This is the biggest change the global railway world has seen since the transition from steam to diesel. Thanks to aggressive carbon-neutrality targets, futuristic urban development and the relentless drive for operational efficiency, today‘s rail transit systems require never before measured levels of performance from the electrical propulsion and infrastructure networks.
A key element that is behind this (often out of view but) non-negotiable high speed revolution is the enameled magnet wire.
To the traction motors of high speed passenger trains drawing thousands of kilowatts, to the signaling transformers which help keep the tracks free of danger, enameled wire is the basic element of rail electrification. Unfortunately, regular commercial grade magnet wire can‘t hold up in the harsh surroundings of the railway environment.
This in-depth technical guide discusses the importance of a dedicated railway enameled wire, the extreme operational stresses it faces, the material advancements being made that are progressing the industry, and the benefits gaining the right partner can have on total cost of ownership (TCO) for OEMs and operators worldwide.
The Critical Role of Enameled Wire in Modern Rail Electrification
In order to understand the demanding constraints to which railway grade enameled wire must be subject, it is first necessary to understand how it is deployed on trackside and rolling stock (the trains themselves).
Traction Motors: The Heart of the Locomotive
The largest end use of highperformance enameled wire in the rail industry is the traction motor. Conventional Electric Locomotives and Electric Multiple Units (EMUs) are being built around an inverter-fed induction motor or a permanent magnet synchronous motor (PMSM), which is fed by a modern VFD and transforms electrical energy supplied by overhead caternary lines and/or third rails, into the torque required to propel railcars costing hundreds of metric tons.
Inside the traction motors the enameled wire is wound into the stator slots under great tension. It must be wound as full of copper as possible to promote a high copper fill factor (percentage of the total slot volume occupied by copper) so that the maximum power can be generated in the minimum space, as the short bogie gives a limited available area.
Transformers and Chokes: Managing the Grid
All electric trains use a variety of electrical standards for supply throughout the world with the most common being 15 k V at 16.7 Hz to 25 k V at 50/60 Hz. Other standards include 750 V, 1.5 k V and 3k V DC. The high voltages supplied on the train are stepped down to a workable voltage by main transformers.
The enameled wire in these transformers and smoothing chokes is required to withstand large continuous currents, high voltage peaks and strong magnetic fields, all without the enamel insulation failing.
Auxiliary Power Units (APUs) and Climate Control
All additional systems provide passenger safety and comfort. This includes air conditioning, heating, ventilation and lights and the onboard diagnostic systems that are all supplied by auxiliary converters and smaller electric motors. These are all less powerful than the traction motors but failure of any of these would result in the entire train coming to a halt so insulation of these magnet wires is essential as well.
Trackside Infrastructure and Signaling
Beyond the rolling stock, enameled wire is vital for trackside infrastructure. This includes:
Substation Transformers Reduce the grid power to the amount used by the railway distribution system.
Balises and Transponders: Passing on key speed and positioning information from the rail line to the train.
Switch Drives: Point-operating ‘motors’ which can physically shift track rails to alter train paths.
The Crucible of Rail: Stresses that Destroy Standard Magnet Wires
What is so special about an industrial quality enameled wire? Why it could not be used in a train? Its so called ‘crucible’ of environmental and electrical stresses in railway operation. Here, an industrial motor may operate at a steady state condition inside a hospital controlled factory. Conversely, a railway traction motor has to survive in a mother of all cocktail of destructive forces.
Electrical Stress: The Menace of High dV/dt and Inverter Surges
Today‘s rail propulsion system uses VFD inverters with Insulated Gate Bipolar Transistor (IGBTs) and, more and more, Silicon SiC MOSFETs. These semiconductor switches turn the power on-off thousands of times per second to achieve very accurate control over a motor‘s rotational speed.
If the inverter is switched very fast in this manner, then the voltage wavefronts are extremely steep (very high d V/dt). When these high frequency voltage pulses propagate down the cables from inverter to motor, then the wavefront encounters impedance mismatches. These result in voltage reflections and transient voltage spikes of twice or three times the magnitude of the “average” DC link voltage.
