Development of High Efficiency Flywheel Energy
Storage System for Power Load-Leveling*Jun-ichi Itoh, *Daisuke Sato, *Tsuyoshi Nagano, *Kenta Tanaka, *Noboru Yamada, and **Koji Kato*Nagaoka University of Technology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp
Abstract - This paper introduces the performance of a power
leveling system with a 3.0-MJ, 3315-r/min flywheel energy
storage. In terms of cost reduction, this system uses low cost ballbearings and general purpose induction motor. Therefore, sucha system configurations occurs large loss during standby mode.
In order to overcome this problem, low loss design algorithm
that focuses on the mechanical loss is applied to the design of the
flywheel. From the experimental results, it is confirmed that the
charge and discharge efficiency of the mechanical part of the
proposed flywheel system is 95.2% (charge 97.9%, discharge
97.5%). Moreover, charging and discharging efficiency aremeasured to evaluate the prototype flywheel system. In addition,a design approach of the outer rotor type motor for improving
the efficiency and the energy density is discussed. From theanalysis, it is confirmed that the loss of the PMSM at the rated
output is reduced by 71.7% as compared with the induction
machine. As a result, it is confirmed that the charging efficiency
is 93.3% and the discharging efficiency is 93.1% if the outerrotor PMSM drives the flywheel.

I.
INTRODUCTION
Renewable energy systems, especially a wind turbine and aphotovoltaic cell, generate a power fluctuation due to themeteorological conditions. Therefore, these systems requireenergy buffers to suppress the power fluctuations. In addition,
an uninterruptible power supply (UPS) is widely used inorder to prevent instantaneous power failure in the data center,the factory and so on. The UPS also needs the energy buffers
because the UPS has to supply the power to loads 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 a short life time. In particular, the lifetime depends
on an ambient and the number of charge and discharge time.
In addition, the battery cannot cope with rapid charge anddischarge by a large internal resistance of the battery. On theother hand, an electric double layer capacitor (EDLC) has ahigh charge and discharge efficiency. Moreover, the rapid
charge and discharge is possible because the internal
resistance is very small. However, similar to the battery, thelifetime is decreased rapidly due to the influence of theambient temperature [1]. On the other hand, the flywheels has
some advantages: (i) environmental friendly, (ii) low
maintenance cost, (iii) long lifetime due to no chemical
structure, and (iv) the charge and discharge characteristic ofhigh cycle are excellent. For these reason, the flywheel has
attracted attention as an energy storage system[2-12].

**Sanken Electric Co., Ltd.
Saitama, Japank.kato@sanken-ele.co.jp
TABLE I. Characteristics of each storage devices.
Flywheel
EDLC
Battery
Kinetic
Chemical
Energy storage
Ion transfer
energyreaction
Charge &Discharge of
Fast
Fast
Slow
short period
Temperature
Limited by
Limited by
Excellentcharacteristic
temperaturetemperature
Energy density
Good
Good
Excellent

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, the magnetic
bearing is required an additional control system [5-7]. In
addition, flexibility of this system is low because this system
is necessary to design in accordance with the capacity of thepower source to be compensated. For these reasons, it canlead to high cost and complexity of the control system of the
flywheel system. Therefore, it is necessary to achieve theflywheel systems that do not use a magnetic bearing. Hence,
multiple parallel operations using small flywheel systems are
assumed in this paper. In this structure, compensation
performance is changed by the number of the combined
flywheel. Then, high flexibility and high scalability of the
flywheel system can be achieved. In this study, generalpurpose flywheel system for parallel operation is developed.
The proposed flywheel system is configured fromcommercial products such as general-purpose ball bearings, a
general-purpose inverters and an inexpensive microcomputer
without the specific device.
In this paper, the effectiveness of the proposed flywheel
system is introduced. First, the proposed power control
method is evaluated by an operation of the power leveling forthe power fluctuation of solar power. The conventional
control method requires special design of the inverter because
the high-speed response performance is required in order todetect the instantaneous power. On the other hand, theproposed method uses only a standard transducer from themarket. After that, the steady state loss is analyzed by using
the loss measurement result. The loss can be separated intomechanical losses and electrical losses. Therefore, a factor of
the loss in the flywheel system is be clarified from the loss
analysis. Next, in order to reduce the bearing loss, theII.

