Abstract: A braking system is disclosed that includes a brake (22), a ram (20) shiftable in a linear direction relative to the brake (22) for actuating the brake (22), and a stop (24) for limiting movement of the ram (20). The system further includes a motor (12) for moving the ram (20) which motor (12) includes a stator and a rotor (13) and a commutation sensor (14) producing an output. A motor controller (28) receives a commutation signal based on the output and controls the motor (12) based on the commutation signal. A processor (31) also uses the commutation signal to generate a position signal indicative of the position of the ram (20) relative to the stop (24). Methods of using this system are also disclosed.

Abstract: A control circuit operates a dual directional DC motor from an AC power source. A controller has first and second position indication inputs adapted to receive respective first and second position indications. The controller also has first and second direction outputs, which respectively energize one of two first relay coils having contacts outputting respective AC voltages responsive to the direction outputs. A full wave bridge rectifier receives the AC voltage and responsively outputs a DC voltage, which energizes a second relay coil. A contact thereof directs that voltage to two sets of first relay contacts, which are also responsive to the direction outputs, and which respectively provide a positive or negative DC voltage to the motor. Another second relay coil contact provides a braking action to the motor responsive to removal of the DC voltage from the rectifier output following removal of the AC voltage to the rectifier.

Abstract: In the motor stop control device utilizable for the reel-type gaming machine 1, there are provided the stepping motor 70 having two pairs of excitation phases for rotating the reel 3 on the outer periphery of which a plurality of symbols are formed, the speed reduction mechanism 700 for transmitting rotation of the stepping motor 70 to the rotation shaft of the reel 3 with a predetermined speed reduction ratio and the main CPU 40 for conducting speed reduction treatment to reduce rotation speed of the stepping motor 70 and excitation treatment by 2-phase excitation against the stepping motor 70 when the motor stop command occurs.

Abstract: Disclosed is a control apparatus for a hybrid vehicle having an engine, a drive motor (3) that regenerates power, and an electric power storage device (6) that gives/receives power to/from the drive motor (3). The control apparatus includes power consumption means (2) for consuming power; a sensor (26, 27) that detects a state of charge of the electric power storage device; means (18, 23, 51) for detecting a driving state of the vehicle; and a controller (9).

Abstract: A device for inrush current prevention and dynamic braking in a motor having a plurality of power inputting terminals, comprises a diode rectifying unit rectifying power supplied from an AC power supplying unit; a capacitor smoothing the power rectified by the diode rectifying unit; a resistor disposed between the AC power supplying unit and the capacitor, being connected to the capacitor; an inverter connected to terminals of the capacitor and the power inputting terminals of the motor, inverting the power from the capacitor into an AC power having multiple phases and supplying the AC power having multiple phases to the motor; a dynamic braking circuit short-circuiting the power input terminals of the motor; and a relaying unit including a first node connecting the dynamic braking circuit to the resistor and a second node connecting the diode rectifying unit to the capacitor in parallel.

Abstract: A device to control a 3-phase motor having an inverter connected to each phase terminal and a brake to arrest the 3-phase motor, includes a power supply to supply power to the brake and the inverter at the same time based on a predetermined synchronization signal. The device also includes a control unit to output an inverter control signal to control the inverter and a power control signal to enable the power supply to supply the power to the brake and the inverter. Thus, the device to control a 3-phase motor has an immediate response to a control signal with a reduced production cost and a system size, by reducing circuit components such as a brake relay and a brake power supply.

Abstract: A motor control circuit includes a forward rotation control circuit, a reverse rotation control circuit, and a brake control circuit. The forward rotation control circuit allows a first current source to supply a first current signal to a motor drive circuit for operating a motor in a forward rotation mode. The reverse rotation control circuit allows a third current source to supply a third current signal to the motor drive circuit for operating the motor in a reverse rotation mode. The brake control circuit allows a second current source and a fourth current source to respectively supply a second current signal and a fourth current signal to the motor drive circuit for operating the motor in a brake mode. The motor control circuit reduces power dissipation in the brake mode since the second and fourth current signals may be smaller than the first and third current signals.

Abstract: An apparatus for decreasing speed of an inverter driven inertial load within a time period for use with a dynamic brake across a DC link including a controller for controlling the inverter to regulate the power returned to the brake such that, when a brake command is received, the power is increased during a power-up period while motor deceleration begins and up to a constant power level after which the constant power level is maintained by increasing torque such that the dynamic brake is fully used during the majority of a braking cycle and a method to accomplish the same.

