Control System for an Electric Locomotive Having AC to DC Converters

The present invention relates to a control system for electric locomotives having AC to DC converters and, more particularly, to a power factor controller for a control system.

In order to regulate the speed of an electric locomotive, thyristors included in AC to DC converters are controlled by firing signals. Since the AC to DC converters generate high harmonic frequency currents which form a psophometric disturbing current, it is necessary to provide a power factor controller to reduce the psophometric disturbing current as well as to improve the power factor of the electric locomotive. Many examples of earlier efforts to provide a power factor controller are described in references such as Japanese Pat. Laid-open Nos. Sho 55-100037, Sho 56-19305 and 56-42819, etc. Although some of these references disclose concepts of merit, currently available power factor controllers require more improvement.

It is desirable for a power factor controller to have three characteristics as follows:

(1) small size because the locomotive has very limited space;

(2) ability to reduce the psophometric disturbing current to under a predetermined value; and

(3) ability to prevent over-compensation which might change a lagging power factor into a leading power factor.

It is an object of the present invention to provide an improved power factor controller.

It is another object to provide a control system having a small size.

It is another object to provide a control system for effectively reducing a psophometric disturbing current.

It is another object of the present invention to provide a method for controlling converters to effectively reduce psophometric disturbing current.

If an electric locomotive is provided with a power factor controller having a capacity for perfectly removing a psophometric disturbing current during transition from no load to 100% load states, the power factor controller will typically be very large in size and will over-compensate and thereby convert the power factor from a lagging to a leading power factor. In an electric locomotive with cascade connections of AC and DC converters, the speed of the locomotive is regulated by control of each of the phase control angles of the converters.

Therefore, in the present invention, regardless of whether all of the capacitance is connected to one converter, or unequally divided between the converters, the power factor controller should be connected to the converter having the greater phase control angle in order to effectively remove the psophometric disturbing current created by the phase control angle of that converter. Since each of the converters is independently supplied with firing signals having different phase control angles by different firing signal generators, connections between the converters and the firing signal generators are switched when a predetermined locomotive status is satisfied.

FIG. 1 shows a diagram of one embodiment of the present invention.

FIG. 2 shows characteristic curves of output voltages for currently available AC to DC converters.

FIG. 3 shows an example of characteristic curves of output voltages for the preferred embodiment shown in FIG. 1.

FIGS. 4 and 5 show characteristic curves explanining the performace of one embodiment of the present invention.

FIG. 6 shows a time chart illustrating wave forms of some important portions of one embodiment.

FIG. 7 shows a diagram of an another embodiment of the present invention.

FIGS. 8 and 9 show some characteristic curves corresponding to FIGS. 4 and 5, but for the embodiment shown in FIG. 7.

FIG. 10 shows some characteristic curves corresponding to FIG. 9.

FIGS. 11 and 12 show a diagram of a main circuit portion and a control circuit portion of an another embodiment of the present invention.

FIG. 13 shows an example of an operational pattern of an electric locomotive.

FIG. 14 shows some characteristic curves of AC to DC converter output voltages during an inverter operation.

FIG. 15 shows a time chart illustrating waveforms of some important portions of the embodiment shown in FIGS. 11 and 12.

FIGS. 16 and 17 show diagrams of main circuit portions of other embodiments of the present invention.

FIG. 18 shows an example of characteristic curves of AC to DC converter output voltages for the circuit of FIG. 17.

