Barbara H. Kenny

Glenn Research Center, Cleveland, Ohio

Roger P. Tokars

Purdue University, West Lafayette, Indiana

Feasibility Study of Jupiter Icy Moons Orbiter

Permanent Magnet Alternator Start Sequence

NASA/TM—2006-214034

February 2006
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Barbara H. Kenny

Glenn Research Center, Cleveland, Ohio

Roger P. Tokars

Purdue University, West Lafayette, Indiana

Feasibility Study of Jupiter Icy Moons Orbiter

Permanent Magnet Alternator Start Sequence

NASA/TM—2006-214034

February 2006

National Aeronautics and

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Glenn Research Center
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Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2006-214034 1

Feasibility Study of Jupiter Icy Moons Orbiter 

Permanent Magnet Alternator Start Sequence 

Barbara H. Kenny 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Roger P. Tokars 

Purdue University 

Electrical and Computer Engineering Department 

West Lafayette, Indiana 47906 

Abstract 

The Jupiter Icy Moons Orbiter (JIMO) mission was a proposed, (recently cancelled) long duration 

science mission to study three moons of Jupiter: Callisto, Ganymede, and Europa. One design of the 

JIMO spacecraft used a nuclear heat source in conjunction with a Brayton rotating machine to generate 

electrical power for the electric thrusters and the spacecraft bus. The basic operation of the closed cycle 

Brayton system was as follows. The working fluid, a helium-xenon gas mixture, first entered a 

compressor, then went through a recuperator and hot-side heat exchanger, then expanded across a turbine 

that drove an alternator, then entered the cold-side of the recuperator and heat exchanger and finally 

returned to the compressor. The spacecraft was to be launched with the Brayton system off-line and the 

nuclear reactor shut down. Once the system was started, the helium-xenon gas would be circulated into 

the heat exchangers as the nuclear reactors were activated. Initially, the alternator unit would operate as a 

motor so as to drive the turbine and compressor to get the cycle started. This report investigated the 

feasibility of the start up sequence of a permanent magnet (PM) machine, similar in operation to the 

alternator unit, without any position or speed feedback sensors (“sensorless”) and with a variable load 

torque. It is found that the permanent magnet machine can start with sensorless control and a load torque 

of up to 30 percent of the rated value. 

Introduction 

In addition to the interesting science objectives, the proposed JIMO mission would also have been an 

opportunity to demonstrate several technologies in the harsh environment of space. A nuclear fission 

reactor was to provide the heat source for a dynamic power conversion cycle that ultimately produced 

electricity for the spacecraft use. The propulsion system was based on electric thrusters. The spacecraft 

electrical power system was approximately two orders of magnitude larger than anything previously 

flown. The entire spacecraft needed to operate reliably and semi-autonomously for over a decade with 

part of the time in the radiation environment of Jupiter. 

One option for the proposed JIMO power system was based on a Brayton cycle with a helium-xenon 

gas mixture as the working fluid. The block diagram of the cycle is shown in figure 1. In this option, once 

at steady state, the alternator is regulated using a parasitic shunt resistor as described in reference 1. This 

keeps the speed of the machine (and therefore the bus voltage) approximately constant regardless of load. 

At start-up, however, there is not enough energy in the working fluid to turn the turbine and 

compressor. This must be accomplished by a motoring action, either through an additional machine 

attached to the shaft and only used at start-up, or by operating the alternator as a motor during the start-up 

transition. A permanent magnet machine is the most likely candidate for the alternator and it can operate 

as either a motor or generator depending on how it is controlled. For optimal operation as a motor, the 

controller requires rotor position information so that the applied voltage and current can be oriented 
NASA/TM—2006-214034 2

Figure 1.—Representative block diagram of Brayton cycle (actual design may differ), 

HX = heat exchanger, “generator” block represents alternator function. 

properly with respect to the rotor field. Typically this is done using either Hall Effect sensors, a resolver 

or an encoder. However, these sensors add complexity and failure modes to the system and they may be 

sensitive to radiation and thus not perform properly in the Jupiter environment. Hence this investigation 

was undertaken to see if the PM machine could be reliably started and operated as a motor without any 

rotor position or magnetic field feedback sensors. 

There are many papers in the literature about sensorless operation of PM machines (refs. 2, 3, 4, and 

5) and mid- to high-speed operation is fairly straight-forward. The difficulty comes at low- and zerospeeds, especially with significant load torques that require large machine currents. One technique 

involves adding an additional voltage signal to the fundamental voltage but requires that the machine 

rotor have a magnetic saliency (inductance variation with rotor position) (refs. 6 and 7). This technique 

may have been a viable candidate to start the JIMO machine because the alternator could have been 

designed and built with a suitable saliency. However, as a baseline case, it was desired to determine if a 

PM machine could be started under load without any additional sensing signal and/or special design. 

