DE3833377C1 - Control method for reducing the power loss of the main thyristor of a D.C. chopper converter during the braking mode of a vehicle motor - Google Patents

Abstract

The intention is to disclose a control method for reducing the power loss of the main thyristor of the D.C. chopper converter, which thyristor is constructed as a turn-off thyristor, during the hazard braking mode of a vehicle motor when the D.C. chopper converter circuit is operating with disconnected brake rheostatic controller and a contact line network is disconnected, during hazard braking mode, from the mains capacitor which is connected upstream of the D.C. chopper converter. The control method is characterised in that the drive level of the brake thyristor is adjusted by means of a control variable (wU) for the voltage (UC) at the mains capacitor, which voltage (UC) is predetermined in terms of its characteristic curve as a function of the rpm (n) of the vehicle motor. Furthermore, the frequency of the ignition signals (ZHT and ZBT) is predetermined in terms of the characteristic curve for the main thyristor of the D.C. chopper converter and for the brake thyristor as a function of the rpm (n) of the vehicle motor. <IMAGE>

Description

The invention relates to a control method according to the preamble of claim 1.

A vehicle engine is in the specified type of circuit arrangement by the DE magazine "electric railways eb", 85th year (1987) Booklet 10, pages 336 to 339 known. It is the main thyristor of the DC chopper a turn-off thyristor (GTO thyristor) is provided.

Such shutdown thyristors are due to the possibility of them Ignite via the control connection and switch it off again, especially advantageous on vehicles, especially on local transport To use locomotives because of the space and the effort for the extinguishing circuits otherwise required for power thyristors are not required.

Fig. 1 shows the basic circuit diagram of the known through the above-mentioned prior art circuit in which the mains voltage U _N NS through an open during emergency brake operation before a choke NDr line contactor from the mains capacitor NK is turned off. FS is a contactor that is also open during braking.

A vehicle motor M , shown here as a DC machine with the terminals AB for the armature and EF for the field winding bridged by a continuous shunt DS , is connected with its one connection via a main thyristor HT designed as a switch-off thyristor to the negative pole and via a free-wheeling diode FD to the positive pole of the Mains capacitor NK connected. MDr is a motor choke coil . The other connection of the motor M is connected to the negative pole of the mains capacitor NK via the series connection of a braking resistor R _V and a braking diode BD which leads in the direction of the motor M.

Instead of the DC machine shown, a three-phase machine can also be used used with a converter as an electronic commutator will.

For braking operation in the case of a network that is not or only partially capable of receiving a load, there is a braking resistor circuit parallel to the network capacitor, which comprises a braking resistor R _BW and a braking thyristor BT (also designed as a cut-off thyristor. The braking resistor R _BW is more common (but not shown in the prior art mentioned above) ) Way a braking resistor freewheeling diode BWFD connected in parallel.

During emergency braking with the line contactor NS open, it must be ensured that the line capacitor NK is not overloaded.

The brake thyristor BT is therefore controlled so that the voltage across the line capacitor NK does not rise above a limit value. It is advisable to regulate the degree of modulation β of the brake thyristor BT by specifying a constant reference variable W _U for the voltage at the line capacitor NK in such a way that a constant voltage U _{C is} established at the line capacitor NK .

This modulation of the Brake thyristor is e.g. B. from "Electric Railways" Volume 44 (1973) Issue 8, p. 172, Section 5.a known.  

FIG. 2 shows the equivalent circuit diagram of the known circuit arrangement according to FIG. 1 for the aforementioned emergency braking operation: a (on average) voltage U _C is applied to the line capacitor. The line labeled E emf of the vehicle engine M drives via the armature resistance R _A of the vehicle engine M a (brake) motor current I _M. A diode current i FD flows through the free-wheeling diode _FD , while a current i _HT flows through it when the main thyristor HT is triggered and a current i _BW flows through it and the conductive braking thyristor BT and the braking resistor R _BW .