FIG. 1 shows a diagram of the preferred embodiment of the present invention. In the drawing, T.sub.r denotes a transformer, one terminal of a primary winding N.sub.1 of which is connected to a trolley wire T.sub.w, which is connected to an AC substation (not shown) provided on the ground through a pantograph, and the other terminal of which is connected to a reference potential, such as a ground, through an electrically conducting wheel (not shown). N.sub.21 and N.sub.22 denote secondary windings of transformer T.sub.r, to which hybrid bridges B.sub.r1 and B.sub.r2 are respectively connected. N.sub.3 denotes a third winding of the transformer T.sub.r. The hybrid bridges B.sub.r1 and B.sub.r2, which include two diodes D.sub.1 and D.sub.2 and D.sub.3, D.sub.4, respectively, and two thyristors Th.sub.1, and Th.sub.2 and Th.sub.3 and Th.sub.4, respectively, are arranged in a cascade connection to which a smoothing inductance L.sub.D and a DC main motor M are connected in series. The bridges B.sub.r1 and B.sub.r2 operate as AC to DC converters. PFC denotes a power factor controller shown surrounded by dashed lines which includes a series circuit of two thyristors T.sub.h5, T.sub.h6 connected in parallel in a back-to-back configuration; a resistance R.sub.s, a smoothing inductance L.sub.s and a capacitance C.sub.s are connected to bridge B.sub.r1. E.sub.d1, E.sub.d2 and E.sub.d denote DC voltages, the former two of which are generated by the bridges B.sub.r1 and B.sub.r2, respectively; E.sub.d is a total of the output voltages of both bridges applied to DC main motor M.

CT denotes a current transformer which detects a pantograph current I.sub.p flowing in primary winding N.sub.1 and provides a current signal I.sub.a. DCCT denotes a direct current transformer which detects a motor current I.sub.m flowing in the DC main motor M and provides a current feedback signal I.sub.mf. The third winding N.sub.3 provides an AC voltage e.sub.a corresponding to the AC voltage applied to primary winding N.sub.1. PCC denotes a PFC on-off control circuit which generates firing signals Th.sub.5G and Th.sub.6G when the power factor of the input power of the locomotive exceeds a predetermined value according to signals I.sub.a and e.sub.a. PI denotes a proportional and integral operational amplifier to which is applied a differential signal, which is an output of an adder AD.sub.1, together with the signal I.sub.mf and a predetermined reference current signal I.sub.mp. APPS.sub.1 and APPS.sub.2 denote automatic pulse phase shifters which generate firing signals having phase control angles based on the AC voltage e.sub.a and corresponding to input signals E.sub.c1, E.sub.c2. The output signal of the PI is applied as an input signal to APPS.sub.1, and a differential signal, which is an output of an adder AD.sub.2, is applied to APPS.sub.2, together with the output signal of the proportional and integral operational amplifier PI and a predetermined bias signal V.sub.B, as an input signal. In turn, the APPS.sub.1, and APPS.sub.2 generate phase control signals T.sub.1 and T.sub.2, and T.sub.3 and T.sub.4 having different phase control angles .alpha..sub.1 and .alpha..sub.2, respectively, corresponding to the output of the PI amplifier. CP denotes a comparator which provides a gate change signal G synchronized with the AC voltage e.sub.a when a measured speed signal v of the locomotive exceeds a predetermined reference speed signal V.sub.Rf.

Log denotes a logical change circuit which includes four AND gates A.sub.1, A.sub.2, A.sub.3 and A.sub.4, four Inhibit gates I.sub.1, I.sub.2, I.sub.3 and I.sub.4 and four OR gates .theta..sub.1, .theta..sub.2, .theta..sub.3 and .theta..sub.4. In the logical change circuit, the gates are combined to give firing signals from corresponding phase control signals as follows:

Where Th.sub.1G, Th.sub.2G, Th.sub.3G and Th.sub.4G denotes firing signals to the gate terminals of the thyristors Th.sub.1, Th.sub.2, Th.sub.3 and Th.sub.4, respectively.

T.sub.1, T.sub.2, T.sub.3 and T.sub.4 denote phase control signals from APPS.sub.1 and APPS.sub.2 ; and + and - symbols indicate whether the phase control signals are positive or negative half waves of the AC voltage e.sub.a, while .alpha..sub.1 and .alpha..sub.2 indicate the values of phase control angles, respectively.

