DE2538043A1 - DC MOTOR DRIVE SYSTEM WITH A FIELD CURRENT REVERSAL IN THE CASE OF A TRANSITION BETWEEN MOTOR AND BRAKE OPERATING MODES - Google Patents

Description

Dr. Horst pupil

PatentarVjvelt
6 Frankfurt / Main 1

Niddasir. 52 26; ^A "f ^USt _{/ h} ¹⁹⁷⁵

Schu./Vo./he.

3635-2O-TT-586

GENERAL ELECTRIC COMPANY

1 River Road
SCHENECTADY, NY / USA

DC motor drive system with a field current reversal when there is a transition between motor and braking modes

The invention relates to DC motor drive systems which are suitable for the optional driving or electrical braking of heavy mechanical loads, such as towing vehicles, and in detail an improved arrangement for exciting the motor field during motor and braking modes.

Electric propulsion systems for large inertial loads, such as towing vehicles, must be designed to propel and electrically brake (commonly referred to as 'electric brake ¹ ') the load according to predetermined torque-to-speed relationships and other selected parameters. DC motors with armature and field windings are usually controlled by changing their armature and field fluxes. For example, U.S. Patent 3,515,970 describes an arrangement for selectively controlling armature current and field flux.

The drive torque of a DC motor is proportional to the product of the armature current and the field strength. During the drive, that is, in the motor operating mode, a maximum rotation

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torque control through separate control of the armature and field currents caused by DC motors. At low engine speeds, the back EMF is very small, which leads to a large armature current. He can at low speeds by controlling the armature supplied Voltage can be limited to acceptable levels, for example by placing a variable resistance device between the source and the armature is switched on. Chopper circuits are commonly used for this purpose, the chopper being periodic is switched so that its duty cycle or its duty cycle is variable and inversely proportional to that in series effective impedance to be switched to the armature. Such chopper or breaker circuits usually also become unilateral Conducting members called freewheeling diodes are used and connected in parallel to the armature circuit, for example to the armature winding and motor choke, and polarized accordingly to the circulating armature current during the switch-off intervals of the chopper switch to direct. By applying a sufficient field current, a sufficient starting torque can be achieved even with separately excited motors can be achieved. The control of the armature voltage and the field current enables suitable torque characteristics to be achieved over a wide speed range.

The control of the electrical braking or deceleration can be achieved in a similar way by an armature and field control. During electrical braking, the motor working as a generator sends an armature current to a braking or consumption load, which in the case of dynamic braking, which is usually referred to as' resistance braking ¹ , is a resistive load. In the case of regenerative braking, commonly referred to as 'regenerative braking', the load is the motor's source of energy. In DT patent application / Ser.-No. 433 409 it is explained how braking can be achieved by controlling the armature current and the field flux. In interruption or chopper systems, electrical braking can be achieved by coupling the motor armature circuit in shunt to the consumer load, for example to the braking resistor or the direct current source. The chopper switch is usually connected in parallel to the armature circuit.

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tet, so that a change in its duty cycle or switched on states the armature current over a speed range and the
controls the resulting range of the armature voltage.

Drive systems of the type mentioned must be easily between the motor and. The drive mode and the deceleration or braking mode can be switched. This requires a change in the operating state, such as a reversal of the field or armature connection.
This is achieved, for example, by appropriately switching the armature terminals in order to reverse the armature polarity with respect to the field polarity. However, excessive currents occur if the switching time is not precisely controlled and, for example, occurs prematurely during an interval of flowing motor currents. Alternatively, you can switch the operating mode by switching the terminal blocks or connections in order to reverse the motor field. Such a switchover must take place in such a way that a suitable field current is built up to overcome the remanent motor flux. Quite
in general, the switching of the commonly used self-excited series DC machines from the drive to the braking mode is temporary voltage surges caused by the change in the
originate inductive armature currents, subject. Also arise
Delays in performing the operating mode switchover.

