Mechanical Loss4

Fig. 4 shows the configuration of the second proposed
FESS in which the system of Fig. 1 is modified in order toreduce the motor loss and the bearing loss. The PMSM
employs the general product due to low production cost. The
ratio output of the motor is 3.7 kW. The SSGB is attached tothe bottom of the flywheel.

32

Discharged Energy

1
0
2950

2850

2750
2650
2550
Rotation Speed[r/min]
(b)

2450

Discharging efficiency

Fig. 3. Measurement results of the charging and discharging
efficiency. The achieved charging efficiency is 80.7 % and the achieved
discharging efficiency is 75.1 %. Therefore, the total achieved
efficiency is 60.7 %.

E

1
J 2
2

(1)

From (1), it is calculated that the amount of the energy change
is 1 MJ when the rotational speed changes from 2300 r/min to2900 r/min. Next, the energy storage efficiency is calculatedusing the experimental results. Note that, the charging
efficiency is the ratio of the input energy to the stored energy.
On the other hand, the discharging efficiency is the ratio of thedecreasing of the kinetic energy to the regenerative energy. If
only the mechanical loss is considered, it is confirmed that theachieved charging efficiency is 97.9% and the achieved
discharging efficiency is 97.5%. Therefore, the total achieved
efficiency is 95.2%. On the other hand, if the motor loss is
considered, the achieved charging efficiency is 80.7% and theachieved discharging efficiency is 75.1%. Therefore, the total
achieved efficiency is 60.7%.
Besides, the discharging efficiency of the energy storage
devices is as follows: the lead acid battery is 75-85%, a redox
flow battery is approximately 70%, and an EDLC isapproximately 90%. Therefore, it is confirmed that the energy
storage efficiency of the prototype FESS is about as same as
the lead acid battery or the redox flow battery. However, theefficiency of the FESS is lower than that of the EDLC.

Fig. 5 shows the analysis result of the steady state loss.
The steady state loss of the first prototype is divided based on
the measurement result. That of the second prototype isestimated. The rotational speed is 2900 r/min and thepressure in the vacuum case is kept at 20 Pa. The mechanicalloss and the electrical loss are separated by a free-run test at
the beginning. The free-run test is a test for electricallydisconnecting the inverter and the motor with the stored
energy in the flywheel. In this state, the stored energy isconsumed only by mechanical loss. Therefore, themechanical loss is calculated from the time until the stored
energy is consumed. The motor loss of the second prototype
is conjectured by the representative value. From the total lossof the first prototype is 436 W. On the other hand, that of thesecond prototype is 274 W. The motor loss of the second
prototype is 30.4% lower than that of the first prototype, andthe bearing loss of the second prototype is 88.0% lower thanthat of the first prototype. In addition, the loss of themagnetic coupling increases slightly by the coil current of theelectromagnet. As a result, it is comfirmed that the second
prototype achieves low loss as compared with the first
prototype. Moreover, the estimated charging efficiency isachieved to 86.9% and the estimated discharging efficiency isachieved to 82.8% at the second prototype. Thus, it isexpected to improve the charge and discharge efficiency byapplying the PMSM and the SSGB.
V. DESIGN OF MAGNETIC COUPLING WITH VARIABLE THRUSTSTRUCTURE
In this section, the magnetic coupling with variable thrust
structure is considered. The required performances of themagnetic coupling are as follows:Permanent magnet
synchronous motor

Variable thrustmagnet coupling

Spherical spiralgroove bearing

Vacuum caseElectromagnetic

Flywheel

For torque transmission

MasterSlave

Fig. 4. Configuration of the proposed flywheel system that comprises
the PMSM, the SSGB, and the proposed magnetic coupling inwhich the system of Fig. 1 is improved.

For variable thrust

For thrust reduction

Fig. 6. Designed magnetic coupling with a variable thrust structure.
Note that the model of the electromagnets and the master side area half model and the one of the slave side is a full model.

500

TABLE III.

