New Applications of Synchronous Generators

Abstract

High-speed synchronous generators rated in the power range of megawatts have been discussed. These generators are nowadays necessary for new aerospace missions. Requirements, ferromagnetic materials, magnetic circuit, electromechanical design and cooling techniques for high-speed multimegawatt generators have been discussed. First prototypes (1 MW and 2.5 MW) have been outlined.

Full text

Jacek F. GIERAS
1
University of Technology and Life Sciences, Bydgoszcz
New Applications of Synchronous Generators
Abstract. High-speed synchronous generators rated in the power range of megawatts have been discussed. These generators are nowadays
necessary for new aerospace missions. Requirements, ferromagnetic materials, magnetic circuit, electromechanical design and cooling techniquesfor
high-speed multimegawatt generators have been discussed. First prototypes (1 MW and 2.5 MW) have been outlined.
Streszczenie. Artykuł dotyczy prądnic synchronicznych o wysokiej prędkosci obrotowej oraz mocy rzędu megawatów stosowanych w aparatach
lataj
ących. Prądnice takie są obecnie konieczne do misji specjalnych w przestrzeni powietrznej. Omówiono wymagania, materiały ferromagnety-
czne, obwody magnetyczne, elementy projektowania elektromechanicznego oraz techniki ch
łodzenia prądnic o wysokiej prędkości oraz mocy rzędu
megawat
ów. Przedstawiono pierwsze prototypy takich prądnic (o mocach 1 MW oraz 2.5 MW). (Nowe zastosowania generatorów synchronicznych)
Keywords: synchronous generators, multimegawatt, aerospace, magnetic circuits, cooling systems, design, first prototypes
Słowa kluczowe: pr
ądnice synchroniczne, moc rzędu megawatów, przestrzeń powietrzna, obwody magnetyczne, układy chłodzenia, projektowanie,
pierwsze prototypy
Introduction
The speed of a.c. machines increases with increase in
the input frequency. The electromagnetic torque is propor-
tional to the electromagnetic power and number of pole pairs
and inversely proportional to the frequency. High speed and
high frequency armature current reduce the dimensions and
mass of electrical machines.
At present, the maximum power of high speed (over 5000
rpm) aerospace synchronous generators does not exceed 500
kW. Several airborne power missions are now evolving that
will require lightweight multimegawatt electrical power systems,
e.g., directed energy weapon (DEW) and airborne radar (AR)
[1, 12, 10, 19]. New high power airborne and mobile military
systems will require 1 to 6 MW of electrical power generated
at speeds 10 to 20 krpm or higher. Potential candidates as
multimegawatt generators are [4, 6, 9, 10, 11, 18, 19, 20]:
• high power density classical synchronous generators
with electromagnetic excitation (wound-field rotor)
• synchronous generators with high temperature su-
perconducting (HTS) excitation winding
• homopolar generators with stationary d.c. HTS excita-
tion winding
• all cryogenic generators (synchronous or homopolar)
Requirements
Basic design requirements for high speed machines
include, but are not limited to:
• high power density (output power-to-mass);
• brushless design;
• compact design;
• minimum number of components;
• ability to withstand high temperature;
• low subtransient time constant (pulse operation);
• minimum cost–to–output power ratio and cost–to–
efficiency ratio;
• high reliability (the failure rate
< 5% within 80 000 h);
• high efficiency over the whole range of variable speed;
• low total harmonics distortion (THD).
It is also desired that modern airborne generators have some
fault tolerance capability. However, generating mode with one
damaged phase winding of a three phase machine and then
normal operation after the fault clears is normally impossible.
Reliability data of airborne high-speed generators are
very scattered and limited to generators rated at maximum
250 kVA. The mean time between failure (MTBF) values up
to approximately 47 000 h as calculated from short-term
maintenance record [18].
The stator laminations are about 0.2-mm thick for fre-
quencies below 400 Hz and about 0.1-mm thick for frequen-
cies above 700 Hz. Low-loss, high-saturation, thin silicon
steel laminations or iron-cobalt laminations are used for sta-
tor and rotor stacks.
To minimize the subtransient time constant, i.e., to elim-
inate eddy-current effects in the rotor pole shoes, the rotor -
similar to the stator - must be in most cases laminated instead
of made of solid steel. The rotor is often protected against
centrifugal forces with the aid of retaining sleeves (cans) [8].
