Motors face a tough life riding the rails

Edited by the Engineeringtalk editorial team Oct 15, 2004

Railway application AC induction motors are required to operate under difficult and demanding conditions, resulting in special demands - both mechanical and electrical.

Railway application AC induction motors, typically employed to drive ancillary equipment, are required to operate under difficult and demanding conditions, resulting in special demands - mechanical and electrical.

According to AEG Electric Motors, which supplies motors for use around the world, they are expected to operate under variable power supply conditions while maintaining high efficiency and power factor levels.

This is to satisfy the increasing demands from train operators to reduce overall loading and improve locomotive efficiency, without sacrificing flexibility.

Motors must also be designed to minimise weight penalties, again to help achieve operating efficiencies.

To illustrate variables in power supply, in cities such as London, New York and Boston, the trains must draw power supply from both overhead wires and third rails.

Also, in Europe, locomotives are equipped to operate using four different voltages - 25 and 15kV AC, as well as 3 and 1.5kV DC.

Although this cross-voltage travel is principally made possible thanks to modern electronics, the resultant supply to the electric motor is far from constant and sinusoidal.

What's more, gaps between rails, although small, cause loss of current.

Although the effect of this can be ameliorated by the use of generators or alternators frequent occurrence causes stress to motors.

In addition, when convertor (DC/AC) supplied, motors can experience hefty switching spikes of up to 1kV and a dV/dt up to 1000V/us as result of consequent power surges.

To counter shock loadings and provide enhanced robustness and reliability, special insulation impregnation techniques are used on the windings, typically to class H standards.

Ambient operating conditions call for motors to be able to perform in temperatures ranging from -30 to +70C in different humidity levels resulting from working in different climate regions and sometimes at heights greater than 1000m above sealevel.

All this is exacerbated by the fact the motor is typically mounted beneath the rolling stock and open to the elements.

To meet these demands, locomotive motors are of all metal construction, using aluminium instead of cast iron wherever possible to minimise weight.

This enables them to withstand shock loadings of between 3 and 10g experienced regularly during such activities as shunting and vibration during the working day.

No flammable components are used to eliminate any risk of fire hazard and the emission of toxic fumes.

In addition, special attention to bearings is required to avoid tolerance problems arising due to operating temperature differentials, particularly high temperatures and thermal expansion - everything, in fact, to ensure a long, maintenance-free, service life.

Applications for the motors include cooling systems for the engine and brakes, and air conditioning for the passenger compartments.