Two four-pole Lundell-type rotors consisting of magnetic and nonmagnetic materials were fabricated by weld-depositing Inconel 625 between two sections of AISI 4617 steel. The rotors had a major diameter of 8.28 cm (3.26 in.). Saturation curves for load and no-load conditions with one of the rotors installed in a 1200-Hz Brayton-cycle research alternator are presented. The other identical rotor was spin-tested to a speed of 63,000 rmp, which was equal to 175 percent of the rated speed

Lumannick, S.

Medwid, D. W.

Tulisiak, G.

English

NASA Technical Reports Server

NASA TECHNICAL
MEMORANDUM
i
X
NASA TM X-2944
CASE FIL
CORY
TEST CHARACTERISTICS OF A WELDED ROTOR
IN A 36 000-RPM LUNDELL ALTERNATOR
by Stacy Lumannick, David W. Medwid,
and George Tulisiak
Lewis Research Center
Cleveland, Ohio 44135
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION • WASHINGTON, D. C. • NOVEMBER 1973
https://ntrs.nasa.gov/search.jsp?R=19740003191 2020-03-23T14:00:32+00:00Z
1. Report No.
NASA TM X-2944
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
TEST CHARACTERISTICS OF A WELDED ROTOR IN A
36 000-RPM LUNDELL ALTERNATOR
5. Report Date
November 1973
6. Performing Organization Code
7. Author(s)
Stacy Lumannick, David W. Medwid, and George Tulisiak
8. Performing Organization Report No.
E-7535
9. Performing Organization Name and Address
Lewis Research Center
National Aeronautics and Space Administration
Cleveland, Ohio 44135
10. Work Unit No.
503-35
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
16. Abstract
Two four-pole Lundell-type rotors consisting of magnetic and nonmagnetic materials were fab-
ricated by weld-depositing Inconel 625 between two sections of AISI 4617 steel. The rotors had
a major diameter of 8. 28 cm (3.26 in.). Saturation curves for load and no-load conditions with
one of the rotors installed in a 1200-Hz Brayton-cycle research alternator are presented. The
other identical rotor was spin-tested to a speed of 63 000 rpm, which was equal to 175 percent
of the rated speed.
17. Key Words (Suggested by Author(s))
Lundell rotor
Rotational stresses
Welded-rotor characteristics
18. Distribution Statement
Unclassified - unlimited
19. Security Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No. of Pages
14
22. Price*
Domestic, $2.75
Foreign, $5.25
* For sale by the National Technical Information Service, Springfield, Virginia 22151
TEST CHARACTERISTICS OF A WELDED ROTOR
IN A 36 000-RPM LUNDELL ALTERNATOR
by Stacy Lumannick, David W. Medwid, and George Tulisiak
Lewis Research Center
SUMMARY
Two four-pole Lundell-type rotors consisting of magnetic and nonmagnetic mater-
ials were fabricated by weld-depositing Inconel 625 between two sections of AISI 4617
steel. The rotors had a major diameter of 8. 28 centimeters (3.26 in.). Saturation
curves for load and no-load conditions with one of the rotors installed in a 1200-hertz
Brayton-cycle research alternator are presented. The other identical rotor was spin-
tested to a speed of 63 000 rpm, which was equal to 175 percent of the rated speed.
The test was terminated at 63 000 rpm because of failure of the test-rig turbine drive
spindle.
INTRODUCTION
A Brayton-cycle space power system being tested at the NASA Lewis Research
Center has a 1200-hertz four-pole Lundell alternator. The rotor (fig. 1) for this alter-
nator consists of two separate magnetic sections with north poles on one section and
south poles on the other section. A third section between the two magnetic sections
consists of a nonmagnetic metal. These three sections are bonded together by brazing
(ref. 1). Overspeed testing of such a brazed rotor to failure at 90 000 rpm (ref. 2)
revealed that the failure occurred in the braze area; the strength of the rotor was more
than adequate for the design speed of 36 000 rpm.
In an attempt to develop an alternative method of fabricating this type of rotor,
Inconel 625 (ref. 3) was weld-deposited between two sections of AISI 4617 steel. When
only a small number of rotors are to be fabricated, welding is generally easier and