Abstract: An electrically controlled, decentralized control system in a vehicle includes at least two energy sources which are independent of one another and at least two control units. In addition, at least one high-current load is allocated to each control unit. The high-current load is permanently allocated to one of the energy sources, while the control units are selectively switchable to the one or the other energy source.

Abstract: A discrete Kalman filter 12 and a disturbance estimating unit 13 are added to a feedback loop for feeding back a detected value of position from a position detector 6. The Kalman filter estimates position and velocity of an object to be controlled, thereafter outputting an estimated value of position and an estimated value of velocity. The disturbance estimating unit estimates from a command value of current and the estimated value of velocity the disturbance that is added to a load 5. The estimated value of position is fed back to a position controller 1, and the estimated value of velocity is fed back to a velocity controller 2. Further, a difference between a target value of current outputted from the velocity controller and the estimated value of disturbance is output as the command value of current.

Abstract: An electrical brake system for a motor vehicle including control modules for controlling the front axle brakes and the rear axle brakes, in which the control modules for the front axle brakes include fault-tolerant behavior, and in which the control modules for the rear axle brakes do not have any external effect in the event of a fault state in the area of the control module.

Abstract: A motor brake releasing device includes an electromagnetic brake for locking a motor shaft and a switch mounted between the electromagnetic brake and a releasing-power-supply for the brake. The switch can be operated by an AND signal produced by a motor driving signal and a motor-brake-release-permitting signal. This structure allows a motor brake to be released when a motor driving system encounters an obstacle, although the motor is left free. The structure also can prevent a final object of the motor shaft from dropping due to its own weight.

Abstract: A blended electrical/friction braking system includes an electric brake feedback monitor which monitors a first signal corresponding to an electric brake effort request of an electric motor of an electrically powered vehicle. The electric brake feedback monitor detects one or more electrical conditions of the electric motor during electric braking and selectively supplies/terminates the first signal to/from a brake controller as a function of the first signal and the one or more electrical conditions. In response to termination of the first signal thereat, the brake controller causes a friction brake of the vehicle to assume the entire braking effort of the vehicle.

Abstract: A sensorless brushless-DC-motor mounted on an electric or hybrid vehicle is powered by an on-board battery through an inverter supplying a three-phase pulse width modulated voltage (PWM voltage). The inverter is controlled to generate the PWM voltage having an average voltage level corresponding to a target motor speed. The PWM voltage level is controlled by changing its duty ratio, so that a difference between the target motor speed and an actual motor speed is minimized. The actual motor speed is determined based on a signal indicating a rotor position detected from the PWM voltage imposed on the motor. When the battery voltage drops and the duty ratio becomes 100%, the target motor speed is temporarily reduced to the level of the actual motor speed. When the battery voltage is recovered and the duty ratio becomes lower than 100%, the target motor speed is gradually increased again to the original level.

Abstract: Conventionally converter-controlled electric motors are allowed to operate in generational mode during braking and to convert the electrical energy so produced into heat. Such motors may also comprise electromagnetically actuated mechanical brakes. In the present invention it is proposed to supply the electrical energy produced by the motor when in a generational mode to an excitation coil of the brake and there store it temporarily as magnetic energy or convert it into heat.

Abstract: A motor drive apparatus is provided in an injection molding machine in order to supply electrical energy to a motor. The motor drive apparatus includes an electricity accumulation unit for accumulating electrical energy; and an electricity accumulation circuit for accumulating in the electricity accumulation unit electrical energy regenerated from the motor during a deceleration period of the motor and for supplying the electrical energy accumulated in the electricity accumulation unit to the motor during an acceleration period of the motor.

Abstract: Electric motors, in particular geared motors, are often equipped with electromagnetically actuated mechanical brakes. In order to ensure that an excitation coil (3) can be operated with alternating-current mains supplies of various voltages, it is proposed to generate a unipolar voltage that varies over time with a certain period by means of a voltage source (2), the output voltage of which is positive and different from zero in a first time segment of this voltage period and in this first time segment has a maximal value, and in a second segment of the voltage period is zero or approaches zero. To pass an excitation current from the voltage source (2) to the excitation coil (3), a current path (15) is provided that is opened substantially at the beginning of the first time segment. The current path (15) is blocked again at a certain point within the first time segment, this point being selected such that the excitation current does not exceed a predetermined maximal value.

