Experimental Verification of Flywheel PowerLeveling System Oriented to Low Cost and General
Purpose Use
Jun-ichi Itoh

Kenta Tanaka

Soya Matsuo

Noboru Yamada

Nagaoka University ofTechnology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Nagaoka University ofTechnology
Niigata, Japank_tanaka@stn.nagaokaut.
ac.jp

Nagaoka University ofTechnology
Niigata, Japan
soya_matsuo@stn.nagaokaut.
ac.jp

Nagaoka University ofTechnology
Niigata, Japan
noboru@nagaokaut.ac.jp

Abstract— This paper discusses the performances of a power
leveling system with a 3.0-MJ, 9500-r/min flywheel energy
storage. In term of the cost reduction, this system uses low cost
flywheel and the general purpose products. Therefore, timedelay that occurs in the measurement circuit can limit thecontrol performance. In order to overcome this problem, a timedelay compensation scheme based on Smith predictor is appliedin order to improve the control performance. In addition,
vibration characteristics during power leveling operation areevaluated in the experiment. From the results, the maximum
value of the vibration velocity is 2.4 mm/s. It was confirmed thatthe vibration level of the proposed system is significantly low.

I. INTRODUCTION
Renewable energy systems, especially a wind turbine or aphotovoltaic cell, generate power fluctuation depending on themeteorological conditions. Therefore, these systems requireenergy buffers, such as the electric double layer capacitors(EDLC), batteries, or flywheels to suppress the powerfluctuations. One of the problems in the battery or thecapacitor energy storage is short of life time. In particular, thelifetime depends on the ambient temperature and the numbersof charge and discharge time. In addition, the battery cannot
carry up with rapid charge and discharge by a large internal
resistance. On the other hand, the EDLC provides high charge
and discharge efficiencies. Moreover, the rapid of charge anddischarge is possible because the internal resistance is verysmall. However, similar to the battery, the lifetime isdecreased due to the influence of the ambient temperature. In
contrast, the flywheels are environmental friendly and low
maintenance cost. Furthermore, the flywheel has a long
lifetime due to no chemical structure, thus the charge anddischarge characteristics have high cycle and highperformance. Additionally, the kinetic energy which stored in
the flywheel is proportional to the square of the rotational

speed. For this reason, ultra high speed rotation using
magnetic bearings have been studied in order to achieve highenergy density [1-6]. The magnetic bearing to hold therotating shaft without mechanical contact has been studied.
However, the structure of the flywheel results in costly.
Therefore, it is necessary to reduce the cost and simplify the
structure of the flywheel system in order reaching towards therealization of a practical system.
This paper presents and discusses a low cost flywheel
system with a regeneration power functions for the power
leveling system. The flywheel system is configured fromnumbers of commercial products; which are consisting of atransducer, a general purpose inverters and an inexpensive
microcomputer without the specific device. However, theinexpensive power detection device has a long time delay. As
a result, the long delay time causes the control becomes
unstable. In this study, in order to improve the response
performance of the regenerative power, time delay
compensation scheme based on the Smith predictor [7] isapplied to the proposed flywheel system. In addition, theproposed system uses low cost ball bearings insteads ofexpensive magnetic bearings. Therefore, the vibration of the
flywheel should be evaluated in terms of environmental
friendly.
In this paper, the effectiveness of the proposed flywheel
system is evaluated from the viewpoint of the following
subjects.
(i)A Response performance of the power control
In the method of controlling the instantaneous power that isconventionally used, exclusive design of the inverter isessential because the high-speed response performance isrequired. On the other hand, in the proposed system, thecustom inverter is not required because a standard transducer
from the market is applied into the power detection. However,the detection of time by inexpensive transducer may lead to
instability of the power control system. Therefore, the control
performance is improved by applying the Smith predictor into
the power control system.
Flywheel(ii) Effectiveness of the power leveling control
Power fluctuation of solar power is reproduced by using thestep-down chopper. In addition, the effectiveness the proposedsystem as a power leveling system is confirmed inexperiments.

Induction
Motor

Vacuumcase(iii) Vibration characteristics
In this study, flywheel is applied to compensate the powerfluctuations, such as solar power and wind power. Therefore,
the flywheel system will install in the vicinity of the place ofresidence areas. Therefore, low vibration and low noise
operation is required. However, since the flywheel is rotatingmachine, it generally produces a larger vibration comparing to
the battery or EDLC. Therefore, the vibration characteristics
during power leveling operation are evaluated by theexperiment.

