in fig1 there is shown a direct current motor control apparatus for determining the average current supplied from a direct current source 10 to a direct current motor load 12 and including a series main thyristor device 14 that repetitively operates in an on condition and an off condition to chop and regulate the voltage applied to the motor load 12 . the control of this voltage is determined by changing the on operation time in relation to the off operation time of the conduction by the main thyristor 14 for controlling the speed of the motor 12 . the speed of a dc series - field traction motor is proportional to the armature voltage and inversely proportional to the field current or field flux in accordance with the relationship that the speed s is proportional to the armature voltage e divided by the field flux φ . to reduce the speed of the motor 12 the armature voltage is decreased and to increase the motor speed the armature voltage is increased . the power supply for a typical traction motor application can be a relatively constant direct current voltage source such as 900 volts from a central power station . as is well known to persons skilled in this art , the main thyristor switch 14 is gated by a conventional conduction control apparatus 16 , in response to the p signal effort request on input 17 , to close and become conductive and is subsequently commutated to open and become not conductive as required to provide a desired average voltage across the motor armature 12 which determines the motor speed . each time the main thyristor switch 14 conducts , the voltage across the motor armature 12 and the motor reactor 18 rises to the voltage e b of the power supply 10 as shown in the motor voltage curve a of fig2 . each time the main thyristor 14 blocks , the voltage across the motor armature 12 and the motor reactor 18 falls to zero as shown by the curve a . the motor 12 responds to the average motor voltage 20 . if the main thyristor switch 14 remains conductive for a longer time period , this raises the average motor voltage and if the main thyristor switch 14 remains blocked for a longer time period , this lowers the average motor voltage . the motor current increases as shown by curve b of fig2 each time the thyristor switch 14 becomes conductive and then decreases each time the main thyristor switch 14 blocks with the inductive effect of the motor armature 12 and the motor reactor choke 18 causing the current wave shape shown in curve b . the average motor current 22 is proportional to the torque of the motor 12 . the freewheeling diode 24 allows the motor current to continue due to the inductance of the circuit after the main thyristor switch 14 blocks and becomes non - conductive to result in the motor current having the wave shape of curve b . a power thyristor switch 26 is made conductive when it is desired for the motor circuit to operate in the power or acceleration mode for the propulsion of the vehicle coupled with the motor armature 12 . a power diode 28 and a motor field winding 30 are connected in the power mode circuit including the power thyristor switch 26 , the return conductor 32 , the voltage source 10 and the main thyristor switch 14 . a brake thyristor switch 34 is made conductive when it is desired for the motor circuit to become operative in the brake or deceleration mode . when dynamic braking operation is desired one or more of the braking resistors 36 , 38 and 40 are included in the brake circuit including the brake diode 42 . a well - known commutation circuit 46 is operative to terminate the conduction of the power thyristor switch 26 when it is desired for the motor circuit to operate in the brake mode with the brake current flowing through the motor armature 12 . as shown in fig3 the power thyristor switch 26 is made conductive when the motor circuit is desired to operate in the power mode for the propulsion of the vehicle coupled with the motor armature 12 . this causes current to flow through the power diode 28 , the motor field 30 and the motor reactor 18 from the main thyristor 14 to theground return conductor 32 connected with the voltage source 10 . the conduction control of the main thyristor 14 can now modulate and determine the average current flow in the circuit including motor armature 12 . when the main thyristor switch 14 is not conducting the energy stored in the motor reactor and the field winding 30 maintains the current flow in the motor armature 12 through the freewheeling diode 24 . as shown in fig4 the brake thyristor switch 34 is made conductive when the motor circuit is desired to operate in the brake mode for dynamic or regenerative braking of the vehicle coupled with the motor armature 12 , with the main thyristor 14 conducting the brake mode current flows from the generating motor armature 12 through any of the brake resistors 36 , 38 and 40 that are not short circuited to determine the desired level of dynamic braking effort as well known to persons skilled in this art , and through the brake diode 42 , the main thyristor 14 , the motor reactor 18 and the motor field 30 through the brake thyristor 34 and up through the motor armature 12 . this provides current through the motor