The object of the present invention is therefore to provide an improved DC motor control system or control system.
-Control system with separately excited fields in motor operation,
whereby the system should be able to be switched easily and quickly between the motor and braking modes. The system is supposed to be a smooth one
or allow bumpless control of the field excitation in the motor and braking modes. There should also be an operating mode switchover
with a minimal number of contactors and achieving the above functions with minimal components as well as parts
enable. It should be suitable as a DC motor control system for towing vehicles and enable a smooth transition between the motor and braking modes. In addition, it should be suitable for regenerative braking as well as resistance braking and for controlling
'' several pull motor groups with common armature and field control circuits are suitable.

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According to the invention, a direct current motor drive system is used to achieve the object set proposed in which a connection of the motor armature only during motor operation by an interrupter switching device coupled to a direct current conductor of the first polarity and by means of a conductive only during braking operation Device is connected to a direct current conductor of the second polarity. Field windings lead from one connection or the one terminal to a controllable DC voltage source, so that the field current in the closed state of the Untcottediere during of the motor operation flows in a first direction and then in a reverse direction if the sub-controller is during the braking operation is open. The field current and therefore the field excitation are determined by changing the output of the adjustable source controlled. The controllable DC voltage source preferably contains switching devices connected in opposite directions, for example Breakers or choppers connected to the conductors in a self-switching arrangement so that they alternate conduct. The field windings are connected to the junction of the switching devices, the voltage being changed by changing the corresponding switching time of the devices is controllable.

The one that can be used to drive and brake electric towing vehicles DC motor drive system has an armature winding, the first connection of which is via a chopper to the first polarity conductor the direct current source and its second connection via a circuit breaker to a conductor of the second polarity of the direct current source are connected. The first and second controlled rectifiers are connected in a self-switching manner and conduct alternately or consecutively. Means are provided for changing their respective gating time intervals to the Control tension on their connection. The field windings are between the connection of the controlled rectifier and the connection of the second armature connection and the interrupter switched. During engine operation with the breaker closed the field current flows in a first direction, during braking operation when the breaker is open and when the second connection of the armature connected to the first conductor by a diode

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the field current flows in an opposite direction, so that the DC motor can work as a braking generator.

In a preferred embodiment, an anchor chopper is between switched the conductor of the second polarity and the other terminal of the armature. Unilaterally conductive and for carrying a freewheeling current polarized means connect the other terminal to the conductor of the first polarity, while unilaterally conducting and for conducting of the armature current generated during regenerative braking is connected between one terminal of the armature and the conductor of the second polarity are. A common armature chopper and a common controllable field voltage source allow several in series, in parallel or motors connected in series and in parallel can be controlled.

The characteristic new features of the present invention emerge from the above statements and the claims. The organization and the manner of operation according to the present invention in connection with further objects and advantages emerge from the following description and the drawings. Show it:
Figure 1 - a simplified circuit diagram of an engine control system

according to a preferred embodiment of the invention, Figure 2 is a schematic circuit diagram of a preferred embodiment of the invention and

Figure 3 is a simplified circuit diagram of another embodiment of the invention.

FIG. 1 shows, in simplified form, an exemplary embodiment according to the present invention. It contains one in the motor armature circuit Chopper circuit used to control armature current during braking and driving is usable without changing the connections. Lines Io and 14 can have positive and negative connections connected to a DC power source. In the case of electric locomotives or transport vehicles, these are usually through Catenary pantographs or a third rail and pantograph systems are formed. A capacitor 12 provides one Part of a filter system that in addition to other features

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will be described in more detail in connection with FIG. Figure 1 shows a DC motor with an armature 16 and a field winding 18. The armature 16 is coupled to a mechanical load 22 / as shown schematically by the line 2o. In the case of towing vehicles the load is usually a driven wheel axle of the vehicle. Multiple motors can be in series, in parallel or in Series-parallel combination can be switched on, as is described, for example, in connection with FIG. A connection of the Armature 16 leads via a motor throttle 24 and an armature chopper 26 to the positive line lo. The other connection of the motor armature is connected to the negative line via a circuit breaker 3o tied together. The interrupter shown is preferably a single-pole lever breaker which is closed during engine operation and can be open during braking. The motor armature is therefore with the interrupter closed during motor operation 3o connected in a conventional manner by a series circuit between lines Io and 14. The series circle contains the Chopper 26, the motor choke 24, the armature 16 and the breaker 3o.