The retaining sleeve can be made of nonferromagnetic met-
als, e.g., titanium alloys, stainless steels, inconel 718 (alloy
based on NiCoCr) or carbon-graphite composites. Good ma-
terials for retaining sleeves have high permissible stresses,
very low electric conductivity, high thermal conductivity, and
low specific density.
The higher the speed (frequency) and more efficient the
cooling system, the smaller the volume and mass. Applica-
tion of direct liquid cooling results in further increase of power
density (output power-to-mass or output-power-to volume).
High efficiency means the reduction of the input me-
chanical power (prime mover) through the reduction of power
losses. The lower the losses, the lower the temperature rise
of a generator and easier the thermal management.
Directed energy weapons
It was believed before the World War II that electromag-
netic waves could be used to destroy aircraft. For example,
in 1935, Sir R.A. Watson-Watt
1
was asked by the Committee
for Scientific Study of British Air Defence to investigate the
ability of transmitting radio frequency waves of high enough
power to boil the blood of German pilots and soldiers.
Directed energy weapons (DEW) take the form of lasers,
high-powered microwaves, and particle beams [10]. They
can be adopted for ground, air, sea, and space warfare.
DEWs irradiate the target with electromagnetic energy. The
so-called fluence is the energy density, i.e.,
(1)
E =
P
dout
ΔtS
A
J/m
2
where P
dout
is the DEW output power, Δt is the duration of
the DEW pulse,
0 ≤ S ≤ 1.0 is the dimensionless transmis-
sion number, also called Strehl
2
ratio, and A is the spot area
1
Scottish physicist Robert A. Watson-Watt (1892-1973), inventor
of radiolocators (British equivalent of radar) to detect airplanes.
2
named after German physicist and mathematician Karl Strehl
(1864-1940).
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on the target. To destroy soft targets, i.e., fabrics, plastics,
etc., approximately
1000 × 10
4
J/m
2
are required, but ex-
tremely hard targets, i.e., tanks, mine resistant vehicles, ar-
mored trucks, etc., might require
100 000 × 10
4
J/m
2
. Once
the target has absorbed this energy, it will begin to heat up
and even burn out.
The only difference between lasers and high-energy mi-
crowaves, which are both made up of photons, is their energy
level. The photon energy
(2)
E = hf = h
c
λ
is a function of the frequency f , where h =6.626 × 10
−34
Js is Planck’s constant, c = 299 792 458 m/s is the speed of
light, and
λ is the length of wave.
The power generation capabilities of electron microwave
tubes (MTs), i.e., klystrons, magnetrons, gyratrons, gridded
tubes and cross-field tubes range from watts to megawatts at
frequencies from 300 MHz to 300 GHz (Fig. 1). Klystrons are
the most efficient MTs and are capable of the highest peak
and average powers. A klystron is a specialized vacuum tube
called a linear-beam tube. The pseudo-Greek word klystron
comes from the stem form klys of a Greek verb referring to
the action of waves breaking against a shore, and the end of
the word electron.
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
0.1 1 10 100 1000
frequency, GHz
average power, W
klystron
gyrotron
crossed-field
amplifier (CFA)
gridded tube
Fig. 1. Average output power versus frequency of state-of-the art
microwave tubes (MTs).
High power microwave
High-power microwave (HPM) weapons produce either
beams or short pulses of high-frequency energy in the
megawatt range. For comparison, a typical microwave oven
generates less than 1.5 kW of power.
The term HPM denotes sources producing coherent
electromagnetic radiation from 1 GHz to over 100 GHz with
an instantaneous power of at least 100 MW. Gigawatt-class
HPM sources include magnetically insulated line oscillator
(MILO), relativistic magnetron, relativistic klystron amplifier
(RKA), relativistic klystron oscillator (RKO), and reltron [12].
Fig. 2 shows a comparison of the peak and average power
characteristics for conventional and HPM sources.
When the microwave energy encounters unshielded cur-
rent conducting bodies, semiconductors or electronic compo-
nents, it induces a.c. current in them. The high frequency
electric current causes the equipment to malfunction without
injuring the personnel (Fig. 3). If the energy is high enough,
the microwaves can permanently burn out the equipment.
The equivalent depth of penetration of electromagnetic
wave into human skin is
Fig. 2. Peak power versus average power domains for microwave
production. CW — continuous wave mode [12].
