Download HIGH SPEED INDUCTION GENERATOR FOR APPLICATIONS IN

2004-01-3174
HIGH SPEED INDUCTION GENERATOR FOR APPLICATIONS IN
AIRCRAFT POWER SYSTEMS
Jay Vaidya, President, Electrodynamics Associates, Inc.
Earl Gregory, Power Generation, Air Force Research Laboratory
Copyright © 2004 SAE International
ABSTRACT
Electric generators have higher
power density when the operating speed is
increased. High speed electric generators
direct coupled to gas turbines provide an
ideal source of electric power for
airborne applications because of reliable
operation and high power density. To
function reliably in the speed range of
60000 RPM to 120000 RPM, the rotor of the
electric generator must be robust.
Examples of the robust rotor technologies
for the generator include: permanent
magnet (PM), induction and switched
reluctance. The objective of the present
paper is to describe the current
activities in the field of high speed
induction generators and associated
controllers. A range of induction
generators and controllers rated from 5 kW
to 200 kW operating at speeds to 62000 RPM
is currently under development. The rotor
is designed using high strength magnetic
materials for the magnetic paths, and high
strength alloys for the conductors that
form the squirrel cage. Tests have been
successfully conducted to demonstrate the
high yield strength of the rotor
construction materials. High power density
of the induction generator designs is
demonstrated by the electromagnetic
weights of the induction generators: 5 kW
weighs 1.7 Lbs., 30 kW weighs 5 Lbs., and
200 kW weighs 37 Lbs. in electromagnetics.
The benefits of induction generator for a
variety of airborne applications are:
Simple, low cost, robust rotor
construction; Electrical excitation allows
de-excitation instantly in case of
internal failure to prevent further
damage; Output voltage can be well
regulated dc or ac, under steady state and
transient conditions; For very high
voltage or for very low voltage
applications, transformers can be used get
the desired voltage output, which helps
improve the controller power density; For
generator control, the rotor speed
information and not the rotor position
information is needed, which simplifies
the design of the sensor itself; Because
no permanent magnets are required, the
generator can function in hostile
temperature environment. The 5 kW and 30
kW generators are designed for operation
at 5000F for location inside the turbine.
Three generators rated at 5 kW, 30 kW, and
200 kW are in various stages of
fabrication and testing. Closed loop
operation using DSP based controllers is
implemented. Test data relating to the
performance of the induction generators
and controllers will be presented. The
test conditions will include the steady
state voltage regulation, transient
voltage regulation, and generator
efficiency analysis.
INTRODUCTION
The electric power generators
convert mechanical energy from a rotating
prime mover, into electrical energy. The
approach for the energy conversion is
based on the interaction between magnetic
fields and electric currents. For airborne
applications, power density in terms of
power per unit weight is the essential
figure of merit. In order to accomplish
the high power density, operation of the
generator at high speed is desirable.
Typical speeds of airworthy turbines for
power up to 200 kW are in the range of
60000 RPM to 120000 RPM. For operating in
this speed range robust construction of
the rotor of the electric generator is a
must. Typical electrical insulators as
well as active components do not perform
reliably in this speed range. Therefore
use of generator technologies that avoid
insulated electrical windings, electrical
connections, and rectifier diodes is
desirable.
The most commonly used wound rotor
synchronous generator of a brushless
design has two stages. These stages
include the main and the exciter with a
rectifier interface between the two. Thus
this generator has two insulated winding,
rectifiers, filter and several connections
in the rotor. This approach is generally
considered unreliable at high speeds of
operation. Other technologies that are
suitable for high-speed operation are:
1. Permanent Magnet
2. Induction
3. Switched Reluctance
4. Homopolar and Rice
Of these several approaches, the
objective of the present paper is to
describe the induction generator
technology and discuss some of the results
of the work done by the authors in recent
years in developing induction generators
for airborne applications.
OPERATING PRINCIPLES OF INDUCTION
GENERATOR
Induction motors are the most
commonly used electric motors from
household appliances to airborne and space
power systems. The reason for this is that
induction motors are robust, can be
controlled easily, and cost less to
produce compared to most other types of
motors. Many of the same features of the
induction motors apply to the induction
generators as well. In construction, the
induction generator is made in the same
manner as an induction motor. It has two
components:
•
A rotor having conducting bars
located inside a slotted iron
core, with the bars short
circuited at two ends, and
• A stator having a three-phase
(or multiple phase) winding
located inside the insulated
slots of an iron core.
Figure 1 shows a cross-sectional view of
the induction generator.