In the embodiment of FIG. 1, phase control signals T.sub.1 and T.sub.2 from APPS.sub.1 are applied to the logic circuit to trigger firing signals Th.sub.1G and Th.sub.2G, which are in turn applied to the gate terminals of thyristors Th.sub.1 and Th.sub.2 of bridge B.sub.r1, and phase control signals T.sub.3 and T.sub.4 from APPS.sub.2 are applied to the logic circuit to trigger firing signals Th.sub.3G and Th.sub.4G, which are in turn applied to the gate terminals of the thyristor Th.sub.3 and Th.sub.4 of the bridge B.sub.r2 when the measured speed signal v is lower than the reference signal V.sub.Rf ; as configured, the output of the comparator CP is "0". However, the output of the comparator CP becomes "1" when the measured speed signal v exceeds reference signal V.sub.Rf, phase control signals T.sub.1 and T.sub.2 of APPS.sub.1 are applied to the logic circuit to trigger firing signals Th.sub.1G, Th.sub.2G which are in turn applied to bridge B.sub.r1, phase control signals T.sub.3 and T.sub.4 of APPS.sub.2 are applied to the logic circuit to trigger firing signals Th.sub.3G, Th.sub.4G which are applied to bridge B.sub.r2. This means that the power factor controller PFC is connected to the bridge to which phase control signals having the larger phase control angle are applied, and which generates the larger portion of the psophometric disturbing current J.sub.p. In other words, the power factor controller PFC most effectively reduces the high harmonic frequency currents, and thereby suppresses the psophometric disturbing current J.sub.p by switching the phase control angle.

FIGS. 2 and 3 show examples of characteristic curves of AC to DC converter output voltages obtained from a conventional control system and the embodiment shown in FIG. 1, respectively. As shown in FIG. 2, in the conventional system, each bridge B.sub.r1 and B.sub.r2 is separately and exclusively controlled by different automatic pulse phase shifters APPS.sub.1 and APPS.sub.2, from maximum control angle to minimum control angle. The starting point in the control of bridge B.sub.r2 occurs after bridge B.sub.r1 has established a predetermined value of DC voltage E.sub.d1, and is determined by the value of the bias voltage V.sub.B being applied to adder AD.sub.2.

Contrarily, as shown in FIG. 3, in the embodiment of FIG. 1, bridge B.sub.r1 is controlled by the APPS.sub.1 and bridge B.sub.r2 is controlled by the APPS.sub.2 while the speed of the locomotive is lower than the predetermined reference signal V.sub.Rf. However, the firing signals triggered by phase control signals T.sub.1, T.sub.2 and T.sub.3, T.sub.4 from APPS.sub.1 and APPS.sub.2 are switched from bridges B.sub.r1 and B.sub.r2 to the bridges B.sub.r2 and B.sub.r1, respectively, when the value of speed signal v exceeds reference signal V.sub.Rf. In the latter instance, the output DC voltage E.sub.d1 of the bridge B.sub.r1 drops because bridge B.sub.r1 is controlled by a larger phase control angle than before the switch precipitated by the value of locomotive speed signal v exceeding V.sub.Rf. This means that bridge B.sub.r1, to which the power factor controller PFC is connected, operates as an AC to DC converter to control the DC voltage E.sub.d and that the amplitude of the psophometric distrubing current J.sub.p, which is generated by the bridge having a large phase control angle, is effectively reduced when that bridge is connected to the PFC.

As is well known, the psophometric disturbing current J.sub.p is defined as follows: ##EQU1## where n: an ordinal number of a harmonic frequency;

I.sub.n : n-th harmonic current included in the pantograph current I.sub.p ; and

S.sub.n : noise factor of n-th harmonic as defined by CCITT (Comite Consultatif International Telegraphique et Telephonique).