(3) δ =
1
√
πfμ
0
μ
r
σ
m
where f is the frequency, μ
0
=0.4π × 10
−6
H/m, μ
r
is the
relative magnetic permeability and
σ is the electric conduc-
tivity. Assuming the conductivity of human tissue
σ =2S/m
at f =3× 10
10
Hz = 30 GHz, and μ
r
=1, the eqivalent
depth of penetration δ =2.05 mm. The tissue is not dam-
aged. Only a burning pain is produced which forces the af-
fected person to escape. Current HPM research focuses on
pulsed power devices that create intense, ultrashort bursts of
electrical energy.
Fig. 3. The HPM uses microwave energy to disable or damage
vehicle electronic control modules/microprocessors that control vital
functions of engines. Image courtesy of Defense Intelligence Agency
(DIA).
Lasers
Lasers produce either continuous beams or short, intense
pulses of light in every spectr um from infrared to ultravio-
let. The power output necessary for a weapons-grade high-
energy laser (HEL) ranges from 1 kW to 10 MW. When a
laser beam strikes a target, the energy from the photons in
the beam heats the target to the point of combustion or melt-
ing. Since the laser beam travels at the speed of light, HELs
can particularly be used against moving targets such as fight-
ers, rockets, missiles, and artillery projectiles (Fig. 4). X-ray
151 PRZEGL ˛AD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9a/2012

lasers may be possible in the not too distant future.
Most HELs being developed and tested for military ap-
plications have laser powers ranging from tens of kilowatts
to 100 kW for tactical-level employment and up to multi-
megawatt for strategic class applications [1]. For compari-
son, a power laser pointer that emits less than 1 W can cause
permanent eye damage in less than 1 s, while average power
outputs of 300 W to 1 kW are commonly used for industrial
laser cutting.
Fig. 4. Russian 1987 mobile HEL defending an airfield. Image cour-
tesy of DIA.
Fig. 5. Russian Beriev A-60 (modified IL-76MD) with HEL turret and
nose mounted radar. 1.1-MW HEL turret, 2 — radar for detecting
aerial targets, 3 — compartment for 2.1 MW turboalternator. Photo-
graph taken at Taganrog Yuznyi Airport in May 2011 by O. Ziminov,
RovSpotters Team.
Fig. 6. GBU-54 laser joint direct attack munition under F/A-18 Hornet
wing [1].
The Beriev A-60 Russian research program which
started in the 1970s was aimed to demonstrate an airborne
HEL DEW capability and provide a baseline for the devel-
opment of operational weapon. So far, two demonstrators
were built, i.e., 1A1 A-60 flying in 1981, 1A2 A-60 flying in
1991 and again in 2009 (Fig. 5). A large bulge on the upper
back of A-60 is a sliding port for a 1-MW laser turret [5]. For
feeding the laser, two turboalternators AI-24WT rated at 2.1
MW were mounted at each side of the fuselage. The nose
mounted turret is equipped with Ladoga-3 radar for detecting
aerial targets [5].
In 2008, the US Army formally recognized the potential
of HEL technology for future weapons by awarding a contract
to Boeing for the HEL technology demonstrator [1]. The abil-
ity of aicraft to conduct counterair warfare is greatly enhanced
by a HEL weapon (Fig. 6). A HEL weapon can automati-
cally identify, acquire, target, and engage an enemy missile
or aicraft.
Particle beam weapon
A particle beam (PB) weapon is a type of DEW which di-
rects an ultra high energy beam of atoms, electrons or pro-
tons in a particular direction by a means of particle projec-
tiles with mass. The target is damaged by hitting it, and thus
disrupting its atomic and molecular structure. In the case of
electric current conductive target, a resistive heating occurs
and an electron beam weapon can damage or melt its target.
Electric circuits and electronic devices targeted by electron
PB weapon are disrupted, while human beings and animals
caught by the electric discharge of an electron beam weapon
are likely to be electrocuted.
The use of space-based PB weapons was first explored
in the late 1960s in the Soviet Union. The high-velocity PB
weapon can target satellites or intercontinental ballistic mis-
siles.
Airborne radar
Airborne radar (AR) systems can be carried by military,
special mission and research aircraft. Military applications of
AR include:
• targeting of hostile aircraft for air-to-air combat;
• detection and tracking of moving ground targets;
• targeting of ground targets for bombing missions.
Both civil and military applications include but are not re-
stricted to:
• accurate terrain measurements for assisting in low-
altitude flights;
• assisting in weather assessment and navigation;
• mapping and monitoring the Earth’s surface for environ-
mental and topological study.
ARs generally operate in the C or X bands, i.e., around 6
GHz or around 10 GHz, respectively. AR includes three major
categories:
• air-target surveillance and cueing radars mounted in
rotodomes (Fig. 7);
• nose-mounted fighter radars ( Fig. 5 and Fig. 8);
• side-looking airborne radar (SLAR) for ground recon-
naissance and surveillance (Fig. 9).