less expensive than other methods of fabrication.
The objective was to meet the magnetic requirements with weldable materials
that could be subjected to high rotational stresses. Spin testing to failure would pro-
vide a strength comparison with other methods of fabrication. A secondary objective
was to develop a method of fabrication that might be applicable to similar rotors in future
space power systems.
DESCRIPTION OF ALTERNATOR AND ROTOR
The 1200-hertz Brayton-cycle research alternator is a three-phase 120/208-volt
machine. Rated output at 36 000 rpm is 14. 3 Mlovolt-amperes with a 0. 75 lagging power
factor (ref. 4). For the test results presented in this report, all the alternator field
coils were connected in series. The brazed rotor for this alternator has magnetic sec-
tions consisting of AISI 4340 steel separated by a nonmagnetic section of Inconel 718.
A tie bolt hole drilled through the entire length of the rotor is included. In the space
power system application, a tie bolt through the hole attaches a turbine on one end and
a compressor on the other end of the rotor.
Magnetic AISI 4617 steel and nonmagnetic Inconel 625 were selected for the welded
rotor. Compared to AISI 4340, the AISI 4617 has a lower magnetic resistance (reluc-
tance) and a lower yield strength. However, the lower yield strength was still more
than adequate for the overspeed specifications for this alternator. Overspeed specifica-
tions were 120 percent of the design speed of 36 000 rpm. Inconel 625 was selected be-
cause it is less susceptable to cracking than Inconel 718 in this type of welding applica-
tion.
ROTOR FABRICATION
The cross-sectional area of the magnetic section of the rotor varies axially. Tem-
plates were made which duplicated these cross-sectional areas at several axial locations.
The templates were then attached to a common base at intervals equal to the rotor-axial-
location intervals. The spaces between the templates were filled with a plastic material
to form a mold with a smoothly varying contour from template to template. With this
mold as a duplicating guide, the contour was machined into two AISI 4617 steel billets
(figs. 2 and 3).
The nonmagnetic section was fabricated by depositing uncoated Inconel 625 wire on
the two machined billets by the gas-tungsten-arc-welding (GTAW) method (figs. 4 and 5).
This depositing of the Inconel 625 was continued until the required distance between the
two magnetic sections was established (fig. 6) and the gap completely filled. Periodic
ultrasonic and fluorescent liquid penetrant inspections were made of the deposited weld
material to check for cracks. The completed billet was then heat-treated at 621° C
(1150° F) for 1 hour and machined to the final configuration. Final machining included
a 1.27-centimeter- (0. 50-in.-) diameter tie bolt hole drilled through the length of the
rotor. Additional ultrasonic and fluorescent inspections were made after final machin-
ing.
APPARATUS
A schematic diagram of the magnetic test setup is shown in figure 7. The test fa-
cility consisted of a dynamometer running at 1440 rpm and two stepup gear boxes. The
externally colled gear boxes each have a stepup ratio of 5:1. A separate system using
MIL-L-7808 oil cooled the alternator. An air-oil mist lubricated the alternator bear-
ings.
A three-phase resistive and reactive load bank was used to absorb the power out-
put of the alternator and to establish the power factor. Short-circuit switches were used
to connect the three-phase fault to the alternator.
The facility used for the spin test consisted of an air-driven turbine mounted in a
vertical position on the top of a vacuum chamber. Reverse nozzles on the turbine were
used for braking. A magnetic pickup on the turbine measured speed. The vacuum cham-
ber had an inner liner of steel plates for shrapnel protection.
The spin test assembly (fig. 8) consisted of a vertically suspended rotor and attach-
ing hardware. A lower catcher bearing with a tapered bore was mounted below the rotor
in case the drive-turbine shaft failed. A journal catcher bearing with a 0. 30-centimeter