Abstract: A method and a device for electrical braking of an all-mains motor by which disconnection of supplied feed current to the motor causes an electric switching operation, whereby electric current arranged to cause a reversed rotary direction for the rotor (1) of the motor is supplie. According to the invention, supplied braking power (P) is controlled and reduced on basis of the successive reduction of rotary speed for the motor until the rotor (1) of the motor has substantially completely ceased to rotate, preferably substantially corresponding to an inverted "soft start" for the motor, whereafter supplied braking current feed is interrupted. It is advantageously monitored whether or not normal start of the motor has been performed, electric switching to braking only being made possible provided that such a start sequence has been indicated.

Abstract: A solid state motor starter, commonly referred to as a soft starter, is constructed in a manner to be easily manufacturable while at the same time combining all the required components in a relatively small package. A solid state power switch is clamped between a pair of bus bars in an offset manner to accommodate a discrete switching relay mounted in an inverted manner between the line input and the bus bar in communication with the load outputs. When in a motor run node, current is shunted away from the solid state power switching device and through the switching relay, a substantially linear current path is achieved to reduce power loss and heat buildup. The arrangement allows for a heat sink mounted to one of the bus bars with adequate spacing between the heat sink and the discrete relay for insertion of a cooling fan. Current sensing is achieved with a Hall effect sensor mounted to the bus bar in a small current sensing region that is created by having a pair of slots in the bus bar to direct current.

Abstract: The wheel brakes of each vehicle axle or side or diagonal are supplied with current by a separate voltage source. To reduce the load on the vehicle's electrical system, the wheel brakes of a front axle are supplied with current first, and then the wheel brakes of a rear axle are supplied with current.

Abstract: A motor drive apparatus used in a motor for controlling a rotor therein to rotate in regular and reverse directions includes a rotating direction detecting circuit for detecting a first direction in which rotor is rotating. A comparator compares a command signal with a reference signal to detect a second direction in which the rotor should rotate. A motor driver comprised of buffer amplifiers and power supply circuits is provided to rotate the rotor based on the command signal. A command signal generator produces a drive signal which is transferred to the power supply circuits through a switch when the first and second directions are both identical to the reverse direction.

Abstract: An electrical system, or a trigger circuit for use in connection with an electrical system, is disclosed. The electrical system includes a circuit element responsive to applied current, a power source for providing current to the circuit element, a substrate, and a substantially clear conductive composition arranged on or in associatin with the substrate for providing an electrical current path between the power source and the responsive circuit element.

Abstract: The wheel brakes of each vehicle axle or side or diagonal are supplied with current by a separate voltage source. To reduce the load on the vehicle's electrical system, the wheel brakes of a front axle are supplied with current first, and then the wheel brakes of a rear axle are supplied with current.

Abstract: A motor control system for controlling the operating speed and direction of a three-phase induction motor. The invention particularly finds use for bringing a rotary printing cylinder precisely into register for processing products in sheet form, such as for printing sheets of corrugated cardboard in a machine for printing corrugated boxes. A three phase induction motor is single phase powered by an AC drive through a first and a second electrical conductor. Direction switching signals and time delay speed switching signals are applied to the appropriate terminals of the AC drive by the second and a third electrical conductor. This allows motor powering and control using only three conductors, such from as the slip rings on industry standard corrugated box printing equipment.

Abstract: A switching signal generator outputs a switching signal which indicates either an accelerating mode or a decelerating mode by comparing a motor control signal and reference voltage. A first activation signal generator and a second activation signal generator output a first activation signal and a second activation signal, respectively, according to the switching signal from the switching signal generator. A switching control signal generator outputs either a switching control signal based on a motor location signal or a desired electric potential according to the switching signal from switching signal generator, a first activation signal from the first activation signal generator and a motor location signal.

Abstract: A vehicle anti-lock braking (ABS) system having a motor-driven pump (10) for providing a hydraulic supply to the ABS system. The pump motor (10) is controlled by a series device in the form of an electronic switch (16), such as a MOSFET, to enable frequent testing of the motor circuit by short pulses which produce no significant noise. Feedback to control motor speed is obtained by measuring the e.m.f. generated by the motor (16) after it has been switched off and after a delay to allow the back e.m.f., caused by switching off the motor, to decay (preferably to zero) before the measurement operation begins. The generated voltage is integrated during a motor switch-off period, the motor speed being established in this period by subtracting the measured integrated voltage when acting as a generator from the main supply voltage (B+) .