Fig. 7. AR mounted in rotodome on E-3B Sentry (Boeing 707
adapted to military functions).
The latter is the smallest sector of the airborne radar mar-
ket and is dominated by synthetic aperture radar (SAR) and
ground moving target indicator (GMTI) sensors. SAR, an ac-
tive all-weather sensor, primarily is used for two-dimensional
(2D) ground mapping. Radar images of an area help detect
fixed targets. GMTI radar picks up moving targets or vehi-
cles. A commercial version of SAR-GMTI, called HiSAR, is
an X-band radar that can see from about 100 km away.
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a/2012 152

Fig. 8. Modified C-18A (converted Boeing 707) as EC-18B advanced
range instrumentation aircraft (ARIA) with 2.1-m diameter radar dish
on the nose.
Fig. 9. Side-looking viewing geometry of imaging radar system.
Sketch drawn by the author on the basis of http://www.radartutorial.eu
Airborne early warning (AEW) systems and weather
radars use megawatt klystrons.
Technology challenges
There are two technically difficult challenges:
• the high voltage continuous electric power required for
DEW systems must be in the range of megawatts;
• a large amount of heat rejected from DEW system dur-
ing operation must be managed.
The thermal management challenge becomes difficult when
the large heat flux is coupled with a small airframe, e.g., a
fighter. The electrical power and thermal management sub-
system of a conceptual generic airborne electrical DEW sys-
tem is shown in Fig. 10. So far, the electrical power and ther-
mal management systems for airborne DEWs are in early
development.
prime mover
turbine
engine
electric
generator
power
conditioning
directed
energy
source
beam
control
thermal management system
heat
heat heat heat
Fig. 10. System block diagram for a generic electrically powered
airborne DEW system.
Classical wound-field synchronous generators in the
range of megawatts for airborne applications are rather
heavy. Synchronous generators with HTS rotor excitation
windings are investigated as a possible solution [9, 19]. Large
power, high speed HTS generators, if available, would be
lighter and more compact than conventional copper wire-
wound or permanent-magnet rotor generators. On the other
hand, windings of HTS generators operate at cryogenic tem-
perature of 77 K, i.e., the temperature of liquid nitrogen. This
requires refrigeration systems, which increase the mass of
airborne power plant. So far, the research is oriented towards
airborne generators of classical construction, i.e., not requir-
ing cryogenic installation.
Ferromagnetic materials
To minimize the overall dimensions of airborne multi-
megawatt generators, high saturation, low core loss ferro-
magnetic thin laminated materials are used. Iron-cobalt al-
loys with cobalt contents ranging from 15 to 50% have the
highest known saturation magnetic flux density, about 2.4 T
at room temperature. They are the natural choice for applica-
tions not only as aerospace generators but also as aerospace
motors [20], transformers and magnetic bearings [3], where
mass and space saving are of pr ime importance. Addition-
ally, the iron-cobalt alloys have the highest Curie temper-
atures of any alloy family and have found use in elevated
temperature applications. The nominal composition, e.g., for
Hiperco 50 from Carpenter, PA, U.S.A. is 49% Fe, 48.75%
Co, 1.9% V, 0.05% Mn, 0.05% Nb and 0.05% Si. Hiperco 50
has the same nominal composition as Vanadium Permendur
and Permendur V. The specific mass density of Hiperco 50 is
8120 kg/m
3
, modulus of elasticity 207 GPa, electric conduc-
tivity
2.5×10
6
S/m, thermal conductivity 29.8 W/(m K) , Curie
temperature 940
◦
C, specific core loss about 44 W/kg at 2 T,
400 Hz and thickness from 0.15 to 0.36 mm. The magneti-
zation curve of Hiperco 50 is as follows: 2.14 T at 800 A/m,
2.22 T at 1600 A/m, 2.26 T at 4000 A/m, 2.31 T at 8000 A/m
and 2.34 T at 16 000 A/m.
Similar to Hyperco 50 is Vacoflux 50 (50% Co) cobalt-
iron alloy from Vacuumschmelze, Hanau, Germany, typically
used for manufacturing very high flux density pole-cores and
pole-shoes of synchronous generators.
Electromechanical design
Multimegawatt generators are typically three-phase
salient-pole synchronous generators with outer stator and in-
ner rotor. The stator double-layer winding is distributed in
slots. The rotor field winding has a form of concentrated-
parameter coils wound on salient ferromagnetic poles.