CONSTRUCTION OF FLYWHEEL ENERGY STORAGE
SYSTEM
Fig. 1 shows the configuration of a proposed flywheel
system that employs the general purpose motor and ballbearings. In this system, it is possible to store the kinetic
energy of the 3.0 MJ at 3315 r/min. Such a low rotation speed
region can be applied to a typical ball bearing and general
purpose motor. In addition, the flywheel vacuum case and the
motor are separated by the magnetic fluid seal. As a result,windage loss can be greatly reduced because it is possible toreduce a pressure in the vacuum case by the vacuum pump.
Moreover, vacuum in the vacuum case does not affect theheat dissipation of the motor because the motor and thevacuum 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 the electrical energy. On the other hand, during
acceleration, the electrical energy is stored as the kinetic
energy, which is working as a motor. Therefore, this system
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. It iscapable of storing sufficient energy for a low rotational speedby the inertia weight is increased. In addition, the windage
loss and the bearing losses are reduced by the reduction inrotational speed. The pressure in a vacuum case is reduced byusing a vacuum pump prior to operation in order to reduce
the windage loss. In addition, the magnetic fluid seal isadopted to maintain the vacuum in the vacuum case.
Table III shows the estimation of the cost of the proposedsystem. Note that the material of the flywheel is usedchromium-molybdenum steel. Here, cost ratio of the material
in flywheel is assumed as 1. On the other hand, cost ratio of
the magnetic fluid seal and other components becomes 5.0 to
be relative to the material cost of the flywheel. The most of
this cost is accounted for the magnetic fluid seal. Therefore, itis necessary to consider the structure which can achieve a
sufficient vacuum without the magnetic fluid seal. From theInduction motor

Motor fanMotor bearing

Ferrofluidic seal
Bearing1

Flywheel

Bearing2

φ1000

65

windage loss and the magnetic fluid seal loss toward higher
efficiency, the flywheel is improved. In addition, the energy
stored efficiency is measured by experiments in order toevaluate the usefulness of the flywheel system as the energy
storage. Next, a design approach of the outer rotor type motor
for improving the efficiency and the energy density is
discussed. The outer rotor type motor can be easily designedwith large output torque as compared to the inner rotor typemotor. Therefore, the reduction in the axial direction of the
motor is achieved. In this case, the losses of an induction
motor and a permanent magnet synchronous motor of equal
size are compared. Finally, the efficiency of the improved
flywheel system is evaluated from loss analysis and loss
estimation.

Fig. 1. Configuration of a proposed flywheel system that employs thegeneral purpose motor and ball bearings.
3Φ200V
3
Power
meter

GeneralPurposeInverterPower
3 meter

Regenerative
converter
Regeneration

VacuumPump

3

3

Ferrofluidic
seal
Bearing

Oil Pump

3

Induction
motor

Flywheel

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 thelow loss design.
Material
SCM440Radius
500 mmThickness
65 mm
Weight of the
Flywheel
392 kgFlywheel
3.0 MJ at
Stored Energy
3315 r/min
Motor/Generator

MLC1115C (Fuji Electric)

Inverter

FRN 37G11S-2 (Fuji Electric)

TABLE III. Estimation of the cost of the proposed system.
Components

Cost ratio

Material cost of Flywheel(SCM440)

1.0

Other material cost

5.5

Power converter

2.9

Induction machine

1.3

Ball bearings

2.6

Other components

5.0

Total cost

18.2

results, it is confirmed that the material cost of the flywheel
in the proposed system is very cheap. However, the total costof the entire system including the processing cost and theother components becomes very expensive. Therefore, it ispossible to achieve a more reasonable system by improving
the manufacturing method and structure.
Next, the cost of the proposed system is compared withother energy storage devices. The comparison condition is thesame as the energy storage. From the reference [12], total
cost of EDLC energy storage systems becomesapproximately 25% higher as compared to the proposed
flywheel system. Note that specification of the capacitor to be
used is as follows: the rated voltage is 2.5 V, a capacitance is2700 F, and the cost per unit is $30. From these results, it isconfirmed that the proposed system can achieve a lower costthan an EDLC energy storage system depending on the
manufacturing cost. However, the reduction of processing
cost and component cost are important issue for practicalapplication of the flywheel.
III. VERIFICATION OF POWER LEVELING CONTROLIn the flywheel system, when induction machine isoperated as a generator during deceleration, kinetic energy isconverted into the electrical energy. On the other hand,during acceleration, the electrical energy is stored as kinetic
energy, which is working as a motor. In general, inexpensive
general purpose inverter can only provide speed command.
Therefore, the charging and discharging power is controlled
by the speed of the flywheel.
The kinetic energy of a body of revolution is expressed by
(1).
E

