Motor starting techniques for 2-15 kW Brayton space power syste

Cantoni, D. A.

Curreri, J. S.

Gilbert, L. J.

English

NASA Technical Reports Server

by %. Y. Gilbert, J. S. 
gy Conversion Enaneesfng Conference 
ugust 3-6, 197% 
https://ntrs.nasa.gov/search.jsp?R=19710015215 2020-03-23T17:12:46+00:00Z
ABSTRACT 
The 2-to-15 kilowatt Brayton space power 
system current ly  under going development and 
tes t ing  a t  Lewis Research Center has been e f f ec t -  
ively s t a r t ed  by motoring the ro ta t ing  uni t .  The 
ro ta t ing  uni t  i s  comprised of a radial-flow tur- 
bine which drives a radial-flow compressor and a 
Lundell-type a l te rna tor .  This Brayton power sys- 
tem generates 120/208volts, 1200 her tz  e l e c t r i c a l  
power a t  a rated speed of 36,000 rpm. The system 
i s  designed f o r  a turbine i n l e t  temperature of 
1600' F (1145 K) and can be operated with a var i -  1 
e ty  of heat sources such as a nuclear reactor  o r  
radioisotope. 
hertz e l e c t r i c a l  power t o  the  a l te rna tor  termi- 
nals  at  s t ands t i l l ,  the ro ta t ing  un i t  can be 
motored t o  a speed of 12,000 rpm i n  about 20 sec- 
onds. 
power, t he  system self-accelerates  t o  i t s  ra ted  
speed, provided tha t  t he  turbine inlet tempera- 
* By applying 20 -volts linell_t_o-neutral, $0 
Upon removal of t$e input e l e c t r i c a l  
ture  _.._ i s  875O . F - ___ (741 .K) or higher. _ -  
r4 
THE BRAYTON POWER SYSTEM (1)" is a closed-loop 
Brayton-cycle engine designed fo r  space applica- 
t ions.  This engine was designed t o  del iver  2 t o  
15 kW of e l e c t r i c a l  power continuously over a t  
l e a s t  a flve-year l i f e  with shutdown and restart 
capabi l i ty  ( f ig .  1). 
The Brayton-cycle technology program at  
Lewis Research Center began in  1963. A t  the  pre- 
sent time extensive ground t e s t s  of a l l  major 
components, subsystems, and the complete engine 
have been o r  are  being conducted ( 2 ) .  This t e s t -  
ing includes invest igat ions of means and methods 
of s t a r t i ng  the engine conveniently. The results 
of motor s t a r t i ng  t e s t ing  on a complete Brayton 
gas loop t e s t  system are  reported here. - The Brayton power system can be s ta r ted-  
t h a t  i s ,  brought t o  a self-sustaining operating 
condition-by e i t h e r  of two methods: 
j ec t ion  o r  (b)  motoring of the a l te rna tor .  
in jec t ing  gas ' ( the  working gas, a mixture of 
helium and xenon) i n t o  a p a r t i a l l y  evacuated 100s 
and ra i s ing  the  turbine inlet  temperature t o  
330° F (440 K),  the . turbine can be driven t o  
bring the system t o  a self-sustaining operating 
condition. 
of the Brayton power system are  described i n  
Ref. 3. 
A second method of obtaining self-sustaining 
operation i s  t o  use the a l te rna tor  as a motor. 
In t h i s  operation the motor spins t h e  compressor, 
c i rcu la t ing  the  working gas which i s  heated by ' 
t he  heat source. When suf f ic ien t  pressure i s  de- 
veloped by the compressor, and suf f ic ien t  heat i s  
absorbed from the heat source t o  provide enough' 
energy t o  the  turbine t o  dr ive the  compressor and 
overcome windage losses ,  the operation becomes 
self-sustaining. 
Mdtor s t a r t i n g  permits a reduction i n  s i z e ,  
and complexity of the gas management system. 
