Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
This chapter presents some concrete electric vehicles. These are currently in traffic and have a typical drive system.
The Hungarian product GANZ articulated tram is a good example for a vehicle operated by series resistance variation, and several runs in Budapest on the BKV (Budapest Transport Company) lines. The Millennium Underground has almost the same drive system as the GANZ articulated tram.
Each vehicle is connected to the 600V DC overhead line network. The vehicle is driven by four series wound traction motor, there are two motors in each bogie. In driving mode motor 1 & 2 and motor 3 & 4 (Fig.4.8) are always connected in series, because the motors were designed for half-voltage (i.e. 300V).
Fig.4.8 illustrates the complexity of the traditional series wound commutator motor driven vehicles operated by series resistance variation. This figure represents the whole circuit diagram of the main circuit.
The diagram contains all the usual elements in the vehicle from the pantograph to the rail: overvoltage arresters, fuses, main switches, connections to the auxiliaries, and the complicated switching system of the motors.
For understanding the circuit, Fig.4.9.a represents the simplified circuit diagram of the driving mode circuit.
The R A and R B series resistors can be varied by sections, moreover two motors can be connected in series or in parallel. In series mode the number of resistance sections is 12, in parallel mode: 10. At the end of the parallel mode sections even two level field weakening can be applied. Therefore to reach the maximum speed (and ω v) of the vehicle, the number of the sections in driving mode is 24. The first six sections – as a pre-section – ensure hitchless start. During the start the tractive force is pulsating in ±9% range. Fig.4.9. represents the characteristic of the motor starting torque with reduced number of sections.
In the first part of the starting, all the four motors (also the series resistors) are connected in series, at first with K 2 switch, then after disconnecting each resistance section, with K 5 switch. When the ω 1 speed is reached the parallel mode of the 1-2 and the 3-4 motors start. (In Fig.4.9.a the current of the first branch is signed with simple arrow, the other is signed with double arrow). The parallel mode is developed by K 3 and K 4 switches, then the R A and R B series resistors are disconnected in section-like mode from the maximal value. In the GANZ articulated tram there is not a D diode connected in series with K 5 switch (can be seen in Fig.4.9.a), but it can be found in several similar circuits e.g. in the drive system of the Millennium Underground. During the switching, the D diode none of the motor currents are interrupted , therefore the transition is continuous from the series mode to the parallel mode. With series mode only ω s speed can be reached, with parallel mode: ω p. The increase of the speed can be achieved by field weakening (with c and d switches).
The whole brake system of the vehicle consists of:
The electric brake of the vehicle is resistive brake; the number of the brake sections is 16. Fig.4.10 represents the electrical circuit of the braking mode.
Motor 1-2 and motor 3-4 form two independent brake circuits with R A and R B resistors. Two motors are cross- or circular-connected, e.g. motor 1 feeds the excitation winding of motor 2. The cross-connection is advantageous, because the braking current is more evenly distributed between the two machines. In both bogies the resistive electric brake is complemented with a mechanical disc drake (SZ) operated by electro magnet. The SZ coil current is proportional to the current of the resistive brake as long as the braking current of the motors vanishes close to the low speed. The current of the disc brake – proportional to the braking current – is achieved by the voltage drop of R le resistor connected in series with R A and R B brake resistors. If the voltage of R le decreases under the battery voltage, than the disc brake uninterruptedly changes to battery supply, and brakes until stop. At the beginning of the brake mode, pre-excitation circuit helps the faster excitation-boost of the motors through R g resistor (current drown with dashed line).
For the lossless control of the DC voltage fed DC motor driven vehicles DC/DC converters are applied, that can be built with thyristor, GTO, and recently IGBT elements.
The Ganz-Ikarus IK 280 trolley is a typical example for a thyristor chopper fed vehicle. Fig.4.11 represents the main circuit diagram of the vehicle.
The traction motor of the vehicle is series wound DC motor with M armature and G excitation winding. The LS smoothing choke is for smoothing the motor current.
The main element of the circuit is the chopper formed by the TFŐ thyristor with turn-off circuit ; this is the main control element for controlling the driving and braking mode of the vehicle. TG, TF and TE thyristors are performing additional functions. The control of these elements is synchronized to the control of the TFŐ chopper, their conduction states end when the TFŐ chopper is turned-off.
The two E switches are operating at driving mode, while the H/F switches are for selecting the braking mode in forward operation. (The braking mode is achieved by reversing the armature current direction). The backward operation is not a normal operation; therefore there is no brake circuit for backward operation. The DC voltage is stabilized by the C capacitor. At start a charging resistor limits the charging current of the capacitor that is short-circuited by a charging contactor in normal mode. Fig.4.12 represents the switching states of the TFŐ thyristor and the time functions of the motor terminal voltage and current in forward operation, traction mode.