1 2
J ............................................................. (1)2where, J is the moment of inertia [kgm ] and  is the angularvelocity of the body revolution [rad/s]. In addition, the speedcommand * which shown in Fig. 2 can be obtained bytransforming (1). On the other hand, the output power of the
flywheel can be obtained by differentiating the both sides of(1) from the relation between the rotational kinetic energy
and power. Thus, the output power is expressed by (2).
2P  Jω

dω...................................................................... (2)dt

Accordingly, the regenerative power of the flywheel is
proportional to the product of the rotational angular velocity
and angular acceleration. The speed command value for theregenerative power of the flywheel is given by (3).
* 

1 P*
dt .............................................................. (3)J  owhere, o is the rotational angular velocity of the previous
sampling time and P* is the output of the PI controller.
Furthermore, the discrete system of equation (3) is given by
(4).

C(s)
Pload * +

PI

-

P*

1
o

1 ω*
Js

G(s)

e-TLs
Pinv
(a) Without smith predictor.

C(s)Pload *
+

+

-

PI

P^s

P*

1
o

1
Js

ω*

G(s)

G(s)

e-TLs

e-TLs

Pd
-

+

Pinv
(b) With smith predictor.

Fig.3. Block diagram of time delay compensation scheme based on
Smith predictor. This compensation eliminates the effect of time delayby estimating the output of the controlled object G(s) after the lapse oftime delay.

 
n

n－1



1 P*
t .............................................. (4)Jn－1where, n-1 is the speed command value of the previous
sampling time and t is the sampling time of the controller.
In this system, the regenerative power is obtained from an
inexpensive power meter that the sampling time is 400 msec.
In such a system, the control may become unstable by setting
a high proportional gain to improve the response. In thispaper, the control performance is improved by the Smith
delay time compensation scheme.
Fig. 3(a) shows the block diagram of the typical feedback
control system. Note that the change of the rotational angularvelocity is sufficiently slower than the processing time of thecontrol system. Therefore, the rotational angular velocity ofthe previous sampling time is treated as a constant value ωo.
The characteristic equation of this control system is given by(5).

1  C(s)G(s)e-TL S  0 ...................................................... (5)Fig. 3(b) shows the block diagram with the Smith predictor
to power control. This compensation eliminates the effect ofdelay time by estimating the output of the controlled object
G(s) after the lapse of delay time. The characteristic equation

Step down
Chopper

Power
meter

3

HPF

2.0

Flywheel

Inverter

1.5
Induction
Motor

Power[kW]PWM rectifier
3Φ200V
3

Pload

1.0
0.5
0.0

-0.5

0

Fig.4. Configuration of the experimental system for the powerfluctuation compensation.

600
Time[s]

800

1000

1200

1000

1200

(a) Fluctuation pattern

1  C (s)G(s)  0 .......................................................... (6)Power[kW]

1.0
0.0

-1.0
-2.0

0

200

400

600
Time[s]800(b) Output power of the Flywheel

2.0
Pgrid
1.5
Power[kW]Pgrid_ref

1.0
0.5
0.0

-0.5

0

200

400

800

1000

1200

600
800
Time[s]

1000

1200

600
Time[s](c) Grid power

7000

Rotation Speed[r/min]Fig. 4 shows the power leveling control system. Note thatthe rotation speed is 5500 r/min at steady state. In theexperiment, the power fluctuation is reproduced by a stepdown chopper. In addition, the flywheel power command
value is extracted from a high pass filter (HPF) that cutoff
frequency is set to 0.0025 Hz. The prototype system isintended to compensate for a short cycle of the powerfluctuation. Therefore, the cutoff frequency of the HPF is set
to low frequency.
Fig. 5(a) shows the power fluctuation pattern which uses apart of generation power by PV panels. From this figure, thegeneration power of the solar cell has a rapid powerfluctuation of approximately 2 kW.
Fig. 5(b) shows the output power of the flywheel and Fig.
5(c) shows the grid power. From these results, the fluctuationof generated power is compensated by the output power ofthe flywheel. Therefore, the sudden fluctuation due to powerfluctuation does not occur on the grid power.
Fig. 5(d) shows the rotation speed of the flywheel. As aresult, it is confirmed that the sudden speed variation does notoccurs. According to the Fig. 5(c), the high frequency ripple
occurs in the grid power. This ripple is caused by the
influence of the power detection delay time. This problem
can be improved by the optimization of the control gain.
Fig. 6 shows the harmonic analysis of the fluctuation
pattern and the grid power. The experimental conditions are
similar to Fig. 5. As the result, it is confirmed that thefluctuation pattern has large frequency component between
0.01 Hz and 0.004 Hz. By contrast, the grid powerfluctuations over 0.004 Hz is reduced 84.6 % by the power
leveling control.
Fig. 7 shows the measurement result of the steady stateloss in experiment with prototype FESS as shown in Fig. 1.
In this experiment, the rotation speed is 3315 r/min, thevacuum case is kept at 500 Pa. The mechanical loss and theelectrical loss are separated by a free-run test at the
beginning. Free-run test is a test for electricallydisconnecting the inverter and the motor with the energy
stored in the flywheel. In this state, the stored energy is