field 30 in the brake mode which remains substantially the same as determined by the conductivity control of the main thyristor 14 as the field current in the power mode of operation of the motor control apparatus for the motor 12 , with the motor acting as a generator in the brake mode and producing an output voltage of opposite polarity in relation to the voltage source 10 . since the field current remains substantially the same in the power mode and in the brake mode this maintains the same back emf voltage polarity for the generating motor 12 . the current flow direction through the motor armature 12 changes in the brake mode as compared to the power mode . in the brake mode with the main thyristor 14 not conducting , the current will flow through a circuit including the voltage source 10 , the freewheeling diode 24 , the motor reactor 18 , the motor field 30 , the brake thyristor 34 , the motor armature 12 and any of the resistors 36 , 38 and 40 that are not short circuited and through the brake diode 42 to regenerate power into the voltage source 10 . it is within the skill of the art to provide a solid - state switch around each of the dynamic brake resistors 36 , 38 and 40 to include the respective brake resistors in or out of the brake current circuit as may be required when it is desired to provide full or partial dynamic braking of the vehicle . depending upon the involved current level , these switches may comprise power transistor devices for lower current levels and thyristor devices for higher current levels , as well known in this art . in reference to the motor control circuit shown in fig1 when the desired vehicle operation requires changing from the power mode with the power thyristor 26 conducting to the brake mode with the brake thyristor 34 conducting , the brake thyristor 34 can be made conducting before the power thyristor 26 is made nonconducting such that both the power thyristor 26 and the brake thyristor 34 are conducting at the same time . this establishes a current path from the voltage source 10 and through the conduction control main thyristor 14 , the motor reactor 18 , the field winding 30 , the brake thyristor 34 , the power thyristor 26 and the ground return conductor 32 back to the voltage source 10 to provide a controlled current path through the motor reactor 18 and the motor field 30 . since the vehicle was previously in the power or acceleration mode and the motor is now running at some speed , the energized field winding 30 produces a magnetic field relative to the motor armature and the motor armature 12 generates a voltage that is a function of the motor speed and this field current . thusly , during a transition period when going from the power mode to the brake mode the motor is caused to operate as a generator of back emf voltage to provide a control of the armature voltage independent of the armature current . in fig5 there is shown the well - known operational characteristic curves for a series - excited direct current motor such as typically employed for the propulsion of transit vehicles . if the generated back emf voltage of the motor 12 , which is now operating as a separately excited motor , could be increased to exceed the sum of the line voltage and the voltage drop of the power thyristor 26 , it would be feasible to have the back emf motor armature current flow in the power thyristor 26 become equal to or exceed the brake thyristor current flow in the power thyristor 26 such that the net current flow in the power thyristor 36 becomes zero or even negative and in this way self - commutates to shut off the conduction of the power thyristor 26 . over a short period of time the current component through the brake thyristor 34 cannot change as fast as the current component through the motor armature 12 because of the inductance provided by the motor reactor 18 and the field winding 30 , and when the net current flow through the power thyristor because of this is zero , the power thyristor 26 will self - commutate . in actual practice however , at reasonably low vehicle and motor speeds the generated back emf voltage of the motor armature 12 will not exceed the line voltage . the curves shown in fig5 illustrate the speed restriction in relation to the line voltage 44 . to provide the desired control of the motor for particularly low vehicle operating speeds , a force - commutation circuit 46 is provided for the power thyristor 26 . when both the brake thyristor 34 and the power thyristor 26 are conducting during the transition from power mode to the brake mode of operation , the commutation circuit 46 is operated to stop the conduction of the power thyristor 26 . the current flow through the inductance of the motor reactor 18 and the field winding 30 will now continue to flow as brake current through the motor armature 12 to provide in effect a very rapid changeover from the power mode to the brake mode of operation . because of the provision of the commutation circuit 46 , brake mode operation is available down to very low vehicle speeds of operation with assured brake current buildup at much lower speeds and lower motor circuit currents than could be provided with prior art control apparatus . in