Fig. 1. Photograph of the prototype Flywheel System. This system
uses low cost flywheel and the general purpose products.
Table 1. Specification of the Flywheel unit.

II. PROTOTYPE FLYWHEEL SYSTEM
Fig. 1 shows the photograph of the proposed flywheel
system of which has the specifications as shown in Table 1.
The general purpose ball bearings is used to reduce the costs
and to improve the performance at the rotational speed within
9500 r/min. Moreover, the outward form of the flywheel is569×511×600 mm, and the energy density is 17.2 kJ/dm3.
Fig. 2 shows the block diagram of the prototype flywheel
system including the accessories parts. Inexpensive power
meters which has a long time delay is applied in this system.
Additionally, the rotational speed command of the flywheel iscontrolled by an inexpensive PIC microcontroller. In addition,
the vacuum pump and the oil cooler are implemented toreduce the windage loss and heat generation. As a part of themechanical features, a small vacuum pump is implemented
because the flywheel has been stored in a sealed container. In
this structure, it is possible to reduce the windage loss which
occurs at high speed rotation region. Furthermore, overtemperature of the bearing and the motor can be prevented by
the oil cooler.
Note that this flywheel system assumes that several
numbers of flywheels are used in one system in order toincrease energy capacity. Therefore, an oil pump and avacuum pump can be used for some flywheels.
In addtion, the loss analysis of the prototype flywheel
system is evaluated.
Fig. 3 shows the analytical result of the steady state losses.
In this experiment, the rotation speed is 9500 r/min, theRated voltage

200V

Rated current

126A

Rated rotation speed

9500rpm

Accumulated energy

3.0MJ

Outward form

569×511×600mm

Energy density

17.2kJ/dm3

Weight of the FW

241kg

Diameter of the FW

45cm
Bearing

3Φ200V
3

Regenerative
converter

GeneralPurposeInverter
ω*
PIC
Micom
Controller

Power
meter

Vacuum case

Induction Flywheel
motor

TL
Oil
Pump

InverterFRENIC5000G 11

Controller

PIC16F887

Sampling Time

20ms

Delay Time TL

400ms

VacuumPump

Fig. 2. Configuration of the Flywheel system including the accessories
parts.

vacuum is kept at 800 Pa. As a result, the flywheel losses inthe steady state are as follows; the bearing loss is 19 %, theiron loss is 76 % for the induction motor. From the analysis, it
is confirmed that the motor loss is dominant in the standby
mode. Therefore, it is necessary to reduce the motor loss inorder to improve the efficiency of the flywheel system.

III.

POWER CONTROL METHOD WITH SMITH
PREDICTOR

In 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).
1
E  J 2 .............................................................. (1)2where, J is the moment of inertia[kgm2] and ω is the angularvelocity of the body revolution[rad/s]. In addition, the speedcommand ω* which shown in Fig. 2 can be obtained bytransforming (1).
On the other hand, the output power of the flywheel can beobtained by differentiating the both sides of (1) from therelation between the rotational kinetic energy and power.
Thus, the output power is expressed by (2).
P  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, ωo 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 givenby (4).
1 P*
t .......................................... (4)J  n－1

 n   n－1 

where,ωn-1 is the speed command value of the previous
sampling time and Δt is the sampling time of the controller.
In this system, the regenerative power is obtained from an
inexpensive power meter that has a sampling time of 400
msec. In such a system, the control may become unstable bysetting a high proportional gain to improve the response. In
this paper, the control performance is improved by the Smith
delay time compensation scheme.
Fig. 4(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 -TLS  0 .................................................. (5)In a typical feedback control system, the system becomes
unstable due to the phase lag. This is caused by the timedelay.
Fig. 4(b) shows the block diagram with the Smith predictor.
This compensation eliminates the effect of delay time byestimating the output of the controlled object G(s) after theC(s)Pload * +

PI

-

3.0

Loss[kW]

2.0

Bearing Loss
Copper Loss

1.0

G(s)
e-TLs

C(s)
Pload *++^
Ps

Iron loss

0.5

1 ω*
Js

(a)Without smith predictor.

-

1.5

1

o

Pinv

Windage Loss
2.5

P*

PI

P*

1

o

1
Js

G(s)

G(s)

e-TLs

-

ω*

e-TLs

Pd

+ Pinv(b)With smith predictor.