As is known, the chopper is essentially a periodically open and closed switch. The control process is effected by an armature chopper control 172, which sends an 'on' key signal to the chopper on line 36 and an 'off ¹ key signal on line 38. The circuit cycle can be controlled in a conventional manner by timing ratio control of the 'on ¹ and' off 'periods of the chopper. In the case of chopping, a controlled rectifier is usually used as a switch, which is switched capacitively by influencing the chopping control circuit. Chopper circuits are described, for example, in the General Electric Company SCR Manual, Issue 4, Section II.2.3 and in US Patent 3,515,970. It should be noted that the present invention can be used in conjunction with various armature circuit arrangements which do not employ such a chopper circuit.

When the engine is running, the chopper 26 is periodically closed,

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_ ₇ _ 2538CU3

so that a current can flow through the armature circuit. In a conventional manner, a unilaterally conductive member 4o is between the Line 14 and the connection 118 of the chopper and the motor choke 24 switched and polarized so that during the switch-off intervals of the chopper an armature freewheeling current can flow.

According to the following description, the arrangement according to the invention provides for a change in the operating state of the drive system between motor and braking modes by actuating the interrupter 3o. The system is activated by opening the breaker switched from motor to braking mode, the DC excitation being removed from the armature and the direction at the same time of field current flow can be reversed in a manner to be described. Because of the field current reversal, the motor works during of braking operation as a generator with armature current flow continued in the same direction as during motor operation, wherein however the armature voltage is reversed.

In the embodiment described, the armature chopper 26 can be switched to the braking mode periodically, with its switching cycle depends on the desired braking torque. A unilaterally conductive member 116 is between the line Io and a connection

119 connected between the armature 16 and the interrupter 3o and poled so that it conducts during the brake reversal and the armature circuit, consisting of the armature 16, the device 116, the line lo, the chopper 26 and the motor throttle 24 completed. Instead of the rectifier 116, a just during the Braking operation closed simple switch can be used. During the shutdown intervals of the chopper, the generated Armature current reduced in a load circuit. A controlled rectifier 122 and a resistor are provided for resistance braking 12o connected in series between the line Io and the connection 118 and thus in parallel with the chopper 26. When switched through In rectifier 122, the armature current generated flows from armature 16, devices 116 and 122, the resistor

120 and the engine throttle 24 existing circuit, during the dynamic braking the armature and drag brake circuit are used to

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loaned off from the DC voltage source, for example by breaking the line Io from the positive terminal of the DC power source. During regenerative braking, the armature current generated can the external direct current source and in that from the armature 16, the device 116, the line lo, the direct current source (not shown in Figure 1), the line 14, the device 4o and the engine throttle 24 existing circuit flow.

A description of the pel energizing arrangement now follows. One Controllable field voltage source 31 has its inputs connected to lines Io and 14 and generated at their connection 244 a controllable output voltage. A field control circuit 33 is used to the output voltage at terminal 244 between to change the potentials appearing on lines Io and 14. The field voltage source 31 can be variable impedance or gate elements which are connected accordingly between the line lo, the output terminal 244 and the line 14. A preferred one and the arrangement described in connection with FIG. 2 uses controlled rectifiers which are in a self-commutation arrangement of the general type as described in 'Principles of Inverter Circuits' by Bedford and Hoft, John Wiley & Sons, 1964, Section 7.4, page 19o. The automatic pulse reversal inverter (auto impulse commutative inverter) has alternating conductive, controlled rectifiers switched in opposite directions, their relative switching time can be changed depending on externally supplied gate signals. The mean voltage at the output terminal 244 can thus be changed between the potential values on the lines Io and by controlling the time of occurrence of the touch or gate pulses passed from the field control to the controlled rectifiers at the field control 33.