The degree of modulation α of the main thyristor HT is with effective braking resistor R _V by the equation

and when the braking resistor R _{V is} short-circuited by the braking resistor thyristor BVWT, by the equation

given.

The line capacitor voltage U _C must be determined in such a way that a stable operating state can be established for the maximum emf E _max and the smallest possible degree of modulation α _{min of} the main thyristor HT ; therefore applies

With decreasing speed of the vehicle engineM it sinksEMK E, and the level of modulationα of the main thyristorHT increases linearly ( α* in the diagram of theFig. 7).

The current emitted to the line capacitor NK via the freewheeling diode FD (average value over a DC regulator clock cycle period) results in

I _FDmitt = (1 - α ) I _M

The mean value of the current I _BWmitt through the braking resistor R _BW at a modulation level β of the braking thyristor is

For an average constant line capacitor voltage U _C , the mean values of the currents through the freewheeling diode FD and the braking resistor R _BW must also be the same (I _FDmitt = I _BWmitt ). The relationship between the degree of modulation β of the brake thyristor BT and the degree of modulation α of the main thyristor HT results from this

The degree of modulation β thus decreases with increasing degree of modulation α and thus with decreasing EMF ( β * in the diagram in FIG. 7).

The forward power loss P generated in the two turn-off thyristors - on average over an actuator period - (with a forward voltage drop u _T ) for the main thyristor HT :

P _HT = α · u _T (I _M ) · I _M

and for the brake thyristor BT :

P _BT = β · u _T (I _BW) · I _BW.

A disadvantage of the use of shutdown thyristor gates is that their Passage and switching losses are relatively high, so that at all modes of operation must be looked for ways, the power losses  to diminish.

The invention is based on the object, in particular during use of turn-off thyristors as the main thyristor and as the braking thyristor in the DC controller circuit with separate Brake resistor controller to specify a control method with in the emergency braking mode, in which the catenary network from the network capacitor and separated from the circuit arrangement connected to it the power loss is reduced.

This object is achieved according to the invention by those characterized in the claim Features solved.

Because the command variable w _U for the line capacitor voltage U _{C is} not kept constant, but can be set by a characteristic curve generator as a function of the machine speed, a lowering of the modulation factor α of the main thyristor HT is possible, so that its forward losses decrease; that means reduction of the ratio U _C / E. Switching losses are also reduced by lowering the reverse voltage at the main thyristor HT . In addition, there is a reduction in the switching losses of both the main and the brake thyristor due to a lowering of the actuator pulse frequency as a result of the additional tracking of the actuator pulse frequency as a function of the machine speed.

The command variable w _U for the voltage at the line capacitor U _C and the actuator pulse frequency must of course not be chosen arbitrarily. Certain criteria must be adhered to in order to achieve a stable operating state at any speed and any motor current. These criteria are characterized as an advantageous embodiment of the control method according to the invention in the subclaims.

The method according to the invention is described below with reference to the drawing  explained in exemplary embodiments.

It shows

Fig. 3 is a block diagram for obtaining the control pulses for the main thyristor and the Bremsthyristor the circuit arrangement shown in Fig. 1,

Fig. 4 shows a current-voltage diagram for determining one of the three to be maintained during the formation of the reference value for the voltage across the capacitor network criteria,

Fig. 5 to be respected according to the three criteria limits for the power capacitor voltage and a possible characteristic curve of the power capacitor voltage in dependence on the speed of the vehicle engine,

Fig. 6, to be observed limits for the power capacitor voltage and a further, simpler characteristic curve of the power capacitor voltage in dependence on the speed of the vehicle engine,

Fig. 7 characteristics of the levels of control of the main and brake thyristor depending on the speed of the vehicle engine and

Fig. 8 characteristics of the levels of modulation of the main and brake thyristor and characteristics of the actuator pulse frequency depending on the speed of the vehicle engine.