Abstract: In an electric vehicle including a transmission provided between a motor and driven wheels, if a shift change detecting means has detected the shift change from a "D" range or an "R" range to an "N" range or a "P" range when a stoppage detecting means has detected that the vehicle is in its substantially stopped state, a torque command value gradually-decreasing means gradually decreases the torque command value for the motor toward zero. This causes the face pressure of meshed portions of gears within the transmission to be moderately decreased, thereby preventing the generation of a striking sound due to a repulsion of the gear.

Abstract: A magnet brake is disclosed. The magnet brake may consist of a stationary magnet assembly and a rotating magnet assembly. The opposing poles on the magnet assemblies have a mutual attraction that biases the assemblies into alignment with each other. The magnet brake may also be combined with Hall-effect switches to provide position and direction detection.

Abstract: A current regulator is disclosed which utilizes a well known driver chip. The driver chip has an input signal known as a brake signal "BRK" which typically is used to stop a standard DC motor. In this invention, the brake signal "BRK" is used to create a low resistance current path in order to sustain the current of the current regulator which is used in conjunction with a stepper motor.

Abstract: A system and methods that provide for battery regeneration in electric vehicles without employing brake blending. Wheel speed sensors provide speed inputs to a controller, for example, that are processed to control the torque commands issued to a motor of the vehicle, which allows a higher level of battery regeneration without causing drive wheels to lock up. In the present invention, speed inputs derived from the wheel speed sensors are sampled. The absolute value of the difference between the input signals derived from the wheel speed sensors is compared to a predetermined difference threshold. If the absolute value of the difference between the input signals is less than the predetermined difference threshold, a regeneration torque command is generated to control the torque output of the motor that is equal to a current value of the regeneration torque command.

Abstract: An electric brake circuit (10) for bringing an alternating current (AC) motor (12) to a rapid halt. When the AC motor is turned on, a braking capacitor (C1) is charged to a preselected voltage. A shunt regulator (18) prevents the braking capacitor from overcharging by shunting charging current to ground once the braking capacitor reaches the preselected voltage. In a preferred embodiment, a limit circuit (20) turns off the charging current on the subsequent positive phase of the AC line voltage powering the electric brake. By shutting off current flow to the circuit when the preselected voltage on the capacitor is reached, power consumption and size of the circuit are minimized.

Abstract: A consist power monitor provides information to the engineer of a multiple unit consist on the ability of each locomotive to provide traction power and dynamic braking. The consist power monitor is a microprocessor based device which communicates with other locomotive consist power monitors, either through the MU cable or over a wireless link, to display consist status to the engineer. Where the communication is through the MU cable, the communications link is provided by a high frequency carrier superimposed on an existing MU cable wire. No modification is required to the MU cable allowing for easy retrofit of existing locomotives. The alternative wireless communication is accomplished by means of a low power, spread spectrum data link now being incorporated into new locomotives. Information exchanged among the consist power monitors pertains to loading and dynamic brake conditions of the locomotives in the consist. Locomotive consist information is presented continuously.

Abstract: An electric drive system in lift trucks has, for driving the lift truck driving wheels, at least one separate excitation electric motor provided with a stator and a rotor which are energized independently of each other by means of an electronic drive and control unit; by this arrangement, space can be saved on the lift truck and the lift truck can be rendered reliable, low in running cost, adaptable to varying load conditions, and adjustable as regards its operational parameters.

Abstract: For the low-loss braking of an electrically driven vehicle with two electric motors connected in parallel and arranged one on each side of the vehicle and which are controlled by a pulse control, it is proposed, for one, to brake the vehicle above a prescribed rpm threshold value by useful-current braking, and below it by means of regenerative braking. Secondly, instead of an rpm threshold value, the battery current can also be used as the criterion for one of the two different braking types. A high heat generation in the motors is avoided and energy is saved by this braking process. An electric switching circuit for the conversion of this process is indicated.