The field winding parts 18 are between the connection point 119 and the output terminal 244 is switched. During engine operation, the interrupter 3o is closed to one end connection of the field winding parts to apply to the negative lead 14. Since the other terminal of the field winding is connected to output terminal 244

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is, the field current flows from the positive line Io through the Source 31 / via output terminal 244, via field winding 18 and via the interrupter 3o to the negative line 14. The magnitude of the field current is a function of the voltage at the output terminal 244, i.e. the field current corresponds to the mean voltage difference between terminal 244 and line 14 divided by the resistance of field winding 18. Thus, the field current flows during of the motor operation in a first direction, and it has a variable that can be controlled by the field controller 33.

During the braking operation, the interrupter 3o is open. Since the armature current generated flows through diode 116 (this current considerably exceeds the field current), the voltage at junction 119 is largely clipped to line Io. The voltage at 119 differs from that on line Io only in the voltage drop across diode 116 in the forward direction. The potential at connection 119 is thus switched from the potential at line 14 to the potential at line Io. if the potential at connection 244 is assumed to be between the potentials on lines Io and 14, is the potential on the field winding 18 to that during engine operation the prevailing state is reversed, and the current flows in the opposite direction. A change between the braking and motor modes for a potential reversal on the field winding and to a reversal of the field current.

During braking, the armature current generated is passed through the diode 116 through the load or chopper circuit, as previously described. In addition, the armature supplies the necessary field current in a self-excited system operating mode. For example, during the conduction intervals of the freewheeling diode 4o via the armature 16 , the field winding 18, the impedance segment of the device 31 between the terminal 244 and the line 14, the diode 4o and the reactance 24, a current path is established. The braking cycle is initiated as follows. The armature chopper 26 is keyed or brought into the non-conductive state to drive the motor torque and current with one of the armature control circuit 172

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- Io -

reduce certain speed to zero. When the anchor, current has reached the value zero or approximately this value, the switch 3o is brought into its open position B according to FIG. Usually lines Io and 14 are in the case of resistance braking switched off from the supply source, while they remain switched on in the event of regenerative braking. First of all, the Considered the case of dynamic braking. After the device 3o is opened, the chopper 26 is switched on and the field control is switched on 33 controlled in such a way that the terminal 244 is essentially connected to the line 14. It can be seen that then a Current from the filter capacitor 12 via the line lo, the chopper 26, the choke 24, the armature 16, the field winding 18 and into the Line 14 flows. This current reverses the residual flux in the field winding 18, whereby the polarity of the armature terminals in relation is caused to reverse on engine operation. This polarity arises due to the current build-up in the field winding 18 the anchor terminal tension is also built up. When the armature voltage is the combined voltage drops of the limb 116 and of the chopper 26, a current flows in the path 116, in the chopper 26, in the throttle 24 and in the armature 16. The current flow commences away from the terminal 119 into the field winding 18, whereby a further increase in the current is justified. When the armature current in the element 116 exceeds the field current in the element 18, it can be shown by the superposition of currents that the current in the field winding 18 mainly from capacitor 12 via line lo, 'backwards' through member 116, as before through member 18 and 18 flows back to line 14. Because of the small DC resistance in the armature-chopper path, the armature current grows faster than usually desired. This causes the chopper to shut down to reduce the current. The ON-OFF time ratio the chopper is subject to constant adjustment in order to regulate the armature current to the desired value, usually can this desired current can assume a ramp function in order to limit shocks to a braking towing vehicle. When the chopper is turned off is, the armature current flows into the line lo, the capacitor 12, the line 14, the member 4o, the throttle 24 and in the armature winding 16. This process results in an energy

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storage in the filter capacitor, which is used during the field current reversal period has been partially emptied. When the voltage across the capacitor 12 reaches a predetermined value, a switch not described and control device not part of the present description by device 122 to convert one of the capacitor to form a different path for the armature current. In the sequence of steps of dynamic or resistance braking, the chopper 26 is switched on first, which depends on the increasing Current switches. The element 122 is then switched on as a function of another control parameter, for example may be the voltage sensed between lines Io and 14. The capacitor 12 is then the real power source for the field winding 18, and the energy is periodically stored as described. Assuming that in a numerical example the excitation current Io ampere and the armature current loo ampere it can be seen that an average armature current component of Io amps into capacitor 12 and finally into the field winding 18 flows while a mean component of 9o amps flows into resistor 12o. Control means can key the Adjust members 26 and 122 to maintain a substantially constant voltage between lines Io and 122.

