Control Strategy for Starter Generator in UAV with
Micro Jet Engine
Jun-ichi Itoh1*, Kazuki Kawamura1*, Hiroyuki Koshikizawa2 and Kazuyuki Abe2
1 Department of Electrical, Electronics and Information Engineering, Nagaoka University of Technology, Niigata, Japan
2 Development Department, YSEC Co., Ltd, Niigata, Japan*E-mail: itoh@vos.nagaokaut.ac.jp, k_kawamura@stn.nagaokaut.ac.jp

Abstract— This paper proposes control strategy of astarter generator connected to a jet engine for an unmanned
aerial vehicle system. Thrust is generated by both the jetengine and propellers which are powered by the jet engine
through the starter generator. A flight range can beextended since energy density of the jet engine in thedeveloped system is higher than battery energy density inthe conventional system. Moreover, the starter generator
directly connects to the jet engine and rotates at high speed
for miniaturization. The proposed control strategy achieves
the starting, the powering and the cooling operations withthe starter generator. It is confirm through an experimentof a 3-kW prototype, that the prototype system achieves the
maximum conversion efficiency of 92.7%. The minimum
generator current THD is 16.5% at 70000 r/min. Further,
the exhaust nozzle temperature is controlled within the
maximum deviation of 2% regarding to the command value
in study state.
Keywords— Starter generator, Jet engine, Unmanned
aerial vehicle(UAV), V/f control.

I.

INTRODUCTION

Recently, unmanned aerial vehicles (UAVs) have been
actively studied for rescue activities in disaster [1–4]. In
particular, the multicopter-type UAV has two advantages.
First, it is easy to approach danger zones because of
unmanned operation. Second, the multicopter-type UAV
does not need a designated landing space. However, themulticopter-type is generally powered by batteries [2].
The flight range and carrying weight are limited becauseof the battery energy density [5]. Therefore, UAV with ajet engine has been developed [6]. In the developed UAV
system, thrust is generated by both the jet engine andpropellers which are powered by the jet engine throughthe starter generator. The flight range can be extended
since the energy density of the jet engine is higher thanthe battery energy density. Furthermore, the developed
UAV system is also be used as an emergency power
supply owing to the starter generator.
An auxiliary power unit (APU) is generally used forstarting and cooling the jet engine [7–8]. However, theuse of APU leads to the increase in cost and size of thesystem. Furthermore, the rotation speed of the generator
in APU is low because the generator is connected to thejet engine through reduction gears [9–10]. Therefore, thegenerator tend to be large in a high power capacity
system.
In this paper, the UAV system with a jet engine andthe control strategy of the starter generator are proposed.
In the developed UAV system, only the starter generator
is used for starting and cooling, which eliminates the useof APU. Furthermore, the starter generator connects
directly to the jet engine and rotates at high speed forminiaturization. The challenge of this paper is theachievement of the stable operation through the proposed
control strategy even when the starter generator transits
among operation modes, i.e., starting mode, poweringmode, and cooling mode without APU and reduction
gears. In particular, the synchronous frequency command
limiter and the output power limiter are used in the
proposed control method. In addition, modulation method
is modified by the estimated intersection phase based onsynchronous PWM. Through the experiments, it isconfirmed that the prototype achieves the maximum
conversion efficiency of 92.7%, the minimum generator
current THD of 16.5% at 70000 r/min. Further, theexhaust nozzle temperature is controlled within the
maximum deviation of 2% compared to the command
value in the steady state.
II.

DEVELOPED UAV SYSTEM

Figure 1 shows the configuration of the developed an
UAV system. The jet engine and the starter generator are
directly connected without reduction gears. The jetengine powers six propellers through the starter generator.
In the aerial applications, weight reduction of the starter
generator is required from the viewpoint of flight range.
Thus, the starter generator is rotated at high speed forminiaturization and weight reduction.
Figure 2 shows the mode transition diagram of thedeveloped UAV system. A host controller selects theoperation mode. The operation modes are described asfollows;
A. All Off Mode
This mode is a stationary state. The power converter
is not operated(gate off).
B. Standby Mode
The DC/DC converter boosts the DC-link voltage
from the battery voltage to 300 V.

