Arthur G. Birchenough

Glenn Research Center, Cleveland, Ohio

The 2kW Mini-BRU Electrical Controls

Concept and Transient Performance

NASA/TM—2006-214043

January 2006
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Arthur G. Birchenough

Glenn Research Center, Cleveland, Ohio

The 2kW Mini-BRU Electrical Controls

Concept and Transient Performance

NASA/TM—2006-214043

January 2006

National Aeronautics and

Space Administration

Glenn Research Center
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

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Available electronically at http://gltrs.grc.nasa.gov
The 2 kW Mini-BRU Electrical Controls 

Concept and Transient Performance 

Arthur G. Birchenough 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The proposed Jupiter Icy Moon Orbiter, JIMO, mission selected a Brayton power conversion system 

as its electrical power generator. Although the JIMO mission power conversion system was expected to 

produce in the order of 100 kW, an available 2 kW Brayton system was used to develop control system 

strategies for the JIMO mission. This report describes the shunt loading voltage/speed regulation control 

concept developed for the 2 kW system, and the transient performance of controls. The 2 kW alternator is 

a permanent magnet alternator as proposed for the JIMO mission, and operates at a similar speed and 

internal impedance, allowing it to be used as an accurate model for performance of the larger system. The 

JIMO mission was cancelled in September 2005. 

Introduction 

The Glenn Research Center has been involved in the development of Brayton cycle power conversion 

systems since the 1960s. A number of control systems have been developed over the years as the program 

has evolved. As the JIMO mission requirements were identified the regulation limits and transient 

response of the power distribution system became important parameters in the design of the spacecraft. In 

order to demonstrate the performance capabilities of a proposed Brayton power conversion system and 

controls, a voltage/speed regulation concept was developed for the 2kW system, and tested to verify its 

performance. The performance data was then used to produce a draft power quality specification for the 

mission. 

The 2 kW Mini-BRU hardware available consists of a complete hot loop consisting of flight quality 

hardware with an electrically heated heat source simulator and an external chiller simulating the heat 

rejection radiator, operating in a vacuum environment, and an identical alternator powered by an air 

turbine. The air turbine-powered alternator was used for the development and performance testing of this 

control concept, while the hot loop system was used with a similar control system for thermodynamic 

performance testing. The parameters of the 2kW alternator closely model the parameters of the proposed 

100 kW alternators of the JIMO system. Both included permanent magnet alternators exhibiting similar 

operating speed, scaled impedance, and scaled inertia parameters. 

Recent Control System Development for the 2 kW Mini-BRU 

The current controls concept, known herein as BRU-J, is the third generation of controls for the MiniBRU since the JIMO project was initiated in fiscal year 2003. The first generation, referred to as the 

“120 Vdc BRU,” was developed to provide a DC power output bus at 120 Vdc. The concept used a 

3 phase rectifier directly off the alternator output, producing 95 to 120 Vdc. The high efficiency Series 

Connected Boost Regulator (SCBR, ref. 1) was used to provide a well regulated 120 Vdc bus for the user 

loads and the parasitic load. The parasitic load was controlled to provide constant alternator speed, with 

an overvoltage limit to reduce speed under light load conditions. This system was developed using an air 

powered Alternator Test Unit (ATU) in a laboratory environment and then operated with a complete 

NASA/TM—2006-214043 1
flight-qualified hot loop Brayton turbine alternator compressor assembly. The hot loop testing was 

documented in NASA/TM—2003–211999 (ref. 2). 

The second generation of controls, referred to as the HV BRU, was based on the first generation, but 

modified to provide an AC power bus to the user load intended to interface with an ion engine. The HV 

BRU concept used a similar rectifier and parasitic load on the rectified output of the machine, with 

identical alternator speed and overvoltage limit controls as the 120 Vdc BRU, although the SCBR was not 

used as there was no need for accurately regulated voltage input to the parasitic load. The ac output of the 

alternator was used as the user power output. The high voltage required for the ion engine, ~1100 Vdc, 

was obtained using a transformer to step up the voltage. The transformer had three 3-phase secondaries, 

providing rectified outputs of approximately 600, 400, and 200 Vdc. The three rectified outputs were 

connected in series, with a regulator on the output of the 200 V segment, to provide a regulated 1100 Vdc 

to the ion engine. This system was also operated with the hot loop Brayton engine after development on 

the ATU, and was used in an integrated test providing the beam power for an operating ion thruster. 