During regenerative or regenerative braking, the circle behaves in a similar way as described above, but with the difference that the filter capacitor is not an important part as before, since the supply source maintains a constant voltage between lines Io and 14. Since the capacitor 12 cannot be charged to a higher potential than that on the line Io, the thyristor 122 would not be turned on will. This would only result in the state of the chopper switched on, with the current in the armature increasing through the path described (the link 116, the chopper 26, the throttle 24 and the armature 16) would be caused while the current is switched off or interrupted chopper via the member 116 into line Io and the source (not shown) through line 14, member 4o, throttle 24 and armature 16 decreases. This rise and fall of the armature current represents the

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The on-off ratio the chopper is controlled to adjust the mean armature current to the desired value.

Two independent special features will now be described. The magnetic energy initially stored in the motor field is not consumed in the switching arc as it is in the case of an application electromechanical means of field reversal would occur. If the contactor 3o is opened while the field current is in the first direction flows from the terminal 244 into the winding 18, the member 116 is caused to apply the induced field voltage to clamp the voltage of the capacitor 12.

A second important characteristic is the ability to control the field in the braking direction by using the control battery shown in Figure 1. If the voltage on capacitor 12 for any Basically collapses before the flux in the field winding 18 has been built up in a suitable manner for the braking event, a Switch 3ol are closed. This allows a current from the battery through a blocking diode 3oo to the connection point 119, through the field winding 18, through the control member 31 and back via the line 14 to the control battery 3o2 flow. If the self-assembly proceeds in a normal way, the diode 300 blocks eventually the reverse current and prevents it from flowing into the battery when the voltage on line Io exceeds the battery voltage exceeds.

Figure 2 shows a preferred embodiment in which the in connection with Figure 1 above-described arrangement and parallel connected sets of two series-connected motor armature circuits to be used. Similar components are therefore assigned common reference numbers.

The positive terminal 2 is connected to the positive line Io via a suitable breaker and line switch 6 and filter chokes 8, 8 '. The filter capacitor 12 is connected to the connection point of the filters 8, 8 ¹ and via a resistor 256 to the

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negative line 14 connected. A filter capacitor 12 'leads from the connection between the coil 8 'and the line Io to the Resistor 256, to which a switch 258 is connected in parallel. Under normal conditions the contactor is closed to the resistance 256 to bridge. However, it is opened when the voltage across capacitor 12 'drops significantly. This can be done by a voltage sensor, not shown and connected via the filter capacitor, takes place. This results in a softer Charging process, i.e. excessive current overloading of the filter during excitation is avoided. Such a soft resp. gradually charging circuit can of course also at another position of the series circuit from the line terminals 2 as well 4 and the filter.

The anchor chopper similar to the type disclosed in U.S. Patent 3,515,970 26 is connected between the line Io and the connection point 118. The chopper contains a chopper head reactance 2oo and a controlled main rectifier 2o2, these links in series between the line Io and the connection point 118 lie. A commutation or switchover capacitor 2o4 lies parallel to the commutation reactance 2o6 and the switchover-controlled one Rectifier 2o8, these elements 2o6, 2o8 being connected in series. The reactances 2o6 and 2oo are connected to the line Io, and the cathode of the rectifier 2o8 is connected to the connection 118 via the coupling diode 2Io connected in series. The controlled rectifiers 2o2 and 2o8 are polarized in the forward direction, and their control connections are with the 'on' terminal 36 and the 'off' terminal 38 connected. A commutation diode 212 is connected in opposite directions in parallel to the rectifier 2o8. A charging network comprising a resistor 214 and a diode 216 leads from the negative line 14 to the connection point of the diodes 21o and 212, the rectifier 2o8 and the capacitor 2o4.

