The Advanced Induction Motor
Clive Lewis, Member, IEEE
Abstract-- The ALSTOM Advanced Induction Motor has
evolved over a period of around 15 years, during which time
function and performance have steadily improved. In the electric
warship, compactness of the propulsion motor is important, but
so also must be reliability, structureborne noise signature, and
shock withstand ability. This has been achieved in the Advanced
Induction Motor through optimization of the electromagnetic
design and material properties to suit the operating conditions of
a propulsion motor. One of the advantages of an induction motor
is its mechanical simplicity. This leads to not only to inherent
reliability, but also to simpler design for shock requirements.
Through careful motor and system design, it has been possible
reduce structureborne noise signatures to levels that permit hard
mounting of the motor to the hull of a surface combatant. The
paper reviews the evolution of the Advanced Induction Motor
from industrial induction motors, and the considerations
necessary to achieve low noise signature and shock requirements.
Index Terms—A C Motors, Electric machines, Induction
motor drives, Marine electrical equipment, Propulsion, Rotating
machine acoustic noise.
T
I. INTRODUCTION
HE Advanced Induction Motor (AIM) has been
developed by ALSTOM over a period of around 15 years.
It was developed as a solution for applications where a low
speed, high torque drive is required. Such applications include
rolling mill motors, mine winders and ship propulsion motors.
The high power density of the AIM has made it particularly
attractive for marine propulsion, and in particular for warships
where space is at a premium. The AIM was selected for the US
Navy’s Integrated Propulsion System program in 1995, and
has been selected as the main propulsion motors for the next
generation of Royal Navy warships, the Type 45 destroyer [1].
The AIM has been the most successful to date of the motor
technologies under consideration for electric warship
propulsion. The alternative motor technologies are discussed
in more detail by Hodge and Mattick in [2]. Based on
induction motor principles, the AIM has an inherently simple
mechanical structure, and hence inherently high reliability and
shock withstand capability.
The evolution of the AIM will be discussed, with particular
regard to the achievements in power density, structureborne
noise signature and shock requirements that are essential for
electric warship applications.
II. POWER DENSITY
In applications where a low speed, high torque drive is
required the design of an induction motor can be optimized to
increase the power density without any penalties in loss of
performance. One of the measures of power density is the
airgap shear stress of a motor. This is the force per unit area on
the rotor surface due to the motor torque, and is defined as:
σg =
τ
2
2π rr l r
(1),
where σg is the airgap shear stress, τ is the motor torque rr is
the rotor radius, and lr is the rotor core length.
A comparison of the airgap shear stress in motors is shown
in table I.
Table 1 – Airgap Shear Stress Comparison
Motor
Standard large industrial induction
motor
High performance industrial 1500
rpm induction motor
Low speed mill motor (origins of the
AIM) (1992)
IPS Motor for US Navy, 19 MW at
150 rpm, (1997)
Current AIM design, 20 MW at 180
rpm, (2002)
Permanent Magnet Motor
High Temperature Superconducting
Motor, 25 MW at 120 rpm
Airgap Shear Stress
(kNm-2)
13
35
45
76
100
120
340
It can be seen that the shear stress in the AIM is now
approaching that of permanent magnet motors.
An example of a low speed, high torque mill motor, which
incorporates some of the early features of the AIM is shown
during assembly in Figure 1.
The current AIM design for a naval propulsion motor is
illustrated in figure 2.
This shock compliant naval propulsion motor develops
20 MW at 180 rpm, with a total mass, including air/water heat
exchangers of 89 tonnes, and with overall dimensions of 3 m
high x 3.6 m wide x 3.3 m frame length.
Some of the developments that have led to the high power
density in the current generation of AIM designs will now be
discussed.