Abstract: An automated tape library employing a motorized robotic picker wherein the electric motor driving the robotic picker is equipped with two brakes that act simultaneously. The first is a mechanical brake which uses a spring to push a fixed brake pad against the rotating motor. During normal motor operation, an electric solenoid is used to overcome the spring and disengage the brake pad from the motor. When the robotic picker is to be mechanically stopped, the solenoid voltage is turned off and the solenoid releases the spring placing the brake pad in contact with the motor. The second brake is a dynamic electrical brake which makes use of the back emf of the motor to drive a current back into its own windings such that the motor decelerates. The circuit further prevents mechanical brake engagement even at lower motor speeds when back emf is relatively low but robot speed is fast enough to cause mechanical damage if stopped via the mechanical brake.

Abstract: A control system for an electric machine powered from a DC source, wherein controlled motoring and controlled braking are achieved with a single current regulator control, even when the DC source cannot accept charging current and/or the induced motor voltage exceeds the DC bus voltage. The motor control circuit comprises a DC source connected to a DC bus, a current control circuit for modulating switching devices connected across the DC bus, an energy exchange capacitor connected across the DC bus, a load resistor, a switching circuit effective when activated to interrupt a connection between the DC source and the DC bus and to connect the load resistor across the DC bus in parallel with the capacitor, and a logic circuit for activating the switching circuit during motor braking under conditions for which the modulation of the switching devices is ineffective to control the braking current.

Abstract: The present invention features an electric motor control system combination with at least one electric motor having a field winding and an armature winding connected in series. The system also includes a first power terminal, a second power terminal, a current-limiting resistor, and a current sensor. The current-limiting resistor and the current sensor are connected in series between the first power terminal and the second power terminal. A contactor has a contact operable to shunt the current-limiting resistor. A microcomputerized control connected to the contactor and the current sensor governs the contractor to shunt the current-limiting resistor when the electric motor is not in a plugged condition. The contactor is operative when the current sensor detects a substantially zero current through the electric motor.

Abstract: According to the invention, a number of sheet materials P1 are successively delivered to and clamped between a plurality of clamp rollers 5. A servo motor 6 is operatively connected to the clamp rollers 5 to rotate the latter. A detector PH1 is disposed upstream of the clamp rollers 5 and detects the position of the sheet material P1. Further, a control device 9 is connected to the servo motor 6 and the detector PH1 to control the servo motor 6 to momentarily decrease its number of revolutions (R.P.M.) in response to the detection signal from the detector PH1 immediately before the sheet material P1 is completely discharged from the clamp rollers 5. The rotative speed of the clamp rollers 5 is therefore momentarily lowered so that the sheet material P1 is discharged at low speed. Accordingly, the sheet material P1 conveniently falls down onto a table 8 so that sheet materials P1 are regularly stacked on the table 8. No backling takes place in the leading end of the sheet material P1.

Abstract: The present invention is a regenerative braking system useful in systems having variable kinetic energy such as vehicles. A DC machine having an independently energized field coil includes a rotor mechanically coupled for rotation at a speed proportional to the velocity of the load. During deceleration the armature is coupled to an inductor and rotational energy provided by the load is converted into magnetic energy stored in the magnetic field of the inductor. To store the energy a short is connected across the inductor, which is preferably operating under superconducting conditions. For acceleration this inductor current is applied to the DC machine to produce a torque for moving the load. The phase reversal required for proper direction of torque is preferably achieved by reversing the direction of field current. The rate of energy transfer and thus the rate of acceleration or deceleration is controlled by controlling the magnitude of the field coil current.

Abstract: An electrical circuit governs the speed and resisting torque of a permanent magnet electric generator which is employed as a means for generating resistance for an isokinetic physical exercise machine to improve the effectiveness of the training. By providing a steep increase of resisting torque to the speed when the torque is below a predetermined value and when the speed exceeds a preset level, it insures a minimum resistance throughout the range of motion and contains the speed within a desired range.

Abstract: A method for operating an electronic control system for an operator controlled, electrically driven vehicle. The vehicle being powered by a direct current electric traction motor. The electronic control system includes a power source, a variable mark-space ratio power regulator responsive to a motor current command signal developed in response to the position of an accelerator, a direction selection means for providing signals indicative of the direction of motion of the vehicle, and a braking signal responsive to brake position. The electronic control system implements operation of the motor automatically in a propulsion mode, a regenerative mode, and a plug mode. The propulsion mode is either a series connected mode or a separately excited mode. The regenerative mode simulates the retarding effects of an internal combustion engine driven vehicle, while recover the kinetic energy of the vehicle by the generation of current to recharge the power source.

