TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to electrically driven rail vehicles powered by an overhead line or a third rail. It relates in particular, albeit not exclusively, to electrical multiple unit (EMU) rail vehicles comprising a plurality of coaches and driven by motors, preferably permanently excited synchronous machines, distributed over the length of the vehicle. More specifically, it relates to a braking method and system, which make extensive use of electro-dynamic braking, and completely or almost completely avoids the use of friction brakes in emergency braking situations.

BACKGROUND ART

[0002] Traditionally, rail vehicles are provided with several different braking means, such as mechanical or electro-dynamic brakes. The mechanical brakes are generally pneumatically operated and transform the kinetic energy of the rail vehicle into heat through friction. The electro-dynamic brakes transform the kinetic energy of the vehicle into electric energy, which can be dissipated as heat in rheostats (rheostatic braking) or fed back to the power line, e.g. an overhead catenary line or a third rail (regenerative braking). Usually, electro-dynamic brakes are provided by the electric motors of the electrically driven rail vehicle, which are operated as alternators or generators. Depending on the kind of traction motor used, e.g. DC motors, permanent magnet AC motors or induction motors, the electric machine must be controlled to transfer energy from the rotor to the stator during braking.

[0003] Mechanical and electro-dynamic brakes are applied jointly or separately for different braking functions, in particular for service braking and for emergency braking.

[0004] For service braking, the mechanical brakes and electro-dynamic brakes are usually blended. More precisely, the electro-dynamic brakes supply the required braking torque up to their full capacity, and are supplemented by the mechanical brakes if necessary, so as to minimize wear and optimize energy regeneration. [0005] On the other hand, electro-dynamic brakes are usually not considered reliable enough to be applied in emergency braking situation. Hence, the whole retarding torque in case of emergency braking is still provided by the mechanical brakes. To provide sufficient performance for emergency braking, the installation of the mechanical brake system requires a number of actuators, which are not permanently used for service braking. The installed mechanical braking means are therefore not used on a regular basis. Increased weight, space for installation of voluminous pneumatic equipment and cost are the consequence.

[0006] A safety braking system for a vehicle with electric traction is disclosed in US 2009/0224706. The safety braking system comprises a first, electric, non-safety brake which is integrated in the traction chain and includes a three-phase permanent magnet motor capable of operating as a voltage generator, a traction inverter capable of being configured as a diode bridge rectifier, an electromechanical switch for connecting the permanent magnet motor to the inverter, and a DC link including a chopper brake resistor, a line filter and a line circuit-breaker. The system further includes a second, safety brake. According to one embodiment, the safety brake includes the diode bridge rectifier of the traction inverter, a terminal load resistor, an auxiliary electromechanical relay connected in series with the terminal load resistor and controlled at an input, the relay and the resistor being interposed in parallel between the chopper and the inverter. A current monitoring device is mounted in series on the DC side of the inverter for monitoring the braking performance of the first brake. Upon detection of a predetermined condition, the current monitoring device triggers the electromechanical relay to switch on the branch of the circuit including the load resistor. In order to avoid motoring, the line circuit breaker is also triggered to open the power line.

[0007] This system is complex as it involves both an electric non-safety brake and an electric safety brake.

[0008] Hence, there is a need for a simpler and more integrated electro-dynamic braking system and method, which make extensive use of electro-dynamic brakes even in the most demanding emergency situations, to avoid the installation of a fully- rated mechanical system. SUMMARY OF THE INVENTION

[0009] The foregoing shortcomings of the prior art are addressed by the present invention.

[0010] According to a first aspect of the invention, there is provided a method for a braking operation of a wheel axle of a rail vehicle, the wheel axle being provided with an electro-dynamic brake system and with a not fully rated mechanical braking system, the method comprising at least a service braking mode, a high integrity emergency braking mode and a parking brake mode, the service braking mode being carried out solely by the electro-dynamic braking system and the parking brake mode being carried out by the mechanical braking system, wherein the high integrity emergency braking mode is carried out solely by the electro-dynamic braking system as long as a low efficiency condition of the electro-dynamic braking system is not met and at least partly by the mechanical braking system, when the low efficiency condition of the electro-dynamic braking system is met. [0011] According to another aspect of the invention, there is provided a method for a braking operation of a wheel axle, in particular a drive wheel axle, of a rail vehicle, comprising at least a service braking mode, a high integrity emergency braking mode and a parking brake mode, wherein the service braking mode is carried out solely by an electro-dynamic braking system from the cruising speed of the vehicle down to standstill, the parking brake mode is carried out by a mechanical braking system which is not a fully-rated mechanical braking system, and the high integrity emergency braking mode is carried out solely by the electro-dynamic braking system as long as a low efficiency condition is not met, and is at least partly carried out by the mechanical braking system when the low efficiency condition is met.

