Permanent Magnet Motor (ALSTOM)
Permanent Magnet Motor (ALSTOM)
Permanent Magnet Motor (ALSTOM)
permanent-magnet
traction motor
The authors
would like to
thank Dr Dr
Harald Neudorfer and Markus
Neubauer of
Traktionssysteme Austria,
and Dr Colin
Goodman of
BCRRE for their
assistance in
the preparation
of this article.
30
being used in the transmissions of hybrid cars, thanks to their low mass and
good controllability. Larger machines
offer a similar potential to enhance
the overall performance of the railway
traction package. The technology is
now beginning to be introduced into
a variety of new rolling stock, but the
integration of PMSMs into traction
packages presents some significant
technical challenges which must be
overcome.
Fundamental requirements
MOTORS | TRACTION
Left: Fig 1. Basic
characteristics of
a traction motor.
Right: Fig 2.
Machine power in
motoring.
Torque control
Manufacture
of permanentmagnet traction
motors at
Alstoms Ornans
factory.
31
TRACTION | MOTORS
Left: Fig 3.
Conventional DC
series-wound
machine.
Right: Fig 4.
Separatelyexcited DC
traction motor.
Water-cooled
375kW
permanentmagnet traction
alternator with
aluminium casing
for diesel-electric
multiple-units.
Below: Fig 5.
Synchronous AC
machine.
Centre: Fig 6.
Squirrel-cage
asynchronous AC
induction motor.
Right: Fig 7.
Permanentmagnet
synchronous AC
machine.
32
In an asynchronous three-phase
machine, the magnetic field rotating
in the stator induces currents in the
rotor cage (Fig 6) that, in turn, generate a magnetic field which interacts
with the stator field to produce either
motoring or braking torque. In motoring, the rotor speed is lower than
the rotating stator field speed set by
the inverter, and in braking it is faster. No torque is produced if the two
speeds are the same. This difference
can be expressed as slip frequency or
percentage slip.
In a PMSM, the rotor field is created
by magnets that are either distributed
on the surface of the rotor or buried in openings in the rotor laminations (Fig 7). The latter arrangement
offers greater mechanical strength
and much lower eddy-current losses
in the rotor. The material with the
strongest magnetic properties is Neodymium Iron Boron (Nd2Fe14B). The
stator field is generated by means of a
relatively standard three-phase multipole winding on a laminated core.
In all electric machines, the rotating magnetic field leads to the generation of voltages that oppose the supply
voltage(s), the so-called back EMF. At
zero speed this is zero, but it grows
linearly with speed. Thus the supply
voltage must be increased to maintain
a constant torque in region 1.
The torque supplied or absorbed
by an electric machine is given by
the product of the magnetic flux and
current. It is the role of the electronic
power converter to condition the
DC or single-phase AC supply voltage such that a suitable current or
currents flow in the motor. Many different types of converters are available,
but most modern traction systems
use insulated gate bipolar transistors
(IGBTs) and some form of pulsewidth modulation.
In the region of constant tractive effort, the voltage (and frequency in the
case of induction machines) applied to
the terminals needs to increase linearly with motor speed so as to maintain
the product of flux and current, that is
the torque, at a constant level. Beyond
the base speed, the applied voltage
cannot be increased further due to
the limitations of the power electronics and the insulation capability of the
machine. However, mechanically, the
machine can go faster.
So region 2 is entered by field weakening, thereby reducing the level of
back EMF or, in the case of a PMSM,
counteracting its influence. In DC
machines this is achieved by reducing
the current flowing through the field
windings (see the resistance RFW in Fig
3) and in a conventional synchronous
machine it is achieved by reducing
the current supplied to the rotor. In
an induction machine, field weakening happens automatically as the supply frequency is increased while the
supply voltage is kept constant. In a
PMSM, field weakening is more difficult to implement because the rotor
field is created by permanent magnets.
In region 3, the flux and current
are reduced at a greater rate than in
the constant power region to avoid
exceeding the machines electrical or
mechanical limits. In the separatelyexcited DC motor, for example, the
armature current is also reduced as a
function of speed.
Advantages and drawbacks
2% higher across 80% of the operating range. The specific power is 30%
to 35% greater, resulting in a machine
that is about 25% smaller and lighter
for the same power rating.
Whereas in an asynchronous motor
heating of the rotor is caused by the
inherent slip power, this is virtually
eliminated with a PM drive, avoiding
the need for rotor cooling. Normally,
PM machine stators are completely
sealed and cooled by means of a heat
transfer fluid, thus leading to potentially more reliable drives. PMSMs also
allow dynamic braking down to very
low speeds and, in theory, it should
be possible to produce a self-controlled retarder by electro-mechanically
short-circuiting the stator windings.
Of course, these benefits are not
available without compromise. There
are seven main drawbacks to the use
of permanent-magnet traction motors, although appropriate mitigation
measures have been developed.
Limitations on the size and cost of
the four-quadrant converter and machine do not allow operation across
the whole speed range by the simple
expedient of supplying the machine
with a voltage that is sufficiently higher than the back EMF to permit the
flow of current required to achieve
the desired torque. This constraint is
solved by means of field weakening,
creating the constant torque and constant power regions. Since the field
generated by the permanent magnets
cannot be adjusted, field weakening
is achieved by injecting currents into
the stator windings which set up fields
to oppose those of the rotating permanent magnets.
These extra currents cause copper losses in the stator windings that
negate, to some extent, the efficiency
gains that are achieved by the use of
MOTORS | TRACTION
Fig 8. Control
loop for a
permanentmagnet
synchronous
machine.
Fig 9. Optimum
efficiency for
a PMSM is
dependent upon
motor speed and
torque.
33
TRACTION | MOTORS
SNCFs first
Citadis-Dualis
tram-train cars
from Alstom
with PM traction
motors are being
commissioned
for service on
the Nantes
Chteaubriand
line.
34
Operator
Trains
Manufacturer
NTV
Alstom
SBB
Bombardier
SNCF
Alstom
SNCF
Alstom
SNCF
Bombardier
Praha
Skoda
Tokyo Metro
Kawasaki
JR East
Toshiba
Siemens
China
CNR Yongji
Sweden
Bombardier
Turkey
Alstom
Japan
Gauge-Changing Train 2
n/a
Reference
1. Schrdl M, Hofer M and Staffler W. Combining
INFORM method, voltage model and mechanical
observer for sensorless control of PM synchronous
motors in the whole speed range including standstill. Elektrotechnik und Informationstechnik 5. 06
pp183190