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Permanent Magnet Synchronous Motor

By on May 12, 2013


Permanent Magnet synchronous motor popularly called PMSM uses rare earth permanent magnet at the rotor as compared to field winding. With recent advancements of permanent magnet, solid state devices and micro-electronics have helped in making use of this motor for traction purpose. To begin with, this motor has found more and more automotive application but opened scope for research and development for Electric Traction. The most important advantages are energy efficiency, high HP/weight, less size making possibility of mounting the motor directly on axle.

Wheel Mounted Traction Motor

Mounting of traction motor directly on the wheel has been wishful thinking since long. PMSM with high specific torque is likely to make this possible shortly. Prototype models have been developed successfully. It consists of a permanent magnet rotor propelled by a rotating magnetic field realised through 3-phase AC supply to stator winding. The rotor will rotate in synchronism with rotating field produced by stator.


  • Application of strong permanent magnetic field resulting small volume and weight. Rare earth permanent magnets Nd-Fe-B is an option.

Mounting Options

  • PMSM can be with option of inner or outer rotor. For inner rotor, the PM is mounted on axle which rotates with application of voltage to stator. For outer rotor, it is directly connected to the wheel and inner stator is fixed. Arrangements are shown below


  • Prototype PMSM has shown 6.3% and 8.6% improvement in energy efficiency and power factor as compared to induction motor.
  • Elimination of speed reduction gear arrangement derives advantage of noise reduction, saving of lubrication, maintenance cost. Initial saving of cost is compensated due to increase cost of PMSM.
  • Due to less loss, fully enclosed motor with surface water cooled arrangement is possible.
  • Small volume and direct drive can facilitate low wheel diameter, low floor, double decked construction.
  • It can open possibilities of increasing speed on narrow gauze by reducing wheel diameter.

It is important to note that all desirable of a traction motors in this case are low i.e.

  • Noise
  • Floor Height
  • Losses
  • Maintenance Cost


With so many advantages, challenges are also obvious

  • While working the locomotive or motor coach dead, high voltages will induce in the stator due to permanent magnet at rotor.
  • Each motor in the vehicle will require independent inverter
  • Permanent magnets will be mounted on the axle thus increasing its weight. The stator is also not supported with any suspension; therefore un-sprung mass will go up.

Options for Indian Railways

Battery operated Narrow gauze trains over hilly terrain

Due to low floor height, higher a speed is possible. Installed battery will be charged at intermediate station or tray system to replace with charged battery. Regenerative braking during downhill journey will charge the battery. Batteries which can be charged quickly are in the offering, a research for automobile requirements is in progress.

Constructional details of mid axle PM alternator:

The rotor consists of a solid core with embedded permanent magnets with no field windings. The stator is in two halves and provided on the axle with the help of split bearings on both sides of the alternator. The stator is held in position with a reaction rod which is secured with the stator on one end and with the bogie on the other end. There is no belt drive in this system.








Advantages of PM mid axle alternator over brushless alternator:

  • Compact in size and lighter in weight.


  • Directly coupled and no belt drive is required hence less maintenance
  • Easy mounting arrangement as the rotor and stator are in 2 halves and can be replaced in less time as compared to existing brushless alternator which requires more than 10 hrs for replacement and coach has to be marked sick.
  • Higher efficiency due to the usage of special magnetic materials.
  • Low leakage inductance resulting in better response.
  • Low voltage and current ripples due to the use of permanent magnets.



Comparison of important parameters is as under:


COST 3.0 lacs 6.0 lacs
WEIGHT 950 Kg 450 Kg
SIZE (L x Dia.) 1019 x  470 588  x  654
EFFICIENCY 55-65% 85-90%
CUT-IN SPEED (rpm) 400 165
MFO (rpm) 800 200


Four Numbers of these alternators were manufactured and put into service. But there were teething problems such as

  • Damage of bearings
  • Leakage of grease
  • Damage/burning of stator windings & insulating material
  • Breakage of terminal box
  • Accumulation of copper dust
  • Repeated control card failures in ERRU etc.


This resulted in the withdrawal of all the four alternators from service. The problems experienced were avoidable with some systematic design improvements jointly by supplier and the firm. Today when world Railways are planning for this arrangement for traction, it is a great opportunity to demonstrate technologies for its application for self generation coaches.

Indian railways has decided for this technology with belt driven arrangement and one such permanent magnet alternator self generating coach is in service. The details of the arrangement are as follows


Belt Driven Permanent Magnet Alternator

In view of the failure of axle mounted PM alternator, Indian Railway decided to go for Belt driven Permanent magnet Alternator.

Rotor of PM Alternator with Permanent Magnets                       Belt arrangement for driving PM


Control Box                                                                                      PM alternator with pulley

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