[0012] The cruising speed of the vehicle is a speed at which the vehicle or wheel axle is designed to run for long period of times between stations. The cruising speed is in any case above 100 km/h and, in the case of a high speed vehicle, higher than 250 km/h. The parking and holding brake system is basically a mechanical brake system and not a fully-rated braking system, i.e. it is thermally not rated to brake the wheel axle from the cruising speed down to standstill. This means that the braking system is substantially less complex and bulky than a fully-rated braking system (which, by definition, must be able to brake the wheel axle from the cruising speed down to standstill by itself). The mechanical braking system is only able to brake the vehicle if the vehicle speed is already below a predetermined threshold, which may depend on the weight supported by the wheel axle. The method takes advantage of this available mechanical braking system to supplement or replace the electro- dynamic braking system at very low speed in the high integrity emergency braking mode, when the efficiency of the passively running static converter and of the electro- dynamic braking system becomes insufficient. [0013] The low efficiency condition of the electro-dynamic braking system is met when the electro-dynamic braking system becomes unable to transform the kinetic energy at very low speed. This may be the case when the current magnitude in a DC link associated of the electro-dynamic brake system drops below a predefined threshold. Alternatively the low efficiency condition is met when a speed of the rail vehicle or of the wheel axle drops below a predefined threshold (e.g. 5 or 2 km/h). The low current and low speed condition can be used simultaneously. The low efficiency condition should be chosen such as to correspond to a vehicle speed that is below the predetermined threshold of the mechanical braking system mentioned above. [0014] Upon deceleration of the wheel axle or rail vehicle from the cruising speed down to standstill in the service braking mode, the electro-dynamic braking system holds the wheel axle and the rail vehicle stationary for a predetermined period of time, e.g. 10s or 1 minute, after which the mechanical braking system takes over and the electro-dynamic braking system is ramped down or faded out. This helps to avoid overloading the electro-dynamic brake converter's switching elements of the electro- dynamic braking system. Hence, in the service braking mode, the electro-dynamic brake system followed by the mechanical brake system are used to bring about the holding brake function, i.e. the function of holding the train at standstill for a short duration. [0015] To prevent reverse movements when the rail vehicle starts moving, the mechanical braking system is released only after building up motoring torque. [0016] After deceleration down to the threshold value in the emergency braking mode, the mechanical braking system progressively takes over. In the high integrity emergency braking mode, a blending of the mechanical brake system and electro- dynamic brake system or the mechanical brake system alone is used for the holding brake function.

[0017] In the high-integrity emergency braking mode, a static converter located between a traction motor and a DC link of the electro-dynamic braking system is run as a passive rectifier, whereas in the service braking mode the static converter is run as an active rectifier. Hence, the risk of a potential malfunction of a control unit in charge of controlling the power semiconductor switches of the static converter is avoided in the high-integrity emergency braking mode.