(0.12-in.) diametral clearance was mounted at the upper end of the suspended rotor.
PROCEDURE
Two rotors were fabricated for the tests. Micrometer measurements of the diam-
eters of the two rotors were made before and after spin testing of the locations shown
in figure 9.
One rotor was proof tested by accelerating at approximately 60 rpm per second to
50 000 rpm in several stop-and-measure steps. After 10 minutes at 50 000 rpm, mea-
surements of the rotor revealed no yielding. This rotor was then installed in a research
alternator for magnetic testing.
During the magnetic testing the following measurements were made: (1) shaft
speed, (2) electrical frequency, (3) voltage and current for each phase, (4) power output,
and (5) field current and field voltage. Data for the three-phase short-circuit test were
taken by connecting a balanced three-phase fault to the alternator terminals.
For the spin-to-destruction test of the second rotor, the catcher bearings were
removed. This rotor was accelerated at approximately 60 rpm per second to 50 000 rpm;
the test was then stopped for measurements of the rotor diameter. The stop-and-mea-
3
sure procedure continued at 3000-rpm intervals between 50 000 rpm and the turbine drive
spindle failure at 63 000 rpm. Spinning time at each interval was 10 minutes.
RESULTS AND DISCUSSION
Magnetic Testing
Saturation curves for open-circuit and for three-phase short-circuit characteristics
are shown in figure 10. For comparison with the original brazed rotor, the dashed line
curve from reference 4 is presented.
The same research alternator was used in these tests and in the tests of reference
4. This research alternator also has two windings per field coil, one for series excita-
tion and one for shunt excitation. In these saturation tests and in the saturation tests of
reference 4, the series and shunt fields were connected in series.
Load-saturation curves are shown in figure 11. For the design point of 120 volts,
the welded rotor required a field excitation of 1850 ampere-turns at unity power factor
and 2150 ampere-turns at a power factor of 0. 75. The dashed-line curve from reference
4 shows the original brazed rotor required a field excitation of 2000 ampere-turns at
unity power factor and 2500 ampere-turns at a power factor of 0. 75. These performance
curves indicate the welded rotor is more than adequate magnetically.
Spin Testing
At 63 000 rpm, the turbine drive spindle (fig. 12) failed. The alternator rotor was
recovered, and, although dented and scratched, the rotor itself was intact. The rotor
had survived at 63 000 rpm (175 percent of rated speed). For comparison, rotor failure
occurred at 95 000 rpm for the brazed rotor and 110 000 rpm for the composite cast
rotor (table I).
The cause of the spindle failure was not apparent, but it may possibly have resulted
from some deformation of the alternator rotor. On the other hand, post -test inspection
of the tie bolt hole revealed no out -of -tolerance growth in diameter or elliptical defor-
mation.
The equation (ref. 5) used to calculate the maximum tangential stress at the periph-
ery of the tie bolt hole was
i jj^ r
4 386.4 L
(3 + v)R2 + (1 _ v)r x 0 69
for SI units, or
2 r l
3. = 1.22— (3 + v)R2 + (1 - v)r21
 4 386.4 L J
for U. S. customary units, where y is the density of the material, u> is the rotational
speed, v is the Poisson ratio, R is the outer radius, and r is the inner radius.
Based only on the calculated tangential stress of 48 100 newtons per square centi-
meter (69 000 Ib/in. ) at 63 000 rpm, the periphery of the tie bolt hole was in the yield
range of the AISI 4617 material. Additional ultrasonic and fluorescent penetrant inspec-
tion revealed no obvious material separation. Hardness tests also showed no abnormal
material situation (fig. 13). Thus, 175 percent (63 000 rpm) of the rated speed was
achieved, but, within the strength limit of the AISI 4617 steel, a higher speed might not
have been achievable.
SUMMARY OF RESULTS
As an alternative to brazing AISI 4340 steel and Inconel 718 for a four-pole Lundell-