These operations demonstrated several concepts: 

• Supplying direct ac power to a load 

• Use of a moderately high frequency, 866 Hz, and therefore light weight transformer to obtain the 

required high voltage without the complexity of dc-dc converters 

• Using a series connection of low voltage rectifier stages to obtain a high voltage without the use 

of very high voltage components 

• Regulation of the high voltage output by controlling only one element of the series connected 

rectifiers 

• Operation with an actual ion engine 

The tests also demonstrated the system stability and robustness during ion engine recycle events, as 

documented in NASA/TM—2004–212960 (ref. 3). 

The third generation of controls, BRU-J, is being developed as an upgrade to the second generation 

system to recognize the need for higher power quality and to test redundant/fault tolerant control 

techniques. The major changes are an improved ac waveform through higher order rectification, more 

accurate bus voltage regulation and increased parallel modularity in the controls for fault tolerance. 

BRU-J Description 

A block diagram of the BRU-J control concept and test bed is shown in figure 1. 

A description of the blocks is as follows: 

Alternator 

The alternator was a 52000 rpm 2 kW nominal unit. The nominal output voltage is 55 V rms L-L, 

866 Hz. The impedance is 260 μ Henries L-N, which is 1.5 Ω reactance at the operating frequency. The 

winding resistance is 160 mΩ L-N, or ~10 percent of the machine inductive reactance. This impedance is 

approximately 1/3 per unit, or a short circuit current of 3 per unit. The nominal load impedance is 4 Ω

L-N. The inertia is such that the unit accelerates approximately 100 percent per second at rated torque. 

Although the frequency and voltage are only about 1/2 and 1/5 the initial JIMO baseline, the 

impedance and per unit impedance remained close to the JIMO parameters at the end of the project. The 

JIMO inertia, per unit, is expected to be greater due to the larger machine rating. A higher inertia is 

expected to have no effect on the transient data shown herein except for figures 8 and 9, where the speed 

changes would be expected to occur more slowly. 

NASA/TM—2006-214043 2
Boom Cable 

The cable in the lab that emulates the initial JIMO boom cable concept is approximately 50 ft of 10–4 

rubber jacketed cable. The parameters have not been measured since it appears as additional alternator 

impedance and is swamped by the magnitude of the alternator reactance.

Bus

The distribution bus was essentially a point on a terminal strip. 

Power Distribution 

The power distribution consisted of a single 3 phase contactor feeding a load, in addition to the feed 

to the Parasitic Load. 

The load switching contained a high speed switch developed to disconnect the ion thruster during our 

integrated testing. It was a FET-based switch which switches on in a couple microseconds, and off in 

about 20 Hs (due to charging of a snubber). The high speed switch was used for all load transients 

experienced in this study. 

Loads 

The loads used for testing consisted of a 12-pulse transformer rectifier set feeding resistive loads. The 

load value was varied in steps up to about 1500 W, or 3/4 rated load. The current transient into the load, 

including the transformer rectifier, usually showed no significant transient due to core saturation. There is 

no filter after the rectifier, except as noted for specific tests shown in figures 4 and 5, so there was no 

filter charging transient. 

Parasitic Load 

The parasitic or shunt load and its controls were the dominant factor in the response of the system. 

The control system parameters were effective in the frequency range from steady state up through several 

kilohertz. Load transients may have contained transition times in the microsecond range, and these 

transients were controlled by the capacitance on the output of the PLR rectifier. 

The cabling for the PLR consisted of several feet of twisted wire and interconnect impedances and 

transformer leakage reactance’s were judged representative of a full scale system. Voltage transients in 

this frequency range were not apparent in the data. 