[0018] The high-integrity emergency braking mode is preferably a second level emergency braking mode and the procedure preferably also includes a first level emergency braking mode. [0019] In the first level emergency braking mode, the vehicle is preferably decelerated from the cruising speed of the vehicle down to standstill solely by the electro-dynamic braking system. In this respect, the first level emergency braking mode is very similar to the service braking mode, except that the set value for the braking power is maximum at any given time from the cruising speed down to standstill, such that a maximum available deceleration is applied by the electro- dynamic brake system at each instant. In both the service and the first level emergency braking mode, a static converter of electro-dynamic braking system associated with the wheel axle is actively controlled. In particular, if the static converter is provided with power semiconductor switches and freewheeling diodes, the power semiconductor switches are triggered to optimise the power transfer from the electro-dynamic brake associated with the wheel axle to DC link and from there to a power line for generative braking and/or to a brake chopper for dissipative braking. The service braking mode and first level emergency braking mode are preferably jerk limited, which is achieved by a proper control of the electro-dynamic braking system, since no mechanical friction braking is involved. [0020] The second, high-integrity emergency braking mode is a braking mode of higher integrity but of lower efficiency, in particular at very low speed, and the mechanical braking system is used to complement or replace the electro-dynamic brakes when the braking power of the electro-dynamic brakes becomes insufficient at very low speed, e.g. below 5 or 2 km/h. In particular, in the case of a static converter provided with power semiconductor switches and freewheeling diodes as discussed above, the power semiconductor switches of the static converter are disabled, the circuit operates as a passive rectifier via the freewheeling diodes to transfer power away from the electro-dynamic brakes. Jerk limitation is realised by progressively fading out the electro-dynamic braking system at low speed and taking over with mechanical braking system.

[0021] The second, high-integrity emergency braking mode is preferably carried out upon a malfunction of the first level emergency braking mode. A malfunction condition in the first level brake control mode is detected when the electro-dynamic braking system does not adequately decelerate the wheel axle in presence of an emergency brake demand based on a comparison between an instantaneous target emergency torque value and an actual torque value. According to a preferred embodiment, the instantaneous target emergency torque value is calculated based on an axle load weight sensor signal and a driving wheel axle revolution speed signal. [0022] The procedure with two emergency braking modes of different priorities ensures that in case of malfunction, the electro-dynamic brake force is not lost completely. Advantageously, the electro-dynamic braking system can be operated both as a regenerative brake and as a rheostatic brake in the service braking mode and in the first level emergency braking mode, and the regenerative brake is given a higher priority than the rheostatic brake. Hence, the procedure takes full advantage of regenerative braking, i.e. of the braking capacity of the power line. The connection to a power line is preferably interrupted in the second level emergency braking mode, such that the electro-dynamic brake performs only as a rheostatic brake.

[0023] According to a preferred embodiment, a wheel slide protection is active during service braking mode, first level emergency braking mode and second level emergency braking mode. The wheel slide protection is preferably applied on a per axle basis. The wheel slide protection may include a first level wheel slide protection associated to the first level emergency braking mode and a second wheel slide protection associated to the second level emergency braking mode.

[0024] According to a preferred embodiment, a load weighing of the car body influences the service braking, first level emergency braking mode and/or the second level emergency braking mode. Preferably, the load weighing is performed locally, on each bogie or each axle, and the service braking and/or emergency braking modes are load compensated according to this local load weighing.

[0025] According to one embodiment, the first level emergency braking mode is activated when an emergency brake loop is de-energised and the second level braking mode is activated when a life signal from the first level brake control unit fails.

[0026] Preferably, the mechanical braking system is an electrically controlled mechanical braking system. Preferably the mechanical braking system is an energise- to-release braking system, and more preferably a spring-applied braking system.

[0027] According to another aspect of the invention, there is provided a method for braking a rail vehicle provided with a plurality of wheel axles, in particular a multiple unit rail vehicle, each of the wheel axles being provided with an electro- dynamic braking system and a mechanical braking system, each of the wheel axles being individually braked as described above. In particular, the low efficiency condition of each electro-dynamic braking system is individually assessed for each electro-dynamic braking system and each axle.

[0028] The procedure is performed on a per axle (or per motor) basis by opposition to a per bogie or per car basis. This means in particular that the potential malfunction of the first level emergency braking mode is preferably detected at the level of each electro-dynamic braking system or each axle, and that switching to the second level emergency braking mode is performed locally, while other well- functioning brake control units are still in the first level emergency braking mode. [0029] The method takes advantage of the fact that on a multiple unit vehicle, the odds of having a simultaneous malfunction of all the local brake control units is almost inexistent. Hence, running some of the electro-dynamic braking systems in a failsafe degraded mode is acceptable, since the well-functioning brake control units will ensure that the overall braking performance of the vehicle is as expected.