C. Startup Mode
The starter generator is driven by the AC/DC
converter in order to assist both the ignition of the jetengine and the acceleration up to 50000 r/min. The hostcontroller controls the starter generator speed in this state.
Further, the jet engine controls exhaust nozzle
temperature.

×6

G

AC/DC

DC/DC

M

DC/AC

Starter
generator

D. Run Mode
The powering operation is performed in the range ofthe rotation speed from 50000 to 70000 r/min, and thebattery is charged. The jet engine controls the speed,
whereas the starter generator controls the output power.

M

DC/AC

Power converter

Jet
engineBattery

Propeller

Fig. 1. Configuration of developed UAV system.
Start(A) All Off Mode
Yes

E. Stop Mode
The engine output is halted, whereas the starter
generator decelerates and cools the jet engine. When theexhaust nozzle temperature of the jet engine is cooled to
50°C or less, the operation of the power converter is
stopped. Then, the operation mode is shifted to the all off
mode.

Shift Command

Shift Command

No

Yes

No
Rotation Speed>50000 r/min(B) Standby Mode

No

Yes
Shift Command

No(D) Run ModeYes(C) Startup Mode

Shift Command

No

Yes

MODULATION METHOD FOR EVEN-ORDERHARMONIC COMPONENTS SUPPRESSION

Injection Port Temperature<50 °CNo

Yes(a) Flowchart of developed UAV system operation.
DC-link voltage

A. Continuous PWM
The starter generator is driven at the rotational speed of
the jet engine, the carrier frequency, and the fundamental
frequency are close to each other. This leads to the loworder harmonic components and the beat components on
generator current. Thus, in the Run Mode, the nine-pulse
synchronous PWM technique is used.
Figure 3 shows the voltage command with amodulation index of 0.8 and a triangular carrier with afrequency ratio of nine when the continuous PWM isapplied. As shown Fig. 3(a), the voltage command vu* iscompared with the triangular carrier to generate a PWM
signal in general. As shown Fig. 3(b), the modulationindex command Vm* and the red carrier umc are comparedto generate a PWM signal. The deformed carrier umc is
calculated byu(  0,180)(1),
umc  msin where  is the phase of inverter voltage command and um
is the triangular carrier.
The proposed modulation method estimates theintersection phase of the modulation signal and thecarrier using a look-up table of the deformed carriers inthe software, then outputs a voltage command according
to the estimated intersection phase. It can be implemented
in a micro-controller because this proposed modulation
method is implemented without changing the hardware.
Table I shows the estimated intersection phase patternsof the continuous PWM. As shown in Table I, theestimated intersection phases of sectors zero and nine aredetermined to be 0° and 180° because of the synchronous
PWM, respectively. The relationship between the phaseand the modulation index command of the deformed
carrier umc of the sectors 1, 2, 3, and 4 is tabulated as(E) Stop Mode

Rotation speed

III.

Mode A Mode B

Mode C Mode D Mode E

Mode A

300V⋍50 V
Powering
operation
Acceleration
assistIgnition
50000 r/min
assistCooling
operation

Time(b) Overview operation of jet generator.
Fig. 2. Mode transition diagram of developed UAV system.

look-up table 1, 2, 3, and 4. Note that 1, 2, 3 and 4
are the estimated intersection phases referred from theselook-up tables. For the sector 5 and later, it is notnecessary to prepare the tables in order to estimate thephase using the symmetry each 90°. The proposed
modulation method acquires the estimated intersection
phase in each sector using the phase referred to fromlook-up table 1 to 4 and the relationship in Table I, andoutputs a voltage command according to the estimatedintersection phase. The generated PWM signal isequivalent to that generated by analog control. Therefore,even-order harmonic components do not occur in PWM
signal.
B. Discontinuous PWM
Figure 4 shows a modulation signal and a carrier withfrequency ratio of 9 and a modulation index of 0.8 when

Number of sector01

Number of sector

2

3

4

5

6

7

1

8

9

0

vm  Vm sin 

(a) 0

2

3

4

5

6

7

8

9

um

(a) 0

Vm{sin  sin(  120)}  1um-1

Vm{sin  sin(  120)}  1

-1
umc 

(b) 1
0
-10°

1

2 3

um
sin 

umd

(b) 1
Vm

4 90°

Vm

0
-10°

190°

1

2 3

90°

190°

Fig. 3. Waveforms of voltage commands and carriers with
continuous PWM.

