Download High Power AC Drive and Motor Applications

ECE 8830 - Electric Drives
Topic 19: High Power AC Drive and Motor
Applications
Spring 2004
Introduction
The main interest of USN engineers related
to AC drives is how to design ship
propulsion systems of the future.
Several advances are taking place in
motors, drives, and system configurations.
Three papers presented in recent
conferences related to this issue will be
discussed - one each related to each of
these three topics.
Introduction (cont’d)
The three topics to be considered are as follows:
1) *High power superconducting magnet
motors.
2) *A low cost, low resolution encoder for
vector-controlled drive applications.
3) +The LHD-8 ship propulsion system.
*American
Society of Naval Engineers (ASNE), Electric Machines
Technology Symposium (EMTS), Philadelphia, PA, January 2004
+International
Naval Engineer's Conference (INEC), Amsterdam, The
Netherlands, March 2004.
Superconducting Magnet Motors
The next evolutionary step in designing high
power PM motors is to replace the high flux
density NdFeB magnets with high TC
superconducting magnets. The zero dc
resistance offered by superconducting wires
allows very high current densities to be
conducted by the wires resulting in very high
magnetic field generation. Furthermore, the
associated zero wire resistance results in low
synchronous reactance (although the
transient and sub-transient reactances are
similar to those for a conventional machine.)
Superconducting Magnet Motors
(cont’d)
In the late 1980’s/early 1990’s a new class
of superconducting materials was developed
based on rare-earth oxide materials. These
materials displayed superconducting
behavior at temperatures as high as ~100K
(compared to conventional superconductors
which displayed superconductivity at ~4K).
This was technologically significant since
liquid nitrogen could be used to cool the
materials to superconducting temperatures
rather than the much more expensive liquid
helium.
Superconducting Magnet Motors
(cont’d)
One of these materials, barium strontium
calcium copper oxide (BSCCO), has been
developed to the point where wires of this
material have reasonably high currenthandling capability and have now been
incorporated into motors for generating
strong magnetic fields.
The paper that we review here describes a
5MW motor employing high temperature
superconducting (HTS) wires for the field
windings.
Superconducting Magnet Motors
(cont’d)
A cutaway drawing of the 5MW HTS motor
is shown below:
Superconducting Magnet Motors
(cont’d)
The rotor assembly includes:
 HTS field winding operating at 32K
 Support structure
 Cooling loop
 Cryostat
 Electromagnetic (EM) shield
Superconducting Magnet Motors
(cont’d)
The stator assembly includes:
 AC stator winding
 Back iron
 Stator winding support structure
 Bearing
 Housing
An external liquid He cryo-cooler module
used to cool the field winding is located
at the non-driven end of the shaft.
Superconducting Magnet Motors
(cont’d)
The field winding is conduction-cooled
through the support structure. A torque
tube transfers the torque from the cooled
environment to the “warm” environment.
The field winding contains 6 pole sets,
each fabricated with BSCCO HTS wire.
The pole sets and support structure are
enclosed in an evacuated chamber to
minimize radiative heat transfer to the
HTS field coils.
Superconducting Magnet Motors
(cont’d)



An EM shield is located at the surface of
the cryostat to:
Protect the field winding by attenuating
asynchronous fields produced by the
stator winding.
Carries high transient torque during a
fault.
Provides damping for low frequency
torsional oscillations.
Superconducting Magnet Motors
(cont’d)
The parameters for this motor are shown
in the table below:
Superconducting Magnet Motors
(cont’d)
Superconducting magnet motors offer
very high efficiency (see figure below)
which results in the motor being approx.
half the size and weight of a conventional
motor of similar power and torque rating
(see next slide).
Superconducting Magnet Motors
(cont’d)
Superconducting Magnet Motors
(cont’d)
Open circuit voltage is found to increase
linearly with current for four speeds ranging
between 60 rpm and 230 rpm as shown below:
Superconducting Magnet Motors
(cont’d)
The losses of the motor are seen to increase
quadratically with line voltage (probably due
to eddy currents) - see figure below:
Superconducting Magnet Motors
(cont’d)
The transient response of the motor to an
instantaneous shorting of the stator terminals
is shown below for a field current of 0.07 p.u.
The time constant is found to match the
motor specs.
Low-Count Encoder for IM Drive
Rather than using an expensive, highprecision encoder for an IM drive, a rotary
encoder with only 64 pulses per revolution
has been considered. Because of the low
precision of the measurement, two different
speed estimation methods have been used
to estimate the rotor speed - one method
used at high speeds (>200 rpm) and one for
low speeds (<200 rpm). The measured
speed contains some errors so a correction
procedure is used to minimize this error.
Low-Count Encoder for IM Drive
(cont’d)
The high-speed method is based on counting
encoder pulses in a fixed time interval. The
rotation angle is time period T is given by:
2

n
4N
where n= # of pulses counted in time T,
and N= # of encoder pulses per revolution.
 rotor speed r is simply given by:
r 