[0030] According to a another aspect of the invention, there is provided a braking system for braking a rail vehicle provided with a plurality of drive wheel axles, each drive wheel axle comprising an associated electro-dynamic braking system and an associated mechanical braking system that is not fully rated, each drive wheel axle comprising an associated first level brake control unit for carrying out at least a service braking mode solely with the associated electro-dynamic braking system from the cruising speed of the vehicle down to standstill, and for carrying out a parking braking mode at standstill solely with the associated mechanical braking system, each drive axle further comprising an associated second level brake control unit for carrying out a high integrity emergency braking mode solely with the associated electro-dynamic braking system as long as a low efficiency condition is not met, and at least partly with the mechanical braking system when the low efficiency condition is met. Preferably all the wheel axles of the vehicle are drive wheel axles as described hereinbefore. Alternatively, the vehicle may also include trailer axles. [0031] The first level brake control unit and the second level brake control unit are preferably each connected to a mechanical brake activation unit associated to the drive wheel axle. In the service braking mode (and, if available in the first level emergency braking mode), the first level brake control unit triggers the activation of the mechanical brake system after a predetermined time at standstill. In the high integrity emergency braking mode, the second level brake control unit triggers the activation of the mechanical brake system as soon as a low efficiency condition is met.

[0032] The not fully rated mechanical braking system is preferably the only friction brake associated with each of the drive wheel axle. In other words, the drive wheel axles are not provided with fully rated mechanical braking system, which is a major advantage in terms of weight and space. [0033] The mechanical braking system is preferably an energise-to-release, preferably a spring applied, braking system. According to a preferred embodiment the mechanical braking system is electrically controlled and actuated, which involves that no pneumatic or electro-pneumatic control system is necessary. Again this is a major advantage in terms of complexity.

[0034] Preferably each mechanical braking system is provided with a mechanical brake activation unit. The first level brake control unit and the second level brake control unit are each connected to the mechanical brake activation unit.

[0035] According to a preferred embodiment, the braking system comprises a power line and, for each of the wheel axles, the associated electro-dynamic braking system comprises at least one traction motor operable as an alternator during braking operation, at least one inverter connected to the motor and operable as a rectifier during braking operation, at least a first wheel slide protection unit, and a signal transmission system with sensors. The first level brake control unit is connected to the inverter and to the first wheel slide protection unit and the second level brake control unit is connected to the first level brake control unit and to a second wheel slide protection unit. The service braking mode and, if available, the first level emergency braking mode are carried out by the first level brake control unit, while the second level emergency braking mode is carried out by the second level, high integrity brake control unit.

[0036] The power line is preferably a DC power line and the braking system comprises a DC link with at least one brake chopper connected to the second level brake control unit and to the inverter, and the motor is a synchronous motor.

[0037] Preferably, the signal transmission system comprises a first connection from the first level brake control unit to an emergency brake loop and a second connection from the first level brake control unit to a load weight sensor.

[0038] Preferably, the signal transmission system comprises a live signal connection from the first level brake control unit to the second level brake control unit and to the line circuit breaker and/or at least one separation contactor. [0039] According to one embodiment, the sensors comprise one or more speed sensors associated to the wheel axle, wherein each of the first level brake control unit, the first wheel slide protection unit, the second level brake control unit and a second wheel slide protection unit is connected to at least one of said one or more speed sensors.

[0040] According to one embodiment, the sensors include one or more current sensors in the DC link, wherein each of the first level brake control unit and the second level brake control unit is connected to at least one of said one or more current sensors. [0041] According to another aspect of the invention, there is provided a wheel axle of a rail vehicle, in particular for carrying out the methods described above, the wheel axle being provided with an electro-dynamic braking system and a mechanical braking system which is not fully rated, the electro-dynamic braking system comprising a traction motor associated with the wheel axle and operable as alternator during braking operation, an inverter connected to the motor and operable as a rectifier during braking operation, wherein the inverter is provided with a first level brake control unit connected to a first wheel slide protection unit and a second level brake control unit connected to the first level brake control unit and to a second wheel slide protection unit. The wheel axle is not provided with any fully-rated mechanical braking system.

