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.
- 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.
- Floor Height
- 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.