In this work, the machine was started from zero speed up to a few hundred rpm without any 

knowledge of the position of the rotor field. In this sense it was an “open loop” start although current 

magnitude was measured and regulated. Then the control was transitioned to a technique known as field 

orientation control or vector control (refs. 8, 9, 10, and 11) where a rotor position estimate, in addition to 

the current magnitude command, was used to determine the commanded voltage to the machine. After 

establishing the field orientation control, a loop was then closed on speed to regulate the rate of start-up. 

Nomenclature 

Lq is the q-axis machine inductance, henries. 

Ld is the d-axis machine inductance, henries. 

x

qy f represents a q-axis scalar quantity where f can be voltage, v, current, i, or flux, λ; x can be the 

rotor reference frame, r, or the stator reference frame, s; and y is either the stator variable, s, or 

the rotor variable, r. 

x

dy f represents a d-axis scalar quantity where f can be voltage, v, current, I, or flux, l; x can be the 

rotor reference frame, r, or the stator reference frame, s; and y is either the stator variable, s, or 

the rotor variable, r. 
NASA/TM—2006-214034 3

x

qdy f represents the d-q vector quantity where f can be voltage, v, current, I, or flux, λ; x can be the 

 rotor reference frame, r, or the stator reference frame, s; and y is either the stator variable, s, or 

 the rotor variable, r. 

* a superscript asterisk represents a commanded value. 

∧ a carrot above a symbol represents an estimated quantity. 

p is the derivative operator, d/dt. 

θr is the angle between the stator q-axis and the rotor q-axis, radians. 

λaf is the flux linkage due to the rotor magnets, volt-sec. 

ωr is the electrical rotor speed, radians/second. 

P is the number of poles of the machine. 

Field Orientation Control 

The basic idea behind field orientation control is to orient the applied stator currents to the rotor 

magnetic field. When this is done, the motor control is simplified because the control variables become dc 

quantities in steady state. The operating point of the machine can then be accurately and dynamically 

controlled so that high efficiency and fast response are obtained. Accurate torque, speed or position 

control of the machine is then possible depending on the outer loop used. A detailed discussion can be 

found in many electric machine textbooks (refs. 8, 9, 10, and 11). 

The key to successful field orientation control is accurate and continuous knowledge of the position 

of the rotor magnetic field. Typically this is done with a position sensor (resolver or encoder) or a 

magnetic field sensor (Hall Effect). In “sensorless” control, this sensor is eliminated and the position of 

the rotor field is estimated using measured machine currents. This section describes basic field orientation 

control, the next section explains the open loop starting procedure without knowledge of the rotor 

position, and the following section describes the technique used to estimate the rotor position. 

A three phase machine, without a neutral connection, can be equivalently described as a two phase 

machine through a transformation from abc coordinates to dq coordinates as follows: 

 f

q

 = fa (1) 

 fd = 

3

1 − fa

 – 3

2 fb (2) 

The reverse transformation is 

 fa = f

q (3) 

 fb = 

2

1 − f

q – 2

3 fd (4) 

 fc = 

2

1 − f

q

 + 

2

3 fd (5) 

The d and q variables described in equations (1) and (2) are in the stator reference frame. From 

equation (1) it can be seen that the q-axis is aligned with the 'a' phase. This means that the 'a' phase 

current is equal to the q-axis current and the 'a' phase voltage (Van) is equal to the q-axis voltage. 

In a permanent magnetic machine, it is convenient to transform these variables to a reference frame 

that is rotating synchronously with the rotor magnetic field. In the rotor reference frame, the d-axis is 

defined to be co-linear with the rotor magnetic field axis. The transformation from the stator frame to the 

rotor frame is given by equation (6) and the inverse transformation is given by equation (7). 
NASA/TM—2006-214034 4

⎥

⎦

⎤ ⎢

⎣

⎡

⎥

⎦

⎤ ⎢

⎣

⎡

θ θ

θ − θ ⎥ =

⎦

⎤ ⎢

⎣

⎡

s

d

s

q

r r

r r

r

q

r

q

f

f

sin cos

cos sin

f

f

(6)

⎥

⎦

⎤ ⎢

⎣

⎡

⎥

⎦

⎤ ⎢

⎣

⎡

− θ θ

θ − θ ⎥ =

⎦

⎤ ⎢

⎣

⎡

r

d

r

q

r r

r r

s

d

s

q

f

f

sin cos

cos sin

f

f

(7) 

The permanent magnetic synchronous machine can then be modeled in the rotor reference frame as 

follows. Equation (8) gives the voltage relationships in the rotor reference frame, equation (9) gives the 

stator flux linkages in the rotor reference frame and equation (10) is the torque expression. 