[0042] According to another aspect of the invention, there is provided a braking system for an electrical multiple-unit vehicle, in particular for carrying out the method described above, comprising a power line, a plurality of carriages, each driven by one or more traction motors each associated with one traction wheel axle and operable as alternator during braking operation, a plurality of inverters each connected to one of the traction motors and one only and operable as a rectifier during braking operation, wherein each of the inverters is provided with a first level brake control unit connected to a first wheel slide protection unit and a second level brake control unit connected to the first level brake control unit and to a second wheel slide protection unit. [0043] The wheel slide protection is carried out as a per axle control and is available during both service brake and emergency brake. Due to low to medium integrity level required for service brake (low to medium integrity) and first level emergency brake (medium level integrity requirement) the wheel slide protection is performed by the first level brake control unit. Whenever the second level emergency brake mode is active the wheel slide protection is performed by a second level wheel slide protection of higher integrity associated to the second level brake control unit.

[0044] Preferably, each traction wheel axle is provided with an electromechanical braking system, preferably a spring-applied braking system. [0045] According to one embodiment, the inverter is connected to a DC-link provided with a current sensor, and the second level brake control unit triggers the application of the electro-mechanical braking system when a current signal detected by the current sensor is below a predetermined, preferably strictly positive threshold.

[0046] According to one embodiment, each driving wheel axle is provided with at least one rotation sensor connected to the second level brake control unit, which triggers the application of the electro-mechanical braking system when a rotation speed signal detected by the rotation sensor is below a predetermined, preferably strictly positive threshold, corresponding e.g. to a ground speed of about 3 to 5 km/h.

[0047] According to another aspect of the invention, there is provided a braking system for a rail vehicle, comprising: an electric power line; an emergency brake loop which is energised in the absence of an emergency brake demand and de-energised in presence of the emergency brake demand; - driving wheel axles each provided with at least one parking brake, at least one electro-dynamic brake, a static converter having AC terminals connected to the electro- dynamic brake and DC terminals, the static converter including a circuit of power semiconductor switches and freewheeling diodes, the circuit being such that when the power semiconductor switches are disabled, the circuit operates as a passive rectifier via the freewheeling diodes to transfer power from the AC terminals to the DC terminals, a DC-link for connecting the DC terminals of the static converter to the power line, the DC-link including one or more braking chopper, a first level brake control unit connected to the emergency brake loop and at least responsive the emergency brake demand to generate trigger signals to control the power semiconductor switches of the static converter and at least one of the one or more braking choppers, and a second level brake control unit for controlling at least one of the one or more braking choppers in presence of an emergency brake demand and of a malfunction condition of the first level brake control unit.

[0048] The braking system takes advantage of the failsafe degraded passive operative mode of the static converter to transfer the braking power of the electro- dynamic brakes back to the DC link and to the braking chopper to achieve a higher integrity level emergency braking whenever the first, lower-level brake control unit proves inefficient.

[0049] According to a preferred embodiment, the braking system further comprises a service brake demand line, the first level brake control unit being responsive to a service brake demand signal on the service brake demand line to control deceleration of the axle down to zero. No friction braking is involved at all in the service braking mode, even at very low speed. Hence, there is no need to equip the vehicle with heavy and bulky full-rate friction brakes and the vehicle does not include such full-rate friction brakes. At standstill, the holding torque delivered by the electro-dynamic brakes is preferably calculated as a function of an assessed or measured load weight, preferably a load weight measured locally on the wheel axle corresponding to the electro-dynamic brake or on the bogie or running gear to which this wheel axle belongs.

[0050] According to a preferred embodiment, each driving axle is further provided with a parking brake, which is preferably a spring applied, electrically released friction brake. It should be understood that unlike service friction brakes, which should be dimensioned such as to dissipate in heat most or all of the kinetic energy of the rail vehicle decelerating from its cruising speed down to standstill, parking brake are dimensioned to merely hold the wheel axle at standstill.

[0051] Preferably, the first level brake control unit automatically activates the mechanical brake and deactivates the electro-dynamic brake when the vehicle has been at standstill for a predetermined time. Preferably, the electro-dynamic brake is progressively decreased down to zero after the mechanical brake has been applied.

[0052] If the second level brake control unit is automatically activated to carry out high integrity emergency braking, the parking brake may be activated when a low efficiency condition of the electro-dynamic braking system is reached because the combination of the passive rectifier and braking chopper does not provide sufficient braking power at very low speed. The low efficiency condition of the electro-dynamic braking system is preferably one condition out of the following set of conditions: the speed of the rail vehicle is less than a predetermined, strictly positive threshold value; - the current amplitude in the DC link is less than a predetermined, strictly positive threshold value.