0.0
Fig. 3. Analytical results of the steady state losses. The rotation speed
is 9500 r/min, the vacuum is kept at 800 Pa.

Fig.4. Block diagram of time delay compensation scheme based on Smith
predictor. This compensation eliminates the effect of time delay by
estimating the output of the controlled object G(s) after the lapse of timedelay.

lapse of delay time. The characteristic equation is given by(6).
1  C(s)G(s)  0 ....................................................... (6)Therefore, the control performance can be improved fromthis equation. In other words, the delay time element has been
removed from the characteristic equation.
In this control system, the speed command ω*^ is obtainedby (2) from the estimation of the output power Ps. Note that
Pinv is detected value of the power meter and Pd is the output^
power after the lapse of dead time. In addition, P s is
calculated by the inertia J and the rotation speed command ω*from (1).
IV.

SIMULATION AND EXPERIMANTAL VERIFICATIONS

Fig. 5 illustrates the simulation results of the regenerative
power from the flywheel system with and without the Smith
predictor. The simulation conditions are as follows; thesteady rotation speed is 6000 r/min and the load is changed
from 0.0 kW to 2.0 kW. From the simulation results of Fig.
5(a), a large overshoot occurs when the Smith predictor is notapplied. By contrast, Fig. 5(b) shows that the overshoot does

not occur when the Smith predictor is applied. Therefore, it isconfirmed that the response performance is improved by theSmith predictor.
Next, the effectiveness of the Smith predictor is verified by
the experiment.
Fig. 6 shows the step response of the output power. Notethat the experimental conditions are similar to the simulation.
In addition, the gain of PI controller is determined by thelimit sensitivity method [8]. The inverter used in theexperiment is FRENIC5000G11 (Fuji Electric), themicrocontroller is PIC16F887 (Microchip Technology), andthe sampling time is 20msec.
According to Fig. 6(a), an overshoots is approximately 500
W because the power measurement system has a time delay.
On the other hand, it is confirmed that the overshoots does
not occur by using Smith predictor in Fig. 6(b). From theseresults, the effectiveness of the compensator is confirmed insimulations and experiments.
Fig. 7 shows the frequency characteristics of the prototype
system. From the results, frequency response after applying
the Smith predictor is 0.18Hz. Power generation system using
renewable energy power generator varies greatly depending

4

33

Pinv

Power[kW]Regeneration
Power[kW]3Power[kW]

Pref

2
1
0
0

5

Time[s]

10

15

22

Pload
Pref

Ts=18.7s

11
PinvPinv

00

-1

0

5

(a)Without Smith predictor

20

25

20

25

Pref

Power[kW]Regeneration
Power[kW]333Power[kW]15
Time[s]Time[s](a) Without Smith predictor.

4

Pinv

2
1
0
0

10

Ts=11.2s22110Time[s]1015(b)With Smith predictor
Fig. 5. Step response of the regeneration power on the simulation
results. The steady rotation speed is 6000 r/min and the load ischanged from 0.0 kW to 2.0 kW.

PPinv
inv

00

-1
-1

5

PPload
ref

5

10

15
Time[s]Time[s](b) With Smith predictor.
Fig. 6. Step response of the regeneration power on the experimental
results. The rotation speed is 6000 r/min and the load is changed from 0.0
kW to 2.0 kW.

2.0

-2

1.5

Gain[dB]

Power[kW]

0

-4
-6

1.0
0.5
0.0

-8
0.01

0.1 0.18
Frequency[Hz]

1

-0.5

0

200

400

600
Time[s]

800

1000

1200

600
800
1000
Time[s](b)Output power of the Flywheel

1200(a)Fluctuation pattern(a)Gain characteristic.

2.0

0
1.0

Power[kW]

-100
-150

0.0

-1.0

-200
-250

-2.0

-300
0.01

0

1

0.1 0.18
Frequency[Hz]

200

2.0

(b)Phase characteristic.

400

Pgrid

1.5

Fig. 7. Bode diagrams of the proposed control system.

Pgrid_refPower[kW]Phase[degrees]

-50

1.0

PWM rectifier
3Φ200V
3

Flywheel

Inverter

0.5

3

Induction
Motor

0.0
-0.5

Power
meter

HPF

0

200

400

800

1000

1200

200

400

600
800
Time[s]

1000

1200

Pload

Fig. 8. Configuration of the experimental system for the power fluctuationcompensation.

on the weather conditions. Therefore, the prototype system
can be applied to compensate for power fluctuations which isseveral tens of seconds [9][10].
V. VERIFICATION OF POWER LEVELING CONTROL
Fig. 8 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 the 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.