FIGS. 4 and 5 show characteristic curves explaining the advantages of one embodiment of the present invention. The horizontal axis of FIGS. 4 and 5 shows the speed v of the locomotive, while the vertical axes show the psophometric disturbing current J.sub.p, the phase control angle .alpha., the DC voltage E.sub.d, the DC main motor current I.sub.m, the field current I.sub.f of the DC main motor M, and the pantograph current I.sub.p and power factor .lambda., respectively. FIG. 4 shows various data which are measured under conditions in which the locomotive is running near the substation supplying electric power to the locomotive. J.sub.p0, J.sub.p1 and J.sub.p2 show the psophometric disturbing currents in instances with the power factor controller PFC being not used, the PFC being connected to bridge B.sub.r1 and the PFC being connected to bridge B.sub.r2, respectively, when bridge B.sub.r1 is first controlled as shown in FIG. 3. .alpha..sub.1 and .alpha..sub.2 show the phase control angles of bridges B.sub.r1 and B.sub.r2. I.sub.p0, I.sub.p1 and I.sub.p2 show the pantograph currents in case of the PFC being not used, the PFC being connected to bridge B.sub.r1 and the PFC being connected to bridge B.sub.r2, respectively. .lambda..sub.o and .lambda..sub.x show the power factor of the locomotive in case of the PFC being not used and the PFC being used, respectively.

As may be understood from FIG. 4, the psophometric disturbing current J.sub.p is greater in the low speed region (for example v<20 km/h) than in the high speed region, and between the low and high speed regions, J.sub.p1 and J.sub.p2 have a reversed relation with J.sub.p0.

When bridge B.sub.r1 is first controlled by the PFC, J.sub.p becomes greater in the high speed region, as is shown by J.sub.p1. Contrarily, where the PFC is connected to bridge B.sub.r2, J.sub.p becomes greater in the low speed region, because the high harmonic frequency currents which are generated by bridge B.sub.r2 are not fully reduced by the power factor controlled PFC which is connected to bridge B.sub.r1.

FIG. 5 shows various data derived from the preferred embodiment under same conditions as the data shown in FIG. 4 was derived. In FIG. 5, v.sub.1 denotes the speed at which the PFC on-off control circuit PCC provides firing signals Th.sub.5G and Th.sub.6G to connect the PFC to bridge B.sub.r1. v.sub.2 denotes the speed at which the comparator CP provides changing signal G to change the phase control signals T.sub.1, T.sub.2 and T.sub.3, T.sub.4 of the APPS.sub.1 and APPS.sub.2 from bridges B.sub.r1 and B.sub.r2 to bridges B.sub.r2 and B.sub.r1. Therefore, in the embodiment, between speed zero and v.sub.1, J.sub.p0, I.sub.p0 and .lambda..sub.0, between the speed v.sub.1 and v.sub.2,J.sub.p1, I.sub.p1 and .lambda..sub.x, and exceeding speed v.sub.2, J.sub.p2, I.sub.p2 and .lambda..sub.x, are gotten, respectively. The phase control angles .alpha..sub.1 and .alpha..sub.2 of bridges B.sub.r1 and B.sub.r2 are changed at speed v.sub.2. As may be understood by comprising FIGS. 4 and 5, the amplitude of psophometric disturbing current J.sub.p is suppressed to the level of J.sub.p2.

This is performed by switching the phase control angles from .alpha..sub.1 to .alpha..sub.2 whereby the phase control angle of B.sub.r1 is changed from .alpha..sub.1 to .alpha..sub.2 while the phase contol angle of B.sub.r2 is changed from .alpha..sub.2 to .alpha..sub.1.

FIG. 6 shows a time chart illustrating the AC voltage e.sub.a, the DC voltages E.sub.d1, E.sub.d2 and E.sub.d, the thyristor firing signals Th.sub.1G, Th.sub.2G, Th.sub.3G and Th.sub.4G, the phase control signals T.sub.1, T.sub.2, T.sub.3 and T.sub.4 and the gate change signal G, plotted as a function of time.

As shown in FIG. 6, at time t.sub.1, gate change signal G is generated and then the phase control angles of bridges B.sub.r1 and B.sub.r2 are changed from .alpha..sub.1 to .alpha..sub.11, and .alpha..sub.2 to .alpha..sub.1. Therefore, although the effective DC voltage E.sub.d does not vary, the DC voltages E.sub.d1 and E.sub.d2 of bridges B.sub.r1 and B.sub.2 are switched each other.