Fig. 4. Waveforms of voltage commands and carriers withdiscontinuous PWM.

TABLE I
Proposed estimated phase patterns of continuous PWM.

TABLE II
Proposed estimated phase patterns of discontinuous PWM.

SectorLook Up
Table

Phase ofIntersection Point

1
2
3
4
4
3
2
1

1
2
3
4

0
1
2
3
4
5
6
7
8

SectorLook Up
Table

Phase ofIntersection Point

Sector9180°180°+1101112
180°+212
180°+33
13
4
180°+4360°-414
4360°-315
3360°-216
2360°-1
17
1
X:Phase by Look Up Table X

0°180°-4
180°-3180°-2
180°-1*xd

 v v*x*
offset,* 1  vmax

if vmax  vmin , 1  vmin

if vmin  vmax ,Phase ofIntersection Point

1
2
3

1
2
3

3
2
1

90°
90°
180°-3180°-2
180°-1

Jet
engine

*
v

id

ibt
Vbt

(2).
fsw=10 kHz or 9 pulse*w(10    60)ic

Battery

fsw=60 kHz

Fig. 5. Configuration of power converter.
Duty command calculate a
*
dc

V

+

PI

ー

1
4

i a* +

PI

ー

v a*

1
Vdc

ia

Vdc
Voltage control

Duty command calculate b
Duty command calculate c
Duty command calculate d

d a*

db*
d c*

1
0
Carrierd d*
10Phase shifted
carrier(3).

Note that in the section of more than 60° in sector 3, theintersection phase is set to 60° when the modulationindex is 0.577 or less.
Table II shows the estimated intersection phasepatterns of the discontinuous PWM. The relationshipbetween the phase and the modulation index command of
the deformed carrier umd of the sector 1, 2, and 3 isdefined as in look-up table 1, 2, and 3. By referring thephases 1, 2, 3 using the modulation index from theselook-up tables, the intersection phase of the carrier andthe modulation signal in each sector is estimated by the
relationship shown in Table II. By using this method, thesymmetry of the PWM signal is secured even in the
discontinuous modulation, and the even-order harmonic

V dc
Vbt

Modulationsin   sin(  120)ib

G

Modulation

umd 

ia
Vdc

Starter
generator

As mentioned in Section A, a deformed carrier umd in
Fig. 4(b) is used. The intersection phases in sectors 0 and
4 are defined as 0° and 90° in advance, respectively.
Therefore, the deformed carriers of sectors 1, 2 and 3 areonly required to estimate the intersection phases. The
deformed carrier umd is calculated byum  1

Interleaved Converteriuv  max[v , v , v ]and  max***
 vmin  min[vu , vv , vw ]*uPhase ofIntersection Point

Look Up
Table

Sector9180°180°+1101112
180°+212
180°+3
3
13
270°
270°
14
360°-315
316
360°-22
360°-1
17
1
X:Phase by Look Up Table X

0°

Inverter x  u , v, w 

voffset  

Look Up
Table

0
1
2
3
4
5
6
7
8

the discontinuous PWM is employed. The discontinuous
PWM signal vxd* shown in Fig. 4(a) is calculated byadding the following offset to the three phase modulation
signal vx.
v

1

1

Gate
signal

Sa,b

2

Gate
signal2

Sc,dFig. 6. Control block diagram of DC/DC interleaved converter.

components do not occur in the PWM signal.
IV.

CONTROL STRATEGY FOR STARTER GENERATOR

Figure 5 shows the configuration of the power
converter. This converter consists of a three-phase
inverter and a four-leg interleave DC/DC converter.
Since the battery voltage is approximately 50 V, theDC/DC converter is required to boost the voltage to 300
V in order to drive the inverter.
Figure 6 shows the control block diagram of DC/DC
interleaved converter. The DC link voltage Vdc isregulated to the command value. The current imbalanceamong four legs is suppressed by the current control ofExhaust nozzle
temperature control

T*

+

I

-

PT*
Power generation *output control

T

*

 g*

+
Output power command*
depend on g
+

ia
ib
ic
id

+
+
+

+i

PI

-

Saturarion

mode

1
s

3

uvw

+

Vbt

+

-

I

+

Discontinuous
PWM
3



0 3

Output power limiter

Pout

bt

10k

g*

vu*,v,w+*

f/V v dconv.