⎥

⎦

⎤ ⎢

⎣

⎡ω λ ⎥ +

⎦

⎤ ⎢

⎣

⎡

⎥

⎦

⎤ ⎢

⎣

⎡

− ω +

+ ω

⎥ =

⎦

⎤ ⎢

⎣

⎡

i 0

i

L R L p

R L p L

V

V r af

r

ds

r

qs

r q s d

s q r d

r

ds

r

qs (8) 

⎥

⎦

⎤ ⎢

⎣

⎡

λ ⎥ +

⎦

⎤ ⎢

⎣

⎡

⎥

⎦

⎤ ⎢

⎣

⎡ ⎥ =

⎦

⎤ ⎢

⎣

⎡

λ

λ

af r

ds

r

qs

d r

ds

r

qs 0

i

i

0 L

Lq0

 (9) 

Te = [ ] r

ds r

qs rqs r

ds i i

4

3P λ −λ (10) 

In field orientation control the d-axis current, ids

r

, is commanded to 0. From (9) and (10) it can be seen 

that this results in a simplified expression for torque, given in (11). 

 Te = [ ] r af qs i

4

3P λ (11) 

Because the magnitude of λaf is constant due to the rotor magnets, the torque of the machine is 

proportional to the q-axis current. This result is similar to the dc motor with a constant field winding 

current where the torque is equal to the armature current times the torque constant. Thus control of the 

torque is achieved in the permanent magnet machine by properly controlling the rotor reference frame 

currents, r

qs i and r

ds i . 

To control the currents in the machine to be the commanded values, a current regulated voltage 

source inverter is used. This means that current errors result in voltage commands to the inverter that 

increase or decrease the applied voltage to increase or decrease the current, respectively. This is 

accomplished through a synchronous frame current regulator (ref. 12) that is basically a PI (proportionalintegral) controller operating on the rotor reference frame currents. The output of the controller is a 

voltage command in the rotor reference frame that is then transformed to the stator reference frame. The 

stator frame voltage commands are then used to calculate the inverter switch duty cycles. 

There are several methods to find the duty cycles from the commanded stator frame voltages (ref. 13). 

In the implementation used here, space vector modulation is used. Space vector modulation is a digital 

technique to calculate the duty cycles directly from the stator reference frame d and q voltages. One 

advantage of space vector modulation is that it increases the dc bus utilization to the maximum value. 

This means that for a given dc bus voltage, the maximum fundamental phase voltage (with minimum low 

order harmonics) is achievable by using space vector modulation. 

Figure 2 shows the overall block diagram of the control structure. As can be seen from equations (6) 

and (7), rotor position is necessary to perform the field orientation transformation. For zero and low speed 

operation, the transformation angle is based on a ramped frequency command. After a few hundred rpm, 

the transformation angle is based on the estimated angle from the estimation algorithm. 
NASA/TM—2006-214034 5

Vqds

s

PM 

Synchronous 

Motor

Stator to 

Rotor 

Frame 

Transform

Rotor to 

Stator 

Frame 

Transform

Space 

Vector 

PWM

Voltage 

Source 

Inverter

abc to dq 

transform

Synchronous 

Frame Current 

Regulator

i

qds

s i

a

i

b

i

qds

r

i

qs

r*

θr

Vqds

r* *

Back EMF 

position and 

speed estimation

i

ds = 0 r*

Open loop 

or back EMF

θr

ωr Open Loop with 

ramped frequency

θr

Speed 

Regulator

ωr

*

ωr

Figure 2.—Basic block diagram of motor control. 

Open Loop Starting 

The principle behind open loop starting is that a slowly rotating magnetic field can be set up in the air 

gap of the machine that the rotor magnets will try to follow. As the speed of the magnetic field increases, 

the rotor speed also increases as it tries to “keep up” with the field. The field must be strong enough to 

pull the rotor along as the frequency increases. The magnitude of the field is related to the magnitude of 

the current in the stator windings. A larger inertia rotor will require more current to accelerate than a 

smaller inertia one. Also, a larger load torque or bearing drag will require more current than a smaller 

one. 

The position of the field is related to the transformation angle, θr. The speed of the field is governed 

by the rate of change of the transformation angle, dt

dθr . The acceleration rate is the second derivative of 

the transformation angle, 2

r 2

dt

d θ . During the open loop portion of the control, the acceleration rate is 

governed by the ramp rate as shown in the block diagram of figure 2. In open loop starting, the current 

magnitude and the acceleration rate of the rotating field are the command variables. Although they must 

work together to keep the rotor accelerating, there is not a one-to-one correspondence between them. For 

example, there is a range of acceleration rates that would work with a particular current command and a 

range of current commands that would work with a particular acceleration rate. The slack variable is the 

efficiency of the motor operation that is not controlled during open loop starting. However, the current 

and acceleration rate commands must be set with some general knowledge of the inertia and the load 

torque of the system. 

The predominant load torque for starting the JIMO alternator was expected to be the bearing drag. 

One Brayton shaft design used foil bearings to minimize bearing wear during the long mission (ref. 14). 