In the range 0≤u k =bU H ≤U H the average value of the motor voltage can be continuously varied by the turn-on ratio (b=t be /(t be +t ki )) of the TFŐ thyristor chopper. The motor current, speed, i.e. the tractive force, the acceleration and the speed of the vehicle can be controlled by varying the voltage. The influence of the voltage change can be seen in Fig.4.2.b. The speed range can be expanded by the field weakening mode that can be achieved by TG thyristor, and relatively to the main thyristor the switching of TG is synchronized but delayed, i.e. the excitation winding is short-circuited periodically.
In traction mode the vehicle consumes p H =U H i H power from the network. The i H current is flowing during the turn-on period of the main thyristor. According to the motor mode, i armature current of the motor has the same direction as the u b internal voltage. The developed power and torque on the motor shaft are: p=u b i=k ϕ ωi=Mω>0 and M=k ϕ i>0.
Fig.4.13 shows the time functions and the switching states of the regenerative braking.
Main feature of the brake circuit is that the motor excitation current is remaining in the same direction during the switching from the driving mode to the braking mode. This is a necessary condition for the excitation boost process that was described in Chapter 4.1.3. Due to the switching of the armature terminals, i armature current of the motor has opposite direction as the u b internal voltage, and the motor operates in braking mode. The measureable brake power and brake torque on the motor shaft are: p=u b i=k ϕ ωi=Mω<0 and the motor torque direction reverses (M=k ϕ i<0), because the armature current direction has changed. If the direction of the network current (i H) is taken according to i direction, it can be observed that comparing with the driving mode the network current direction reverses and the recuperated power is: p H =U H i H. The i H network current only flows during t ki turn-off time.
Similarly to the driving mode, the braking current is controlled by the turn-on ratio of the TFŐ chopper. If u b <U H than TE thyristor is continuously in turn-on case, as it is represented in Fig.4.13.a. The regenerative braking is operating if u b >U H caused by e.g. too high speed, but is this case the TE should be turned-off, that connects the R E series resistor into the braking circuit. The voltage of R E resistor has an opposite direction to u b, therefore it allows that the circuit of Fig.4.13 is operable with the u b -iR E <U H conditions.
If the network is not suitable for regenerative braking, then the circuit is switching to resistive braking, as in Fig.4.14. The operation of the circuit is similar to the regenerative braking operation, but the braking current closes through TF thyristor and R F braking resistor. The breaking kinetic energy is converted into heat in R F braking resistor.
The braking current can be controlled by the turn-on ratio, if iR F <U H. The D diode prohibits the current that would flow towards the network, i H=0.
This trolley equipped with thyristor chopper is driven by a single motor ; the electric circuit is clear and simple. On the other hand it has a disadvantage. For the appropriate operation of the different driving and braking modes (can be seen in Fig.4.12…4.14) the main thyristor should possess safety turn-off circuit - that cannot be given in the previous figures - i.e. it must not remain unduly in turn-on state. The TFŐ main thyristor sign means the thyristor system with built-in turn-off circuit, its first control input starts the turn-on process, while the second input starts the turn-off process.
At junctions of two trolley lines, the poles can temporary switch reversed polarity voltage to the vehicle that cause problems at trolley supply. The vehicle circuit of Fig.4.11 would fail by the effect of reversed polarity, therefore before the junctions it must be switched-off from the overhead line. In modern vehicles a rectifier is built-in between the poles and the C filter capacitor. The inverter fed trolley (Fig.5.9) is an example for this, where the rectifier is amended with two IGBT switching elements to achieve the regenerative braking at normal polarity despite the rectifying.
The still operating DC motor driven vehicles: trams, trolleys, metros are successively modernized to IGBT chopper fed drive system. Fig.4.15 represents the main circuit diagram of a DC motor driven, IGBT chopper fed T5C5K type tram (T5C5K is the modernized version of the Czechoslovakian Tatra T5C5 type tram).
The elements of the IGBT chopper perform functions similar to the thyristor chopper, and the same notations are used as in Fig.4.11. The operation of the chopper is similar to Fig.4.12 … 4.14 in driving mode and in several electric braking mode, but IGBT switching elements are used instead of the thyristors.