600
Time[s](c) Grid power

7000

Rotation Speed[r/min]

Step down
Chopper

6000
5000
4000
3000
0

(d)Rotation speed
Fig. 9. Experimental results of the power fluctuation compensation
control when applying Smith predictor.
(Rotation speed is 5500r/min)Fig. 9(a) shows the power fluctuation pattern which uses asa part of generation power by PV panels. From this figure, thegeneration power of the solar cell has a rapid powerfluctuation of approximately 2 kW.

0.5

Grid power

Power[p.u.]

0.4
0.3
84.6%

0.2

Fluctuation pattern

0.1
0.0
0.001

0.1
0.01
Frequency[Hz]1

Fig.10. Harmonic analysis during the power fluctuation compensation.
The experimental conditions are similar to Fig. 9.

Fig. 9(b) shows the output power of the flywheel and Fig.
9(c) shows the grid power. From these results, the fluctuationof generated power is compensated by the output power ofthe flywheel. Therefore, the power fluctuation is noticed notoccurring in the grid power.
Fig. 9(d) shows the rotation speed of the flywheel. As aresult, it is confirmed that the sudden speed variation does notoccurs. According to the Fig. 9(c), the high frequency ripple
occurs in the grid power. This ripple is caused by the
influence of the time delay error. This problem can be
improved by the optimization of the control gain.
Fig. 10 shows the harmonic analysis of the fluctuation
pattern and the grid power. The experimental conditions are
similar to Fig. 9. 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 by 84.6 % in the power
leveling control.
VI.

VIBRATION ANALYSIS

In this study, the flywheel is applied as a powercompensator for the solar power and wind power. The
flywheel system will be installed in the residence areas as
therefore low vibration and low noise operations are
preferable. The vibration characteristics during power
leveling operation are evaluated by the experiment.
Fig. 11 shows the mounting position of the acceleration
sensor used for vibration analysis. In the experiment,
acceleration in the radial direction and axial direction aremeasured with respect to the axis of rotation of the flywheel.
In addition, DS-3000(Ono Sokki) is used as the acceleration
sensor in the analysis and data acquisition.
Fig. 12(a) shows the vibration velocity in the radial
direction, Fig. 12(b) shows the vibration velocity in the axial
direction. where, the experimental conditions are the same as
the operation of the power fluctuation compensation. In

addition, the vibration velocity is calculated by integrating theacceleration measured from the acceleration sensor. Fromthese results, the vibration velocity in the axial direction islarger than the radial direction of the flywheel. The maximum
value of the vibration velocity is 2.4 mm/s. According to thestandard of JIS, the prototype system is classified as zone B
which vibration velocity is between 1.12 mm/s to 2.8 mm/s.
In this zone, long-term operation is possible without any
restriction. In addition, the vibration velocity is increased
rapidly at a rotational speed of 5200 r/min in the axial
direction. The vibration becomes large at the number of
revolutions of the same when changing the power fluctuation
pattern. Therefore, it is a resonance phenomenon due to thenatural frequency of the system.
Next, the causes of vibration are considered and thefollowing four points show the reasons of vibration in the
flywheel [11].
(i) The vibration caused by the imbalance of mass distributionof the rotor axis,(ii) The vibrations due to the deviation of the center line of
the rotary shaft which is connected by a shaft coupling,(iii) The vibration caused by the electromagnetic force of theinduction machine,and (iv) The vibration caused by the rolling elements of the
bearing.
From the past studies, the element of (i), (ii), (iii) appears
prominently in the vibration velocity. The vibration caused by
the imbalance of mass distribution of the rotor appears as therotational frequency component in the radial direction. In
addition, the vibrations due to the deviation of the center lineof the rotary shaft which is connected by a shaft coupling
appear to be larger as the order component and the rotation
frequency in the axial direction are increasing. Moreover, thevibration caused by the electromagnetic force of the motor isgenerated by the magnetic attraction force of the rotor and thestator. This vibration appears as a frequency component of
twice the power supply frequency. Therefore, the causes ofvibration that occurs in the prototype system are analyzed byharmonic analysis.
Fig. 13 shows the harmonic analysis results at maximum
vibration velocity in each of the direction. From Fig. 13(a),the radial direction is dominated by the vibration of therotational frequency component. This is the vibration caused
by unbalanced mass of rotating axis. In addition, the vibrationof 13 kHz is a vibration due to torque ripple because it isequal to the carrier frequency. Moreover, the vibration of 550
Hz is the natural frequency component of the system. From
Fig. 13(b), the axial direction is dominated by the rotationalfrequency component. This is the vibration caused by thedisplacement of the center shaft coupling. From these results,
the component of twice the power supply frequency is 0.2
mm/s or lesser in both the directions. Therefore, the influenceVibration velocity[mm/s]