FIG. 7 shows a schematic diagram of another embodiment of the present invention. The embodiment is similar to the embodiment shown in FIG. 1 except for power factor controller PFC and the PFC on-off control circuit PCC. The power factor controller PFC is divided into two power factor controllers PFC.sub.11 and PFC.sub.12 ; the total capacitance of both controllers is the same as that of the PFC of FIG. 1. The capacitance of one of the two power factor controllers (e.g., PRC.sub.11), however, is larger than that of the other. The PFC on-off control circuit PCC provides firing signals Th.sub.51G and Th.sub.61G by which the power factor controller (i.e., PFC.sub.11) having a greater capacitance associated with bridge B.sub.r1 is connected across bridge B.sub.r1 when the power factor of the locomotive exceeds a first predetermined value, and provides firing signals Th.sub.52G and Th.sub.62G by which the power factor controller (i.e., PFC.sub.12) having a smaller capactiance associated with bridge B.sub.r2 is connected across bridge B.sub.r2 when the power factor of the locomotive exceeds a second predetermined value which is greater than the first predetermined value.

FIG. 8 and FIG. 9 show characteristic curves corresponding to FIGS. 4 and 5 for the embodiment shown in FIG. 7. Only the psophometric distrubing current J.sub.p and the power factor .lambda. are shown in order to simplify the drawings as compared with FIGS. 4 and 5. FIG. 8 shows various data which are measured under conditions with the locomotive running at a place about 10 km from the substation. J.sub.p0, J.sub.p3, J.sub.p4 and J.sub.p5 show the psophometric disturbing currents with both power factor controllers PFC.sub.11 and PFC.sub.12 not being used; PFC.sub.11 being connected to bridge B.sub.r1 ; both PFC.sub.11 and PFC.sub.12 being connected to bridges B.sub.r1 and B.sub.r2, respectively; and both PFC.sub.11 and PFC.sub.12 being connected to bridges B.sub.r2 and B.sub.r1, respectively, with bridge B.sub.r1 being first controlled as shown in FIG. 3. .lambda..sub.0, .lambda..sub.3, .lambda..sub.4 and .lambda..sub.5 show the power factors of the locomotive with both PFC.sub.11 and PFC.sub.12 not being used; PFC.sub.11 being connected to bridges B.sub.r1 ; both PFC.sub.11 and PFC.sub.12 being connected to bridges B.sub.r1 and B.sub.r2, respectively; and both PFC.sub.11 and PFC.sub.12 being connected to bridges B.sub.r2 and B.sub.r1, respectively.

FIG. 9 shows data for the embodiment shown in FIG. 7 derived under the same conditions as the data shown in FIG. 8. In FIG. 9, v.sub.3 denotes the speed at which the PFC on-off control circuit PCC provides firing signals Th.sub.51G and Th.sub.61G to connect PFC.sub.11 to bridge B.sub.r1. v.sub.4 denotes the speed at which the PCC further provides firing signals Th.sub.52G and Th.sub.62G to connect PFC.sub.12 to bridge B.sub.r2. The speed v.sub.4 is equal to the speed v.sub.1. v.sub.5 denotes the speed at which the comparator CP provides gate change signal G to change the phase control signals T.sub.1, T.sub.2 and T.sub.3, T.sub.4 of APPS.sub.1 and APPS.sub.2 from the bridges B.sub.r1 and B.sub.r2 to bridges B.sub.r2 and B.sub.r1, respectively. Therefore, in this embodiment, between speed zero and v.sub.3, J.sub.p0 and .lambda..sub.0 occur; between speed v.sub.3 and v.sub.4, J.sub.p3 and .lambda..sub.3 occur; between speed v.sub.4 and v.sub.5, J.sub.p4 and .lambda..sub.4 occur; and above speed v.sub.5, J.sub.p5 and .lambda..sub.5 occur. The phase control angles .alpha..sub.1 and .alpha..sub.2 of bridges B.sub.r1 and B.sub.r2 are changed at speed v.sub.4. As may be understood by comparing FIGS. 8 and 9, the amplitude of psophometric distrubing current J.sub.p is suppressed by the amplitude of J.sub.p5. This is performed by changing of the phase control angle between .alpha..sub.1 and .alpha..sub.2. In the embodiment, additional merit is obtained as follows. The power factor controller PFC.sub.11 which has a smaller capacitance than that of the PFC shown in FIG. 1 is connected to bridge B.sub.r1 at speed v.sub.3, which is a lower speed than speed v.sub.1. Then between speed v.sub.3 and v.sub.1 (=v.sub.4), the power factor .lambda. is improved in comparison with the embodiment shown in FIG. 1 because the two power factor controllers allows the PFC.sub.12 to be connected to bridge B.sub.2 at a lower speed than is possible in embodiments where the total capacitance is to be connected to a single bridge.