BRAYTON POWER SYSTEM 
(a)  gas in- 
By 
Results of t e s t ing  in jec t ion  s t a r t i ng  
\ 
The core of t h i s  power system ( f ig .  1) i s  
the Brayton ro t a t ing  uni t  (BRU) shown i n  Fig. 2. 
It consis ts  of a gas turbine, an a l te rna tor ,  and 
a compressor mounted on a single shaf t ,  The 
shaf t  i s  supported by pivoted three-pad gas jour- 
n a l  bearings located j u s t  outboard of the a l t e r -  
nator, and a gimbal-mounted douhle-actin: step- 
sector  gas thrus t  bearing a t  the compressor end 
of the shaft. Star tup and shutdown of the unit 
are  accomplished while the  bearings and hydro- 
"Numbers' i n  parentheses designate Refer- ' 
ences at  end. of paper. 
1 
s t a t i c a l l y  supported by jacking gas external ly  
supplied t o  the bearings a t  100 t o  150 p s i a  (70 
t o  100 N/cm2). For normal design-speed operation, 
the jacking gas i s  turned off  and the bearings, 
both thrus t  and journal, operate completely hy- 
dro dynami c a l l y  . 
Major subsystems ( f i g .  3 )  of the Brayton 
power system are  the e l e c t r i c a l  subsystem and the 
heat re ject ion subsystem. The e l e c t r i c a l  subsys- 
tem regulates and d is t r ibu tes  the generated elec- 
t r i c a l  power as  well  a s  providing a l l  control  and 
log ic  functions required t o  operate the system. 
The heat re jec t ion  subsystem removes waste heat 
and r e j e c t s  it t o  space from a radiator .  A 
400 her tz  motor-pump assembly c i rcu la tes  l i q u i d  
coulant (Dow Corning 200) through the system. 
The coolant i s  pumped through the a l te rna tor  and 
through a s e r i e s  of  four  cold p la tes  on which the 
e l e c t r i c a l  subsystem components are mounted. 
i t y  of operating the  a l t e r n a t o r  of the Brayton 
power system as a mot,or was demonstrated i n  
earlier t e s t i n g  described i n  R e f .  4. The Brayton 
a l te rna tor  i s  a brushless s ta t ionary-coi l  soi id-  
ro tor  modified-Lundell machine. In  order t o  de- 
termine i t s  charac te r i s t ics  as a motor, the al- 
te rna tor  w a s  t es ted  with a var ie ty  of applied 
voltages. The no-load armature current and tor -  
que charac te r i s t ics  determined f o r  motoring vol t -  
ages of 20 and 30 v o l t s  ( l ine- to-neutral) ,  400 
hertz  were within bounds t h a t  warranted t h e i r  use 
as s t a r t i n g  voltages f o r  motor s t a r t i n g  the 
closed loop system. These values are  indicated ' 
i n  Figs. 4 and 5. 
Motor s t a r t i n g  the Brayton power system was 
f irst  demonstrated i n  a closed rrhotrr gas loop i n  
a t e s t  r i g  es tabl ished fo r  evaluation of the BRU 
only. 'In t h a t  i n s t a l l a t i o n  the BRU w a s  the only 
f l ight- type engine hardware under t e s t .  The ' , 
t e s t s  were intended t o  exercise the u n i t  over i t s  
complete operating range, determine any opera- 
t i o n a l  l imitat ions,  and evaluate the behavior of 
the BRU during s ta r tup  and shutdown. 
brought t o  self-sustaining system operation. 
This operating condition w a s  a t ta ined  by motoring 
the BRU t o  6600 rpm with turbine i n l e t  tempera- 
tu res  above 980° F (800 K) .  The t e s t s  are  f u l l y  
described i n  Ref. 5. 