Interrupt cycle
calc.

M
U
X

*

1 du,v, w
Vdc 3

3
*
voffsetModulationP* + Pout +M + Rate Limit UX +v g* = 0 gd
*
g

 g*
M
U
X

f c*

Gate
signal3

Su,v, w

1
0
Carriermode

Saturarion

Fig. 7. Control block diagram of 3-phase inverter.

V.

STABILIZATION ANALYSIS OF POWER GENERATIONOUTPUT CONTROL

Figure 8 shows the characteristics of jet engine thrust
against the rotation speed. As shown Fig. 8, a thrust of
68.0 N is obtained at the rotation speed of 70000 r/min.
Under the atmospheric pressure, the atmospherictemperature, and the air density are constant, the thrust of
the jet engine depends only on the rotational speed
regardless of the output power. Since the thrust of the jetengine is proportional to the cube of the rotational speed,
the thrust F obtained by the measured value isapproximated by the cube of the rotational speed asfollows;
F  kF 3(4),
where  is rotation speed of the jet engine and kF is
coefficient obtained from the measured value.
Figure 9 shows the block diagram of the output power
control system with a jet engine. In this system, thegenerator synchronous angular frequency g is produced

8.00E+01
80
7.00E+01

6.00E+01
60

Jet engine thrust F [N]each leg. Moreover, the carrier of each two legs is phaseshifted by half a period compared to the other two legs.
As a result, the switching frequency is equivalently
doubled and the current ripple is reduced to half [11].
Figure 7 shows the control block diagram of the threephase inverter. The power control is operated in the runmode, whereas the V/f control is employed in the othermodes. The rotation speed of the jet engine is suddenly
reduced because the output power of the jet engine is not
sufficiently high when the inverter control switched fromthe V/f control to the output power control or exhaust
nozzle temperature control for the run mode. In otherwords, the self-sustained operation of the jet engine isdifficult at the low speed. In order to solve this problem,a synchronous frequency command limiter is applied. As
a result, the speed is kept constant until the jet engine
output becomes sufficiently high. During the startup
mode, the generator torque suddenly changes. In order toprevent overcurrent in this operation, an output power
limiter is introduced. Consequently, the synchronous
frequency command is compensated in order to avoid thesudden change in the torque. Furthermore, around therated speed, discontinuous PWM is employed to deal
with overmodulation region.

5.00E+01

4.00E+01
40
3.00E+01

2.00E+01
20
1.00E+01

0.00E+00
0

00

20
20

10

30

40
40

60
60

50

70

80
80

Rotation speed [r/min](×103)

Fig. 8. Characteristics of jet engine thrust.

Controller
*
Pout

1
1  sT I+

1  sTI
KP
sTI

Plant

g s 3kvf 2  1 
+
0

r2

Js

eq.(8)

Pjet
Pth_s+ Pout

Fig. 9. Block diagram of output power control system with jetengine.

by the difference between the output power command
Pout* and the output power detection value Pout. Further, it
is assumed that the response of the rotation speed control
for the jet engine is sufficiently slower than that of theoutput power control. By ignoring the loss, the total
power of the jet engine Pjet is calculated by
Pjet  Pth  Pout
(5),
where Pth is the thrust power of the jet engine. This thrust
power is added to the shaft power Pout that drives thepropeller. This shaft power is determined by the flight
speed and the thrust of the aircraft. However, if theaircraft is stationary as in the test, the shaft power cannot
be calculated from the flight speed. In this case, thestationary shaft power Pth_s is calculated byF(6).
Pth _ s 
 736
11.2
Substituting (4) into (6) and setting the coefficient askth, the stationary shaft power [12] is calculated by
736kF 3
(7).
Pth _ s   kth 3
11.2
In order to analyze the stability of the control, therotation angular velocity is linearized around the steadystate points.
2
Pth _ s  3kth0 (8).

Imaginary partiu vuv

No.2

Vout

Fig. 12. Configuration of experimental system.
TABLE IIISpecification of starter generator.