C. D. Lewis is with ALSTOM, Rugby, UK.
0-7803-7519-X/02/$17.00 © 2002IEEE
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B. Cooling
An efficient cooling mechanism is a necessity in motors
with high power density. Heat conduction and transmission
path between the source of the heat, predominantly the copper
conductors, and the ultimate removal of the heat should be as
short as possible. The AIM is fundamentally an air cooled
machine, with air/water heat exchanger when required. It uses
stator and rotor core construction incorporating radial
ventilation ducts that are provided using a patented ALSTOM
method known as “pin-vent”. This provides an efficient means
of removing heat from the windings. A typical pin-vent
lamination is illustrated in Figure 3.
Figure 1 – Mill Motor, 3.7 MW at 40 rpm
Figure 3 – Typical pin-vent lamination
III. STRUCTUREBORNE NOISE SIGNATURE
Figure 2 – AIM Propulsion Motor, 20 MW at 180 rpm
A. Electromagnetic Optimization
A standard industrial induction motor is normally required
to start from a fixed frequency supply, either at full line
voltage or at reduced voltage. In order to achieve satisfactory
starting performance, the performance of the induction motor
at full load must be compromised. When a motor is driven by a
variable speed drive new degrees of freedom are available to
the motor designer [3]. The electromagnetic design of the
motor, and particularly the rotor can be optimized for
operation at low slip. Further improvement to the performance
can be obtained by the appropriate choice of the motor pole
number and frequency. If a relatively low fundamental
frequency is chosen (2 – 20 Hz) material properties can be
optimized for the low frequency operation. In standard
induction motors, low speeds and high pole numbers normally
result in a poor power factor, and it is necessary to use as small
an air gap as possible to maximize the power factor. In the
case of a 12-pole AIM it is possible to obtain power factors in
the region of 0.85-0.9, even with the relatively large airgap of
about 8 mm necessary for naval shock and structureborne
noise signature requirements. This size of airgap is equivalent
to that used in traditional DC naval propulsion motors. The
high power factor reduces the rating requirements for the drive
power electronics, and also results in reduced losses in the
power electronics, resulting in higher overall system
efficiency.
In naval applications a low structureborne noise signature is
important. A standard induction motor is not normally
regarded as suitable where low noise signature is required,
since the combination of a slotted stator and rotor together
with a small airgap produce significant levels of force on the
stator teeth, and hence high noise levels. Careful attention to
all aspects of the design, together with some of the advantages
of the AIM discussed above, result in a structureborne noise
signature compatible with hard mounting of an AIM in a
surface warship.
In a low noise application, all aspects of the sources,
transmission and emission of noise need to be considered.
These are summarized in Figure 4.
Firstly, the forces produced by the sources of the noise must
be minimized. In the AIM minimization of electromagnetic
forces is achieved by the appropriate selection of the numbers
of stator and rotor slots, poles and phases. This can minimize
the number of forcing frequencies in addition to minimizing
the amplitude of each forcing frequency. Secondly, the
structural dynamics of the motor and its mounting must be
designed such that any sources of noise are not amplified by
resonances within the structure. The variation of the forcing
frequencies over the operating speed range of the motor must
be considered.
In a ship propulsion application, in steady state operation,
the power required by the propeller is proportional to speed3.
The noise signature of the propulsion motor is only significant
at lower speed, below the speed of cavitation inception on the
propeller. Additional improvement to the motor noise
signature can be achieved by taking advantage of this power
curve. An optimized strategy for motor flux control is
employed at reduced speed to minimize electromagnetic noise,
and decreasing the quantity of cooling air at reduced speed and
power can reduce ventilation noise. The noise signature that
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can be achieved for a 28 MW, 150 rpm AIM, operating at
1.8 MW, 60 rpm, which would be at a ship speed of
approximately 12 knots on a typical surface combatant are
shown in Figure 5.