Between junction 118 and negative lead 14 are four motors connected in series-parallel connection. A first circuit has, connected in series, a motor choke 24, brake

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Resistors 23 and 25, which can be bridged by contacts 27 and 29, a first motor armature 218, a second motor armature 16 and an interrupter switch 3o. A second motor armature circuit connected in parallel to the first contains a motor choke 24 ', braking resistors 23' and 25 ', which can be bridged by contacts 27' and 29 ', a third motor armature 218', a fourth motor armature 16 'and an interrupter switch 3o '. The interrupter switches 3o and 3o * are each shown as single-pole and mechanically coupled circuit-breakers, which can thus form a single-pole switching arrangement. A freewheeling diode 4o is connected between the line 14 and the connection point 118. A one-sided conductive member 116 leads from the connection of the armature 16 and the switch 3o to the line lo, while a one-sided conductive member 116 is connected between the connection of the armature 16 'and the switch 3ο ¹ and the line Io. The members 116 and 116 'are polarized so that they conduct the armature current generated during the braking operation. The above-described arrangement from FIG. 2 is similar to that which was explained in connection with the aforementioned US patent application.

A dynamic or resistance brake circuit, which contains a controlled rectifier 122 and a braking resistor 12o, is located between the line and the connection point 118.

The controllable field voltage source 31 connected between the lines Io and 14 has an output terminal 244. The first motor field winding 22o and the second motor field winding 18 are connected in series between the terminal 244 and the point 119, which establishes the connection between the armature 16, the interrupter 3o and represents the unilaterally conductive member 116. Similarly, fourth and third motor armature windings 18 'and 22ο ^{1 are} connected in series between terminal 244 and point 119' which represents the connection between members 16 ', 3o' and 116 '.

The field voltage source used in the preferred embodiment 31 represents a complementary pulse-commutated inverter or inverter of the type described in Section 7.4 of the above-mentioned literature

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represent the type mentioned by Bedford and Hoft. The use of a similar circle for other purposes is disclosed in the aforementioned patent application. A series connection between the Lines Io and 14 contain controlled rectifiers 236 and 238 and a center-tapped inductance 242. The anode of member 236 is connected to line Io while its cathode leads to an end terminal of the coil 242. The anode of member 238 is with the other end terminal of coil 242 connected, while its cathode leads to line 14. Between line Io and junction 244 is a feedback diode 245 and a small resistor 248 connected in series. Similarly, there is a feedback diode 246 and a resistor 25o in series between line 14 and junction 244. Commutation capacitors 252 and 254 are between the connection point 244 and the corresponding lines Io as well as 14 switched. The diodes, which are connected in opposite directions in parallel to the controlled rectifiers, ensure on after commutation Returning the energy stored in the coil 242, which is consumed in the resistors. The controlled rectifier 236 and 238 are triggered successively at a predetermined frequency by means of periodic ignition pulses from the field control circuit 33 Figure 1 keyed, the control terminal 256 of the rectifier 236 and the control terminal 258 of the rectifier 238 are excited. The arrangement described ensures self-commutation of the controlled rectifiers 236 and 238 and thus their alternating switching. By changing the timing of the control signal from the field controller 33 to the Terminals 256 and 258 is routed, the corresponding switching times of members 236 and 238 can be modified. Through this there is an average potential at the terminal 244. When the element 238 is switched through and the element 236 is blocked, is the terminal 244 is at the potential of the line 14. If the element 236 is switched through and the element 238 is blocked, points the terminal 244 to the potential of the line Io. The mean potential at terminal 244 therefore corresponds to the ratio of the switching time of member 236 for the entire conduction time of members 236 and 238 per cycle.