Foil bearings are very low loss beyond some minimal speed, however at low speed and zero speeds; there 

is direct contact between the shaft and the bearing raceway. This contact generates heat as the shaft turns 

that can damage the bearings if the minimal “lift off” speed is not attained quickly. The problem for the 

open loop portion of the motor control is that an exact bearing drag torque model for foil bearings has not 

been developed. The drag torque depends on several factors as the rotor accelerates. Some of what is 

known is based on empirical data from testing foil bearings and additional tests would have been 

conducted on the final JIMO machine design but at the time of this work, there was no exact model of the 

foil bearing drag torque during start-up conditions. So the open loop motor control needed to be robust 

enough to start the machine even though the exact drag torque could not yet be predicted. However, once 
NASA/TM—2006-214034 6

the machine control changed to field orientation, the rotor speed could be controlled to the commanded 

value regardless of the drag torque or any other load torque. 

Position Estimate 

The rotor position (and speed) estimate is based on a technique known as back EMF (electro-motive 

force) estimation. The “back EMF” technique derives its name from the fact that the spinning rotor 

magnets will generate a voltage (the back EMF) at the terminals of the machine. This voltage can be 

integrated to find the stator flux (ref. 4). From the stator flux vector and an estimate of the torque angle, 

the rotor flux position can be estimated (ref. 3). This will be described next. 

The stator reference frame equation describing the relationship between the stator voltage, current and 

flux linkages for the fundamental excitation of the machine is given in equation (12). 

s s qds s

qds sqds v = i R + pλ (12) 

The stator flux can be estimated by integrating the stator voltage less the IR drop as shown in 

equation (13). 

( ) ∫ = − dt s s

qds sqds s λqds v i R (13) 

In the simplest implementation (used in this work), the stator voltage, vqds

s , is assumed to be equal to 

vqds

s* , the commanded voltage, which is known in the controller. This is equivalent to assuming a constant 

dc bus voltage and no voltage loss in the inverter. Improvements in the flux estimate by using either better 

estimates of stator voltage or by actually measuring stator voltage are possible and described in the 

literature. 

Additionally, the open loop integration described by equation (13) was actually implemented as a low 

pass filter with a 25 Hz bandwidth. This low pass filter implementation requires an adjustment of the 

estimated angle at very low speeds (below about 1500 rpm) due to the phase shifting properties of the low 

pass filter. However, this is easily accomplished with a look-up table. The technique was found 

experimentally to result in a reliable rotor position estimate at speeds above about 300 rpm. 

The stator flux vector can be found from equation (13) but it is knowledge of the position of the rotor 

flux, λaf, which is necessary for proper field orientation. This can be found by expressing the stator flux in 

the rotor reference frame wherein the d-axis component of the stator flux vector is aligned with the rotor 

flux, λaf. 

r q qs r

qs λ = L i (14) 

af r d ds r

ds λ = L i + λ (15) 

The angle between the stator flux vector, λqds, and λaf is the torque angle which is found from equations 

(14) and (15). 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

δ = −

af r d ds

r q qs 1 tan

L i λ

L i

 (16) 
NASA/TM—2006-214034 7

qs

ds

qr dr

λaf

iqs

r Lq

ids

r Ld

λqds

δ

θr

λqs

s

λds

s

 

ω^r ^ Kp

Ki

___ Kd

J^

J^

___1

1

p

___

1

p

+ ___ + + +

+

+ 1

p

___ θr

Te

^

+

-

^

θr,bemf

These relationships can be best shown graphically as in figure 3. For field orientation control, the 

d-axis of the rotor reference frame is defined to be coincident with the rotor flux. To transform from the 

stator frame to the rotor reference frame, the rotor angle, θr, must be known. This is the angle between the 

stator frame d-axis and the rotor frame d- axis (or, equivalently, the angle between the stator frame q-axis 

and the rotor frame q-axis) as shown in figure 3. Mathematically, this angle can be found from 

equation (17) as shown. 

− δ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

λ

λ

θ = − s

ds

s

qs 1 r tan ˆ (17) 

The estimated rotor angle, r

ˆ

θ , is used for the reference frame transformations as shown in figure 2 

and is also used as an input into a speed observer as shown in figure 4. A torque estimate can also be used 

to improve the response of the speed estimate reference 15. 

At 300 rpm, the position estimate is accurate enough to use as the transformation angle shown in 

figure 2. At this point the open loop control can be discontinued and field orientation control can be used 

reliably. Once under field orientation, the acceleration of the machine is completely controlled by the 

commanded current value. The speed loop is closed next (using the estimate shown in figure 4 as 

feedback) and then the current command becomes the result of the speed loop error rather than being 

commanded explicitly. 

Simulation Results 

The simulation used a model of a representative permanent magnet machine based on the expected 

power requirements of the alternator. The assumed parameters are given in table 1. The control consisted 

of a closed loop current regulator, the open loop starting sequence, field orientation control, and the rotor 

position/speed estimation algorithm described in the previous section. The start-up bearing drag torque 

was modeled as a linear function that decreased with speed to a small constant value at 4500 rpm. That 

function is given in equations (18) and (19). The simulation was written in Matlab/Simulink. The results 

are presented in figures 5 to 8. 

