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.

• R&D in railways (DB, JR East and others) and railway manufacturers.
• JR East is presently manufacturing test vehicles of Advances commuter train for Tokyo Metropolitan Area using wheel-mounted permanent magnet synchronous motors.
• Engineers from Technical University Darmstadt have recently made a feasibility study (Koch et al. 2002) examining the potential of wheel-mounted permanent magnet synchronous motors for main line locomotives. The study showed no principal constructive barriers for such an option.

• LCC reduction
• Constructive advantages

Maintenance

Maintenance reduced (no gears and flexible couplings)

Constructive advantages

Small volume and direct drive facilitate low-floor and double-decked construction, independent wheel drives offer potential for gauge-adjustable vehicles.

Motor cooling

Due to lower losses, heat removal can be realised by an exterior water-cooling rather than ventilation. This reduces dirt contamination of motor and thus maintenance and wear.

Table 2: Features of wheel-mounted direct drive and its applications (Japanese perspective)

Source: Matsuoka, Kondoh, Hata 1997.

Safety

Wheel-mounted permanent magnet synchronous motors may cause safety problems: If power failure occurs and the train has to be removed quickly to clear the track, the induced currents in the motor have to be handled (since induction effects cannot be switched off, due to permanent magnetism). The solution of this problem requires additional effort.

Complexity

Whereas 4 asynchronous motors can be fed by one inverter, permanent magnet synchronous motor need one individual inverter each.

Unsprung mass

In a wheel-mounted construction the heavy rotor magnets contribute to unsprung mass and put additional strain on wheel-track system.

Acceptance

Asynchronous motors are a successful standard technology. Consequently, reluctance to take the risk to introduce a new technology is high.

Tokyo commuter train

JR East is presently manufacturing test vehicles of Advances commuter train for Tokyo Metropolitan Area using wheel-mounted permanent magnet synchronous motors.

A wheel-mounted permanent magnet synchronous motor has a higher efficiency than the system consisting of asynchronous induction motor drive, mainly due to missing transmission gears.

• Tests made on the RMT1A (an experimental motor developed at the Railway Technical Research Institute (RTRI) in Tokyo) revealed, that efficiency is improved from 87,9 % for a conventional asynchronous induction motor to 94,2 % for the RMT1A wheel-mounted synchronous motor. This means a relative improvement of 7 %.
• Koch et al. 2002 studying the feasibility of wheel-mounted permanent magnet synchronous motor for a main-line locomotive assumed an efficiency of conventional asynchronous motor gears of 93,6 % at payload and calculated for the efficiency of the wheel-mounted synchronous machine a value of 95,6 %. This is a relative improvement of only 2 %.

We heuristically assume that an intermediate value of 4 % is realistic for many applications. This is used in the following elasticity table.

Noise

Noise is reduced compared to conventional drive solutions due to elimination of gears and elimination of ventilation.

Lubricants

Through the elimination of gears the loss of lubricants into the environment can be avoided.