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FIG. 3 shows an alternative embodiment which is different from the ones in FIG Connection with Figures 1 and 2 described examples differs in that the anchor chopper to the supply line 14 instead of the Supply line Io is connected. The interrupter 3o leads from the positive line Io to the connection between the motor armature 218 and the unilaterally conductive diode 116. During operation of the engine, a circuit results via the interrupter 3o, which are in series switched motor armature 218 and 16, the motor choke 24 and the armature chopper 26 to the line 14.

The one containing resistor 12o and controlled rectifier 122 Resistance brake circuit is parallel to the armature chopper 26. The braking resistor 23 is between the freewheeling diode 4o the terminal of the motor choke 24 and the chopper 26 and the line Io connected in series. The switch is located in a known manner or contactor 27 parallel to the braking resistor and is closed except for certain periods of regenerative braking. That one-sided Conductive member 116 leads from the line 14 to the connection point 119 between the armature 218, the interrupter 3o and the field winding 18. During regenerative braking, there is a conduction path via the armature windings 218 and 16, the motor choke 24 and the braking resistor 23, the diode 4o, the line lo, the external direct current source (not shown), the line 14 and the one-sided conductive member 116. The field voltage source 31 can be of the same type as in the embodiments of FIGS is used. The field windings 22o and 18 are connected between the output terminal 244 of the field voltage source 31 and the connection point 119 in the same manner previously described.

Various types of control can be used during the motor and brake modes be used. For example, the same modes of operation as in the above US patent application and the like can be used Control circuit configurations can be used. The embodiments explained above are to be regarded only as examples, and numerous changes and modifications are possible within the scope of the present invention.

- patent claims -

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Claims (9)

Claims l \ driving means for DC motors with the armature and field windings, the engines during engine operation to drive a mechanical load, and are driven during braking operation by the load / and are connected during motor operation the armature windings between conductors of the first and second polarity connected to a DC power source can be connected, the direction of the current through the field windings being reversed during braking operation and the armature windings being coupled to feed a circuit for consuming or reducing the armature current generated, characterized by first and second conductors (lo, 14), by armature windings (16, 218, 16 ¹ , 218 ') with first and second terminals (118, 119), of which the first (118) can be connected to the first conductor (lo, 14) via armature impedance elements (26, I2o, 122) is, by interrupter switching means (3o), which open during braking and closed during engine operation are, and are connected between the second terminal (119) and the second conductor (14, lo), by means (116) which are conductive only during braking operation and which are connected between the second terminal (119) and the first conductor (lo, 14) are connected, through first and second impedance elements (31), which are connected between the first and second conductors (lo, 14) and form an output terminal (244) of controllable voltage at their connection point, and through between this output terminal (244) and the second Terminal (119) connected field winding means (18, 22o) so that the field current flows in a first direction during motor operation and in a second direction during braking operation. 2. Device according to claim 1, characterized in that the first and second impedance elements each switch elements (236, 238), which can lead consecutively and alternately. 3. Device according to claim 2, characterized by means (33) 609812/0691 - ι ο - for changing the switching time of one of the switching elements with respect to the other switching element to the mean potential the controllable output terminal (244). 4. Device according to claim 3, characterized in that the conductive means comprise a unilaterally conductive member (116), which is polarized accordingly in order to conduct the armature current generated during braking. 5. Device according to claim 4, characterized in that the armature impedance elements have chopper switching means (26). 6. Device according to claim 5, characterized in that the first and second switching means have controlled rectifiers which are in a self-commutating push-pull chopper circuit (26) are arranged. 7. Device according to claim 6, characterized in that on one side conducting means (4o) from the second conductor to the first terminal (118) of the anchor means. 8. Device according to claim 2, characterized in that capacitor elements (12, 12 ¹ ) are connected between the first and second conductors (lo, 14). 9. Device according to claim 2, characterized in that means to amplify the current flowing through the field windings during braking operation, a voltage source (3o2) and a one-sided have conductive member (3oo), these parts being connected in series with the field windings (18, 22o) and on one side conductive member is polarized accordingly to the current in braking operation from the voltage source through the field windings conduct. 609812/0691