[0053] When the condition is reached, the mechanical brake release system becomes de-energised and the mechanical brake automatically applies. [0054] According to one embodiment, the first level brake control unit includes a first level wheel slide protection controller for setting an instantaneous torque limit as a function of at least the revolution speed of the axle and for preventing that the actual braking torque exceeds the instantaneous torque limit. [0055] Preferably, the second level brake control unit includes a second level wheel slide protection controller for setting an instantaneous torque limit as a function of at least the revolution speed of the axle and for preventing that the actual braking torque exceeds the instantaneous torque limit.

[0056] According to a preferred embodiment, the first level brake control unit of each driving axle is connected to a load weight sensor for measuring the load weight on said driving axle or on a couple of axle including said driving axle, and determines a target service deceleration torque based on a load weight value measured by the load weight sensor.

[0057] The first level brake control unit of each driving axle is connected to a load weight sensor for measuring the load weight on said driving axle or on a couple of wheel axles including said driving axle, and determines a target service deceleration torque based on a load weight value measured by the load weight sensor.

[0058] The electro-dynamic brake is using a permanently excited synchronous motor, in particular a permanent-magnet motor used as traction motor for the rail vehicle. Thanks to the operative principle of this kind of motor, an automatic locking protection is achieved, since at 100 % slide (wheel locked) the motor braking torque is zero.

[0059] The power line is provided with at least one line circuit breaker. According to a preferred embodiment, a trigger circuit of the line circuit breaker is connected to the first level brake control unit or to the supervision unit to open the line circuit breaker if a regenerative mode abortion condition is met. In practice, motoring of the electro-dynamic brakes during braking will trigger the opening of the line. [0060] Each driving axle is advantageously provided with one or more sensors included in a list of sensors consisting of a driving wheel axle revolution speed sensor, an axle load weight sensor and an actual brake torque sensor.

[0061] According to an embodiment, a malfunction condition of the first level brake control unit is detected when the first level brake control unit does not adequately decelerate the wheel axle in presence of an emergency brake demand based on a comparison between an instantaneous target emergency torque value and an actual torque value. According to a preferred embodiment, the instantaneous target emergency torque value is calculated based on an axle load weight sensor signal and a driving wheel axle revolution speed signal.

SHORT DESCRIPTION OF THE DRAWINGS

[0062] Other advantages and features of the invention will become more clearly apparent from the following description of specific embodiments of the invention given as non-restrictive example only and represented in the accompanying drawings in which:

Fig. 1 is a schematic view of a rail vehicle provided with a braking system according to one embodiment of the invention,

Fig. 2 is a schematic view of the braking system of Fig. 1.