The output electric power, as a function of dimensions,
electromagnetic loads, winding parameters, and rated pa-
rameters, according to the author
3
,is
(4)
P
out
=
π
2
6
√
2
1

k
w1
k
fill
(k
2
y
− 1)n
s
D
3
1in
L
i
j
a
B
mg
η cos φ
where n
s
= f/p is the synchronous speed, f is the sta-
tor current frequency, p is the number of pole pairs, k
w1
is
the stator winding factor for the fundamental harmonic,
D
1in
is the stator core inner diameter, L
i
is the effective length
of the stator core, j
a
is the stator (armature) winding cur-
rent density,
B
mg
is the peak value of the stator magnetic
flux density, k
fill
is the stator slot fill factor, k
y
is the stator
core outer diameter-to-stator yoke inner diameter (measured
at the bottom of slots) ratio,  is the EMF-to-voltage ratio, η is
the efficiency, and
cos φ is the power factor. For 1 ≤ p ≤ 20
the coefficient k
y
can be estimated as
(5) k
y
≈ 1.05 +
1
1.5p
For example, for n
s
= 15 000 rpm = 250 rev/s, 2p =8,
k
w1
=0.9, D
1in
=0.21 m, L
i
=0.22 m, j
a
=16.1 × 10
6
A/m
2
, B
mg
=0.9 T, k
fill
=0.48,  =1.07, η =0.95,
the output power according to eqn (4) is
P
out
=1.5 MW.
The coefficient
k
y
estimated on the basis of eqn (5) is k
y
=
1.217
. Assuming the airg gap (mechanical clearance) as g =
3
unpublished work
153 PRZEGL ˛AD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9a/2012

2.6 mm, the rotor surface linear speed is v = π(D
1in
−2g)n
s
= π(0.21 −2 ×0.0026) ×250 = 160.85 m/s. The allowable
rotor surface linear speed for compact wound-field salient-
pole synchronous machines with retaining sleeves is about
200 m/s.
The magnetic flux distribution in the cross section area
of a salient pole multimegawatt generator is shown in Fig. 11.
Rotor pole cores are highly saturated, because the volume
envelope of an airborbne generator must be as small as pos-
sible.
Fig. 11. Magnetic flux distribution in the cross section area of a 12-
pole, 72-slot multimegawatt generator as obtained from the 2D FEM.
The stator slots are semi-closed trapezoidal or semi-
closed oval slots. The number of stator slots per pole per
phase is from 4 to 10. Large number of stator slots per pole
per phase and double layer chorded windings allow for re-
ducing the contents of higher space harmonics in the air gap
magnetic flux density waveform. At high speeds (high fre-
quencies) coils have a low number of turns and a large num-
ber of parallel wires. At speeds 15 000 rpm and higher, very
often single turn coils must be designed in order to adjust
the induced EMF to the terminal voltage. Parallel paths are
common.
The outer surface of the stator core is sometimes corru-
gated to improve the heat transfer from the stator core sur-
face to the stator enclosure or the liquid jacket.
The number of salient rotor poles is typically from 4 to 12.
Pole shoes have round semi-closed slots to accommodate
the damper. The rotor core is made of the same material as
the stator core, i.e., iron-cobalt thin laminations. Rotor coils
are protected against centrifugal forces with the aid of metal
wedges between poles, which also participate in the cooling
system of the rotor [17]. Sometimes, in addition to wedges,
rotor retaining non-magnetic sleeves are used [8]. The rotor
outer diameter and shaft diameter depend, amongst others,
on the rotor cr itical speed. Problems of rotor dynamics are
much more serious than in low speed synchronous machines
[16].
Cooling techniques for high speed electric machines
High speed multimegawatt generators are cooled by the
following media:
• air;
• oil;
• aircraft fuel;
• water/glycol mixture;
• refrigerant.
In a direct air cooling system the air enters through the hous-
ing or end bell and passes through the ducts between con-
ductors, air gap and sometimes through rotor channels. The
air is exhausted through a perforated screen around the pe-
riphery of the housing or special air outlet. Direct air cooling
in high power density generators is rather insufficient.
Table 1. Heat transfer properties of some materials.