A s  a r e s u l t  of these t e s t s ,  a s e t  of l i m i t -  
ing conditions for  self-sustaining operation for 
given speed and turbirie i n l e t  temperature a t  a 
constant compressor i n l e t  temperature (80' F) 
(300 K) was established for the  BRU in  t h i s  t e s t  
loop. The function representing these conditions 
I ' 
SYSTEM STARTING EXPERIENCE - The pract ical-  
In s ta r tup  t e s t s  the BRU was motored and 
is  i l l u s t r a t e d  i n  Fig. 6. The so l id  r i g h t  h a l f  of 
the curve represents combinations- of temperature 
and speed which were s table  operating points for 
the loop under t e s t .  A t  any speed and tempera- 
tu re  combination above 15,000 rpm and 680' F 
(633 K) the system reverted t o  conditions ind i -  
cated by the curve. A t  speeds below 15,000 rpm 
the  system accelerated or  decelerated depending 
upon whether temperature and speed conditions 
were represented by a point above or below the 
l e f t  (or  dashed) side of the operating l i n e .  
From a consideration of t h i s  operating l ine ,  an 
accelerat ion t o  design speed i s  predictable i f  
the BRU i s  spun t o  a speed of 12,000 rpm a t  tem- 
peratures above 800' F (700 K). 
TEST OBJECTIVE - The alternator-motoring 
s t a r t i n g  described i n  Ref. 5 was  performed by 
the engine. 
a s t a r t i n g  inverter ,  it i s  desirable  t o  start the  
Brayton power system with 20 vol t s ,  400 hertz.  
A t  t h i s  l e v e l  the ba t te ry  voltage (+28 vol t s )  i s  
high enough so t h a t  no voltage boost i s  needed, 
and no converter o r  transformer stage i s  required 
i n  the inverter .  
termine t h e  f e a s i b i l i t y  of using 20 vol t s l ine- to-  
neutral ,  400 hertz ,  are  described i n  t h i s  report .  
TEST FACILITY - The motor s t a r t i n g  t e s t s  
were performed i n  a complete Brayton gas loop 
test  system. The BRU i s  i n s t a l l e d  with the 
f l ight- type Brayton heat exchanger u n i t  (BHXU) . 
The BHXU consis ts  of a recuperator, waste heat 
exchanger, and connecting ducting t o  the BRU, and 
i s  constructed a s  a single uni t .  The BRU and the  
BHXU, i n  conjunction with an e l e c t r i c  (nonflight-  
type) heat source, form the complete Brayton gas 
loop. 'Reference 6 describes the features  of t h e  
f a c i l i t y  . 
heaters  on the piping between the  system e l e c t r i -  
c a l  heat source and the s c r o l l  of the turbine in-  
l e t  were used t o  elevate the temperature at the 
turbine i n l e t  t o  the desired running value. This 
procedure was followed i n  order t o  r a i s e  the t e m -  
perature of the  BRU and t o  heat by natural  con- 
vection the gas inventory so t h a t  the turbine in- 
l e t  temperature was constant during the s tar tup.  
The gas inventory was adjusted t o  se t  the desired 
i n i t i a l  system pressure. 
A few moments before energizing the a l te rna-  
to r ,  the gas bearings were pressurized and the 
system heat source was turned on. 
motor startin!: operation, t h i s  heat source con- 
t r o l  was adjut ted t o  maintain the turbine i n l e t  
- 
applying 2 8  vol t s  l ine-to-neutral ,  - 400 hertz,  t o  
In order tos impl$fy  the design of 
The motor s t a r t i n g  t e s t s  performed t o  de- 
TEST PROCEDURF - Prior  t o  motoring, guard 
During the 
2 
temperature at the desired running value. 
connected across the f i e l d  c o i l  of the alternator 
i n  order t o  l i m i t  the voltage induced in  the c o i l  
by the motoring e lec t r ica l  machine. The selected 
alternating voltage was then applied t o  the t e r -  
minals of the alternator t o  drive it as a 3- 
phase sol id  rotor induction motor. A t  synchro- 
nous speed the motoring power was disconnected, 
the f i e l d  loading res i s tor  removed, and the BRU 
e lec t r ica l  system reconnected. While the system 
was bootstrapping, the voltage regulator-exciter 
(VR-E) was turned on a t  about 18,000 rpm so tha t  
the integral  system e lec t r ica l  controls would be 
effective. 
