mass (t)

Fig. 4  Pressure relay/load-sensing device

On freight trains, where the difference between the tare and fully loaded vehicle masses could be in the order of 300 7• (from 20 to 30 I/vehicle for the empty wagon to 90 t/vehicle for the fully loaded one), load sensing is very important. For high-speed nains, such as EMU V250, the difference between VOM and CE loading conditions, as visible in Table 2, is not in general lower than 10 7»—20 7o.

As a consequence, the corresponding variation in terms of deceleration and dissipated power on disks is often numerically not much relevant and is partially tolerated by regulations in force [10] for high-speed trains with more than 20 axles, in emergency braking condition or in other backup mode, where the full  functionality  of  the  plant should not be completely available.

@ Springer

For the reasons of safety, the correct implementation of the double stage pressure ensuring that lower pressure is applied on cylinders for traveling speed of over 170 km/h is much more important. This is important because the energy dissipated during a stop braking increases approx- imately with the square of train traveling speed and, as a consequence, a reduction of disk clamping forces may be fundamental to avoid the risk of excessive thermal loads. Furthermore, the adhesion limits imposed by [l0J prescribe a linear reduction of the braking forces between 200 and 350 kin/h, according to a linear law which corresponds to a reduction of the braking power of about one-third in the above-cited speed range.

2.2 Electrical braking and blending

Electrical or electro-dynamical brakes are a mandatory trend for a modern high-speed train. Most of the more modern EMUs have the traction power distributed over a high number of axles. On EMU V250 train, nearly 50 7« of the axles is motorized and nearly 55 7r of the total train weight is supported by motorized bogies.

As a consequence, a considerable amount of the total brake effort should be distributed to traction motors, by performing regenerative or dissipative bra1‹ing, according to the capability of the overhead line for managing the corresponding recovered power. In panicular, not only regenerative but also dissipative electric braking is quite attractive, considering the corresponding reduction of wear

J. Mod. Transport.  (2013) 21(4):247—257

Design and preliminary validation of a tool

of friction braking components such as pads and disks. Since electric braking is applied in parallel with the con- ventional pneumatic one, an optimized ltiixing strategy in the usage of both systems, usually called blending, has to be performed.

In Fig. 5, the electric braking effort available on a motorized coach as a function of the train traveling speed and of the electrihcation standard of the overhead line is shown. Three different operating conditions can be recognized:

• Maximum pneumatic braking force: under a certain traveling speed, the corresponding operating frequen- cies of the traction system are too low. On the other hand, also the demanded braking power is quite low, and so it can be completely managed by means of the pneumatic braking system.

• Minimum pneumatic braking: in this region,  the electric braking effort is limited to a maximum value, often related to the motor currents. If a higher braking effort is required, then the pneumatic brake is activated to supply the difference.

• Pneumatic  braking  increases  to  supply insufficient

electric power: as  speed  increases,  the performances of the motor drive system are insufficient to manage the corresponding power requirements, limiting the maxi- mum braking effort to the associated iso-power curve. As a consequence, the contribution of the pneumatic braking power tends to increase with  speed.