 TL (N-m) = 7.0 – 0.0015 * speed (rpm) for 0≤speed<4500 rpm (18) 

TL (N-m) = 0.2 for speed≥4500 rpm (19) 

Figure 3.—Vector diagram showing flux, 

current, angles, and reference frame axes. 

Figure 4.—Speed observer. 
NASA/TM—2006-214034 8

TABLE 1.—MACHINE PARAMETERS USED IN SIMULATIONS 

Machine Parameter Value 

J 0.01 kg-m2

Ls 34 HH 

Rs 0.03 Ω

λaf 0.023 volt-sec 

Poles 6 

Rated torque 21 N-m 

Rated current 144 amps (rms) 

0 2 4 6 8 10 12 0

1000

2000

3000

4000

5000

time, seconds

speed, rpm

start speed

loop control

change current command

start vector control

actual machine speed

estimated machine speed

commanded machine speed

Figure 5.—Machine speed during start-up: 

actual, estimated, and commanded. 

0 2 4 6 8 10 12 −500

0

500

1000

1500

time, seconds

power, watts

Figure 6.—Machine power during start up. 

The simulation begins with an acceleration ramp rate of 400 rpm/second and a current magnitude 

command of 75 amps (peak). At t = 2 seconds (machine at ~850 rpm) the control switches from open loop 

to field orientation. At t = 2.2 seconds, the current command is reduced to 55 amps and at t = 2.5 seconds 

the speed loop is closed. Acceleration continues based on a ramped speed command of 400 rpm/second 

until the commanded value of 4500 rpm is reached at t = 9 seconds. 

Figure 5 shows the commanded speed, the estimated speed and the actual speed of the machine. The 

estimated speed can be seen to agree with the actual speed very closely although the actual speed 

demonstrates some ringing during the open loop portion of the control. If the control is not switched over 

to field orientation in time, the machine will reach a speed where this ringing becomes large enough to 
NASA/TM—2006-214034 9

0 2 4 6 8 10 12 0

1

2

3

4

5

6

7

time, seconds

Load torque, N−m

Figure 7.—Load torque characteristics. 

0 2 4 6 8 10 12 0

1000

2000

3000

4000

5000

6000

7000

8000

time, seconds

energy, joules

Figure 8.—Energy required to start machine. 

cause the machine to cease accelerating and to lose synchronism; the starting procedure fails. Also, it 

appeared during the simulation trials that lower speeds were more tolerant of variations in the ramp rate 

and the commanded current level than higher speeds. So for an actual implementation, it would be 

important to switch over to field orientation control as soon as practical to guarantee good performance. 

Figures 6 and 7 also show ringing during the open loop start portion of the control and this is because 

the actual machine speed was used in the simulation to calculate the load torque (eq. (18)) and the 

machine power, respectively. 

Experimental Set-Up 

The experimental set-up consisted of a 4 pole PM machine, a Magtrol hysteresis dynamometer, an 

1103 dSpace Controller and a Semikron IGBT inverter switching at 20 kHz. The PM machine was 

designed in a “back-to-back” configuration—there are two stators and two rotor pieces mounted on the 

same shaft with a small steel flywheel in between. The machine has an inertia of approximately 

4.654 e-4 kg-m2

. The dynamometer has an inertia of 1.49 e-3 kg-m2. A picture of the set up is shown in 

figure 9 and the machine parameters are given in table 2. Only one stator of the PM machine was 

connected to the output of the Semikron inverter and the other stator was left open. The machine is rated 

for 60,000 rpm operation and the peak power at full speed is 9 kw. 
NASA/TM—2006-214034 10

TABLE 2.—MACHINE PARAMETERS USED IN EXPERIMENTAL WORK 

Machine Parameter Value 

J 4.65×10–4 kg-m2

Ls 110 HH 

Rs 0.06 Ω

λaf 0.0144 volt-sec 

Poles 4 

Rated torque 1.5 N-m 

Rated current 24 amps (rms) 

The machine used in the experiment was available from a previous program and was used only to 

demonstrate the feasibility of the starting procedure on a PM machine. It was not intended to be 

completely representative of a potential JIMO alternator machine. 

Dynamometer 

The Magtrol dynamometer is a hysteresis brake. The load torque is produced by energizing a 

magnetic pole structure on the stationary portion of the dynamometer. This causes a magnetic field in the 

air gap that restrains the dynamometer rotor from spinning. The magnetic pole structure is energized with 

a field coil. The more current in the field coil, the larger the load torque produced. The amount of current 

in the field coil can be controlled from the Magtrol controller. A certain amount of current is provided to 

the field coil for each setting from 0 to 100 percent of total available load torque. The dynamometer can 

provide load torque at all speeds, including zero. 