The block diagram shows a 24 pulse rectifier for the PLR. In actuality the BRU-J system used two 

12 pulse rectifiers, phase shifted 15° to give a 24 pulse effect. Each 12 pulse rectifier in turn consisted of 

two 6 pulse rectifier sets paralleled thru interphase reactors. While the full scale system would use 

multiple transformer rectifier sets for redundancy, the response of the BRU-J system should be 

representative. 

The block diagram shows a filter of 120 μFs. On a load dump the excess energy has to charge this 

capacitor up to raise the line voltage. The 120 μF capacitor would slow the charge rate to about 

0.1 percent overshoot per microsecond for a 100 percent load dump. This is a major factor in how fast the 

control loop needs to respond. The value of this capacitor could be increased to reduce the rate of 

overshoot, but the data in this document and the JIMO mission voltage specification were developed 

based on this value. In the BRU-J configuration the 120 μF capacitor is distributed as 12 separate 10 μF 

capacitors, one for each of the 12 segments of the PLR, identical to the current concept for the full scale 

system. 

The power switch was an FET operating in a PWM mode. Although there was a slight filter used for 

EMI it was ineffective in the frequency range of the control loop. 12 individual switches were used for the 

12 segments of the BRU-J implementation. 

The PLR itself would be sized to load the machine 100 percent, plus additional margin to overload 

the machine for shutdown and failure scenarios. The 4 Ω (equivalent) load was capable of nearly 

NASA/TM—2006-214043 3
80 percent overload, somewhat greater than required, but it was easily implemented as 12 to 50 Ω

resistors and not important to the dynamic response during load transients. 

The 12 phase modulator is an area where modeling this control system as a linear lumped single PLR 

element might not be sufficiently accurate for some analyses. The BRU-J test model had 6 separate 

modulation signals, each controlling 2 elements. Each of these elements is powered from a 12 pulse 

rectifier output, but the second rectifier’s phase is shifted 15° to maintain balanced 24 pulse loading. The 

6 signals are time sequenced such that the 6 signals, each at a 1733 Hz (twice alternator frequency) rate, 

essentially appear as a 10 KHz (actually 10.4 KHz) modulation rate. The segmenting and modulation 

scheme implemented are representative of what might be expected to be used in the JIMO mission. 

Control Loop 

Voltage sensing: The voltage sensing consisted of squaring each phase of the alternator output and 

adding the instantaneous value of the squares. The result was a dc level and an unfiltered output which 

contained (assuming a balanced system) no components below approximately 10 KHz, the 6th harmonic, 

and no time delay. It was effectively the rms value of the total three phase waveform, without the need for 

filtering at the fundamental frequency. The measurement did not implement the square root function on a 

true rms measurement, but over the voltage narrow range of the regulation band the lack of the square 

root function only appeared as a gain change, not as a significant non-linearity. 

Voltage regulator: The regulation loop compared the measured output to the setpoint, and integrated 

the error, a simple integral control loop. However, current feedforward from sensing the load current was 

added to the integrator output. The gain of the feedforward loop is important in how much control effect 

the integral loop has to exert, but not critical to the system stability. For these tests the gain was 

intentionally reduced approximately 10 percent to generate an easily viewable error at the integrator 

output to check the system response. 

BRU-J Performance 

The focus of this report is a series of load step transient tests that were performed on the BRU-J test 

bed. The test conditions and an explanation of steady state system responses are as follows: 

The alternator controls were configured to operate in a constant voltage (all data except fig. 9) or 

constant speed mode. Within the load capacity of the system, in steady state operation, the voltage or 

speed regulation was pretty much within the ”noise” in the system, on the order of 0.1 percent. Thermalor time-induced drifts were not studied, and as these would generally be eliminated by commands from 

the spacecraft computer, will not be reported further herein. 

The air turbine power BRU was operating at nominally 1200 to 1500 W and the load step was from 

0 to 1000 W user load, or 1000 to 0 W user load. The user load was a transformer rectifier set connected 

to a resistive load, representative of the expected JIMO loads. JIMO loads will typically also contain 

some filter capacitance, but these data, except for figures 4 and 5, are without any capacitance to allow 

clear identification of the load transient. 

