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   Wheel-mounted permanent magnet synchronous motor  evaluated  
The wheel-mounted permanent magnet synchronous motor is an interesting alternative to conventional AC asynchronous motors with gear transmission. In a wheel-mounted construction the torque of the motor is directly transmitted to the wheel.
Technology field: Optimisation of traction technologies
close main section General information
  close sub-section Description
   

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

Direct Drive 1.gif

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

Direct Drive 2.gif

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.

close main section General criteria
  close sub-section Status of development: prototype
   
  • 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.
  Time horizon for broad application: 5 - 10 years
    Manufacturers claim that production could start quickly if there was a demand from railways. The power class of permanent magnet synchronous motors currently produced for busses is not too far away from the ones needed for light rail applications. Even so, broad application would take some years since refitting of existing vehicles is not an option.
  Expected technological development: dynamic
    (no details available)
    Motivation:
   
  • LCC reduction
  • Constructive advantages
  Benefits (other than environmental): big
   

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)

Direct Drive 3.gif

Source: Matsuoka, Kondoh, Hata 1997.

  Barriers: medium
   

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.

    Success factors:
    (no details available)
  Applicability for railway segments: high
    Type of traction:  electric - DC, electric - AC
    Type of transportation:  passenger - main lines, passenger - high speed, passenger - regional lines, passenger - suburban lines, freight
    In principle, wheel-mounted permanent magnet synchronous motors could be applied to all segments of electric fleet.
    Grade of diffusion into railway markets:
  Diffusion into relevant segment of fleet: 0 %
  Share of newly purchased stock: 0 %
    Wheel-mounted permanent magnet synchronous motor is not marketable yet.
  Market potential (railways): high
    Barriers could impede introduction of wheel-mounted permanent magnet synchronous motors. Otherwise market potential could be very big in long-term perspective.
    Example:
   

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.

close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: 2 - 5%
  Energy efficiency potential throughout fleet: 2 - 5%
   

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.

 

Traction

Brake energy recovery

Effect on efficiency of power train

Elasticity with regard to efficiency of power train

Effect on
total energy consumption for traction

High speed train

electric

no

4 %

1,00

4 %

 

 

yes

1,11

4 %

Intercity train

electric

no

1,00

4 %

 

 

yes

1,12

4 %

Regional train

electric

no

1,00

4 %

 

 

yes

1,33

5 %

Suburban train

electric

no

1,00

4 %

 

 

yes

1,42

6 %

Freight

electric

no

1,00

4 %

Range:

4 - 6 %

  Other environmental impacts: positive
   

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.

close main section Economic criteria
  close sub-section Vehicle - fix costs: low
    At present, the price is difficult to predict but experts estimate that permanent magnet synchronous motor will be more expensive than asynchronous motor due to expensive rare earth magnets. However higher motor price will be roughly compensated by saved costs for gears. Some authors (cf. Koch et al. 2002) even assume lower overall investment costs through eliminated gears. This could be true if scale effects can be realised.
  Vehicle - running costs: significant reduction
    Both maintenance and energy costs are appreciably reduced.
  Infrastructure - fix costs: none
    (no details available)
  Infrastructure - running costs: unchanged
    There might be an almost negligible increase in infrastructure costs due to increased wear and tear on track (unsprung mass!).
  Scale effects: medium
    (no details available)
  Amortisation: (no data)
    (no details available)
no data available Application outside railway sector (this technology is railway specific)
close main section Overall rating
  close sub-section Overall potential: promising
  Time horizon: long-term
    Wheel mounted permanent magnet synchronous drives are an attractive option for future vehicle layout since they offer further increase in power density and overall traction efficiency. Principal technological or economic barriers do not exist or seem solvable in mid-term. Permanent magnet synchronous technology is more mature at present than transversal flux technology which is also discussed for a wheel-mounted drive.
References / Links:  Koch 2000;  Matsuoka et al. 1997
Attachments:
Related projects:  High performance motor for direct drive
Contact persons:
 date created: 2002-10-09
 
 
© UIC - International Union of Railways 2003
 
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