According to FIG. 3, a first characteristic curve generator 1 is provided for obtaining the ignition pulses Z _BT for the brake thyristor BT (designation as in FIG. 1), which, depending on the speed n of the vehicle engine M, is a reference variable w _U for the voltage U _C at the mains capacitor NK specifies when operating the emergency brake. This reference variable is compared in the usual way with the control variable given by the actual voltage U _C at the mains capacitor NK , and the control deviation is fed to a first control amplifier 4 , at the output of which a signal voltage U _StBT occurs which is proportional to the desired modulation level β of the brake thyristor BT is. The signal voltage U _StBT controls a first control unit 6 , which forms the ignition pulses Z _BT to be applied to the control connection of the brake thyristor BT .

In order to form the ignition pulses for the main thyristor HT , a reference variable w _IM for the vehicle engine current is compared with the actual value of the engine current (here: braking current) I _M in the manner customary in a DC chopper. The control deviation is fed to a second control amplifier 3 , at the output of which a signal voltage U _StHT occurs which corresponds to the desired degree of modulation α for the main thyristor HT .

The signal voltage U _StHT controls a second control unit 5 , which forms the ignition pulses Z _HT to be applied to the control connection of the main thyristor HT .

A second characteristic curve generator 2 specifies the actuator pulse frequency f _{St of} the ignition pulses Z _HT and Z _BT for the control units 5 and 6 as a function of the speed n of the vehicle engine M.

As already mentioned, a number of criteria must be observed to determine the optimal course of the characteristic curves for the actuator pulse frequency f _St and the reference variable w _U for the line capacitor voltage U _C as a function of the speed n .

In the embodiment of the method according to the invention, three minimum values are of outstanding importance for the command variable w _U.

To determine the criteria for the minimum values the performances are first considered that in detail in FIG. 1 and FIG. 2 circuit elements shown are implemented.

The power P _M generated by the motor M is on the on the rotational speed n dependent electromotive force E and the motor current (braking current) I _M:

P _M = E · I _M.

The power converted in the resistors for the armature resistor R _{A is} :

P = _RA I _M ² R _A

and for the braking resistor R _V :

_RV = P I _{M V} R ².

The power to be absorbed by the braking resistor R _{BW is} determined for a short-circuited braking resistor R _V :

P _RBW = P _M - P _RA

and for an effective braking resistor R _V too

P _RBW = P _M - (P _RA + P _V ).

The first criterion for determining the characteristic curve for the command variable w _{U of} the line capacitor voltage U _C is the question: Which line capacitor voltage U _Cmin is required so that the braking resistor R _BW with the maximum modulation level b _{max of} the braking thyristor BT consumes the required power P _RBW ? In general:

The minimum capacitor _voltage U _cmin is then

Depending on the speed n , curves can thus be calculated at a predetermined maximum modulation level β _max (for example β _max = 0.95 or β _max = 1). FIG. 5 (and likewise FIG. 6) shows the curve profiles of the minimum line capacitor voltage U _Cmin determined in this _way as curve A 1 with effective braking resistor R _V and as curve A 2 with short-circuited braking resistor R _V.

The second criterion to be observed for the reference variable w _U is that the capacitor voltage must be large enough that the EMF characteristic is below the adjustable counter-voltage characteristic. That means the following must apply:

E (n ; I _M ) (1 - α _min ) U _C + I _M (R _A + R _V ).

(If the braking resistor R _V is short-circuited, use R _V = 0 in this respect).

If the minimum modulation level α _{min of} the main thyristor is fixed (for example α _min = 0.05), values can be created for the individual speeds n as parameters with the vehicle engine that satisfy this relationship.

In Fig. 5 (as well as in Fig. 6) the limiting characteristic curves B 1 for the effective Bremsvorwiderstand R _V and B 2 is applied to the short-circuited braking resistor R _V with respect to this second criterion are.

