Development of Magnetic Assist System in Flywheel
Energy Storage System for Power Load-LevelingJun-ichi Itoh, Takumi Masuda, Daisuke Sato, Tsuyoshi Nagano, Takeo Suzuki and Noboru Yamada*Nagaoka University of Technology
Nagaoka, Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract— This paper presents a flywheel energy storage
system (FESS) for load-leveling. In order to improve theefficiency and the lifetime, a permanent magnet synchronous
motor, a spherical spiral groove bearing (SSGB) and a magnetic
assist system are applied to the prototype. The variable thrustby the proposed magnetic assist system is evaluated. As a result,the variation of thrust becomes 16.4%. In addition, it isobserved that the loss of the SSGB relates to the precession
movement of the flywheel. Besides, it is observed that the loss of
the SSGB depends on the temperature of the lubricating oil.
Keywords—Energy storage; Flywheel; Magnetic assist

TABLE I.

Energy storage
Charge &Discharge ofshort period
Temperature
characteristic
Energy density

CHARACTERISTICS OF ENERGY STORAGE METHODS.
Flywheel
Kinetic
energyBattery
Chemical
reaction

Fast

Slow

Fast

Limited bytemperature
Excellent

Limited bytemperatureGood

Excellent
Good

EDLC
Ion transfer

I. INTRODUCTION
Renewable energy systems, especially a photovoltaic celland a wind turbine, generate a power fluctuation depending on
the meteorological conditions. Therefore, these systems
require energy buffer to suppress the power fluctuations.
Table I depicts the characteristics of each energy storage
method. Batteries can achieve a high energy density at low
cost. However, one of the problems in the battery energy
storage is a short life time. In particular, the lifetime depends
on the ambient and the number of charge and discharge time.
In addition, the batteries cannot cope with rapid charge anddischarge due to a large internal resistance. On the other hand,
EDLCs have a high charge and discharge efficiency.
Moreover, the rapid charge and discharge are possible because
the internal resistance is very small. However, similar to thebattery, the lifetime decreases rapidly due to the influence of
the ambient temperature [1]. On the other hand, flywheel
energy storage systems (FESS) have following advantages
compared to chemical batteries: (i) the excellent charge anddischarge characteristic with short period, (ii) long lifetime
due to no chemical structure, (iii) low maintenance cost, and(iv) environmentally friendly. Therefore, the flywheel has
attracted many attentions as an energy storage system [2-8].
In recent years, the flywheel with an ultra-high speed
rotation has been studied [2-4]. The kinetic energy which isstored in the flywheel is proportional to the square of therotational speed. Therefore, the magnetic bearings have beenstudied in order to achieve high energy density. However, theconventional five-axis magnetic bearing system requires anadditional control system [7]. Furthermore, the flexibility ofthis system is low because this system is necessary to designin accordance 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 isalso proportional to the inertia. However, when the typical ballbearing is applied, the bearing loss increases due to theincrease in the mass of the flywheel. Therefore, it is more
effective to apply a spherical spiral groove bearing (SSGB) as
the bearing loss reduction method because the SSGB can float.
Furthermore, the SSGB has longer lifetime than the ballbearing [9].
In addition, the magnetic assist system is proposed in orderto reduce the bearing loss of the flywheel. Because theflywheel rotates around only one point in the SSGB, theprecession movement occurs. This increases the loss of the
flywheel. Therefore, the touchdown bearing is employed inorder to avoid the precession movement. Consequently, it isnecessary to verify the loss of the SSGB using touchdown
bearing. Another method to suppress the precession
movement is the use of the magnetic assist system which cangenerate the thrust onto the flywheel. Therefore, the bearing
loss characteristic of the SSGB needs to be evaluated in theterms of the thrust of flywheel. Furthermore, the loss in the
SSGB is also dependent on the temperature. Therefore, it isalso necessary to verify the relationship between the bearing
loss and the temperature characteristics of the SSGB, which
depends on the weight of the SSGB onto the bearing.
In this paper, in order to improve the efficiency and thelifetime, the PMSM, the SSGB and the proposed magnetic
assist system are employed in the prototype FESS. This paper
evaluates the proposed magnetic assist system as the first stepin order to apply the SSGB. Therefore, the thrust of the
proposed magnetic assist system is evaluated by theexperiment. In addition, the relationship between the loss andthe temperature characteristics in the SSGB is also verified by
the experiment.