Figure 9.—Experimental demonstration hardware. 

Magtrol 

Dynamometer PM machine 

Semikron 

inverter 

dSpace 

control screen 

dSpace 

controller

Magtrol 

controller
NASA/TM—2006-214034 11

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

Dynamometer Setting, %

q ,

wrench measurment

inch-oz. (static)

current calculated

inch-oz (dynamic)

Figure 10.—Dynamometer load torque as a function of open loop setting. 

The load torque as a function of percentage setting was measured statically and dynamically as 

follows. For the static measurement, a torque wrench was used. The percentage settings were gradually 

increased to a peak value of 24 percent and the torque measurement recorded. The results are shown in 

figure 10. For the dynamic measurement, the machine was held to a constant speed of 2000 rpm by the 

dSpace motor controller. Then the dynamometer percentage settings were increased and the peak current 

necessary to hold 2000 rpm for each value of setting was noted. The dynamometer torque can be 

computed from the peak current measurement by assuming perfect field orientation control and from 

knowledge of the back EMF constant according to equation (11) where λaf is the back EMF constant, r

qs i 

is the magnitude of the phase current and P is the number of poles of the machine. 

r af qs i

4

3P

τ = λ (11)

Both torque calibration results are shown in figure 10. It can be seen that at the higher levels of 

percentage settings the dynamic measurement indicates a higher torque value than the static 

measurements. The dynamic measurement is actually measuring the torque produced by the PM machine 

and assuming that it equals the torque produced by the dynamometer. In reality, the machine torque is 

equal to the dynamometer torque plus any mechanical losses in the system. The difference between the 

static measurements and the dynamic measurements shown in figure 10 is most likely due to these losses. 

Experimental Results 

An open loop start with a transition to field orientation and then to closed loop speed control was 

demonstrated for four values of constant load torque corresponding to the percentage load settings on the 

dynamometer of 0, 12, 18, and 23 percent, respectively. Based on the measurements reported in figure 10, 

this corresponds to approximately 0, 10, 20, and 30 percent of rated torque for this machine. The 

controller parameters remained the same for each load value. From t = 0 to ~30 seconds, the machine was 

accelerating using a ramped frequency command with a ramp rate of 10 rpm/second. When the speed 

reached 300 rpm, the control switched from the open loop ramp to the field orientation (vector) control. 

The machine then accelerated more quickly under field orientation because the current in the stator 

windings of the machine was optimally placed with respect to the rotor magnetic field. When the machine 
NASA/TM—2006-214034 12

reached 2,000 rpm, the speed loop was activated and the speed was regulated to the set point of 

2,000 rpm. Once the speed loop was engaged, higher speeds could be achieved by changing the set point 

value. This is shown with a ramped speed command (100 rpm/sec) to 4,000 rpm for each case. The results 

are shown in figures 11 through 18. 

0 20 40 60 80 0

500

1000

1500

2000

2500

3000

3500

4000

4500

time, seconds

speed, RPM

change

commanded speed

closed loop

rotor speed

estimated

open loop commanded speed

transition to

vector control

transition to closed loop

speed control

speed command

Figure 11.—Speed of PM machine using sensorless 

starting with no dynamometer load torque. 

0 20 40 60 80

−20

−10

0

10

20

time, seconds

phase current, amps

Figure 12.—Phase current of PM machine during start 

sequence with no dynamometer load torque. 

(red: commanded magnitude, blue: measured) 
NASA/TM—2006-214034 13

0 20 40 60 80 0

500

1000

1500

2000

2500

3000

3500

4000

4500

time, seconds

speed, RPM

change

commanded speed

closed loop

rotor speed

estimated

open loop commanded speed

transition to

vector control

transition to closed loop

speed control

speed command

Figure 13.—Speed of PM machine using sensorless 

starting with ~10 percent of rated load torque. 

0 20 40 60 80

−20

−10

0

10

20

time, seconds

phase current, amps

Figure 14.—Phase current of PM machine during start 

sequence with ~10 percent of rated load torque. 

(red: commanded magnitude, blue: measured) 
NASA/TM—2006-214034 14

0 20 40 60 80 0

500

1000

1500

2000

2500

3000

3500

4000

4500

time, seconds

speed, RPM

change

commanded speed

closed loop

rotor speed

estimated

open loop commanded speed

transition to

vector control

transition to closed loop

speed control

speed command

Figure 15.—Speed of PM machine using sensorless starting 

with ~20 percent of rated load torque. 

0 20 40 60 80

−20

−10

0

10

20

time, seconds

phase current, amps

Figure 16.—Phase current of PM machine during start 

sequence with ~20 percent of rated load torque. 

(red: commanded magnitude, blue: measured) 
NASA/TM—2006-214034 15

0 20 40 60 80 0

500

1000

1500

2000

2500

3000

3500

4000

4500

time, seconds

speed, RPM

change

commanded speed

closed loop

rotor speed

estimated

open loop commanded speed

transition to

vector control

transition to closed loop

speed control

speed command

Figure 17.—Speed of PM machine using sensorless 

starting with ~30 percent of rated load torque. 