Finally, as a third criterion for the creation of the characteristic curve for the speed-dependent command variable w _U , it should be noted that the characteristic curve fields are covered when the braking resistor R _V is short-circuited.

In FIG. 4, the voltage characteristic curves in dependence on the motor current (braking current I _M) are applied in the emergency braking. It is important to ensure that the overlap area Δ U is not chosen too small at maximum motor current I _Mmax (for example, Δ U = 100 V).

Therefore must apply

In Fig. 5 (and also in Fig. 6) a corresponding limit curve C is drawn for the line capacitor voltage U _C , above which the transition from the effective to the short-circuited braking resistor must take place.

Taking into account the three criteria, it is advantageously possible to set the capacitor voltage U _C as a function of the speed n of the vehicle engine, as shown by the curve U _C (n) in FIG. 5.

In FIG. 5, the curve of the electromotive force E is also shown for a predetermined motor current value I _M. In addition, it is also shown at which speed the braking resistor R _{V is} short- circuited by firing the braking resistor thyristor BVWT .

The course U _C (n) shown in FIG. 5 requires a relatively large outlay in the specification of the characteristic curve. If the control is to be simplified, the profile of the line capacitor voltage U _C shown in FIG. 6 should therefore be aimed for. Thereafter, when braking to a certain speed (for example up to the speed at which the braking series resistor R _{V is also} short-circuited), the mains capacitor voltage is regulated to a constant value, in order then to drop linearly to a standstill.

After defining the characteristic curves for the line capacitor voltage U _{C in} accordance with FIG. 5 or FIG. 6, the modulation levels of the main and brake thyristors can be determined by simple calculation:
The following applies to the degree of modulation of the main thyristor

( R _V = 0 should be used if the braking resistor is short-circuited).

Regarding the value of the armature resistance R _A , it should be noted in this equation that the greatest, that is to say the most unfavorable, values for the degree of modulation α result when the machine is warm. The warm resistance value of the armature resistance R _A is therefore to be expected here.

The degree of modulation of the brake thyristor BT is given by the relationship:

In FIG. 7, the corresponding characteristic curve for the degrees of modulation as a function of the speed n of the vehicle engine is plotted for the speed-dependent profile of the line capacitor voltage U _C given by FIG. 6. For the main thyristor, curve α 1 shows the degree of modulation with an effective braking resistor and curve α 2 shows the degree of modulation with a short-circuited braking resistor. In an analogous manner, the course of the degree of modulation β 1 with an effective braking series resistor and the course of the degree of modulation β 2 with a short-circuited braking series resistor are shown for the brake thyristor.

As already mentioned above, the curves of the modulation levels α * for the main thyristor and β * for the braking thyristor are shown in broken lines with a constant line capacitor voltage over the entire speed range.

The profile of the braking resistor current is also proportional to the profile of the line capacitor voltage U _C

shown in Fig. 7.

The diagram in FIG. 7 shows that the degree of modulation α is lower when the line capacitor voltage is guided as a function of the speed than when the line capacitor voltage is constant ( α *). This reduces the main thyristor leakage.

Although increases, however, the level of at β of Bremsthyristors, but despite more than three times as large Aussteuerungsgrades β increase the conduction losses P _BT (resulting from the relationship

P _BT = β · u · _T I _BW

If the forward voltage drop u _T known for the individual switch-off thyristors as a function of the current I _BW can be calculated) only by about 50%. If the reduction in switching losses is taken into account, the difference is even smaller. Otherwise, the braking thyristor can tolerate a momentary increase in the power loss, since on average it is less loaded than the main thyristor over the duty cycle "accelerating - braking" and is cold at the start of the braking operation.

If the line capacitor voltage U _{C is} reduced, the actuator pulse frequency f _{St can} also be _reduced as a function of the engine engine speed n while maintaining the maximum motor current _ripple Δ I _Mmax , so that the power loss of the main and brake thyristors is also reduced as a result.