II. CONSTRUCTION AND STRATEGY OF FLYWHEEL ENERGYSTORAGE SYSTEM
Fig. 1 shows the block diagram of the FESS including themeasurement system and the auxiliary devices. In the FESS,
the PMSM is operated as a generator during acceleration,
where the electrical energy is stored as the kinetic energy. On
the other hand, during deceleration; the kinetic energy isconverted into the electrical energy. Therefore, this system
employs a regenerative converter. Furthermore, anoverheating of the bearing and the motor can be prevented by
the cooling of the oil pump.
Table II shows the specification of the flywheel. It can beobserved that the shape of the flywheel is disk-shaped. The
stored energy is calculated by (1) from the rotational angularvelocity  and the moment of inertia of the flywheel J.


E

1 2
J 
2

3Φ200V
3
Power
meter

Permanent magnet
synchronous motorPower
3 meter 3

GeneralPurposeInverter

Regenerative
converter
Regeneration

Magnetic
coupling

Bearing

Flywheel
Oil Pump

3

VacuumPump

3

Vacuumcase

Fig. 1. Block diagram of FESS including measurement system andauxiliary devices.



The flywheel is capable of storing energy up to 0.15 MJ in
the prototype to evaluate the fundamental operation. It isnoted that the stored energy of the final FESS will be 3 MJ.
In addition, the pressure in the vacuum case is reduced byusing the vacuum pump in order to further reduce thewindage loss. The magnetic coupling is adopted to maintainthe vacuum in the vacuum case.
Fig. 2 shows the configuration of the second proposed
FESS, the system of the first prototype FESS in which ismodified in order to reduce the motor loss and the bearing
loss [10]. The low efficiency of the first prototype FESS iscaused mainly by the additional loss when the induction
motor is used to drive the flywheel. Besides, the PMSM
employs the general product due to low production cost. The
ratio output of the motor is 3.7 kW. The cause of themechanical loss comes from the ball bearing. Thus, it isnecessary to apply a PMSM and a SSGB to achieve highefficiency for the entire FESS. In addition, SSGB has longer
lifetime than the ball bearing. In addition, the flywheel of thesecond prototype is smaller than that of the first prototype
because the purpose of this paper is basic operation
verification of the SSGB and the magnetic assist system. The
proposed magnetic assist system controls the thrust of the
SSGB.
Fig. 3 shows the conceptual diagram of SSGB. The
features of SSGB have small friction loss, long lifetime andhigh reliability. The flywheel axis rotates in the lubricant oil
stored inside SSGB. This rotation creates a thin oil sick aroundthe flywheel axis, which makes the flywheel float whenrotating. This structure helps the rotation of the flywheel
eliminate the mechanical contact with other parts.
Consequently, the high efficiency and the abrasion reduction
are expected to reduce the cost. In addition, the lighter the
flywheel is, the easier it can float. Therefore, the magnetic
assist system is proposed in order to reduce the weight of the
flywheel.

TABLE II.

SPECIFICATIONS OF FLYWHEEL WHICH IS CALCULATEDFROM LOW LOSS DESIGN.

Flywheel

Motor/Generator

SCM440177 mm
65 mm
50 kg
0.15 MJ at
Stored Energy
6000 r/min
MM-BF337 (Mitsubishi Electric)
Material
RadiusThickness
Weight of the Flywheel

Magnetic coupling

Permanent magnet
synchronous motor

Vacuum caseElectromagnet for variable thrust

Spherical spiralgroove bearing
Flywheel

Magnet for weight reduction

Fig. 2. Configuration of proposed FESS that comprises PMSM, SSGB,
and proposed magnetic assist system.

GrooveLubricating oil

Fig. 3. Conceptual diagram of SSGB.

III. PROPOSED MAGNETIC ASSIST SYSTEM
The magnetic assist system is considered in this section.
The required performances of the magnetic assist system are
as follows:

Shaft
Electromagnet

Flywheel When the variable thrust structure does not operate, theweight of the flywheel is reduced to approximately
97% by the permanent magnet.
 When the variable thrust structure operates, themaximum variation of the thrust is approximately 10%of the weight of the flywheel.
Fig. 4 shows the designed magnetic assist system. The
proposed magnetic assist system consists of the passive type
and the active type. There are magnets in bottom of the
flywheel and beneath the flywheel in order to reduce theweight of the flywheel by the magnetic repulsion, i.e. thepassive type magnetic assist system. On the other hand, thevariable thrust structure consists of the electromagnets and themagnet at the top of the flywheel, i.e. the active type magnetic
assist system. Note that the coil current is same in allelectromagnets. Therefore, 16 electromagnets are operated as
one large electromagnet. The current in the electromagnets
generates the magnetomotive force (MMF) F, which is
obtained by (2).
F  NI

Magnet(for weight reduction)(a)Magnet assist system

Magnet(for variable thrust)(2),

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
inverter in order to change the MMF direction. Therefore, thethrust can be controlled indirectly by the current. Then, thethrust characteristic of the designed magnetic coupling isanalyzed by the finite element method.