0 20 40 60 80

−20

−10

0

10

20

time, seconds

phase current, amps

Figure 18.—Phase current of PM machine during start 

sequence with ~30 percent of rated load torque. 

(red: commanded magnitude, blue: measured) 

The various controller gains were set with the maximum load torque applied (30% of rated). The 

gains included the current regulator gains, the speed observer gains (see fig. 4) and the speed regulator 

gains. In addition, the commanded values of the open loop ramp rate and the current value were selected. 

In general, setting the gains appropriately for any controller is a combination of experience, analysis and 

testing and this was no exception. A few remarks can be made here about the settings but when the final 

JIMO alternator hardware is designed and built, the controller will have to be appropriately tuned during 

testing. 

The current regulator was tuned for a lower bandwidth than is typical for a field orientation 

application. Typically, a 1 to 2 kHz bandwidth is achievable for field orientation but during open loop 

starting, a fast response could cause the machine to lose synchronism as it was accelerating. So the 
NASA/TM—2006-214034 16

bandwidth was reduced to the 200 to 500 Hz range. The speed observer bandwidth must be fast enough to 

accommodate any necessary acceleration rates. This was particularly noticeable in simulation (where high 

acceleration rates were used) but experimentally a bandwidth of ~100 Hz worked well. Both the current 

regulator and the speed observer gains worked well for all values of load torques. 

The other settings: open loop ramp rate, commanded current value and speed regulator gains, did 

make a difference on performance as the load torques were changed. In particular, for a lighter load 

torque, the ramp rate could be increased and the commanded current value could be decreased. For 

example, as the load was increased, the machine would not start in open loop with a commanded current 

value that was too low. Additionally, if the ramp rate was set too high for a particular load, the machine 

would lose synchronism as it started to accelerate. In general, heavier load torques required a slower ramp 

rate and a higher commanded current value than lighter load torques. 

The speed oscillations seen in figures 11, 13, 15, and 17 occurred at the transition when the speed 

loop was closed (at 2000 rpm) and were heavily affected by the speed regulator gains. All of the gains 

were tuned for the 30 percent load case and then those gains were used for the other cases. The 30 percent 

loading was selected because that was similar to what has been observed experimentally with the 2 kW 

Brayton unit (ref. 1). For the 0 percent load case (fig. 11) the machine accelerates very quickly once field 

orientation (at t ≈ 30 seconds) is achieved due to the high current command. The high acceleration rate of 

the machine in conjunction with the speed regulator gains causes it to overshoot the 2000 rpm set point 

when the speed loop is closed and then oscillate back to the commanded value. As the load is increased, 

the response improves (the magnitude of the oscillations decreases) because the system gains were set 

using the 30 percent load case. 

It can be seen that the PM machine will start under all four conditions even though it was only tuned 

for the highest load torque case. This is an encouraging result because it shows that the foil bearing drag 

does not have to be predicted exactly in order to tune the controller to start the machine. The controller 

can be tuned for the expected bearing drag torque during testing but it is robust enough to operate with a 

different load torque. It also should be noted that the actual speed was not measured. The speed results 

shown on the plots are the estimated ones. In the future the actual speed should be measured to see if 

there is ringing in the speed during open loop starting as observed in the simulations. 

Conclusions 

A permanent magnet machine would be a likely candidate for the alternator for a JIMO-type mission. 

This machine could also operate as a motor during start up to begin to circulate the helium-xenon gas of 

the Brayton cycle. This work has shown in simulation and experimentally that position- and speedsensorless starting of a permanent magnet machine under varying load torques is feasible. Experimentally 

it was shown that the machine could start with one set of gains in the controller over a range of load 

torques from 0 to 30 percent of rated. This is important because it demonstrates the robustness of the 

control in the presence of significant load variations. 

In the future, more tests should be conducted with a load torque profile during start up that mimics the 

drag that would expected due to the foil bearings. Additionally, the speed should be measured and 

compared to the estimated value. Finally, the controller should be applied to the demonstration 2 kW 

Brayton unit that has a PM alternator and foil bearings to demonstrate the feasibility on a more realistic 

experimental model. 

References 

1. Hervol, David, Mason, Lee, Birchenough, Art, and Pinero, Luis; “Experimental Investigations from 

the Operation of a 2 kW Brayton Power Conversion Unit and Xenon Ion Thruster,” NASA TM—

2004-212960, Proceedings of the Space Technology and Applications International Forum (STAIF2004), Albuquerque, NM, Feb. 8–12, 2004. 
NASA/TM—2006-214034 17

2. Vas, Peter; Sensorless Vector and Direct Torque Control, Oxford University Press, New York, NY, 

1998. 