400

Loss [W]

Magnetic
coupling loss
300

Windage loss

200

Bearing loss

100

Motor /
Generator loss

PARAMETERS OF MAGNETIC COUPLING.
Pole number
Coil number
Turn number of coil
Rated coil current
Yoke and core
MagnetOuter diameter
0First prototype

Second prototype(Estimation)

Fig. 5 Loss analysis results at the rated energy storage. The steadyrotation speed is 2900 r/min and the pressure of the vacuum caseis 20 Pa.

MasterSlave

Inner diameter
Thickness of magnet
Air-gap length

Between master and slave
Between coil and slave

16
16
40
10 A
S45C
N45M
155 mm
255 mm
45 mm
4 mm
10 mm
7 mm

 The allowable torque is over than the maximum output
torque of the PMSM (7.4 Nm).
 When the variable thrust structure does not operate, thethrust is same as the gravity of the flywheel (490 N).
 When the variable thrust structure operates, themaximum variation of the thrust is approximately 20%.
Fig. 6 shows the designed magnetic coupling with thevariable thrust structure. Table III shows the parameter of themagnetic coupling. Note that the model of the electromagnets
and the master side are a half model and the one of the slave
side is a full model. The magnets for the torque transmission
are 16 poles. The magnets in the innermost layer reduce
excessive thrust by the magnets for the torque transmission.
The variable thrust structure consists of the electromagnets andthe outside magnet in the slave side. Note that the coil current
is same in the all electromagnets. Therefore, the 16
electromagnets are operated as one large electromagnet. The
current in the electromagnets generates the magnetomotive
force (MMF) F. The MMF is obtained by (2).
F  NI(2),

Fig. 7. Analysis model of the magnet coupling. The element count is
212,691 exclusive of the air region.

where N is the turn number of the coil and I is the current.
From (2), it is understood that the MMF can be controlled by
the current. In addition, the current is controlled by the inverterin order to change the MMF direction. Therefore, the thrust canbe controlled indirectly by the current. Then, the thrust
characteristic of the designed magnetic coupling is analyzed by
the finite element method.
Fig. 7 shows the analysis model of the proposed magnet
coupling. It is 45 degrees model because the coupling has

Fig. 8 shows the analysis result of the thrust and the torquewhen the variable thrust structure does not operate. From Fig.
8(a), the thrust is almost same as the design value (487 N)
when the rotation angle is 0 degree and F = 0A. In addition, theslave side decreases the thrust by 11.1% when the MMF
changes from 0A to -400 A. This is caused by the repulsion
force between the coils ant the magnet for the variable thrust.
In addition, the repulsion force is always constant with therotation angle because the magnet for the variable thrust is one
pole. From Fig. 8(b), the maximum torque is 16.7 Nm at 11.25
degree. Therefore, the allowable torque of the proposedmagnetic coupling is twice maximum output torque of the
PMSM. In addition, the torque at F = 0 A is same as that at F =-400 A because the direction of the MMF by the electromagnet
is axial direction.
Fig. 9 shows the characteristic between thrust and MMF
when the rotation angle is 0 degree. The thrust at F = 0 A isalmost same as the gravity of the flywheel (487 N). The change
in the thrust is 111 N between F = -400 A and F = 400 A.
Therefore, the magnetic coupling can be designed as required.
In addition, the thrust varies linearly with the MMF. Thismeans that the thrust is controlled easily by the current.

600

Thrust [N]

400

0
-200
-400
-600

(a)

Fig. 10 shows the 2D analysis model of the prototype
electromagnet. It is built by the EI-core (PC40EI60/TDK). The
coil is placed in the E-core side. The turn number of coil is 140.
In addition, the element count is 5,898 which include the air
region. Next, the thrust of the I-core side is analyzed andmeasured when the MMF and the air-gap length are changed.
Fig. 11 shows the characteristic between thrust of the I-coreside and MMF. The air-gap length is kept constant at 4.8 mm.
From Fig. 11, the analysis result is almost same as theexperimental result. In addition, the thrust is proportional to thesquare of the MMF. When the magnetic resistance is constant,the flux is the proportional to the MMF. Therefore, the thrust is
the proportional to the square of the flux.
Fig. 12 shows the characteristic between thrust of the I-coreside and air-gap length. The MMF is kept constant at 140 A.
From Fig. 11, it is confirmed that the thrust is inverse
proportion to the square of the air-gap length. As a reason forthat the flux in the electromagnet is almost inverse proportional
to the air-gap length.