The user load was configured as a 12 pulse rectifier, and the parasitic load was operated in a 24 pulse 

configuration. Therefore the power factor at the alternator terminals changed as a function of the loading, 

from 0.995 at no user load to 0.98 with 1 kW user load. Since the waveforms were not pure sinusoids, the 

power factor was defined as the watts divided by the volt-amps. 

The reduction in power factor due to the user load caused the steady state alternator speed to increase 

approximately 2 percent to provide a constant voltage, or alternatively the alternator voltage decreases 

approximately 2.5 percent if the system was operating in the constant speed mode. These effects are not 

visible in the data showing millisecond time spans, but are shown in figures 8 and 9. All data shown in 

this document was taken in the constant voltage mode except for the data in figure 9. 

NASA/TM—2006-214043 4
The load switching was performed with a solid state switch that switched all three phases 

simultaneously. The switch turned on in a few microseconds, and turned off in approximately 20 μsec due 

to snubber capacitance. 

Figures 2 and 3 show the instantaneous output voltage transient for a load step and load removal, 

respectively. All three phase voltages were shown, and the instantaneous current was in one phase. 

The most obvious effect of the load switching was the change in the voltage waveform since the PLR 

was based on a 24 pulse rectifier and the load was a 12 pulse system. Neither the load step nor removal 

caused a significant notch in the output voltage waveform. Figures 4 and 5 showed the effect of using a 

capacitive input filter on the output of the rectifier. The effect of the load changes on the voltage 

regulation loop is shown in figures 6 and 7. 

The filter for transients in figures 4 and 5 consisted of a series inductor of 40 μH and 1 μF for figure 4 

(20 μF for figure 5) across the load resistance of 250 Ω. In terms of impedance at the ripple frequency of 

10 KHz (12 times the 866 Hz fundamental), the inductive reactance was 24 Ω, the reactance of the 1 μF 

capacitor was 15 Ω. The load current showed a 150 percent inrush spike for the filter with the small 

capacitance, and approximately 3 per unit rated-current (2 kW base) with the large capacitance. Although 

the bus voltage showed significant notches due to these inrushes, the system recovered in a couple of 

cycles. The load current is shown as the instantaneous Phase A current in figure 4, but as a voltage 

representing the rectified sum of all three currents in figure 5. 

The response of the voltage regulator to the resistive load transients is shown in figures 6 and 7. The 

data is plotted as the direct scope readings of signals in the regulator, and require some explanation. The 

“Sum of Phase Voltages Squared” is essentially a root mean square (rms) signal except that the square 

root function has not been performed. For small signal changes as shown in the data, this results in an 

expanded scale so that an apparent 10 percent change is really only a 5 percent change in the rms voltage. 

Also, there is essentially no filtering in the calculation so high frequency data is not filtered out. The 

“Sum of Phase Currents” is similarly the signal voltage from the three-phase rectification of current 

transformers on the load current. This signal is used for the feed forward compensation in the regulator. 

The “Integrator Error” trace is the output of the integral stage of the control loop, and is shown to 

illustrate the duration of the transient more clearly than trying to interpret small changes in the output 

voltage signal. 

The final two traces show the longer term response to load transients in the constant voltage and 

constant rpm modes. In figure 8 the alternator speed varied approximately 2 percent in response to the 

load changes. The cause of the change is the reduced power factor of the 12 pulse load compared to the 

power factor of the 24 pulse PLR. In order to maintain output voltage with the reduced power factor, the 

voltage regulator decreased the total load on the machine, allowing the alternator speed, and therefore 

internal alternator voltage, to increase. Figure 9 shows the corresponding response in the constant rpm 

mode. The initial transient response is identical between these modes since high speed inner voltage loop 

dominates the initial response. 

Conclusions 

A 2 kW Brayton cycle alternator was constructed as a prototype development test bed for the 

proposed JIMO mission. The alternator characteristics were similar enough to the expected JIMO 

machine characteristics that performance of the 2 kW test bed could have been representative of actual 

JIMO performance. Load transient data taken on this test bed, used to specify the draft JIMO Power 

Quality Specification, showed significantly faster response than typical motor generator sets using field 

control and source throttling. 