RESULTS AND DISCUSSION 
Prior t o  motoring, a 50-ohm res i s tor  was 
The time for a motor start of the Brayton 
power system i s  the time required t o  accelerate 
the BRU from s tands t i l l  t o  rated speed. 
time includes motoring time (or  the time t o  reach 
the synchronous speed for  the applied e lec t r ica l  
frequency) and bootstrap time (the time t o  accel- 
erate from synchronous speed t o  rated speed): 
' 
speed characterist ics of the alternator function- 
ing as a motor. 
system pressures and temperatures. The torque 
developed by the motoring machine accelerates the 
l i g h t l y  loaded BRU t o  synchronous speed. When 
the motoring power i s  removed, if the system con-' 
dit ions (speed, pressure, and temperature) a re  
adequate, the turbine develops a torque i n  excess 
of the ccmpressor, bearing, and windage load 
torques. The BRU bootstraps and speed i s  l imited 
only by the system speed control circuitry.  
Asset of motor starting t e s t s  were conducted 
by applying 20 vol ts  RMS line-to-neutral, 400 
hertz t o  the alternator.  The gas loop was i n i -  1 
t i a l l y  pressurized t o  15 ps ia  (10 N/cm2) and pre- 
heated t o  turbine in le t  temperatures of 875O F 
(741 K), 950' F (783 K) ,  and 1200° F (922 K). , 
' In each t e s t  hydrostatic jacking gas was 
supplied t o  the bearings prior t o  motoring t o  
ensure against bearing rub. This gas was ex- 
ternal ly  supplied continuously u n t i l  design speed 
was reached. Thus, there was an excess gas in- 
ventory as  well as an increase i n  gas temperature 
and pressure as a resu l t  of system operation. 
The resultant system pressure was allowed t o  
build up t o  18 psia (12 N/cm2) a t  the compressor 
discharger before excess gas was bled. 
ture of 875' F (741 K) is shown in Fig. 7. . Syn- 
This 
Motoring time i s  determined by the torque-1 
Bootstrap t i m e  i s  a function of 
The speed curve for a turbine iriLet tempera- 
chronous speed (12,000 rpm) was reached i n  SO 
seconds and motoring power was disconnected imre-  
diately. Shaft speed continued to increase, d e  
t o  the bootstrapping action, t o  about 18,ooO rpm 
at '  which speed the voltage regulator-exciter 
(VR-E) was turned on, The load of the VR-E was 
sufficient t o  cause the shaft  t o  decelerate no- 
ticeably. 
i n  about 280 seconds. 
Figure 8 shows the speed curve obtained a t  
950° F (783 K). A t  t h i s  condition, synchronous 
speed was reached i n  25 seconds. The effects  of 
the VR-E on shaft  speed are noticeably l e s s  than 
a t  the lower temperature conditions of Fig. 7. 
Time t o  reach design speed decreased t o  142 sec- 
onds. 
A t  1200° F (922 K) turbine i n l e t  temperature 
(fig.  9), the time t o  reach synchronous speed was 
20 seconds and the effects  of the VR-E were neg- 
l ig ib le .  
72 seconds. 
One motor s t a r t  was performed with the i n i -  
tial gas pressure i n  the systFm at  25 psia (17 N/ 
a n 2 ) .  
Fig. 10. Although the evidence i s  limited, the 
result indicated tha t  increased system pressure 
produced a faster start. 
reach synchronous speed remained a t  twenty sec- 
onds, but the bootstrap time decreased t o  t h i r t y  
seconds for  a turbine i n l e t  temperature of 
These data are combined i n  Fig. 11 t o  show1 
t o t a l  motor s ta r t ing  time as a function of tur-, 
bine i n l e t  temperature. The data includes timei 
t o  reach synchronous speed plus bootstrap tim. I a 
The slope of the curve i s  very steep t o  about ' 
1000° F. 
changes',more gradually. 
pressure of 15 psia, a reasonable selected mini- , 
mum turbine in le t  temperature i s  i n  the order of 
1000° F (ell K) . This temperature results i n  a I 
t o t a l  motor s tar t ing time of about 2 minutes. 