Noise sources
Time Harmonics
Permeance
Harmonics
MMF Harmonics
Electromagnetic
Forces
Ventilation
forces
harmonics in the motor current will cause additional noise in
every type of motor. An AIM naval propulsion motor would
normally be supplied by a PWM converter with a carrier
frequency of 2 kHz, which will production noise in the motor
at 2 kHz and multiples of 2 kHz. Since this convertor noise is
at a relatively high frequency, it would be relatively easy to
attenuate it by means of metallic attenuating mounts. These
mounts would have a resonant frequency of around 50 Hz, and
provide more than 40 dB attenuation at 1 kHz and greater.
Because of their high stiffness, the static deflection would be
small, so that solid metal stops could be provided to limit
movement under fault or shock conditions. Such a system can
be designed to fit almost entirely within the existing motor
dimensions.
The effect of a PWM convertor with a 2 kHz carrier
frequency, and the attenuation with metallic attenuating
mounts is shown in figure 6
Structureborne N oise Levels with Convertor
With and Without Meta llic Attenua ting Mounts
Stiffness
Mass
Damping
90
Mode
Shapes
Acceleration (dB re 1E-5 ms-2)
Structural
Dynamics
Frequency
Response
Transmission
Isolation
70
60
50
Unattenutated
Attenuated
40
30
20
10
0
Geometry
Material
properties
16
Emission
Radiation
Efficiency
Transmission
Medium
31.5
63.5
125
250
500
1000
2000
Octa ve Ba nd (Hz)
Figure 6 – Noise with Convertor
IV. SHOCK
Noise into Water
Compared to other types of rotating electrical machine, an
induction motor is inherently robust. The rotor in particular is
of simple construction, with solid copper conductors, without
insulation, contained in slots in an iron core. AIM designs
have been qualified to US Navy and Royal Navy shock
standards. This has been achieved by the use of detailed FE
analysis using design response spectra. An example of such an
FE model is shown in Figure 7.
Figure 4 – Sources and Transmission of Noise
Structureborne N oise Levels
90
Acceleration (dB re 1E-5 ms-2)
80
80
V. CONCLUSIONS
70
60
50
40
30
20
10
0
16
31.5
63.5
125
250
500
1000
2000
Octa ve Ba nd (Hz)
Figure 5 –Noise Levels at Part Load
These levels do not include any effect due to time
harmonics from the convertor supply. Convertor time
The Advanced Induction Motor has been developed over a
period of about 15 years from it origins in traditional induction
motor designs. It has evolved into a compact power dense
motor that is particularly suitable for naval ship propulsion.
The structureborne noise signature has been refined to allow
the motor to be hard mounted in a surface warship. Further
reductions in structureborne noise transmission could be
achieved by means of stiff, metallic mounts. AIM designs have
also been shock qualified to US Navy and Royal Navy
standards.
Further improvements to power density could be possible in
the future, particularly through improvements to heat transfer
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by the use of advanced stator winding insulation systems that
are currently under development.
Figure 7 – FE Model for Shock Analysis
VI. REFERENCES
[1]
[2]
[3]
Vanderpump, D, Benatmane, M, Murray, P, “The Type 45 Destroyer
power and propulsion system,” Presented at INEC 2002, Glasgow, UK,
23-25 April 2002.
Hodge, CG, Mattick DJ, “The electric warship IV” Trans ImarE, Vol
113, Part 2, pp 49-63
Grieve DW, Cross DM, “Recent developments in the design and
application of large, high performance cycloconvertor drive systems.”
Third international conference on power electronics and variable
speed drives (conf publ no. 291), London, UK, IEE, 1988, pp 309-314
Clive Lewis (M 1999) was born in Hereford,
UK in 1957. He received a BSc(Eng) from
Imperial College, London in 1979. He has
been with ALSTOM or its predecessor
companies in Rugby, UK since 1980. He has
been involved in the development of large
rotating machines throughout this time, and
has particular experience in the measurement
and modeling of structureborne noise in
various types of AC and DC motors for naval
applications. He experience also includes
electromagnetic, thermal and mechanical
analysis of machines.
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