(b)Upper side of flywheel

Table III shows the parameters of the designed magnetic
assist system. When the variable thrust structure does notoperate, the analysis result of the thrust applied to the flywheel
is 477 N. In other words, the weight of the flywheel is reduced
to 97.3%.
Fig. 5 shows the analysis result of the variable thrust. The
variable thrust is 57.5 N (11.7%) at F = 290 A. Therefore, theactive type magnetic assist system can be designed as
requirement. In addition, the thrust monotonically increases
with the MMF. This means that the thrust is controlled easily
by the current.
IV. EXPERIMENTAL RESULTS
The variable thrust structure is evaluated with the second
prototype. Note that the thrust is not measured directly. In
particular, the weight of the flywheel is measured andconverted to the thrust.
Fig. 6 shows the measurement result of the variable thrust.
The variable thrust is 80.2 N (16.4%) at F = 290 A. The
measurement result is larger than the analysis result because
the analysis model is not perfectly considered as the magnetic
materials composing the FESS.

Magnet(for weight reduction)

(c)

SSGB

Lower side of flywheel

Fig. 4. Designed magnet assist system. There are the magnets in bottomof the flywheel and under the flywheel in order to reduce the weight of
the flywheel by the magnetic repulsion. The variable thrust structure
consists of the electromagnets and the magnet in top of the flywheel.

Next, it is necessary to evaluate only the effect of thepassive type magnetic assist system for the FESS. In case of
only the passive type applied, the oscillation of the flywheel
can occurs on the z-axis, because the passive type magnetic
assist system can only control the flywheel weight on the zaxis. By using the touchdown bearing, the movement of the
shaft of the flywheel on the x-axis and the y-axis can besuppressed. Therefore, it is necessary to evaluate the effectsof the use of the touchdown bearing.

TABLE III.

PARAMETERS OF MAGNETIC ASSIST SYSTEM.
(a)

(b)

Magnet for variable thrustMaterial
N45M
Thickness
3 mm
Outer diameter
300 mmInner diameter
234 mmElectromagnet for variable thrustMaterial (Core)
S45C
Number of coils
16
Turn number of coil
29

60
50

Variable thrust [N]Magnet for weight reduction
Material
N45M
Thickness
5 mm
Outer diameter
160 mmInner diameter
120 mm

40
30
20
10
0

(c)

(d) Air gap
Between the magnets for weight
reduction
Between the electromagnet andthe magnet for variable thrust

50

100

150

200

250

300

M agnetomotive force [A]Fig. 5. Analysis result of the variable thrust. The variable thrust is 57.5
N (11.7%) at F = 290 A.

8 mm
100

5 mm

90

Fig. 8 shows the loss analysis results of the flywheel when
the rotation speed of the flywheel is 6000 r/min and thepressure of the vacuum case is 3.10 Pa. The weight of the
flywheel using the active type magnetic assist system isreduced by 2.92 kg as compared with the weight of the
flywheel without using the magnetic assist system. However,
the mechanical loss of the flywheel using the active typemagnetic assist system is 26.2 % higher than that of the
flywheel without using the active type magnetic assist system.
As a conclusion, the loss of the flywheel is not improved by
the constant thrust of the active type magnetic assist system.
One of the reason is that the precession movement changes
continuously. Therefore, the precession movement cannot be
suppressed by the constant thrust. Besides, the precession
movement of the flywheel can be suppressed by designing thecenter of gravity of the flywheel closer to the SSGB.

70
60
50
40
30
20
10
0

50

100

150

200

250

300

M agnetomotive force [A]Fig. 6. Measurement result of the variable thrust. The variable thrust is
80.2 N (16.4%) at F = 290 A.

250

200

Loss of flywheel [W]

Fig. 7 shows the loss analysis results of the flywheel when
the steady rotation speed of the flywheel is 6000 r/min and thepressure of the vacuum case is 3.10 Pa. The mechanical lossof the flywheel without the touchdown bearing becomes
30.4 % higher than that of the flywheel using the touchdown
bearing. This is because the precession movement iseliminated when using the touchdown bearing. Therefore, thetouchdown bearing of the flywheel is necessary for achieving
the low mechanical loss. Nevertheless, when the flywheel
uses the touchdown bearing, the mechanical loss of thetouchdown bearing still occurs. Consequently, the cancel ofthe precession movement without the touchdown bearing isnecessary in terms of the mechanical loss reduction of thetouchdown bearing and also the cost reduction. Therefore, theflywheel using the magnetic assist system is proposed to thehigh efficiency FESS as a solution for the flywheel without
the touchdown bearing. Next, the effect of the active typemagnetic assist system is evaluated for the cancel of theprecession movement.

