Introduction

The present market trend has influenced many conglomerates and start-ups to shift their focus on electric propulsion technology. One such inclination has resulted in design a permanent magnet synchronous motor (PMSM) for electric traction applications. The motor classified under a subset in which it is not alone, the other motors such as Switched reluctance motor and Synchronous reluctance motor share the common sir name “Special machines”.

Specification

To get the experiment started a market study helped to conclude and arrive at the required motor specifications.

Rated Power (kW)	12
Rated Voltage (V)	72
Max Torque (Nm)	23@5000, 5@7500
Speed (RPM)	5000
Dimensions (d x l)	180 x 120
Weight (kg)	15
Efficiency (5)	96

The motor is designed to be a conventional, air-cooled traction motor that can comfortably sit to power moderate commercial wagons.  To break down more into the construction of motors we can begin with the stator and rotor design, the materials used will be discussed alongside and then we can touch upon the simulation results towards the tail of the blog and mark an end there.

 

The figure on the left showcases the silicon steel stamping of stator and rotor used in the construction of this motor, The motor is designed with 24 Slots and 10 Pole (24S 10P) configuration to extract maximum performance which is a very specific and dominant requirement for traction motors.

 

An optimized grade of High permeability silicon steel has been used to reduce hysteresis and eddy current loss without trading off the cost factor. Neodymium magnets slid into a distinct with ‘V’ shaped cuts provided magnetic field necessary for the motor action, the distinct ‘V’ shape cuts are chosen to yield high CPSR (Constant Power Speed Ratio). The other parameter to quantify will be the cogging torque and was made to be <2% which will result in very low voltage harmonics at the motor terminals.

 

Moving into the winding part a distribution winding pattern is followed, this enables the voltage to get spatially distributed/ displaced equally unlike a concentrated winding which is in existence otherwise. The production-oriented decision here is to allow the windings to be slid easily into the stator slots, with this demand in place the fill factor is restricted to 40%. This factor can be increased closed to 60% but the decision can be made only after the production feasibility is evaluated.

 

Performance Curves

 

The machine has been designed to extract peak performance at 23Nm@ 6000RPM(Fig. 2) guzzling 12kW of power. Generally, traction machines are positioned to operate at moderate power and do sprints at peak power. Hence the design consideration is made to have peak efficiency at a moderate operating condition rather than at peak power. This statement made here hold good only for motor design pertaining to traction application.

 

Optimizing the efficiency of traction motor is a crucial part of machine design since it needs to effectively utilize the battery power. The designed machine here can operate at efficiency as high as 97%. Moreover, torque ripple needs to be minimized for better riding comfort which is a measure of the quality of machine back-EMF

   

   The picture to the right gives you a glimpse of magnetic flux distribution at full load capacity on the motor laminations. The geometry has to be meticulously designed to avoid saturation at various loaded conditions considering utilizing the laminations to the maximum extent.  A calculated percentage of overrating can be possible without getting into saturations, such a margin has been taken into account while designing as we always need a machine to perform beyond its limits, if it is necessary.