Thermal Specific heat Specific
Material conductivity capacity density
W/(m
◦
C) J/(kg
◦
C) kg/m
3
Water at 20
◦
C 0.63 4184 997.4
Ethylene glycol 0.25 2380 1117
Engine oil at 20
◦
C 0.15 1880 888
Transformer oil at 25
◦
C 0.131 1870 879
Jet fuel (Jet A, Jet A1
and JP-8) at
50
◦
C 0.122 2050 784
Jet fuel (Jet A, Jet A1
and JP-8) at
100
◦
C 0.111 2300 735
Air at 20
◦
C 0.025 1005 1.177
Hydrogen 0.175 980 0.084
In most airborne multimegawatt generators liquid cooling
must be employed. Heat transfer properties of liquids and
gases are compared in Table 1. In general, liquids are much
better coolants than air in terms of ther mal conductivity and
specific heat capacity.
Hollow conductors and direct liquid cooling seem to be
economic solution for generators rated at 1 MW and above.
A typical liquid loop consists of an air-liquid heat ex-
changer, which is used to dump the heat load being car-
ried by the liquid into the air conditioning system, a pump
and resorvoir [13, 15]. Fig. 12 explains an oil cooling sys-
tem for a multimegawatt wound-field synchronous generator
with heat exchanger. The stator is cooled with the aid of an
oil jacket and the rotor is cooled by pumping oil through the
shaft. This hydraulic circuit employs a pressure sensing re-
lief valve to regulate external circuit oil flow at minimum 38
liter/min
=0.63 liter/s.
Fig. 12. Simplified diagram of oil cooling system for a multimegawatt
aicraft synchronous generator with heat exchanger. 1 — stator of
synchronous generator, 2 — rotor of synchronous generator, 3 —
air-oil heat exchanger, 4 — pump, 5 — primary coolant (oil), 6 —
secondary coolant (air). Oil reservoir is not shown.
The liquid coolant, i.e., oil or water is pumped through
the stator jacket or through the stator hollow conductors (di-
rect cooling system) and cooled by means of a heat ex-
changer system. Since the current density in the rotor field
winding is high, the rotor also must be cooled by pumping the
oil through the hollow shaft. The heat exchanger is a compo-
nent that keeps two coolants separate, but allows transfer of
heat energy between them. The primary coolant has lower
temperature than machine parts. The secondary coolant has
lower temperature than the primary coolant.
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Table 2. Typical current densities for electrical machines with differ-
ent cooling systems.
Cooling system Current density, A/mm
2
Totally enclosed machine,
natural ventilation
4.7 to 5.4
Totally enclosed machine,
external blower 7.8 to 10.9
Through-cooled machine,
external blower 14.0 to 15.5
Liquid-cooled machine
23.3 to 30.0
The cooling can be intensified by adding a fuel-oil heat
echanger also called fuel-oil cooler (Fig. 13). A fuel is used
to cool the generator oil and then the hot fuel can be pumped
back to the wing fuel tanks, providing partially the wing de-
icing. When the fuel flow is low, the fuel temperature will rise
significantly, so recirculation lines are used to pipe the hot
fuel back into the fuel tank [15]. Fuel is much better cooling
medium than the air (Table 1).
Fig. 13. Simplified diagram of oil cooling system for a multimegawatt
aicraft synchronous generator with two heat exchangers. 1 — sta-
tor of synchronous generator, 2 — rotor of synchronous generator,
3 — oil-air heat exchanger, 4 — pump, 5 — primary coolant (oil),
6 — secondary coolant (air), 7 — fuel-oil heat exchanger, 8 — fuel
(secondary coolant), 9 — fuel temperature control. Oil reservoir is
not shown.
Similar to high-speed compressors and microturbines, a
refrigerant, e.g., R-12 or R-134a can also be used for cooling
multimegawatt generators. Refrigerant is directed to cool the
stator core outer surface and/or stator core inner surface (air
gap).
With increase in the output power, the rotor cooling prob-
lems become very difficult. One of methods is to use alu-
minum cold plates between the rotor coils and rotor pole core
[14].
Typical current densities for electrical machines with dif-
ferent cooling systems are given in Table 2. Those values
must be verified with calculation of internal temperature dis-
tribition using the thermal equivalent circuit or better – with
the finite element method (FEM).