In the embodiment of FIG. 7, a minor variation may be made. Namely, the capacitance of the power factor controller PFC.sub.11 may be made smaller than that of the PFC.sub.12. Of course, the total capacitance does not change. In this case, the psophometric disturbing current J.sub.p and the power factor .lambda. are changed from J.sub.p3 and J.sub.p4 and .lambda..sub.3 and .lambda..sub.4 to J'.sub.p3 and J'.sub.p4 and .lambda.'.sub.3 and .lambda.'.sub.4, as is shown in FIG. 10. As shown in FIG. 10 by cross hatched lines, both characteristic curves become less desirable by this minor variation.

FIG. 11 and FIG. 12 show a diagram of a main circuit portion and a control circuit portion for the main circuit of another embodiment. The circuit shown in FIG. 11 is similar to the circuit shown in FIG. 1 except that diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 have been replaced by thyristors Th.sub.11, Th.sub.21 Th.sub.31 and Th.sub.41. Therefore, with this embodiment, a locomotive can operate in a regeneration operation mode while the locomotive is breaking to a stop. FIG. 12 shows a main portion of the control circuit for the main circuit shown in FIG. 11. Some gate circuits for the thyristors of bridge B.sub.r2 and the input circuits for the automatic pulse phase shifters APPS.sub.1 and APPS.sub.2 shown in FIG. 1 are omitted in FIG. 12 to simplify the latter drawing. In FIG. 12, .beta.c denotes a minimum advanced control angle (.beta..sub.min) calculator which provides a minimum advanced control angle signal (.beta..sub.min) for a steady regeneration operation with an AC voltage v.sub.p applied to the bridge and a predetermined commutation margin angle signal .gamma.. PD denotes a polarity detector which provides polarity signals of the AC voltage e.sub.a. P denotes a terminal to which a signal "1" is given when the locomotive is under powered operation. COMP.sub.1 and COMP.sub.2 denote comparators which provide a gate change signal G.sub.1 when the measured speed signal v exceeds the reference signals v.sub.Rf, and a gate change signal G.sub.2 when the measured speed signal v becomes lower than the reference signal v.sub.Rfb, respectively. One of the signals G.sub.1 and G.sub.2 is selectively used as gate change signal G when the locomotive is under either powered operation or regeneration operation. For some gates, prefix letters A, I and O denote AND gates, Inhibit gates and OR gates, respectively, as FIG. 1. Formation of the firing signals Th.sub.1G, Th.sub.2G, Th.sub.11G and Th.sub.21G is shown in FIG. 15.