1.0

Accelerometer(Radial)Accelerometer(Axial)Rotation frequency component

0.8
0.6
0.4

Natural frequency
component

Carrier frequency
component

0.2
0.0
10

100

1000
Frequency[Hz]10000

Fig.11. Mounting position of the accelerometer.

2.0

4000

1.5

Vibration velocity

1.0

2000

0.5
0.0

2.5

0

200

400
600
Time[s]

800

0
1000

Vibration velocity[mm/s]

Rotation Speed

2.5(a)Radial direction

6000

Rotation Speed[r/min]

Vibration velocity[mm/s]

3.0

Rotation frequency component

2.0
1.5
1.0
Bearing Vibration
0.5
0.0
10

100

1000
Frequency[Hz]10000(a) Radial direction

2.5

Vibration velocity 4000

2.0
1.5

2000

1.0
0.5
0.0
0

200

400
600
Time[s]800(b) Axial direction

6000

0
1000

(b) Axial direction
Fig.12. Analysis of the vibration velocity during the power fluctuationcompensation control.

of the vibration caused by the electromagnetic force of the
motor is very small.
Fig. 14 shows the harmonic analysis result at maximum
vibration acceleration. In general, the bearing vibration
appears as the acceleration of the high frequency component.
Therefore, the factor of high frequency vibration is evaluatedby the acceleration analysis. The frequency component of thevibration that is caused by the bearing can be calculated fromthe rotational frequency and dimensions of the bearing [12].
The frequency component of the vibration generated by therolling elements of the bearing is expressed by (7).

Fig.13. Harmonic analysis results at maximum vibration velocity in eachdirection.

4.0

Vibration acceleration[m/s2 ]Rotation Speed

Rotation Speed[r/min]

Vibration velocity[mm/s]

3.0

3.0
2.0
1.0
0.0
100

1000
Frequency[Hz]10000

Fig.14. Harmonic analysis result at maximum vibration acceleration.
(Axial direction)

fc 

fr
d
(1 - cos a) ................................................. (7)2D

where, fr is rotational speed of the shaft, d is the diameterof the rolling element, D is the pitch circle diameter of the
bearing and α is the contact angle.
In this system, the vibration of the bearing appears as acomponent 10.57 times, 12.43 times and 14.92 times of therotation frequency. From the harmonic analysis, the vibration
appears as a component 11.88 times, 12.90 times, 15.50 times

as large as the rotation frequency. Therefore, the analysis
results are almost identical with the theoretical value. The
amplitude of the high order component is greater due to theresonance with the natural frequency of the system. Fromthese analysis, it is confirmed that the main factor of the highfrequency component is the vibration of the ball bearings.
VII.

CONCLUSION

This paper discussed a low cost power leveling system witha 3 MJ, 9500 r/min flywheel energy storage. In this system,
the control performance is improved by applying the Smith
compensator. From the step response of power, it isconfirmed that the overshoot of the power does not occur andthe settling time is reduced.
The loss analysis of the system is demonstarated. From theanalysis, it is confirmed that the motor loss is dominant in thestandby mode. Therefore, it is necessary to apply thepermanent magnet synchronous motor with low loss forfurther efficiency of the flywheel system.
Furthermore, the effectiveness of the control for the
prototype system is evaluated in term of vibration analysis.
From the analysis, it is confirmed the main factors that cause
the vibration are (i) the unbalance of the flywheel and (ii) the
rolling element of the ball bearings. Therefore, it is possible
to further suppress the vibration by adopting a pivot bearings
to reduce the mechanical contact and improve the machining
accuracy.
ACKNOWLEDGMENT
A part of this study was supported by Industrial
Technology Grant Program in 2011 from New Energy andIndustrial Technology Development Organization (NEDO) of
Japan.
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