[0063] Corresponding reference numerals refer to the same or corresponding parts in each of the figures.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0064] Referring to Fig. 1, a rail vehicle, in this example an electrical multiple unit vehicle comprises a number of coaches 10, each of which runs on one or more two- axle bogies 12. Each bogie 12 is provided with a secondary suspension installed between the coach body and a bogie frame 13. The secondary suspension may include a levelling system 14 and a load weighing sensor 16. The local load weighing sensor is e.g. a pressure sensor associated to the pneumatic or hydro-pneumatic levelling system 14. A pressure sensor attached to the gas reservoir delivers the load signal. [0065] At least some of the wheel axles 18, in the example all of them, are drive axles each individually linked to a dedicated permanently excited synchronous motor 20. Each wheel axle is also equipped with an electrically controlled mechanical parking brake system 22 consisting of one or more friction brakes 22.1. Each parking brake 22.1 is biased towards the braking position by a spring 22.2 and released by an electrically controlled actuator 22.3. The parking brakes are not rated for braking the vehicle above a very-low speed limit of e.g. 3 to 5 km/h. Remarkably, the wheel axles 18 are not provided with other friction brakes rated for mechanically transforming into heat all the kinetic energy of the rail vehicle. [0066] Each permanently excited synchronous motor 20 is individually connected to a dedicated static converter 24 which can be operated alternatively in an inverter mode for powering the associated permanently excited synchronous motor 20 and in a rectifier mode for braking the permanently excited synchronous motor 20. As will explained in more detail below, a first level brake control unit 26 controls the static converter in both service braking and emergency braking situations. The first level brake control unit 26 is connected to an emergency braking loop 28 and to a service braking line 30. A second level brake control unit 32 of higher integrity takes over the control of the electro-dynamic braking in case of failure of the first level brake control unit 26. [0067] The permanently excited synchronous motor 20, static converter 24, first level brake control unit 28 and second level brake control unit 32 together constitute an electro-dynamic braking system 33 associated to the wheel axle 18. As illustrated in Fig. 2, each static converter 24 has AC terminals 34 connected to the associated permanently excited synchronous motor 20 via a three-phase circuit breaker 36 and DC terminals 38 connected to a DC-link 40 associated with the first level brake control unit 26. The rotor of the motor 20 is mechanically linked to a wheel axle 18 of the rail vehicle via a gearing 42. The steel wheels 44 of the wheel axle 18 close the electric circuit and allow the current to flow back to the feeder station via the track 46 connected to the earth. The static converter 24 includes a circuit of power semiconductor switches 48 and freewheeling diodes 50. The power semiconductor switches 48 of the static converter 24 are controlled by the first level brake control unit 26 and can be operated alternatively in an inverter mode for powering the permanently excited synchronous motor 20 and in a rectifier mode for feeding back energy from the permanently excited synchronous motor 20 to the DC terminals 38 of the static converter 24. The circuit of the static converter 24 is such that when the power semiconductor switches 48 are disabled, the circuit operates as a passive rectifier via the freewheeling diodes 50 to transfer power from the AC terminals 34 to the DC terminals 38.

[0068] The rail vehicle includes one or more current collectors 52 for connecting the vehicle to a DC catenary line 54 or a third rail. Each static converter 24 is individually connected to the current collector 54 via the DC link 40 and a line circuit breaker 56. The DC link 16 is connected to the DC terminals 38 of the static converter 24 and includes a first level braking chopper unit 58 and a second level braking chopper unit 60, each of which includes a braking chopper 62, resp. 64 and a braking chopper resistance 66, resp. 68 connected in parallel with the static converter 24. The DC link further comprises a DC link capacitor 70 connected in parallel with the braking chopper units 58, 60 and with the DC terminals 38 of the static converter 24. The braking chopper 62 or the first level braking chopper unit 58 is controlled by the first level brake control unit, while the braking chopper 64 of the second level braking chopper unit 60 is controlled by the second level brake control unit 32. [0069] The first level brake control unit 26 receives inputs from emergency brake commands, e.g. a binary input connected to the emergency brake loop 28, a brake pipe or an emergency pushbutton. The first level brake control unit 26 also receives a load weight signal from the load weight sensor 16 of the bogie 12 corresponding to the drive wheel axle 18. Additionally, the first level brake control unit 26 receives a revolution speed signal from a rotation sensor 72 associated to the drive wheel axle 18.

[0070] The same revolution speed signal is fed to the second level brake control unit 32. Each of the first level and second level brake control units 26, 32 is provided with a wheel slide protection unit 74, resp. 76, which is also fed with the revolution speed signal. Both the first and second level brake control units 26, 32 are connected to a life line 78 which triggers the line circuit breaker 56 and to a current sensor 80 that measures the current flowing in the DC link between the DC terminals 38.

[0071] When the static converter 24 operates in the inverter mode and the permanently excited synchronous machine 20 runs as a motor to propel the vehicle, the electric current flows between the DC link 40 and the DC terminals 38 of the static converter 24.

[0072] When a service brake signal is present on the service brake demand line 30, braking is performed by the permanently excited synchronous machines 20 only and combines regenerative and rheostatic braking, with priority to the former. More specifically, the first level brake control unit 26 receives the signal from the service brake demand line and controls the power semiconductor switches of the static converter to operate the static converter 24 in the rectifier mode such that the permanently excited synchronous machine 20 runs as alternator to deliver power through the static converter 24 to the DC link 40. The DC voltage of the static converter 24 is controlled via the first level brake control unit 26, based on the revolution speed signal, the load weight signal, the service brake signal and a slide protection signal from the wheel slide protection unit 74. If the power line allows it, power is fed back to the power line. Otherwise, the first level brake control unit 26 opens the line circuit breaker 56 such that the power is directed through the closed braking chopper 62 and braking chopper resistance 66 of the first level braking chopper unit 58. Remarkably, the control of the DC voltage during service braking is carried out locally on a per axle basis.