FIG. 13 shows an example of an operational pattern of locomotive operation. v, I.sub.p, E.sub.d, I.sub.f, I.sub.mp and PI show a speed pattern, a pantograph current pattern, a DC voltage pattern, a field current pattern of the DC main motor M, a reference current pattern of the DC main motor M and an output pattern of the proportional and integral operational amplifier, respectively. In FIG. 13, the powered operation is a period between t.sub.11 and t.sub.14 and the regeneration operation is a period between t.sub.14 and t.sub.16. In FIG. 13, v.sub.Rf and v.sub.Rfb indicate the reference signals applied to the comparators Comp.sub.1 and Comp.sub.2. The output of the proportional and integral operational amplifier PI is reset to zero when the operational mode of the locomotive is changed from powered operation to regeneration operation at time t.sub.14. Further, the field current I.sub.f of the main DC motor M is changed to reverse polarity so that the DC voltage E.sub.d of the main DC motor is changed to reverse polarity at time t.sub.4.

FIG. 14 shows some characteristic curves of bridge output voltages when the bridges B.sub.r1 and B.sub.r2 are operated under regeneration operation. When the locomotive speed v falls below the reference speed v.sub.Rfb, the phase control signals to the bridges B.sub.r1 and B.sub.r2 are switched from APPS.sub.1 and APPS.sub.2 to APPS.sub.2 and APPS.sub.1, respectively.

FIG. 15 shows a time chart in which waveforms of some important portions of the operation of the embodiment shown in FIGS. 11 and 12 are shown. In the FIG. 15, the waveforms are divided to illustrate both the powered operation mode and the regeneration mode, respectively, with the waveforms in each mode during two cycle portions being centered upon the time at which the changing signals G.sub.1 and G.sub.2 are given. There is a rest period shown between the control signals during which all firing signals are interrupted, between the powered operation and the regeneration operation (this rest period corresponds to the time t.sub.14 shown in FIG. 13).

FIG. 16 shows a diagram of a main circuit portion of another embodiment of the present invention. This is similar to the embodiment shown in FIG. 1 except that another circuit for a second DC main motor M.sub.1 is provided, each circuit being separately provided with a factor controller PFC, and PFC.sub.1. The control circuit for both power factor controllers of the embodiment are similar to the control portion of the circuit shown in FIG. 1.

FIG. 17 and FIG. 18 show a diagram of a main circuit portion of another embodiment and characteristic curves of output voltages of its AC-DC converter. In FIG. 17, a DC voltage is applied across a DC main motor M by cascade connection of four rectifying bridges B.sub.r1, B.sub.r2, B.sub.r3 and B.sub.r4. As shown in FIG. 17, the DC voltage applied to the DC main motor is controlled by two bridges B.sub.r1 and B.sub.r2. As described in FIG. 3, and shown in FIG. 18, when the locomotive speed v exceeds speed v.sub.rf1, the phase control signals to bridges B.sub.r1, B.sub.r2 are switched form APPS.sub.1, APPS.sub.2 to APPS.sub.2, APPS.sub.1. As is further shown in FIG. 18, when the locomotive speed v exceeds speed v.sub.rf2, the outputs of bridges B.sub.r2, B.sub.r1 are shifted to bridges B.sub.r3, B.sub.r2 and when the locomotive speed v exceeds speed v.sub.rf3, the outputs of B.sub.r2, B.sub.r1 are shifted to B.sub.r4, B.sub. r2. Namely, majority of the DC output voltage is always controlled by bridge B.sub.r1 to which power factor controller PFC is connected. Therefore, the psophometric disturbing current J.sub.p is effectively reduced in comparison to currently available controllers.

In the above explanation, although the gate change signal G, by which the phase control signals to the bridges are changed between two automatic pulse phase shifters APPS, is obtained from a comparison of a measured speed v of the locomotive with a speed reference signal v.sub.Rf, various kinds of other signals which represent the amplitude of the psophometric disturbing current J.sub.p, may be obtained as follows.

A comparison of:

(i) the amplitude of a measured load of the locomotive with a reference value, or

(ii) the amplitude of a DC current flowing through a converter with a reference value, or

(iii) the amplitude of an AC current supplied to a converter with a reference value, or

(iv) the amplitude of the psophometric disturbing current flowing through a converter with a reference value.