The magnitude of the al ternator  current dur- 
ing motoring i s  shown i n  Fig. 12. The curve rep-, 
resents the line-to-neutral RMS current per phase' 
for  the 20 volts,  400 hertz starts. The results 
are typical for  all temperature investigated. 
The s ta r t ing  current reaches 50 amperes and grad- 
ually decreases u n t i l  jus t  before reaching syn- 
chronous speed. Upon reaching synchronous speed, 
the current steps down t o  1 7 . 5  amperes. 
Design speed (36,000 rpm) was reached 
The shaft  speed reached 36,000 rpm i n  
The result ing acceleration i s  shown i n  
The motoring t h e  t o  
U0Oo F (866 K) . i '  
Above t h i s  temperature the slope 
For an i n i t i a l  loop 
I 
SUMMARY OF RESULTS 
Motor s tar t ing the Brayton gas loop t e s t  
3 
1 1 '  
1 ,  , .  - -- 
.. S. Valerino, R. P. Macosko, A. S. 
Asadourian, T. P. Kecker and R. &uchawJr,- ;-A 
"Preliminary Performance of a Brayton-Cycle- 
Power System Gas Loop Operating with Krypton 
Over a Turbine Inlet Temperature Range of 
1200' to 1600' F." NASA TM X-32769, 1970. ,e 
by higher turbine in l e t  temperatures. The motor- 
ing time t o  synchronous speed varies only s l ight-  
l y  from 20 seconds, but the acceleration of the 
1. J. L. KLann, "Two-to-Ten Kilowatt Solar or  
1 
I ference, Vol. I, New York, 1968, pp. 407- * 
2. A. 3, Valerino and L. W. Ream, "Performance , 
Radioisotope Brayton Power System. Inter?  
society Energy Conversion Engineering Con- 
415. 
of the Major Components i n  a Closed Loop 
2-to-15 kM Brayton Parer System." IECEC, . 
19 71. ' 
"Injection Start of a Brayton Cycle Turbo- 
canpressor Operating on Gas Bearings i n  a . 
I 
3. R. Y. Wong, R. C. Evans and D. J. Spackman, 
Closed LOOP." TM X-1590, May 1968. i 
4. D. S. Repas and R. J. Frye, "Motor Starting 
Characteristics of a Modified Lundell Alter- 
nator." NASA TM X-2200, March 1971. 
S ta r t  of a 2- t o  10-kilowatt Brayton Ro- 
t a t ing  Unit Operating on Gas Bearings i n  a 
Closed Loop." NASA TM X-2266, 1971. 
5. R. C. Evans, R.  Y. Wong and C. Winzig, "Motor 
. -  
'6. A 
I 
. a  
i 
! 
i '  
i -  
E-6291 
Figure 2. - Brayton rotating unit. 
Figure 1. - Brayton power system test engine. 
Figure 3. - Schematic diagram, Brayton power system. 
1% VOLTAGE-TO-F REQU ENCY r 
LIME-TO-NEUTRAL 3 
2 
1 
0 
(a) APPLIED FREQUENCY, 400 HERTZ; 
SYNCHRONOUS SPEED, 12 OOO RPM. 
'I- 
(b) APPLIED FREQUENCY, 800 HERTZ; 
SYNCHRONOUS SPEED, 24 O00 RPM. 
5r 
0 20 40 60 80 100 
SPEED, PERCENT OF SYNCHRONOUS 
(c) APPLIED FREQUENCY, 1200 HERTZ; 
SYNCHRONOUS SPEED, 36 000 RPM. 