ARMATURE CORE
a rotating magnetic field is created in
the induction machine. This rotating
magnetic field causes currents to flow in
the short-circuited conductors of located
in the rotor. The interaction between the
magnetic field and the current in the
rotor conductor causes production of a
mechanical force or torque. This in turn
causes the rotor to begin rotating. As the
rotor rotates, the difference between the
speed of rotation of the rotor (ωr) and
the speed of rotation of the magnetic
field (ωm) reduces. This difference is
called the slip (s). As the slip
approaches zero, the torque produced by
the electromagnetic interaction reduces
rapidly, until it reaches zero when the
slip s is zero.
Figure 2 shows torque speed
characteristics of the induction machine
as the speed rises from zero to ωm, and
the slip falls from ωm to zero. Note that
the rotational speed of the magnetic field
is called the ‘synchronous speed”. And the
rotor is said to rotate “asynchronously”.
This range of operation is the “motoring”
range when the electrical energy is
converted into mechanical energy at the
shaft of the induction machine. It should
be noted that the torque production near
the synchronous speed varies in a linear
fashion as shown by the stable region
indicated in Figure 2.
If the induction machine is driven
by a prime mover at a speed higher than
the speed of the magnetic field (ωm), then
the electromagnetic interaction begins to
produce torque in the opposite direction.
Or to view this more correctly, it absorbs
the mechanical energy from the prime mover
and produces electrical energy. This is
what is called the “super synchronous”
operation of the induction machine, or the
generation mode.
Figure 2: Induction Motor Torque v/s Speed
ARMATURE
WINDING
100
90
80
SQUIRREL
CAGE
ROTOR CORE
Torque, %
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
Speed, % of Synchronous Speed
Figure 1: Induction Generator Cross
Section
When a 3-phase ac electric power
source is applied to the stator windings,
It should be noted here that the
useful range of operation of the induction
generator is in the linear region marked
as stable region in Figure 3. If the speed
of operation exceeds the limit, then the
output power starts falling rapidly.
discussed here briefly prior to the
discussion relating to the control of the
induction generator.
Figure 3: Induction Generator Torque v/s Speed
Torque, %
0
-10 100
-20
-30
-40
-50
-60
-70
110
120
130
140
150
160
170
180
190
ISOLATED OPERATION
The airborne power system must
function independently and has only a
small battery power source to draw from
for initialization and emergency. It must
function reliably to provide the power in
a stable manner.
200
-80
-90
-100
Speed, % of Synchronous Speed
Stable Region
The torque-speed curves shown in
Figure 2 and 3 are typical of the
induction machines. These apply to
specific designs at specific terminal
voltages. The torque itself is a function
of the voltage and varies as a square of
the voltage under unsaturated magnetic
conditions. Thus the power output is a
function of the square of the voltage. As
can be seen from Figure 3, it is a linear
function of the slip in the stable region.
In a typical generator application,
the terminal voltage must be held nearly
constant. Thus to vary the power output as
dictated by the load requirements, the
slip must be adjusted. Figure 4 shows how
the change of slip at a given speed of the
prime mover affects the change in the
output power for a given terminal voltage.
To change the slip, the electrical
frequency of the voltage applied to the
generator stator must be varied in
relation to the rotational speed of the
rotor.
Figure 4: Torque Speed Curves with Varying Slip
0
-10
-8
-6
-4
-2
0
2
T o rq u e , %
-20
-40
-60
-80
-100
CONSTANT VOLTAGE OPERATION
For typical high voltage dc
applications, the generator must provide a
reasonably constant voltage under steady
state load and overload conditions, with a
permissible drop under external short
circuit.
SPECIAL APPLICATIONS: DIRECTED ENERGY
In directed energy applications for
high power microwave weapons, voltage step
up using an ac 3-phase transformer is
desirable. In certain weapon applications,
the load appear more like a capacitor to
be charged from say 10% of the nominal
voltage to 100% of the voltage, with a
constant current rather than constant
voltage.
ENGINE STARTING
In most airborne applications, it is
also desirable that the generator
functions as a starter to bring the prime
mover speed to the level when it produces
the necessary torque.
HIGH SPEED OPERATION
Operating requirement of 60000 RPM
to 120000 RPM requires a careful design of
the rotor structure using high strength
materials as well as containment design to
limit the stresses in the rotating
components.
THERMAL
Cooling of the stator and rotor is
typically accomplished by airflow or by
use of engine lubricant oil. In certain
integrated starter generator applications,
the generator is located on the same shaft
as other turbine components and may be
subjected to a high temperature
environment of up to 5000F or more.