0

5
10
15
20
Rotation angle [degree]

25

Thrust at F = 0 A and 400 A. The slave side decreases thethrust by 11.1% when the MMF changes from 0A to -400 A.

20
100
Master (0 A)
Slave (0 A)
Master (-400 A)
Slave (-400 A)

-10
-20
(b)

0

5
10
15
20
Rotation angle [degree]

25

Torque at F = 0 A and 400 A. the torque at F = 0 A is same asthat at F = -400 A because the direction of the MMF by theelectromagnet is axial direction.

Fig. 8. Analysis results of thrust and torque.

600
Thrust [N]VI. EVALUATION OF THRUST ANALYSIS
In this section, the prototype electromagnet is developed.
Then, in order to apply the thrust control by the magnetic
coupling into the proposed FESS, the thrust analysis result of
the electromagnet is evaluated in the experiment.

Master (0 A)
Slave (0 A)
Master (-400 A)
Slave (-400 A)

200

Torque [Nm]rotational symmetries. The element count is 212,691 exclusiveof the air region. In addition, the upper side of purple magnets
is N pole and the upper side of blue ones is S pole. The magnet
temperature is 20 degree Celsius. The current polarity isdefined as positive when the current flows through the coils ina counterclockwise direction. Therefore, the upward MMF (zaxis direction) polarity is defined as positive when the current
polarity is positive. In this analysis, the thrust and the torqueare calculated at each rotation angle when the slave side isfixed and the master side is rotated.

550
500
450
400

-400

-200
0
200
Magnetomotive force [A]400Fig. 9. Characteristic between thrust and MMF when the rotation angle
is 0 degree. The change in the thrust is 111 N between F = -400 A
and F = 400 A.

From Fig. 11 and 12, the analysis results agree well withthe experimental results. Therefore, the validity of the thrust
analysis by the FEM is confirmed.
VII. CONCLUSION
In this paper, the FESS (3.0-MJ, 2900-r/min) using low costball bearings and general purpose induction motor is evaluated.
From the experimental results, the charge and discharge
efficiency of the FESS is 60.7% (charge 80.7%, discharge
75.1%). Therefore, the second prototype FESS is applied to the
PMSM, the SSGB and the magnetic coupling with variablethrust structure in order to achieve high efficiency and longer
lifetime. The estimated charging efficiency is achieved toFurthermore, the prototype is developed to evaluate the validityof the thrust analysis. As a result, it is confirmed that theexperimental results are almost same as the analysis results.
Coil (Copper)

The future work is the control of the magnetic coupling andthe measurement of the charge and discharge efficiency for thesecond prototype FESS.
REFERENCES

Air-gap[1]
Core(Ferrite)Fig. 10 Analysis model of the prototype electromagnet. The turnnumber of coil is 140 and the core is PC40EI60/TDK.

Air-gap length: 4.8 mm

0.35
0.3

Thrust [N]

0.25
0.2
0.15
0.1

Analysis result
Experimental result

0.05
0

50

100

150
200
250
300
M agnetomotive force [A]

350

400

Fig. 11 Characteristic between thrust of the I-core side and MMF. The
experimental result are almost same as the analysis result.

M agnetomotive force: 140 A

0.35
0.3

Thrust [N]0.25
Analysis result
Experimental result

0.2
0.15
0.1
0.05
0

1

2

3
4
5
Air-gap length [mm]

6

7

8

Fig. 12 Characteristic between thrust of the I-core side and air-gap
length. The thrust is inverse proportion to the square of the air-gap
length.

86.9% and the estimated discharging efficiency is achieved to82.8% at the second prototype. Thus, it is expected to improvethe charge and discharge efficiency by applying the PMSM andthe SSGB. In addition, the thrust of the proposed magnetic
coupling can be changed 11.1% by the electromagnet.

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