type rotor, two rotors were fabricated by weld-depositing nonmagnetic Inconel 625 be-
tween two sections of magnetic AISI 4617 steel. One rotor was proof tested at 50 000 rpm
and then installed in a research alternator. The magnetic characteristics of this welded
alternator exceeded those of the original brazed-construction rotor.
The second identical rotor was spin-tested to 63 000 rpm (175 percent of the rated
speed) and was still intact, compared to the brazed and composite rotors, which failed
at 95 000 and 110 000 rpm, respectively.
At 63 000 rpm, the turbine drive spindle of the spin rig failed and the testing was
discontinued. At speeds above the 60 000 rpm level, the calculated stresses at the pe-
riphery of the tie bolt hold were in the yield range of the AISI 4617 steel, which suggested
that the rotor burst speed may not have been substantially higher than the achieved test-
ing speed.
Lewis Research Center,
National Aeronautics and Space Administration,
Cleveland, Ohio, August 3, 1973,
503-35.
REFERENCES
1. Dunn, James H.: The 1200-HZ Brayton Electrical Research Components. Rep. APS-
5286-R, AiResearch Mfg. Co. of Arizona (NASA CR-72564), Mar. 19, 1969.
2. Davis, J. E.: Design and Fabrication of the Brayton Rotating Unit. NASA CR-1870,
1972.
3. Anon.: Handbook of Hunington Alloys. Hunington Alloy Products Div. , International
Nickel Co., Inc.
4. Repas, David S.; and EdMn, Richard A.: Performance Characteristics of a 14. 3-
Kilovolt-Ampere Modified Lundell Alternator for 1200 Hertz Brayton-Cycle Space-
Power System. NASATND-5405, 1969.
5. Roark, Raymond J.: Formulas for Stress and Strain. Third ed., McGraw-Hill Book
Co., Inc., 1954.
6
TABLE I. - PUBLISHED VALUES OF ROTOR AND MATERIAL CHARACTERISTICS
Rotor
Welded
Brazed
(ref. 2)
Composite
cast (ref. 2)
Failure
speed,
rpm
63000
95 000
110 000
Magnetic
material
AISI 4617
AISI 4340
AISI 4340
Yield strength
of magnetic
material4
N/cm2
b42 700
45 100
b42 700
lb/in.2
b62 000
65 400
b62 000
Nonmagnetic
material
Inconel 625
Cast Inconel 718
Cast MAR M2 11
Yield strength
of magnetic
material1
N/cm2
b44 800
75 800
b86 200
lb/in. 2
b
 65 000
110 000
b!25 000
aYield strength varies with optimized heat treatment for each material combination.
Handbook value.
C-68-1C42
Figure 1. - Alternator rotor.
C-70-2076
Figure 2. - Machined billet of AISI4617.
j [Mff$^^
C-70-2075
Figure 3. - Relative positions of AISI 4617 billets before welding.
8
C-70-2121
Figure 4. - Machined billet with deposited Inconel 625.
C-70-2124
Figure 5. - Relative positions of billets with deposited Inconel 625 before joining.
9
C-70-2184
Figure 6. - AISI4617 billets joined by Inconel 625.
Test facility
Gear Gear
box box
Dynamometer — | | f~
J S — H T — HTT TT
External External
cooling cooling
i_.
S
s
MIL-L-7808
coolant
1 t
36 000-rpm
alternator
ho rt -circuit
witches
Three -phas
fault
Three-phase
load bank
Figure 7. - Magnetic test setup.
10
Hardware
 t
used for test /[|
purposes only~(
N
Figure 8. - Spin-test assembly.
I, J, K H
Figure 9. - Rotor diameter measurement locations. (Dimensions in centimeters
(in.).) (From ref. 2.)
11
200,r— 200
16o— 160
e 120- -^  120
IBI
)— £ 80
40— 40
O1—
} Open circuit
"I Three-phase
_f short circuit
Rotor
Welded
B razed (ref. 4)
1000 2000 3000 4000 5000
Field excitation, At
Figure 10. - No-load saturation curves for 1200-hertz Lundell alternator with
welded rotor compared with those for brazed rotor of reference 4.
2001—
160
z 120
§
E
E 80
40
Power
factor
Design points
0.75
Rotor
Welded
Brazed (ref. 4)
1000 2000 3000
Field excitation, At
4000
Figure 11. - Load-saturation curves for 1200-hertz
Lundell alternator with welded rotor compared
with those for brazed rotor of reference 4. Rated
current, 39.7 amperes.
12
C-72-2058
Figure 12. - Failed spindle of drive turbine.
C-72-1326
Figure 13. - Cross section of spin-tested rotor.
NASA-Langley, 1973 15 E~7535 13
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Test characteristics of a welded rotor in a 36,000-rpm Lundell alternator

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