-120

-100

-80

-60

-40

-20

0

Parameter

Real partFig. 10. Roots locus when the initial angular velocity 0 isincreased.

0.6

+

G

0=0p.u.

0=1p.u.

-40

M

No.1

0=0.7p.u.

0Battery

PWM rectifier

+

40

-80
-140

Output power [p.u.]

Inverter

Unstable region800=0.7p.u.

0=0.4p.u.

0=0.1p.u.

2

Rated rotary field speed

70000 r/min

Rated speed

68271 r/min

Rated voltage

200 V

Rated current

15.3 A

Rated power

4 kW

Rated torque

0.6 N・m
3.0 kg
110 mm
192 mmWeightDiagram
Full length0.5

0=1p.u.

Value

Poles

0.4

Pout*
0.3

1.05
0.9

1.0

1.1

1.2

1.3

1.4

1.5

Time [s]Fig. 11. Step response of output power when output powercommand is changed from 0.3 p.u. to 0.5 p.u.

Note that o is the initial angular velocity at the steadystate point. Consequently, the transfer function from
input to output of this control system is expressed by
9 K p kvf 2 kth0 2
Ki r2 J
(9),G(s) 
2
9 K p kvf kth0 2 s 9 K p kvf 2 kth0 2
2
s 

r2 J
Ki r2 J
where Kp is the proportional control gain, Ki is theintegral control gain, kvf is the voltage coefficient in thev/f control, J is the total inertia of the jet engine andgenerator, and r2 is the secondary winding resistance ofthe generator. Furthermore, Kp and Ki are expressed asfunctions of the damping coefficient  and the response
angular frequency n.
2 n r2 J
(10)
Kp  
2
9k vf k th0 '2
Ki 

2
n

(11)

Note that 0' is the initial angular velocity. This angularvelocity should be set accordingly to the detection valueof the angular velocity. However, the angular velocity
detection is not employed in the test; therefore, this value
is predetermined as following.
Figure 10 shows the roots locus when the initial
angular velocity 0 is increased. In this system, thepowering operation is performed in the rotation speed
range from 0.7 to 1.0 p.u. Therefore, the initial angularvelocity setting value 0' is 0.7 p.u. The damping
coefficient  is set to 0.7. The response angular frequency
n is set; thus, the overshoot time is 0.05 seconds, whichis 1/10 of the jet engine control period of 0.5 seconds. As
shown in Fig. 10, when the rotation speed is 0 p.u., thecontrol system is at the stability limit because the poles
locate on the imaginary axis. The control system becomesstable because the poles move to the negative half plane
when the rotation speed is larger than 0 p.u.
Figure 11 shows the step response of output power
when output power command is changed from 0.3 p.u. to
0.5 p.u. As shown in Fig. 11, at a rotation speed of 0.7
p.u. and 1.0 p.u., the response is equal to or larger thanthe design response time. The response time is delayed
and a large overshoot occurs in output power at therotation speed of 0.4 p.u. and 0.1 p.u., which is the lowspeed range. However, such large overshoot does notoccur since the power generation operation is performed
only in the high-speed range in this system.
VI.

EXPERIMENTAL RESULTS

A. Modulation method for even-order harmoniccomponents suppression
Figure 12 shows the experimental system. Table III
shows the specification of the starter generator. In thistest, two motors shown in Table III are connected insteadof the jet engine. In addition, a small capacity DC
regulated power supply is connected to supply the
excitation current at the time of starting since the starter
generator is an induction generator.
Figure 13 shows a block diagram of the PWM
converter. This control system is adjust the slip anglefrequency and control the DC-link voltage. In the highspeed range, switching from the asynchronous PWM tothe synchronous PWM. Furthermore, around the rated
speed, discontinuous PWM is employed to deal with theovermodulation region.
Figure 14 shows the operation waveforms of thecontinuous PWM at frequency ratio of nine and therotation speed of 0.8 p.u. The modulation index is 0.871,
and both the conventional method and the proposed
method control the output voltage to be constant at 300 V.
Figure 15 shows the harmonic analysis results of thegenerator current of the continuous PWM. As shown Fig.
15(b), the proposed method suppresses low even-orderharmonic components, such as second, eighth, and tenthorder, which are generated by the conventional method.
Vg* = 0
*
out

V

+

 s*+

PI

 g*

ー

+

Vout

 *j

1
s

Voltage control

gd

vu*,v,w*

f/V V d
conv.