400

2.0

is given by (6).

Therefore, the control performance can be improved fromthis equation. In other words, the delay time element has been
removed from the characteristic equation.

200

6000
5000
4000
3000
0

200

400

(d) Rotation speed
Fig. 5. Experimental results of the power fluctuation compensation
control when applying Smith predictor (Rotation speed is 5500r/min).

consumed only by mechanical loss. Therefore, themechanical loss is calculated from the time until the stored
energy is consumed. From the results, it is confirmed thatthe bearing loss accounts for 48.6% of all mechanical loss.

0.5

Induction
Motor

Grid power

Power[p.u.]

0.4
0.3
0.2

VacuumCase

84.6%

Magnetcoupling
FlywheelFluctuation pattern

0.1
0.0
0.001

0.1
0.01
Frequency[Hz]1

Fig.6. Harmonic analysis during the power fluctuation compensation.
The experimental conditions are similar to Fig. 5.

800
700

Loss[W]

600
500
400
300
200
100

Magnetic Fluid
Seal Loss
Windage Loss
Bearing Loss
Motor
Mechanical Loss
Iron Loss
Harmonic
Copper Loss
Copper Loss0
Fig. 7. Loss analysis results at the rated energy storage. In theexperiment, the steady rotation speed is 3315 r/min and the pressure of
the vacuum case is 500 Pa.

Additionally, the electrical losses of the motor are accounted
for 41.7% of the total loss. Therefore, it is necessary to
improve the motor efficiency and reducing the bearing loss
toward higher efficiency.
IV. IMPROVEMENT OF THE FLYWHEEL SYSTEM
A. Improvement toward higher efficiency
The previous chapter clarifies that the bearing loss, thesum of the windage loss and the magnetic fluid seal loss
account for 57% of the total loss of the proposed flywheel
system. In order to reduce the bearing loss, the windage loss
and the magnetic fluid seal loss toward higher efficiency, theflywheel is improved.
Fig. 8 shows the configuration of a proposed flywheel
system after the system of Fig. 1 is improved in order toreduce the bearing loss, the windage loss and the magnetic
fluid seal loss. Table IV shows the specification of the
improved flywheel system. In this system, it is possible tostore the kinetic energy of the 3.0 MJ at 2900 r/min because
the shape of the flywhell is changed in order to achieve themoment of inertia with lighter weight of the flywheel. In
addition, the flywheel vacuum case and the motor are
separated by not the magnetic fluid seal but the magnetic
coupling. The higher pressure in the vacuum case can be

Fig. 8. Configuration of a proposed flywheel system that comprises a
general purpose motor and ball bearings after the system of Fig. 1 is
improved.
Table IV. Specifications of the flywheel which is calculated from thelow loss design.
Material
SCM440Radius
500 mmThickness
65 mm
Weight of the
Flywheel
392 kgFlywheel
3.0 MJ at
Stored Energy
2900 r/min
Motor/Generator

MLC1115C (Fuji Electric)

Inverter

FRN 37G11S-2 (Fuji Electric)

kept by using the magnetic coupling than the magnetic fluid
seal because the magnetic coupling is used. As a result,windage loss can be reduced as compared with the magnetic
fluid seal because the magnetic coupling connects
magnetically the flywheel vacuum case with the motor.
Moreover, the diameter of the ball bearing is changed inorder to reduce the bearing loss.
B. Analysis of the energy storage efficiency
In this section, the energy storage efficiency of the
improved prototype system (Fig. 8) is analyzed. This property
is divided into a discharging efficiency and a charging
efficiency. The charging efficiency is the ratio of the energy
consumed for storing the desired energy. On the other hand,the discharging efficiency is the ratio of actual energy can bereleased with respect to stored energy.
Fig. 9 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
to 2900 r/min. In the discharge test, the flywheel isdecelerated from 2900 r/min to 2300 r/min. The stored energy
is calculated using (6) from the rotational angular velocity
and the moment of inertia of the flywheel J.
E