3. Patel, N., T. O'Meara, J. Nagashima, and R. Lorenz, “Encoderless IPM Traction Drive for 

EV/HEV's,” Conference Record of the 2001 IEEE Industry Applications Conference, Chicago, IL. 

4. Wu, Rusong and Slemon, Gordon; “A Permanent Magnet Motor Drive Without a Shaft Sensor,” 

IEEE Transactions on Industry Applications, Vol. 27, No. 5, September/October 1991, pp. 1005–

1011. 

5. Matsui, Nobuyuki; “Sensorless PM Brushless DC Motor Drives,” IEEE Transactions on Industrial 

Electronics, Vol. 43, No. 2, April, 1996, pp. 300–308. 

6. Jansen, Patrick and Robert D. Lorenz, “Transducerless Position and Velocity Estimation in Induction 

and Salient AC Machines,” IEEE Transactions on Industry Applications, Vol. 31, No. 2, March/April 

1995 pp. 240–247. 

7. Schroedl M., “Sensorless control of AC machines at low speed and standstill based on the 

“INFORM” method,” Conference Record of the Thirty-First IEEE Industry Applications Society 

Annual Meeting, October 6–10, 1996, Vol. 1, pp. 270–277. 

8. Krishnan, Ramu, Permanent Magnet Synchronous and Brushless DC Motor Drives: Theory, 

Operation, Performance, Modeling, Simulation, Analysis and Design, Virginia Tech., Blacksburg, 

Virginia, 1999. 

9. Krause, Paul, Analysis of Electric Machinery, McGraw-Hill Book Company, New York, New York, 

NY, 1986. 

10. Novotny, Donald; Lipo, Thomas; Vector Control and Dynamics of AC Drives, Oxford University 

Press, New York, NY, 1996. 

11. Mohan, Ned; Advanced Electric Drives, MNPERE, Minneapolis, MN, 2001. 

12. Rowan, T. and R. Kerkman, “A New Synchronous Current Regulator and an Analysis of CurrentRegulated PWM Inverters,” IEEE Transactions on Industry Applications, Vol. IA-22, No. 4, 

July/August 1986, pp. 678–690. 

13. Holtz, Joachim, “Pulsewidth Modulation for Electronic Power Conversion,” Proceedings of the 

IEEE, Vol. 82, No. 8, August, 1994, pp. 1194–1214. 

14. DellaCorte, C.; Lukaszewicz, V.; Valco, M.J.; Radil, K.C.; Heshmat, H.; “Performance and 

Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery,” NASA/TM—2000-

209187/Rev. 1, Proceedings of the Society of Tribologists and Lubrication Engineers Annual 

Meeting, Nashville, TN., May 7–11, 2000. 

15. Lorenz, Robert D., K.W. Van Patten, “High-resolution velocity estimation for all-digital, AC servo 

drives” IEEE Transactions on Industry Applications, Vol. 27, No. 4, July/August 1991, pp. 701–705. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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NASA TM—2006-214034

E–15391

WBS–22–982–10–50

23

Feasibility Study of Jupiter Icy Moons Orbiter Permanent Magnet Alternator

Start Sequence

Barbara H. Kenny and Roger P. Tokars

Permanent magnet machine; Sensorless control; Alternator; Sensorless starting

Unclassified -Unlimited

Subject Categories: 44 and 20

Barbara H. Kenny, e-mail: Barbara.H.Kenny@nasa.gov, NASA Glenn Research Center; and Roger P. Tokars, e-mail:

RTokars@purdue.edu, Purdue University, Electrical and Computer Engineering Department, 1 Purdue University,

West Lafayette, Indiana 47907. Responsible person, Barbara H. Kenny, organization code RPE, 216–433–6289.

The Jupiter Icy Moons Orbiter (JIMO) mission was a proposed, (recently cancelled) long duration science mission to

study three moons of Jupiter: Callisto, Ganymede, and Europa. One design of the JIMO spacecraft used a nuclear heat

source in conjunction with a Brayton rotating machine to generate electrical power for the electric thrusters and the

spacecraft bus. The basic operation of the closed cycle Brayton system was as follows. The working fluid, a heliumxenon gas mixture, first entered a compressor, then went through a recuperator and hot-side heat exchanger, then

expanded across a turbine that drove an alternator, then entered the cold-side of the recuperator and heat exchanger and

finally returned to the compressor. The spacecraft was to be launched with the Brayton system off-line and the nuclear

reactor shut down. Once the system was started, the helium-xenon gas would be circulated into the heat exchangers as

the nuclear reactors were activated. Initially, the alternator unit would operate as a motor so as to drive the turbine and

compressor to get the cycle started. This report investigated the feasibility of the start up sequence of a permanent

magnet (PM) machine, similar in operation to the alternator unit, without any position or speed feedback sensors

("sensorless") and with a variable load torque. It is found that the permanent magnet machine can start with sensorless

control and a load torque of up to 30 percent of the rated value.