The switching-on is more complicated than in the trolley, because these trams are equipped with four motor drive system similarly to the GANZ articulated tram (Fig.4.8.). In this drive two motors are always connected in series similarly to the GANZ articulated tram, because the motors are designed for half voltage (600V/2=300V). In multi-motor drives - consist of series wound DC motors – the cross- or circular connection is commonly applied as in Fig.4.15, where in driving mode the E switches while in braking mode the F switches should be turned-on.
Fig.4.16 explains the cross connection of the motors in driving and braking mode. (For the easier understanding, in the figure the two always series connected motor is signed with I and II index.) According to the figure, in driving mode I and II motors are connected in parallel, in braking mode these are connected in series, the current of one motor is the same as the excitation current of the other motor. This ensures simple braking circuit and smooth load distribution. Upto 95% duty-cycle the chopper operates with constant 1000 Hz frequency, further increasing the traction motor voltage is performed by decreasing the operational frequency of the chopper.
The V43 type diode locomotive (nickname in Hungary: Szili) has the simplest structure among the vehicles equipped with semiconductor converters, and still a large number of these are running in Hungary. This locomotive is driven by two motors. Fig.4.17 represents its schematic circuit diagram.
The motor terminal voltage cannot be varied by a diode rectifier, only if the AC voltage of diode bridge input side is varied. For this purpose a transformer - and the associated split switch - is built-in that has several splits on the high-voltage side. The faultless and interrupt-less transition between the splits is performed by K1…K3 auxiliary switches, synchronized to the moving sliding contacts. By handling the split-switch equipment, the driver of the locomotive can achieve such tractive characteristic that can be seen in Fig.1.5. The regenerative braking is not possible because of the diode rectifier. The output of the diode rectifier is a pulsating DC voltage that pulsates with 100Hz frequency. In these vehicles the aim is to smooth the current instead of filtering the voltage. The pulsation of the current remains relatively high (±20%), despite of the LS smoothing choke - built before the motors - that should be taken into account at the motor design ( laminated stator motors design to pulsating current).
In the schematic circuit diagram of the V43 locomotive a 16kV split can be seen on the primary side of the main transformer. The reason is that in the past 16kV overhead line voltage was used instead of the currently used 25kV.
Relatively large number or thyristor converter fed locomotives were manufactured. The most common is the V63 type locomotive (nickname in Hungary: Gigant). Fig.4.18 represents its schematic circuit diagram.
The vehicle is driven by parallel connected compound wound DC motor, and there are three motors in each bogie. Fig.4.18 presents a drive of one bogie. The driving mode can be selected with E switch, while the braking mode can be selected with F switch. In driving mode the main circuit of the three motors is fed by a series connected TH-1 and TH-2 thyristor bridges. To reduce the pulsation of the DC voltage the two bridges are half-controlled, i.e. one branch of each bridge contains diodes that perform the function of null-diodes. The thyristors of TH-1 and TH-2 bridges are controlled with a shift. If less than half-voltage is required for controlling the motors, then only the thyristors of TH-1 bridge are controlled, and the circuit closes through the diodes of TH-2 bridge. Close to half-voltage the controlling of TH-2 bridge’s thyristors starts with a small overlap. If higher voltage is required the thyristors of the TH- 1 bridge are operating with full control (as a diode bridge), the voltage control is performed by the thyristors of the TH-2 bridge. With this so called “follow-up control” method the phase angle of the consumed current can be improved.
The circuit does not allow regenerative braking; only resistive braking can be achieved by closing F switches. Simple braking force control can be achieved by the controllable separate excitation current . The separate excitation windings of each motor are connected in series, and their common current can be controlled by the THG excitation thyristor bridge. Generally Chapter 4.1.4 deals with the functions which can be achieved by the separate excitation of a compound wound motor. The resistive braking can be applied with limitation; the limits should be defined because of the heat generated on the braking resistor. Basically, the braking is mechanical.
In electric cars the separately excited DC motor drives were also used because of the simple controllability, e.g. with two quadrant chopper as in Fig.4.19. In the circuit there is a separate chopper in the excitation circuit (with TG, DG elements), a separate chopper for driving mode (with TM, DM elements), and a separate chopper for regenerative braking mode (with TF, DF elements). The change between driving- and braking mode is performed electronically, without separate switches and without any excitation boost problems. The control operates according to the conventional GP accelerator pedal and FP braking pedal functions, i.e. basically it defines torque control. The forward or the reversed direction can be selected by E and H switches of the excitation circuit in standing position of the car.
The circuit is clear and the functions can be easily separated from each other. If field weakening is applied by the excitation controller (as in Fig.4.6.a), in driving and braking mode the mechanical boundary curves of Fig.4.19.b can be achieved.
(The literature used for this chapter: …)