1 2J ............................................................... (6)2

From this equation, it is calculated that the amount of theenergy change is 1 MJ when the rotational speed changes
from 2300 r/min to 2900 r/min. Next, the energy storage
efficiency is calculated using the experimental results. Note

6
Mechanical
Loss

Power[kW]5

Table V. Comparison of the characteristics between inner rotor
type motor and outer rotor type motor.

Electrical Loss

4

Evaluation criteria

Inner rotor type

Outer rotor type3

Limitation of magnet size

LimitedNo Limited

Winding coils

Difficult

Easy

Heat dissipation

Easy

Difficult

Inertia moment of motor

Small

Large

2
Stored Energy
1

0
2300

2500
2600
2700
Rotation Speed[r/min]

2400

2800

2900

(a) Charging efficiency
6

Power[kW]5

Mechanical
Loss

Electrical Loss

Rotor

4

Rotor bar
3
2
Discharged Energy
1

0
2950

2850

2750
2650
2550
Rotation Speed[r/min]

Stator2450(b) Discharging efficiency
Fig. 9. Measurement results of the charging and discharging
efficiency.

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 the decreasing of the kinetic energy
to regenerative energy. If only the mechanical loss is
considered, it is confirmed that the achieved 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, theachieved charging efficiency is 80.7 % and the achieved
discharging efficiency is 75.1 %. Therefore, the total
achieved efficiency is 60.7 %. This cause of low efficiency ismainly the additional loss caused by introducing theinduction motor driving the flywheel. Thus, it is necessary toapply the PMSM to achieve high efficiency of the entire
flywheel system.
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%, an EDLC is approximately
90%. Therefore, it is confirmed that the energy storage
efficiency of the prototype flywheel is about the same as thecurrent main power storage device.
V. IMPROVEMENT OF MOTOR EFFICIENCY
Table V shows comparison of the characteristics between
inner rotor type motor and outer rotor type motor. In oreder toimprove the efficiency and the energy density of the proposed
flywheel system, a design approach of the outer rotor typemotor is discussed in this chapter. The general purpose
induction motor can be used when the motor is locatedoutside of the vacuum case. However, due to the limitation of
the magnet size, the inner rotor type motor is relatively large
in the rotation axis direction. Thus, the energy density of the
flywheel system becomes lower. In contrast, the outer rotor(a) Induction motor model

Rotor

MagnetStator(b) Permanent magnet synchronous motor modelFig.10. Analysis model.

type motor can be easily designed with large output torque ascompared to the inner rotor type because the magnet size is
not limited. Therefore, the reduction in the axial direction of
the motor is achieved. For these reasons, an outer rotor typemotor can be applied in order to improve the energy densityand efficiency of the flywheel system.
Fig. 10 shows the induction machine model and PMSM
model. In this flywheel system, it is possible to store thekinetic energy of the 3.0 MJ at 6000 r/min. Moreover, theshape of the stator core is the same in the PMSM and theinduction machine type. Table VI show the specification of
the proposed system.
Fig. 11 shows the three-dimensional analysis of (a) the
induction machine and (b) PMSM. In the three-dimensional
analysis, the magnetic flux flowing in the axial direction andthe inertial body are analyzed. An analysis condition is a sine
wave source both in the analysis of the PMSM and theinduction machine. In addition, the magnetic material SS400
is applied to the flywheel and the rotor support. From the

0.25
0.20

Loss[p.u.]