Abstract: A method and apparatus for controlling a direct current motor of an elevator during emergency braking by conducting a current generated by the motor through a braking resistor are disclosed, in which a semiconductor switch connects the braking resistor across the motor during the emergency braking and a mains voltage to the motor is simultaneously interrupted.

Abstract: A brake control system for a railway car having a primary electric brake and a secondary friction brake that supplements the electric brake as the electric brake effectiveness fades. The secondary friction brake is normally applied in accordance with a first supplementary control signal provided by an electrical component, such as an operational amplifier, the output of which is the difference between a brake command signal and the actual braking produced by the electric brake. A second supplementary control signal corresponding to the brake command signal is effective to control the secondary friction brake, in response to either anomalies in the power supply or failure of the operational amplifier. A switch connects the brake command signal to an electric-pneumatic converter to provide pneumatic pressure to the friction brake corresponding to the brake command, and to concurrently terminate operation of the electric brake, when either one or both of the foregoing abnormalities are detected.

Abstract: A battery powered electric vehicle d.c. motor control includes a first potentiometer actuable by an accelerator pedal and a second potentiometer actuable by a brake pedal. The motor current is controlled in accordance with the signals at the sliders of these potentiometers and contactors determine the connection mode of the motor under the control of a comparator which is sensitive to the signal at the slider of the first potentiometer only. This causes braking mode to be selected except when the accelerator pedal is depressed. A resistor is connected in series with the second potentiometer to ensure that a minimum braking demand signal is always present, but means is provided for cancelling this minimum braking demand signal in certain vehicle operating conditions and further means is connected to the comparator for overridingly selecting motoring mode in these conditions.

Abstract: The electric braking of a passenger vehicle is described, wherein the current generated by the propulsion motor in the brake mode of operation is distributed between the power supply line and a dynamic brake resistor circuit in response to the filter capacitor voltage, and with the operation of the dynamic brake resistor circuit being controlled relative to the chopper thyristor with a predetermined frequency of operation.

Abstract: There is disclosed apparatus for controlling the regenerative braking of a transit vehicle by sensing the operation of the power source third rail for that transit vehicle for the purpose of preventing regeneration of power by a moving vehicle into that power source rail when it has been determined that the power source third rail is not receptive during the regenerative braking effort of that vehicle.

Abstract: The circuit effects a changeover from the traction configuration to the braking configuration or vice-versa in less than one millisecond. It includes the field winding (3) of the motor (1) disposed between a traction thyristor (12) and a main chopper (13) followed by a traction diode (14) and the armature of the motor, the field winding being in parallel with a free-wheeling diode (D5) and a shunt thyristor (THS).

Abstract: A chopper and a series circuit of a dynamic braking resistor and a thyristor are connected in parallel with a d.c. motor which is operated as a generator during braking operation. A series circuit of a diode and an inductance element is connected in series with the d.c. motor. A condenser connected between the ground and the junction of the inductance element and the diode forms a filter circuit together with the inductance element. Current generated by the d.c. motor flows into a d.c. source through the diode and the inductance element during regenerative braking operation. An over-voltage detector fires the braking thyristor when the voltage across the condenser becomes greater than a predetermined limit value, as a result the dynamic braking operation is performed.A chopper controller varies the interval between an ON pulse to turn the chopper into ON state and an OFF pulse to turn the chopper into OFF state for adjusting ON time and OFF time of the chopper in response to the current through the d.c.

Abstract: In a system which controls the speed of a DC motor and in which the motor is driven by connecting the motor field winding and a series rectifier across the motor armature, a braking force is applied by connecting the armature across a braking resistor and the series rectifier. Regenerative braking is accomplished by connecting the armature across the line when the line voltage is low, and non-regenerative braking is accomplished through a series resistor when the line voltage is high. Suitable diodes or control rectifiers connect the armature across the line or the braking resistor.

Abstract: Apparatus for increasing the available regenerative braking energy and for enabling low current control without frequency variation in a time-ratio controlled main thyristor power circuit for a direct current traction motor. A variable impedance element is serially inserted in a motor armature free-wheeling current path between a free-wheeling diode and the motor armature, the variable impedance element preferably comprising a power resistor and parallel connected thyristor. During electrical regenerative braking the thyristor is used to modulate the power resistor to thereby present a continuously variable impedance in the armature current path. During motoring at very low speeds the thyristor can be operated to modulate the power resistor and thus reduce the minimum controllable level of motor current normally limited to commutation current level without the necessity of reducing the main thyristor operating frequency.