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Successful completion of the French State Railways SNCF project

Maintenance of 160 locomotive motors over the last three years.

A 1470 kW traction motor overhauled for SNCF | Output inspection
A 1470 kW traction motor overhauled for SNCF | Output inspection

Every day, over 14,000 trains carry several million passengers on the SNCF (French National Railway Company) rail network. With its headquarters in the Paris suburb of Saint-Denis, the state-owned company operates almost the whole of rail transport in France and Monaco, and is relied on by many for their daily journey to work, school or university.

To ensure that the public can continue to enjoy safe and problem-free travel, maintenance works are performed at regular service intervals in an effort to avoid any loss of service.

Search for a competent co-operation partner

In 2012, SNCF (via its Lyon maintenance works) set out to find a competent partner to maintain the DC motors in their trains. Possessing a sound reputation for successful completion of rail transport projects, plus extensive large DC drive units, know-how was vital, and BENNING Electrical Machinery Section [“BeM”) fulfilled the criteria.
BENNING’s established working relationship with SNCF, plus a successful onsite audit of BENNING, Bocholt, by four SNCF Technology and Quality Department employees were decisive factors too.

Collection, maintenance and delivery

SNCF prepared a three-year action plan. During this period, 160 traction motors (type TAB660 for locomotives BB25500) were to be collected from Lyon, overhauled at the BENNING works and delivered back to Lyon.
For BENNING, the maintenance of the DC drive units represented an exciting but demanding challenge. The drive units each weigh 5.3 tonnes and, to avoid even the slightest damage (for example, during transport), they are fixed to steel transport frames that individually weigh 800 kg.
Within a maximum of 10 working days, the DC drive units had to be thoroughly overhauled, checked and painted. In order to stay within this time frame, BENNING divided the motors into five subunits during initial disassembly. This was possible because each subunit had been stamped with a number at the factory, which allowed for work to be carried out and monitored, including documentation, in parallel without gaps.

Commutators of up to 1000 mm diameter are machined on this CNC controlled commutator mica undercutter.
Commutators of up to 1000 mm diameter are machined on this CNC controlled commutator mica undercutter.

Thorough cleaning with subsequent vacuum drying

The BENNING staff members adopted a similarly fast but thorough working method for cleaning the rotor and stator. Since these components normally suffer a high degree of contamination, the cleaning process was immediately followed by vacuum drying. For this, the items to be dried are subjected to negative pressure, which reduces the boiling point of water and allows it to evaporate at a lower temperature. In this case, the required component temperature was 90°C.

Determining the degree of drying and pollution

Directly after drying at a room temperature of 20°C, several electrical measurements were carried out. To do this, it was necessary to establish the so-called polarisation index (PI), which allows assumptions to be made regarding the degree of drying and pollution, and therefore the insulation conditions of the different subunits. In order to arrive at a representative result for this and to achieve the PI > 2 value required by SNCF, the process was not always successful at the first attempt – despite considerable efforts – and it was necessary to demonstrate through repeated cleaning and vacuum drying operations with subsequent measurements that the value PI > 2 could indeed be achieved.
“Through the long-term application of the direct voltage, the insulation molecules become aligned and polarised. The polarisation index more or less measures the degree of movement of the molecules, and it is measured by dividing the value obtained after 10 minutes by the value obtained after one minute,” explains Johannes Dyhringer, Project Manager for the Traction Motor Department in BENNING BeM.

The same principle applies to the stator, for which the PI value must also be measured: the stator is cleaned, dried and cooled to room temperature to achieve a limiting value of PI > 2, so that it can be subjected to further machining. For the end shields at the drive end and the rear end, or for the brush unit, the insulation resistance is an important reference value for components with insulated construction. These components must also fulfil at least the given limit values.

As soon as the rotor had attained the PI value > 2, the commutator could be machined. The latter consists of a total of 516 segments, which must all be milled out to a depth of 3.2 mm, and de-burred. After being successfully overhauled, and final fitting of all components, each motor was put through a thorough functional test in the test bay before receiving a new coat of paint and being sent on its return journey to Lyon.

Assessed as “very good”

During the overhaul of the 160 traction motors, BENNING remained in close contact with SNCF. Every week the client required a detailed status report on the current condition of all the locomotive components in the BENNING works. Similarly, once per year, the French national railway company carried out an audit lasting several days for checking quality on site. Thanks to the implementation of suggestions from the auditors, BeM was assessed as ‘very good’ at the last audit in 2015, gaining 86 out of a possible 100 points.
According to Dyhringer: “For BeM, this result is both a blessing and a curse. We’re proud of our achievement; for example, the fact that during those three years there wasn’t a single complaint about any motor, but BENNING won’t rest on its laurels with this brilliant result. On the contrary: for 2016, we have set ourselves the goal of overhauling SNCF’s largest traction motor. This is a further challenge for BeM, and we would like to master it just as successfully.

Further Information

contact: Johannes Dyhringer
telephone: +49 28 71 / 93 427

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