Slip, %
POWER CONDITIONING
REQUIREMENTS FOR HIGH SPEED AIRBORNE
GENERATOR
The airborne electric power systems
have peculiar and perhaps stringent
requirements, which affect the design,
operation, and control of the induction
generator. These requirements are
It is clear from the earlier
discussion that the induction generator
needs an ac source to magnetize the
magnetic circuit with a certain frequency
that can be adjusted. This is accomplished
by use of a 3-phase inverter typically
built using IGBT’s in the configuration
shown in Figure 5.
Hysteretic Control
1.4
Induction
Machine
Current
+
0
0
10
20
30
40
50
60
70
80
90
Angle, Degree
_
Figure 5: Three Phase Inverter Connected
to Induction Machine
The IGBT’s are switched to provide
ac currents in the generator stator
windings at the desired frequency. There
are two techniques commonly used for this:
i)
Pulse width modulation (PWM)
ii)
Hysteretic current control
In the PWM technique, the IGBT’s are
switched at a fixed switching frequency
and turned off when the desired level of
current is reached. This is shown in
Figure 6. Thus what is controlled is the
“ON” time of the pulse.
Pulse Width Modulation
Figure 7
The control of the induction machine
in the generator mode as well as in the
start mode is accomplished by control of
the current supplied to the 3 phases. The
relationship between the torque at the
shaft and the current can be visualized
better in the 2-phase frame of reference,
called the D-Q frame. From the fundamental
electromagnetic relationships, it is known
that the torque T is proportional to the
product of the magnetizing current (ID)
and the current IQ. In the non-salient
machine such as the induction machine, the
D-Q axes can be arbitrarily selected so
that ID= IQ. Then we express the torque
equation as:
2
τ rωs
)
τ r 2ω s2 + 1
T = kT ( I DQ )(
Where KT = Torque Constant, Nm/A2
IDQ = √2 ID, A
1
A m p litu d e
τr =
ωs =
Rotor time constant, s
slip frequency, radians/s
For optimum torque,
τr =
1
ωs
, and
2
0
0
30
60
90
Angle, Degree
Figure 6
In the hysteretic current control,
the command current is compared to the
actual current, and the switching of the
IGBT’s is accomplished using a hysterisis
band around the commanded current as shown
in Figure 7.
T = 12 kT I DQ ………………………………………(1)
GENERATOR CONTROL
Here we use the energy storage
equation for the generator-load systems
shown in Figure 8.
2
DC
Bus
3 Phase
Transistor
Bridge
Generator
( s + s0 ) 3 = s 3 + 3s0 s 2 + 3s0 s + s03
ki =
( s03τ em )
ηω
(3s0 τ em ) − 2 /( RC )
2
,kp =
ηω
, kd =
τ em (3s0 − 2 /( RC )} − 1
ηω
R
Here
τem = electromechanical
η = machine efficiency
ω = shaft speed, rad/s
C
For constant voltage operation, the
command voltage vc is held constant. For
constant current charging of a capacitor,
the command voltage is ramped up from the
initial low voltage to the final peak
voltage of the capacitor.
Figure 8: Generator and Load
Cv
Wr = r
2
2
………………………………………. (2)
START MODE OPERATION
In this case, the motor current in
the DQ frame of reference is controlled
and the output speed of the induction
machine is compared to a commanded speed
or a speed ramp. An IP control is used as
shown in Figure 10.
Where
Wr = stored energy in the capacitor, J
C = capacitance, F
vr = voltage across the capacitor, V
Equation (1) and (2) are used to
develop, a PID control algorithm to
control the square of the voltage (V2r) by
comparing it to a command voltage (vc)
squared. The result is a third order
transfer function derived from Figure 9
for the control loop.
RC / 2
RC
(
+ 1)
2s
vr
ωa
_
T
g /(τ rr s + 1)
ηω
τs + 1
+
time constant, s
_
ωr
WL
1
/
s
ki
+
+
W
_
Wr
ki
1
s
+
kp+kds
Figure 9: Generator Control Loop
kp
+
_
Figure 10: Motor Control Loop
The transfer function and IP
parameters are as follows:
gk i / τ rr
s 2 + s ( gk p + 1 ) / τ rr + gk i / τ rr
Compare the denominator to
The transfer function is:
k iηω / τ em
, where
s 3 + as 2 + bs + c
1 + k pηωRC / 2
τ + RC / 2 + ηωkd RC / 2
a = em
,b =
, c = kiηω / τ em
τ em RC / 2
τ em RC / 2
Comparing the denominator with
(s + s0 ) 2
ki =
τ
rr
s0
g
2
,k
p
=
2 s 0τ
rr
−1
g
Where
g = unit speed change per unit torque
change, (rad/s)/Nm
τrr = rotor time constant, s.