Variable thrust [N]80

Mechanical loss
150

Copper loss

100

Iron loss
50

0With touchdown bearing

Without touchdown bearing

Fig. 7. Analysis result of the loss of flywheel when the steady rotation
speed of the flywheel is 6000 r/min and the pressure of the vacuum caseis 3.10 Pa. The mechanical loss is increased by 30.4 % without thetouchdown bearing.

Therefore, the improvement of the loss reduction due to themagnetic assist system can be expected by adjusting the centerof gravity of the flywheel closer to the SSGB.

Fig. 10 shows the measurement results of the temperature
characteristic of the SSGB under the same experimental
conditions as the measurement results of Fig. 9. The
temperature of the lubricating oil increases dependently on therotation speed of SSGB and the increase of its weight onto thebearing. In addition, the viscosity of the lubricating oil
decreases with the increase in its temperature, which leads tothe decrease in the fiction between the bearing and the
lubricating oil. Consequently, the bearing loss decreases when
the weight of the SSGB onto the bearing increases as shownin Fig. 9. However, the decrease in the viscosity of the
lubricating oil makes the SSGB difficult to levitate. If theviscosity of the lubricating oil drops below the level when the
SSGB cannot levitate, the bearing loss increases notably. As aconclusion, the temperature of the lubricating oil needs to beobserved in order to both reduce the bearing loss and make the
SSGB levitate.
V. CONCLUSION
In this paper, the prototype FESS was applied to the
PMSM, the SSGB and the magnetic assist system in order toachieve high efficiency and longer lifetime. The prototype
was possible to improve the charge and discharge efficiency
by applying the PMSM and the SSGB. In addition, thevariation of thrust by the proposed magnetic assist system was
16.4%. However, the loss reduction due to the magnetic assist
could not be achieved. It was because the complication of theprecession movement of the flywheel. The precession
movement of the flywheel could be suppressed by bringing
the center of gravity of the flywheel closer to the SSGB.
Therefore, the improvement of the loss reduction due to themagnetic assist system was expected by adjusting the centerof gravity of the flywheel closer to the SSGB. In addition, theprecession movement of the flywheel and the temperature of
the lubricating oil need to be controlled to reduce the loss of
the SSGB.
The future work focuses on the control of the magnetic
assist system and the measurement of the charge anddischarge efficiency for the prototype FESS. In addition, it isLoss by magnet assist system

Loss of flywheel [W]300
250
Mechanical loss

200
150

Copper loss

100

Iron loss

500Magnetic assist system(OFF)

Magnetic assist system(ON)

Fig. 8. Analysis result of the loss of flywheel when the steady rotation
speed of the flywheel is 6000 r/min and the pressure of the vacuum caseis 3.10 Pa. The mechanical loss is increased by 26.2 % by the magnetic
assist system.

300

Bearing loss of SSGB [W]

Fig. 9 shows the measurement results of the bearing loss
characteristic in another prototype of the SSGB for the bearing
loss consideration when the rotation speed of the SSGB is
500-4000 r/min. Note that in this prototype, the precession
movement is canceled completely, and only the bearing lossof the SSGB is directly measured by a torque meter. The
bearing loss increases due to the light weight of the SSGB
onto the bearing. In other words, the bearing loss can be
reduced by increasing the weight of the SSGB onto thebearing. This is because the bearing loss of the SSGB depends
on the viscosity of the lubricating oil.

350

150 [kg]
200 [kg]
300 [kg]

250
200
150
100
50
0
0

500

1000

1500 2000 2500 3000
Rotation speed [r/min]

3500

4000

4500

Fig. 9. Measurement result of the bearing loss of the SSGB.

300 [kg]
200 [kg]

90.0

Temperature of SSGB [Celsius]

On the other hand, it is considered the increase in the
bearing loss of the SSGB by the constant thrust of the active
type magnetic assist system. Therefore, it is necessary toevaluate only the bearing loss characteristic of the SSGB in
the case of no precession movement.

80.0
70.0
60.0

150 [kg]

50.0
40.0
30.0
20.0
10.0
0.0
0

Fig. 10.
SSGB.

500

1000

1500 2000 2500 3000
Rotation speed [r/min]

3500

4000

4500

Measurement result of the temperature characteristic of thenecessary to measure the effects of the precession movement
and the temperature of the lubricating oil.
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