NASA/TM—2006-214043 5
References 

1. Button, R.M., “An Advanced Photovoltaic Array Regulator Module,” NASA Technical 

Memorandum 107304, NASA Lewis Research Center, Cleveland, OH, August 1996. 

2. Hervol, D., Mason, L., and Birchenough, A., Glenn Research Center, Cleveland, OH, “Experimental 

Results From a 2 kW Brayton Power Conversion Unit,” NASA/TM—2003-211999, January 2003. 

3. Hervol, D., Mason, L., Birchenough, A., and Pinero, L., “Experimental Investigations From the 

Operation of a 2 kW Brayton Power Conversion Unit and a Xenon Ion Thruster,” NASA/TM—2004-

212960, February 2004. 

 

Figure 1.—BRU-J 2 kW block diagram. 

NASA/TM—2006-214043 6
-150

-100

-50

0

50

100

150

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50

Time (msec)

Phase Voltage

-15

-10

-5

0

5

10

15

Amps

Phase A 

Current

Phase C 

Volts

Phase B 

Volts

Phase A 

Volts

Figure 2.—Load step. 

-150

-100

-50

0

50

100

150

-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25

Time (msec)

Phase Voltage

-15

-10

-5

0

5

10

15

Amps

Current

Phase C 

Volts

Phase B 

Volts

Phase A 

Volts

Figure 3.—Load removal. 

NASA/TM—2006-214043 7
-150

-100

-50

0

50

100

150

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50

Time (msec)

Phase Voltage

-25

-20

-15

-10

-5

0

5

10

15

20

25

Phase A 

Current

Phase A 

Volts

Phase B 

Volts

Phase C 

Volts

Figure 4.—Load step (with 1 μF capacitance). 

-100

-80

-60

-40

-20

0

20

40

60

80

100

-0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00

Time (msec)

Volts

0

2

4

6

8

10

12

Load Current (no units)

Load 

Current

Phase C 

Voltage

Phase B 

Voltage

Phase A 

Voltage

Figure 5.—Load step (with 20 μF capacitance). 

NASA/TM—2006-214043 8
0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

-1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Time (msec)

Volts

Sum of Phase Voltages 

Squared

Sum of Phase 

Currents

Integrator Error

Figure 6.—Load step. 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

-2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00

Time (msec)

Volts

Sum of Phase Voltages 

Squared

Sum of Phase 

Currents

Integrator Error

Figure 7.—Load removal. 

NASA/TM—2006-214043 9
0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

Seconds

Volts

51500

51700

51900

52100

52300

52500

52700

52900

53100

53300

53500

Sum of the Phase 

Voltages Squared

Alternator 

RPM

Sum of Phase

Currents

Figure 8.—Constant voltage mode. 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Seconds

Volts

49000

49200

49400

49600

49800

50000

50200

50400

50600

50800

51000

RPM

Load Current

Alt Speed

Output Voltage 

(Sum of Squares)

Voltage Trim

Figure 9.—Constant rpm mode. 

NASA/TM—2006-214043 10
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16

The 2kW Mini-BRU Electrical Controls Concept and Transient Performance

Arthur G. Birchenough

Brayton cycle; Alternator controllers; Parasitic loading; Voltage regulation

Unclassified -Unlimited

Subject Category: 33

Responsible person, Arthur G. Birchenough, organization code RPE, 216–433–6331.

The proposed Jupiter Icy Moon Orbiter, JIMO, mission selected a Brayton power conversion system as its electrical

power generator. Although the JIMO mission power conversion system was expected to produce in the order of 100 kW,

an available 2 kW Brayton system was used to develop control system strategies for the JIMO mission. This report

describes the shunt loading voltage/speed regulation control concept developed for the 2 kW system, and the transient

performance of controls. The 2 kW alternator is a permanent magnet alternator as proposed for the JIMO mission, and

operates at a similar speed and internal impedance, allowing it to be used as an accurate model for performance of the

larger system. The JIMO mission was cancelled in September 2005.