T
Low-Count Encoder for IM Drive
(cont’d)
The main problem with this approach is
quantization error which can be reduced by
increasing N (more expensive) or using
digital filtering techniques (result in delays
=> not suitable for vector-controlled drives
(at low speeds).
 A different approach is used for low
speeds.
Low-Count Encoder for IM Drive
(cont’d)
For the low speed region, output pulses of
a high frequency clock are counted
between two consecutive encoder pulses.
If the # of counted clock pulses is n and
the frequency of the clock is fc , the rotor
speed is given by:
2 f c
r 
4 Nn
Low-Count Encoder for IM Drive
(cont’d)
This method also has a quantization error
due to fc/n. This is only a problem at
higher speeds but the approach is good at
low speeds.
An error minimization method applicable
at both low and high speeds is described
in the paper.
Also, parameter sensitivity of the field
orientation control and a self-tuning
procedure are described in the paper as
well as Matlab/Simulink simulation results.
Hybrid Ship Power System
The third paper describes a hybrid power
system for a US Navy ship in which a steambased power/propulsion system is being
replaced with a hybrid gas turbine/auxiliary
motors for propulsion and electric powered
auxiliary systems.
Hybrid Ship Power System (cont’d)
These systems are designed for the LHD
8 ship which is in the Wasp class of
amphibious assault ships (40,500 ton,
844 ft.) designed to support the USMC
and perform amphibious assaults against
defended shore positions.
Hybrid Ship Power System (cont’d)
The design goal for the shipboard power
system was to replace two independent
steam boilers and two 35,000 hp steam
turbine engines. A GE 25,000 hp gas turbine
engine was available and qualified for USN
ship propulsion, but was deficient for this
application. Also, gas turbines require
ducting through the island structure of the
ship. For these reasons (and the volume
required for two gas turbine engines per
shaft) made this approach unfeasible.
Hybrid Ship Power System (cont’d)
Recently, the GE LM25000+ gas turbine
engine capable of delivering 35,000 hp has
been developed and using one such engine
per shaft, the ship propulsion requirements
are met. Therefore, this was the technology
that was incorporated into the LHD 8. In
addition to the main gas turbine engines, an
auxiliary propulsion motor (5,000 hp) driving
a 2-stage reduction gear into a controllable
pitch propeller was used (see next slide).
Propulsion is provided by either the main gas
turbine engine or the auxiliary motor - but
not by both simultaneously.
Hybrid Ship Power System (cont’d)
Propulsion Brakes
Line Shafting
MRG
Propulsion Gas
Turbine
Motor
Propulsion
Clutches
Main Thrust
Bearing
Turning
Gear
FIGURE 3 - Conceptual diagram of shaft propulsion power train
arrangement for LHD 8
FIGURE 4 - Sketch of shaft propulsion
power train design concept for LHD 8
Hybrid Ship Power System (cont’d)
Several options were considered for the
electric plant design for the LHD 8 which
considered factors such as ship construction
cost, life cycle cost, survivability, etc. The
electrical load analysis is shown in the table
below:
Table 1 - Predicted electrical load analysis summary with anticipated service life growth margin
DAY TEMP
ANCHOR
CRUISE
F
MW
MW
MW
10
16.6
18.6
17.4
90
10.0
11.5
11.0
0
DEBARK
Hybrid Ship Power System (cont’d)
A traditional USN 450V AC electric power
distribution system was considered unsuitable
for the system because of the limitations of
USN 450 VAC circuit breakers at the expected
power loads, and so it was decided to increase
the system voltage to 4160 VAC. Since most
ship loads are derived from 450 VAC, a power
distribution system architecture for routing the
4160 VAC power, converting the 4160 VAC to
450 VAC, and then distributing the 450 VAC
power, was required. An AC Zonal Electrical
Distribution System (AC ZEDS) was selected
(see paper for details).
Hybrid Ship Power System (cont’d)
For the auxiliary propulsion system (APS) 5
different motor types were considered. Their
advantages/disadvantages are provided in
the table below:
TABLE 2
COMPARISON OF PROPULSION MOTOR TYPE OPTIONS
MOTOR
TYPES
RELATIVE
SIZE
DC
max dia
AC SYNC
near max dia
AC INDUC
near min dia
AC PM
near min dia
AC SUPER-C
min dia
RELATIVE
COST
near min
near min
min
near max
max
RELATIVE
RISK
min
min
min
near max
max
Hybrid Ship Power System (cont’d)
A 5,000hp AC induction motor was selected.
With the two auxiliary motors on the two
shafts, a total of 10,000 hp was available for
propulsion which could propel the ship at a
steady state speed of 12 knots while at full
power (70,000 hp) the ship could move at
over 20 knots.
The next design issue related to the APS
was how to start the ship only using the
auxiliary motors.
Hybrid Ship Power System (cont’d)
The first approach considered was to get the
shaft spinning using the gas turbine engine
prior to switching to the auxiliary motor.
However, this did not allow for completely
independent operation of the two systems.
A variable speed drive (VSD) rather than a
fixed speed drive approach was selected to
allow full torque to be provided by the
induction motor at start-up.
Hybrid Ship Power System (cont’d)
Having decided on a VSD approach, the
particular VSD topology had to be
selected. The range of design solutions is
indicated in the table below:
Hybrid Ship Power System (cont’d)
A transformerless, 6-pulse rectifier requires
substantial filtering in order to meet the
stringent power quality requirements for
naval shipboard use. Passive filtering takes
up a lot of room and active filtering would
need some development time. An
alternative approach would be to use a PWM
rectifier, but this too would require some
development time.
Hybrid Ship Power System (cont’d)
For VSD designs incorporating
transformers, 12-, 18-, and 24-pulse
rectifiers are viable options and can provide
the requisite power quality without
additional filtering. However, the limited
space in the machinery room could not
accommodate a large single transformer or
even two smaller transformers (and the
additional required switchgear). Further
work is needed to define the details of the
VSD design.
Hybrid Ship Power System (cont’d)
The rest of the paper focuses on the
concept of operations (CONOPS) description
for the shipboard power system, including a
conceptual layout of the machinery and a
description of the power management
priorities. The paper concludes with a
description of the APS control system.