Table 3 shows a comparison of selected cooling tech-
niques for high speed electric machines. The current den-
sity in the windings depends on the class of insulation, cool-
ing system and duty cycle (continuous, short time or inter-
mittent). The current density values given in Table 3 are for
220
◦
C (maximum operating temperature of windings). The
Table 3. Selected techniques for enhancing heat dissipation in elec-
tric machines
direct cooling system with hollow conductors is the most in-
tensive cooling system (up to 30 A/mm
2
). Spray-oil cooling
(28 A/mm
2
) is almost as intensive as direct cooling. Using
cold plates between pole cores and coils, the estimated max-
imum current density should not exceed 22 A/mm
2
. Thus, the
spray oil-cooled rotor windings allows for maintaining higher
current density than cold plates. Spray cooling of the rotor
wire together with intensive cooling of the stator winding will
theoretically lead to smaller size and weight than application
of cold plates.
First prototypes
Air-cooled 2.5-MVA, 13 600-rpm synchronous generator
The first lightweight, high-speed, multimegawatt synchronous
generator for helicopter application was built and tested
in 1976 by Aerojet Electrosystems Company, Azusa, CA,
USA, under contract with Westinghouse Electric Corporation,
Lima, OH, USA [11]. It was a three-phase, 2.5-MVA, 13 600-
rpm, 2887-5000-V, 6-pole, continuous-duty, air-cooled syn-
chronous generator (Fig. 14). The generator was designed
to be driven by the T64 GE turboshaft engine (prime mover).
The architecture was the same as that of a conventional
brushless aicraft synchronous generator, i.e., main gener-
ator, brushless exciter, and permanent magnet generator
(PMG) – the so-called subexciter. The 650-mm long stator
stack was made of 0.127-mm thick punchings. The 6-pole
rotor was also laminated. The frequency was 680 Hz, full-
load current 421/271 A, power factor 0.84/0.92, mass 680
kg, and power density 3.15 kW/kg.
Fig. 14. 2.5-MW, 13 600-rpm, air cooled lightweight generator [11].
1-MW synchronous generator with cold plates
Innovative Power Solutions (IPS), Eatontown, NJ, USA has
recently announced a lightweight megawatt-class airborne
generator [6, 7]. Effective rotor cooling system with cold
plates has been used to reduce the size of the generator [14].
The IPS megawatt airborne generator is a synchronous
generator with salient-pole wound rotor (electromagnetic ex-
citation) and conventional stator with laminated core and
155 PRZEGL ˛AD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9a/2012

winding distributed in slots. A new patented method of cool-
ing the rotor poles and conductors has been implemented
[14]. This method uses cold plates disposed between each
rotor pole and field coils. A cooling medium (liquid or gas) cir-
culates in the rotor. Each cold plate serves to conduct heat
from both the pole core and winding. The cooling medium
enters the rotor through the shaft and is distributed between
cold plates via manifolds, transfer tubes and plugs. The cool-
ing medium after exiting the rotor (through the shaft) is then
conducted to a heat sink or heat exchanger where its tem-
perature is reduced.
Fig. 15. Construction of rotor poles and winding. 1 — pole core,
2 — pole shoe, 3 — field winding, 4 — cold plate identical elements,
5 — cold plate passageways, 6 — V-shaped wedge, 7 — top wedge
[14].
Fig. 16. Rotor of 1 MW IPS generator with cooling system [14].
According to IPS, the lightweight airborne 1 MW gener-
ator is 406 mm in diameter, 559 mm long and weighs 210 kg.
The power density is 4.76 kW/kg.
To design the rotor field winding, IPS has used flat wires
with rectangular cross section in an edge-winding fashion
similar to how a slinky toy looks [6, 7]. The wire is in con-
tact with cooling media along the entire per imeter of the coil.
The smaller dimension of the wire is disposed toward the
pole core lateral surface and the larger dimension is paral-
lel to the pole shoe, as shown in Fig. 15. Since a rectangular
cross section wire has bigger area of contact between adja-
cent wires than an equivalent round wire, the heat transfer
characteristics for rectangular wires are better.
The rotor may have one or more cold plates surrounding
each pole core. Fig. 15 shows a rotor with a pair of identi-
cal cold plates per pole. Each cold plate has passageways
for conduction of a cooling medium (Fig. 16). Either liquid
(oil) or gas cooling medium can be used. The end region of
each cold plate matches the bend radius of the field excitation
Fig. 17. Longitudinal section of IPS’ lightweight megawatt generator.
Arrows show cooling locations. Courtesy of IPS, Eatontown, NJ,
USA.
coils. The proposed shape of cold plates does not increase
the length and diameter of the rotor.
For fabrication of cold plates high thermal conductivity
materials are used, i.e., aluminum, copper or brass [2]. The
cold plate preferably includes its own insulating layer, e.g.,
in the case of aluminum, the insulating material is aluminum
oxide with its thickness of 0.125 to 0.25 mm.
