Conventional electric rail vehicles transmit the driving force of the
traction motor to the wheelset via flexible couplings and reduction gears. The
torque of the traction motor is multiplied by the gear ratio.
As an alternative to conventional AC asynchronous motors with gear
transmission, a wheel-mounted permanent magnet synchronous motor is currently
developed by a number of railways and manufacturers. This is favoured by
technological developments of recent decades such as
- advanced power electronics facilitating variable speed drive of AC motor
through inverters being adjustable in frequency
- rare earth permanent magnets permitting the construction of light and
efficient permanent magnet synchronous motors
Wheel-mounted construction
In a wheel-mounted permanent magnet synchronous motor the torque of the motor
is directly transmitted to the wheel without using gears and flexible couplings.
This leads to reduced transmission losses (and thus energy consumption), noise,
maintenance, mass and volume.
If the transmission gears are eliminated the motor has to supply a higher
torque. Asynchronous motors do not meet this requirement and can therefore not
be realised as a wheel-mounted construction.
Permanent magnet synchronous motor
Besides transversal flux motors, the permanent magnet synchronous
motor is a promising candidate for a wheel-mounted construction due to high
specific torques. It consists of a permanent magnet rotor driven by a rotating
magnetic field realised through 3-phase AC-fed coils. It is called synchronous,
because the rotor will rotate at a constant speed which is synchronous with the
rotation of the magnetic field.
Technological realisation
In order to achieve strong permanent magnetic fields with magnets with small
volume and weight, rare earth permanent magnets are used (e.g. Nd-Fe-B).
Constructive options:
There are several constructive options for wheel-mounted permanent magnet
synchronous motors.
Figure 1: Constructive options
Source: Matsuoka, Kondoh, Hata 1997.
(i) Inner vs. outer rotor motor
Electric motors can be of inner or outer rotor type. Usually traction motors
are of inner rotor type, i.e. the outer part is fixed and the inner part
rotates.
Inner rotor type motors direct drive traction motor, have an inner rotor
directly connected to the axle and an outer stator. In the case of an outer
rotor type direct drive traction motor, the outer rotor is directly connected to
the wheels, and the inner stator is fixed to the axle. Whereas the inner rotor
type traction motor requires its own bearings, the outer rotor type does not,
which leads to a simpler construction.
(ii) Dual wheel drive and individual wheel drive (independent wheel drive)
In conventional rail vehicles, both wheels are connected by the axle, and
therefore rotate simultaneously ("dual wheel drive"). Outer rotor type traction
motor may be divided into two motors, each wheel rotating independently. Thus an
independent wheel driving bogie may be realized allowing adjustability to
different rail gauges.
Technical data
Table 1 gives a comparison of some technical key features of wheel-mounted
permanent magnet synchronous motor and a conventional induction motor.
Table 1: Comparison between wheel-mounted permanent magnet synchronous motor
of RMT1A type and conventional induction motor
Source: Matsuoka, Kondoh, Hata 1997.
Fields of application
Railways.
Manufacturer
R&D in railways and manufacturers. Magnetmotor (Starnberg, Germany)
produces permanent magnet synchronous motors for bus sector. |