The motor current ripple is given by the relationship:

where f _St the actuator pulse frequency and L _A denote the inductance in the armature circuit of the vehicle engine.

For constant mains capacitor voltage, here U _C = U _Cmax and nominal frequency f _Stnnn , the maximum ripple results with a modulation factor α = 0.5 of the main thyristor

The characteristic curve of the lowered actuator pulse frequency f _{St is then} calculated (under the aforementioned condition that the same motor current _ripple Δ I _{Mmax should} occur) according to the relationship:

Even if it is expected that a degree of modulation of α = 0.5 will occur transiently, the frequency can still be according to the equation

be lowered so that switching losses are saved.

In FIG. 8 is again a characteristic of a possible represented by the rotational speed-dependent specification of the n power capacitor voltage U _C. This characteristic curve initially runs constant in the higher speed range, in order to then fall parabolically when the vehicle engine brakes further as the brakes are applied.

The modulation levels α 1 and β 1 for the main and braking thyristor with effective braking resistor and α 2 and β 2 for main and braking thyristor with short-circuited braking resistor are necessary to achieve this profile of the line capacitor voltage U _C.

Furthermore, two speed-dependent control pulse frequency characteristic curves are shown, in which the frequency f _{1 St} according to the previously specified relationship

and the frequency f _{2 St} according to the formula also listed above

are lowered.

Claims (7)

1. control method for reducing the power loss of the main DC thyristor during braking operation of a vehicle engine,

• one connection is connected to the other pole of the mains capacitor both via a free-wheeling diode directed away from the vehicle motor with one pole of a mains capacitor which is disconnected from the mains during braking and also via the main thyristor of the direct current regulator also directed away from the vehicle motor,
• - Its other connection both via a brake diode polarized in the direction of the vehicle engine and a series-connected braking resistor as well as via a polarity polarized in the direction of the vehicle motor, bridging the series connection of the brake diode and braking resistor in conductive state also with the other braking resistor thyristor Mains capacitor is connected and
• which is connected to a braking circuit which is formed by the free-wheeling diode, a series circuit comprising a braking resistor and a braking thyristor and arranged between the two poles of the line capacitor, and either the series circuit comprising a braking resistor and a braking diode or the braking resistor thyristor,

characterized by

• - That the degree of modulation of the brake thyristor is corrected via a reference variable for the voltage at the line capacitor, which is specified in a characteristic curve as a function of the speed of the vehicle engine and
• - That the pulse frequency of the ignition signals for the dc main thyristor and for the brake thyristor characteristic is also specified depending on the speed of the vehicle engine.

2. Control method according to claim 1, characterized in that the speed-dependent characteristic of the reference variable for the voltage at the line capacitor obeys the following criteria:

• a) The minimum value of the command variable is chosen at least so large that the braking resistor can absorb the required braking power with the maximum degree of modulation of the braking thyristor.
• b) The minimum value of the command variable is chosen so large that the EMF characteristic of the vehicle engine is below the adjustable counter-voltage characteristic.
• c) The minimum value of the command variable is chosen so large that the characteristic fields overlap when the braking resistor is short-circuited in the brake circuit.

3. Control method according to claim 2, characterized, that the reference variable for the voltage at the line capacitor at Braking is first kept at a constant value and then off a predetermined speed drops with the speed.   4. Control method according to claim 3, characterized, that the decrease in the reference variable is linear or in sections linear (polygon) can be selected. 5. Control method according to one of claims 1 to 4, characterized, that the pulse frequency of the ignition signals for the DC regulator Main thyristor and accordingly for the brake thyristor a characteristic curve, which is the ratio of through the reference variable given voltage at the line capacitor possible maximum value of the voltage at the line capacitor follows. 6. Control method according to claim 5, characterized, that the characteristic of the speed-dependent pulse frequency reduction is additionally corrected by a factor which is the degree of modulation of the DC chopper main thyristor for the case takes into account that a maximum motor current ripple too is maintained in the frequency reduction.