TABLE VI. Estimation of the cost of the proposed system.
(a) Induction motor
Outer diameter of rotor
460 mmOuter diameter of stator
300 mm
Air gap length
1 mm
Number of slots
48
Number of rotor bars
40
Pole number
4
Number of coil turns
12
Iron
35JN230
Rotor bar
CopperCopper loss
(Secondary)

0.15
0.10

Iron loss(Stator)Iron loss(Rotor)Copper loss
(Primary)

0.05(b) Permanent magnet synchronous motor
Outer diameter of rotor
460 mmOuter diameter of stator
300 mm
Air gap length
1.5 mmNumber of slots
48
Pole number
4
Number of coil turns
3
Iron
35JN230
MagnetNMX-S45SH

0.00

PM
motor
25%

PM
PM
IM
motor
motor
50%
100%
Load[%]
Fig. 12.Comparison of loss with respect to the load conditions
IM

IM

800
Magnetic Fluid Seal Loss

Rotor support material

700
Windage LossFluxdensity[T]Loss[W]
Rotor

Bearing Loss

400

Windage Loss
Bearing Loss

Motor Mechanical Loss

300
Iron Loss

100
0.0

(a)

500

Motor Mechanical Loss
Iron Loss200

Flywheel
Stator68%

Magnet coupling

2.0

1.0

44%

600

Induction motor model

Magnet coupling
Windage Loss
Bearing Loss

Copper Loss
Copper Loss0
Before improvement

After improvement

Motor Mechanical Loss
Iron Loss
Copper Loss

After improvementwith outer rotor PMSM
(Estimation)

Rotor support material
Fluxdensity[T]2.01.0
Flywheel
Stator

Rotor
0.0

(b)

Permanent magnet synchronous motor modelFig.11. Analysis result

analysis, it is confirmed that the magnetic flux leaking fromthe rotor flows in the support material and the flywheel.
Therefore, the analysis of the proposed FESS is necessary toconsider the flywheel and the rotor support.
Fig. 12 shows the comparison of loss with respect to theload conditions. Note that, the loss is normalized by theoutput power. From the analysis, it is confirmed that theelectrical loss of the PMSM at the rated output is reduced by
71.7% as compared with the induction machine. In particular,
the effect of reducing the copper loss is significant. The causeof the loss reduction is that the secondary copper loss does
not occur in the PMSM. This is because the secondary
magnetic flux is generated by the rotor magnets in the
PMSM. Similarly, it is confirmed that the PMSM can achievea low loss as compared with the induction machine in theFig. 13. Loss analysis results at the rated energy storage. In theexperiment, the steady rotation speed is 2900 r/min and the pressure of
the vacuum case is 20 Pa.

entire load range. From the analysis, the effectiveness of the
PMSM type flywheel is verified by the simulation.
Fig. 13 shows the analysis result of the steady state loss
in the experiment with prototype flywheel system before andafter the improvement. In the experiment of the flywheel
system after the improvement, the rotational speed is 2900
r/min and the pressure in the vacuum case is kept at 20 Pa.
The mechanical loss and electrical loss are separated
respectively by a free-run test as well as Fig. 7. From theanalysis results using the induction machine, the motor loss
contributes 68.1% of the total loss. On the other hand, thetotal loss after the improvement is 44% lower than before
the improvement because the mechanical loss is reduced by
the improvement. As a result, it is comfirmed that flywheel
after the improvement achieves low loss as compared withthe flywheel before the improvement. In addition, the loss
when the motor is changed to the outer rotor PMSM isestimated. From the results, the total loss is reduced by 68%
to the loss before the improvement. Moreover, thedischarging efficiency be achieved to 93.3% and thecharging efficiency be achieved to 93.1% if the motor ischanged to the outer rotor PMSM.

VI.

CONCLUSION
This paper introduces the performance of a power
leveling system with a 3.0-MJ, 3315-r/min flywheel energy
storage. In this proposed system, simplification of the systemis achieved by applying the general purpose induction
machine and the ball bearings. From the loss analysis, it isconfirmed that the bearing loss accounts for 48.6% of allmechanical loss. In addition, in order to reduce the bearing
loss, the windage loss and the magnetic fluid seal loss, theproposed flywheel system is improved. Moreover, the energy
storage efficiency is measured by experiments. Towards
higher efficiency, authors will consider the design approachof the outer rotor type motor. From the analysis, it isconfirmed that the loss of the PMSM at the rated output isreduced by 71.7% as compared with the induction machine.
Similarly, it is confirmed that the PMSM can achieve a low
loss as compared with the induction machine in the entire
load range. From the analysis, the effectiveness of
flywheelsystem with the outer rotor type PMSM is verified
by the simulation. As a result, it is confirmed that the
charging efficiency is 93.3% and the discharging efficiency is
93.1% if the outer rotor PMSM drives the flywheel.
Therefore, it is confirmed that the energy storage efficiencyof the prototype flywheel is about the same as the current
main power storage device.
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