To provide adequate mechanical integrity of the rotor at
high speeds and maintain good contact between the winding
and cold plates, V-shaped wedges press the winding against
cold plate surfaces (Fig. 15). Top wedges are used to se-
cure V-shaped wedges in their positions (Fig. 15). Cooling
locations fro rotor and stator are shown in Fig. 17.
Cold plates can be designed as two-part or single-part
cold plates. In the first case both parts are identical. A pair of
transfer tubes with plugs at each end of a cold plate provides
hydraulic connection with manifolds located at opposite ends
of the rotor [14]. This forms a closed system for circulation of
cooling medium.
The overall cooling system can be improved by adding
radial fans to the rotor and fins to the internal housing. Such
a design, although increases windage and ventilation losses,
can help to remove heat from the air within the generator and
transfer heat to the aluminum housing (Fig. 17). The rotor
and stator cooling technique implemented by IPS leads itself
to compact generator design. However, the cold plate cooling
system is less effective than spray oil-cooled end windings.
Oil-cooled 2.5 MW synchronous generator
Electrodynamics Associates, Inc., Oviedo, FL, has demon-
strated in 2011 a 2.5-MW, 15 000-rpm, 1500-Hz, oil cooled
multimegawatt synchronous generator ( Fig. 18). The prime
mover is planned to be a Rolls Royce T406 (AE 1107C-
Liberty) turboshaft engine that would be capable of provid-
ing full power at altitude. The load would be a rectifier and
ultimately some kind of DEW.
The generator is a conventional salient-pole (
2p =12),
wound-rotor synchronous machine with an exciter embed-
ded in the rotor and oil-cooled rotor and stator. Details are
given in Table 4 and Table 5. The power density claimed is
14.1 kW/kg for hogged aluminum housing and 16.7 kW/kg for
magnesium housing.
The duty cycle is 6 min on and 12 min off. The generator
is oil-cooled with maximum oil flow rate of 115.5 liter/min. The
inlet oil temperature is
65.5
◦
C. The oil cools the stator back
iron with some incidental end turn wetting. The rotor field
winding end turns are oil-sprayed.
PRZEGL ˛AD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9
a/2012 156

Table 4. Mechanical specifications of 2.5-MW, 15,000-rpm syn-
chronous generator.
Speed, rpm 10,000 to 15,000
Mass, kg 177 (Al housing, without oil)
Dimensions, mm 343 L
× 559 W × 456 H
Rotor
welded
sleeve inconel
Design life, h 1000
First critical
speed, rpm
8000
Laminations Hiperco Co alloy
Rotor squeezed
damper film damper
Table 5. Electrical specifications of 2.5-MW, 15,000-rpm syn-
chronous generator.
Rated power, MW 0.5 to 2.5
Number of phases
3
Number of poles 2p =12
Frequency, Hz 1000 to 1500
Line voltage, V
620/1240
Voltage
regulation, %
±1
Phase current, A 2500/1250
Power factor
0.95
Efficiency, % 85 to 95
Conclusion
High speed synchronous generators in the megawatt
power range are necessary for new airborne missions as
DEW and AR. Potential candidates are (a) wound-field,
salient pole synchronous generators, (b) synchronous gener-
ators with HTS field excitation system, (c) homopolar gener-
ators with stationary HTS winding, and (d) all cryogenic gen-
erators. There are two difficult challenges in construction of
high speed multimegawatt generators:
• high power density, low envelope volume and low mass;
• thermal management and heat dissipation.
Intensive oil cooling system of both the stator and rotor is
required to minizmize the size and mass of the generator.
Cooling systems with primary and secondary cooling media
and heat exchangers are required. It is necessary to point
out that high current density in the stator and rotor winding
reduces the effciency. The desired efficiency should be at
least 95%. At present time, classical rather than HTS air-
borne multimegawatt generators are preferred.
Fig. 18. Multimegawatt oil-cooled synchronous generator rated at
2.5 MW, 15,000 rpm, 1500 Hz. Photo courtesy of Electrodynamics
Associates, Inc., Oviedo, FL, USA.
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Authors: Prof. Jacek F. Gieras, Ph.D., D.Sc., IEEE Fel-
low, Department of Electrical Engineering, Electrical Ma-
chines and Drives, University of Technology and Life Sci-
ences, Al. S. Kaliskiego 7, 85-796 Bydgoszcz, Poland, email:
jacek.gieras@utp.edu.pl
157 PRZEGL ˛AD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9a/2012