Figure 4. - No-load motor starting arma- 
ture current characteristics of 1200- 
hertz Lundell alternator. One per-unit 
current, 39.7 amperes, 
\ T R &  
LO 
0 
(a) APPLIED FREQUENCY, 400 HERTZ; 
SYNCHRONOUS SPEED, 12 W RPM. 
(b) APPLIED FREQUENCY, 800 HERTZ; 
SYNCHRONOUS SPEED, 24 OOO RPM. 
.5 
20 40 60 80 100 0 
SPEED, PERCENT OF SYNCHRONOUS 
(C) APPLIED FREQUENCY, 1200 HERTZ; 
SYNCHRONOUS SPEED, 36 000 RPM. 
Figure 5. - No-load motor-torque char- 
acteristics of 1200-hertz Lundell alter- 
nator. One per-unit torque, 2.09 
pound-feet (2 83 N-m), 
SPEED, 
R P M  
PRESSURE, 15 PSlA (18 MICM2) 
APPLIED VOLTAGE, 28 V L-N 
FREQUENCY, 4W HZ 
1100 
5000 ioooo 15000 20000 25000 30000 
SPEED, RPM 
Figure 6. - Minum turbine inlet temperature and speed re- 
quired for self-sustaining operation. 
35 
30 
25 
IVR-EON 6 O6 
0- 
0 
0 
0 s" 
.o 
" 
O 0  20 
15 
0 
TIME, SEC 
test system, turbine inlet temperature of 8 7 9  F (741 K). 
otor starting characteristics of Brayton gas loop 
E-6291 
40Xld 
30 5 ~  
V R-E ON-1 
a- 20 600 
O0 
% 
II 
v) 
o o  15 
10 
ooo 0 
0 
0 
0 
0 
0 
0 
TURBINE INLETTUVIPER- 
ATURE, 9500 F (783 K) 
15 PSlA (10 NICMZ) 
20 V L-N 
SYSTEM PRESSURE, 
APPLIED VOLTAGE, 
APPLIED FREQUENCY, 400 Hz 
oh 20 1u) 60 80 io0 120 14 16o 180 
TIME, SEC 
Figure 8. - Motor starting characteristics of Brayton gas loop test 
I I I I I I I I I 
system. Turbine inlet temperature of 9 5 8  F (783 K). 
40x103 
35 
30 
25 
20 
SPEED, 
RPM 
15 
000 
0 
0 
0 
APPLIED VOLTAGE, 20 V L-N 
APPLIED FREQUENCY, 400 Hz 
0 20 40 60 80 100 
TIME, SEC 
Figure 9, - M d o r  starting characteristics of Brayton 
gas loop test.system, turbine inlet temperature, 
lZOOo F (922 K). 
SPEED, 
RPM 
APPLIED VOLTAGE, 20 V L-N 
0 10 20 3 0 4 0  50 60 
TIME, SEC 
Figure 18. - Motor starting characteristics of Braytongas 
loop test system, inlet temperature, 11M0 F (866 K). 
280r- Y 
TOTAL 
START IN G 
TIME, 
S EC 
200 
120 
l i  80 
I 0 J 
1460 1660 
40 
700 
TURBINE INLET TEMPERATURE, K 
< 
800 1000 1200 
TURBINE INLET TEMPERATURE, OF 
Figure 11. - Starting time of Brayton gas 
loop test system as a function of turbine 
in let tern peratu re. 
60 
NG APPLIED VOLTAGE, 20 V L-N 
APPLIED FREQUENCY, 400 HZ 
rSYNCHRONOUS SPEED 
40 
I 
ARMATURE 
CURRENT, / 
AMP 
20 
0 10 20 30 40 
MOTORING TIME, SEC 
Figure 12. - Motor start ing c u r r e n t  for  Brayton 
gas loop test system. 
NASA-Lewis-Corn'l 


Motor starting techniques for the 2 to 15 kW Brayton space power system

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