+

3

uvw



3

+

*
voffsetDiscontinuous
PWM
3

03

B. Control strategy for starter generator
Figure 18 and Table IV shows the prototype of the jetgenerator and the specifications of the jet engine. As

f c*Interrupt cycle
calc.

Also, the eighth harmonic component was reduced by
99.2% compared to the conventional method. In addition,
the generator current total harmonic distortion (THD) is
reduced by 9.99% compared to the conventional method.
Figure 16 shows the operation waveforms of the
discontinuous PWM at a frequency ratio of nine and arotation speed of 1.0 p.u. The modulation index is 1.08,
and both the conventional method and the proposed
method control the output voltage to be constant at 300 V.
Figure 17 shows the harmonic analysis results of thegenerator current of the discontinuous PWM. As Fig.
17(b) shown, the proposed method suppresses low evenorder harmonic components, such as second, eighth, andtenth order, which are generated by the conventional
method. Also, the eighth harmonic component was
reduced by 99.1% compared to the conventional method.
In addition, the generator current THD is reduced by
7.14% compared to the conventional method. Therefore,
this method is effective also in the discontinuous PWM.

Fig. 13. Block diagram of PWM converter.
Input voltage vuv [250 V/div]

Input voltage vuv [250 V/div]

Input voltage vuv [250 V/div]

Input voltage vuv [250 V/div]

0

0

0

0

0

0

0

0

Generator current iu [10 A/div]

Generator current iu [10 A/div]

Generator current iu [10 A/div]

Output voltage Vout [250 V/div]

Output voltage Vout [250 V/div]

Output voltage Vout [250 V/div][400 µs/div]

0[400 µs/div]

Generator current iu[p.u.](a) Conventional method.
(b) Proposed method.
Fig. 14. Experimental results of continuous PWM.
Fundamental
7

THD:86.5%
th

17

th

103

104

105

Frequency [Hz][400 µs/div]

Fundamental(1149 Hz, 8.07 A)

100

2nd
10

(912 Hz, 7.80 A)

10th
7th 11 17th

THD:77.1%
19th

10-2
10-4

103

5th

th
th
7th 8 10 11th

THD:74.2%

4th

10-4

103

104

105

Frequency [Hz](a) Conventional method.

104

Frequency [Hz](b) Proposed method.
Fig. 15. Frequency analysis results of continuous PWM.

105

Generator current iu[p.u.]

Generator current iu[p.u.]

Fundamental[400 µs/div]-2(a) Conventional method.

0

Output voltage Vout [250 V/div]0(a) Conventional method.
(b) Proposed method.
Fig. 16. Experimental results of discontinuous PWM.

2nd

10-2
10-4

8th 10th 11th

(912 Hz, 8.18 A)

100

0

Generator current iu[p.u.]

0

Generator current iu [10 A/div]

Fundamental(1149 Hz, 7.79 A)

100

7th

11th

17th

THD:68.9%

10-2
10-4

103

104

Frequency [Hz](b) Proposed method.
Fig. 17. Frequency analysis results of discontinuous PWM.

105

Jet engine

Starter
generator

Fig. 18. Prototype of jet generator.
TABLE IV
Specification of jet engine.
Parameter

Value

Weight2.9 kgDiameter

131 mmFull length
Rated thrust

281 mm165 N

Rated speed

100000 r/min

DC voltage Vdc [250 V/div]
0
Battery current Ibt [40 A/div]00Generator voltage vuv [500 V/div]