[0073] If the service brake signal is maintained, the vehicle is decelerated down to standstill by the permanently excited synchronous motors 20 as described above without applying any friction brake.

[0074] After the vehicle has reached standstill, a holding torque is maintained by the permanently excited synchronous motors 20. The holding torque is controlled by the first level brake control unit 26. The holding torque is preferably a function of the load weight locally measured by the load weight sensor 16. [0075] After a predetermined time at standstill, e.g. 10 seconds or 1 minute, the parking brakes 22 are applied and the holding torque applied by the permanently excited synchronous motors 20 is faded out to avoid overheating of the power switches 48 of the static converter 24. [0076] As will be understood to those skilled in the art, the holding brake function in a standard mode, i.e. when the vehicle decelerates according to a service brake mode, is initially performed by the electro-dynamic brakes only (i.e. the permanently excited synchronous motors 20), before the mechanical brakes 22 take over.

[0077] The friction brake 22.1 is also automatically applied whenever the electrical auxiliary energy feeding the actuator 22.3 drops below a certain threshold. In that case the auxiliary energy is not sufficient to keep the parking brake in release status. Subsequent application of auxiliary energy releases and the parking brake and reinstate normal operation.

[0078] The friction brake 22.1 can also be applied or released by the driver actuating a parking brake push button.

[0079] To prevent reverse movements when the rail vehicle starts moving, the mechanical braking system is released only after building up motoring torque. When motoring torque is present the parking brake 22 receives release signal and the electromechanical release device of each mechanical brake 22 is actuated to build up counterforce against the spring to move and keep the friction elements in the release position.

[0080] The emergency brake loop 28 is energised in the absence of an emergency brake demand. If the emergency brake loop is de-energised, the first level brake control unit 26 switches to a first level emergency brake mode, which is comparable to the service brake mode, except that the deceleration is the highest achievable. The first level wheel slide control unit 74 is still active in the first level emergency braking mode.

[0081] In case of malfunction of the first level control unit 26 in response to an emergency braking demand, the second level brake control unit 32 takes over. Remarkably, the switching from the first level emergency braking mode to the second level emergency braking mode is performed on a per axle basis. The malfunction corresponds to an actual braking torque lower than a given threshold, which is calculated at each instant. In the second level emergency braking mode, the gate drive units of the power semi-conductor switches 48 are blocked and a life signal becomes zero. The static converter 24 operates in a passive rectifying mode via the freewheeling diodes 50. While the braking efficiency is less in the second level emergency braking mode than in the first level emergency braking mode, a degraded deceleration performance with high integrity is still available. The second level brake control unit controls the braking chopper 64 of the second level braking chopper unit 60.

[0082] When the speed decreases below a given threshold, the second level braking chopper unit 60 linked to the static converter 24 which operates as a passive rectifier cannot provide sufficient braking torque. Hence, the second level control unit is linked to the electromechanical parking brake and de-energises the electromechanical brake (i.e. applies the brake) to decelerate the wheel axle from a low speed, e.g. 3 to 5 km/h down to standstill by friction. The DC current in the second level braking chopper unit 60 is proportional to the braking torque of the permanently excited synchronous motor 20. Hence, the application of the friction brake 22 can be triggered by a current threshold measured by the current sensor 80 located in the DC link 40, or by the rotation sensor 72.

[0083] A number of modifications can be envisaged. The overhead power line or third rail can be an AC power line.

[0084] The combined electric drive and brake system of the rail vehicle can be provided with a single line circuit breaker or with one common line circuit breaker and a line circuit breaker in each DC link.

[0085] Obviously, the speed sensor 72, current sensor 80 and/or line circuit breaker 56 can be duplicated for higher reliability, with different sensors of each type being associated to the first level and second level brake control units or with all sensors being associated to the first level and second level brake control units 26, 32. [0086] A single, common braking chopper unit can replace the first level and second level braking chopper units. In that case, the first level brake control unit and second level brake control unit both control the common chopper.

[0087] Technical features disclosed in connection with one aspect or embodiment of the invention can be combined with other aspects or embodiments.