_
SOFTWARE IMPLEMENTATION
Implementation of the control
algorithms is made in the software using
MathWorks XPC modules or by using Texas
Instruments DSP’s.
HARDWARE IMPLEMENTATION
Electrodynamics has implemented the
design approach discussed here in the
hardware for the 200 kW induction
generator operating at speeds up to 62000
RPM. Similar approach is taken for the 5
kW and 30 kW induction generators, which
are currently under development. Figure 11
shows the generator hardware
Figure 13: Closed Loop Control of Voltage
Figure 14 shows the test results of
the current control in a closed loop
system. The current command is shown by
the blue line and the actual current is
shown by the green line. As the command
for the increase in current is applied,
the actual current lags behind the command
but reaches the desired current level in a
few milliseconds.
Figure 11: 200 kW Generator
Figure 12 shows graphically the
electromagnetic weights of the three
generators to provide approximate scaling
of the induction generators in 5 kW to 200
kW range.
Weights of 60000 RPM Induction Generators
40
Figure 14: Closed Loop Current Control
Weight, Lb.
35
30
25
Figure 15 shows the operation of the
generator in the start mode. The generator
speed follows the speed ramp command to
reach the desired speed in 90 seconds as
commanded.
20
15
10
5
0
0
50
100
150
200
Pow er, kW
Figure 12
TEST RESULTS
Figure 13 shows the test results of
the closed loop control of voltage. The
voltage at the dc bus is shown by the blue
line and the current is shown by the green
line. As the load is applied, the voltage
dips somewhat but recovers in a few
milliseconds to the original level.
Figure 15: Start Mode Operation
TECHNOLOGY BENEFITS
Based on the experience in
developing the induction generators and
controllers in 5 kW to 200 kW range, we
list below the benefits that the
technology provides for a variety of
airborne applications requiring DC bus
voltage.
ROBUST CONSTRUCTION
The rotating component has no
insulated windings or rectifiers, which
are the weak members of electromagnetic
guts.
LOW COST
Since no permanent magnets are used,
the cost of materials and labor are
reduced.
FLEXIBLE CONTROL
Either voltage or current can be
regulated. Start mode motor operation and
generator mode operation are accomplished
easily with software based controls.
HIGH VOLTAGE APPLICATIONS
For applications such as high power
microwave, the ac output of the induction
generator is stepped up using a small
transformer. The rectified dc voltage from
the transformer is controlled by methods
discussed earlier at the low voltage
level.
SPEED FEEDBACK
Use of speed feedback rather than
position feedback, simplifies the
controller hardware and reduces the cost
as well.
HOSTILE THERMAL ENVIRONMENT
Since no permanent magnets are used,
operation in high temperature environment
is feasible.
INSTANT CONTROLLER RESPONSE
In case of internal failure, the
excitation and hence the production of
power can be shut down within
milliseconds, thus preventing further
hazard to the equipment and the
surroundings.
CONCLUSION
High-speed operation of the
generators for airborne applications is
desirable for reaching high power density
goals. Induction generator technology
offers an ideal approach to operation
under hostile conditions of temperature
and speed. It also provides the benefits
of low cost robust construction with
simple, flexible control. The approach
taken by Electrodynamics Associates, Inc.
in developing the induction generator
technology is presented. Hardware and test
data to support the capability claims for
the technology are also presented.
ACKNOWLEDGMENTS
Major portion of the work described
in this paper is supported by the funding
from AFRL under contract number F33615-00C-2018.
CONTACTS
Jay Vaidya is the President and Principal
Engineer at Electrodynamics Associates,
Inc., 409 Eastbridge Drive, Oviedo, FL
32765. He specializes in design,
development and manufacture of Electric
Motors and Generators as well as
Controllers using state of the art
technologies including PM, synchronous,
induction, and SR. He may be contacted at
[email protected], or by phone at (407)
977 1825.
Earl Gregory is with Air Force Research
Laboratory, Power Generation Branch,
Wright-Patterson AFB, OH 45433. He is the
Program Manager for the AFRL contract
cited above. He may be contacted at
[email protected], or by phone at
(937) 255 6205.