0
Generator current iu [20 A/div][400 µs/div]

Fig. 19. Experimental waveforms of Run Mode.
グラフ タイトル

Generator current iu [%]shown in Fig. 18, the starter generator is connected to thejet engine without a speed reduction gear.
Figure 19 shows the experimental waveforms of Run
Mode at the rotation speeds of 70000 r/min, where theoutput power to the battery is 2.98 kW. As shown in Fig.
19, the DC link voltage is regulated to 300 V.
Furthermore, the stable power generation operation isachieved since the battery current is constant at any
rotation speeds.
Figure 20 shows the harmonic analysis results of thegenerator current in Fig. 19. As shown in Fig. 20, evenorder low harmonic components are less than 0.2% which
is sufficiently smaller than the fundamental component.
Figure 21 shows the characteristics of the generator
current THD against output power. As shown in Fig. 21,
the minimum current THD of 16.5% is achieved at therotation speed of 70000 r/min and the output power of
2.98 kW. This is because the fundamental component of
the generator current increases as the output powerincreases.
Figure 22 shows the efficiency characteristics of thepower converter. As shown in Fig. 22, the maximum
efficiency of 92.7% is achieved at the rotation speed of
70000 r/min and the output power of 2.98 kW.
Figure 23 shows the temperature characteristics of theexhaust nozzle against rotation speed and the output
power when the ambient temperature is 26°C. As shownin Fig. 23, as the rotation speed of the jet engine increases,
higher the output power is obtained at the same exhaust
nozzle temperature. Further, this system has the highest
efficiency when the exhaust nozzle temperature is around
800°C. Therefore, an output power command depends onangle frequency command g* is as shown in Fig. 23.
Figure 24 shows the experimental results the jetgenerator operation with the exhaust nozzle temperature
control. The exhaust nozzle temperature command is
800°C. The jet engine is accelerated to 60000 r/min,
65000 r/min, and 70000 r/min in the run mode. Then thejet engine is decelerated to 60000 r/min. As shown in Fig
24, even when the rotation speed accelerates ordecelerates, the exhaust nozzle temperature converges tothe command value. The exhaust nozzle temperature canbe controlled within the maximum deviation of 2%
compared to the command value in the steady state.
Furthermore, the exhaust nozzle temperature at 70000
r/min drops to 700°C because the inflow current of thebattery is limited. In addition, the proposed control
method achieves among the starting, the powering andthe cooling operations. The transition without thedeceleration is achieved by the synchronous frequency
command limiter when the startup mode changes to therun mode. The output power gradually approaches zeroafter this transition, because the starter generator
maintains the rotation speed until the output power of thejet engine becomes sufficiently high. The acceleration
without overload is achieved in the startup mode by an
output power limiter, which limits a command up to 1

100

100000Fundamental(13.1A 1114 Hz)

THD:16.5%

10

10000

1

1000

0.1
100

0 1 2 3 4 55 6 7 8 9 10
10 11 12 13 14 15
15 16 17 18 19 20
20
Harmonic number
Fig. 20. Harmonic components on the generator current.

kW. Further, the starter generator decelerates by a freerun when the operation mode transitions to the stop mode
from the run mode. Then the inverter restarts at a rotation
speed of 1000 r/min. The starter generator simultaneously
performs the cooling operation.
VII. CONCLUSION
The control strategy for UAV with the jet engine were
proposed in this paper. The stable transition without
decelerating and overcurrent between the operation
modes of the starter generator was achieved by thesynchronous frequency command limit and the output
power limiter. In addition, the even-order low harmonic

300030.8
8060000 r/min
70000 r/min

3
1
1.5
2
2.5
1000
1500
2000
2500
3000
Output power [kW]Fig. 21. Characteristics of generator current total harmonics
distortion.
00

0.5
500

0.8
80

60000 r/min
0.660

Output power
[kW]

1.5
1500

2
2000

2.5
2500

3
3000

Output power [kW]Fig. 22. Characteristics of efficiency of power converter.

components are suppressed by the modulation method
using the estimated intersection phase for thesynchronous PWM. The 3-kW prototype system achieved
the maximum conversion efficiency of 92.7%, theminimum generator current THD of 16.5% at 70000
r/min. Further, the exhaust nozzle temperature wascontrolled within the maximum deviation of 2% of thecommand value.
ACKNOWLEDGMENT
This paper is based on results obtained from a projectsubsidized by the New Energy and Industrial Technology
Development Organization (NEDO) of Japan.
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[4]

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Run mode

Stop
mode

3000 30[3]Startupmode

4000 4

0.2
20

1
1000

650 °C

55
60
65
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55000
60000
65000
70000
Rotation speed [r/min](×103)
Fig. 23. Characteristics of exhaust nozzle temperature against
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70000 r/min

0.4
40

0.5
500

700 °C

0
50
50000
0

50000 r/min

00

°C
850
°C
800
C
750 °

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