addition , the main thyristor 14 can determine the magnitude of this brake current before the power thyristor 26 is commutated to establish at least the initial brake effort provided by the motor 12 . in addition , the motor control circuit of fig1 provides much faster power mode to brake mode transfer than could be obtained with prior art transit control apparatus that require mechanical brake power to changeover switches which had to be physically positioned and coordinated to determine the motor control apparatus operation . the prior art control apparatus such as shown in fig6 when controlling an automated transit vehicle running with tight speed regulation required mechanical switches , as described in greater detail in the above - referenced published article , was continually changing back and forth between the power mode and the brake mode in an effort to provide a narrow tolerance in relation to the signal block desired command speed for the vehicle . in addition , the manufacturing cost of these mechanical switches is increasing as a function of time such that it is lower in cost at the present time to provide and maintain the desired power mode to brake mode and vice versa changeover operation in accordance with the solid - state switching apparatus of the present invention . the motor control circuit as shown in fig1 in addition provides a coast mode of operation that was not available with the prior art transit motor control apparatus , where neither one of the power mode and the brake mode of motor circuit operation is provided . the prior art transit motor control apparatus required operation in the power mode to build up enough motor armature current to permit the brake mode of operation to occur with an adequate brake effort buildup and with enough residual field flux to sustain the armature current and field flux in the motor . with the transit motor control apparatus of fig1 a coast mode is practical with both the power thyristor 26 and the brake thyristor 34 not conducting . when it is desired to operate the motor control circuit in the brake mode without previous operation in the power mode , both of the power thyristor 26 and the brake thyristor 34 can be fired to become conducting and then the main thyristor 14 is used to regulate the current flow to a desired level through the field winding 30 and through those two thyristors 26 and 34 as previously described . when the brake mode of operation is desired , the commutation circuit 46 can terminate the conduction of the power thyristor 26 and the brake mode of operation begins as previously described . thusly , when in the coast mode of operation it is not required to go in the power mode before going into the brake mode of motor operation . this permits a saving of considerable energy by not having to run in the power mode before going into the brake mode , for example , when the vehicle is operating on a downhill grade where periodic brake mode of operation might be desired to control the vehicle speed as desired in relation to a provided command speed for the vehicle . when going from the power mode operation as shown in fig3 to the brake mode of operation as shown in fig4 a transition time period can be provided when the brake thyristor 34 is made conducting while the power thyristor 26 remains conducting . the brake thyristor 34 has a forward voltage drop in the order of 2 volts with the motor back emf voltage usually being greater than such a voltage drop . this results in a reverse bias for the power diode 28 so the current path is now established from the voltage source 10 through the conducting main thyristor 14 , the motor reactor 18 , the field winding 30 , the brake thyristor 34 , the power thyristor 26 and the return conductor 32 back to the voltage source 10 . when the main thyristor 14 is not conducting , the freewheeling diode 24 completes the current path from the return conductor 32 to the motor reactor 18 . the main thyristor 14 in this way regulates the current flow through the field winding 30 during this transition period with the armature current path being eliminated . in fig7 a and 7b , there is shown the operational relationships of the various voltage waveforms to illustrate the operation of the motor control apparatus of the present invention . in fig7 a , there are shown current waveforms to illustrate the power mode and brake mode operations of the apparatus shown in fig1 in accordance with the present invention . when the effort request p signal 17 provided to the conduction control apparatus 16 has a value above a predetermined level , such as 60 milliamps , the power mode of operation is initiated , with the power thyristor 26 being fired to be conductive at time 60 . the armature current of the motor 12 is shown by curve 62 , and when the power thyrister 26 becomes conductive , this armature current normally builds up as determined by the jerk limiter to a regulated desired value as determined by and corresponding to the value of the p signal . the corresponding motor field current is shown by curve 64 . at time 66 and when the p signal has a value below a predetermined level , such as 60 milliamps , the brake mode is initiated , and the p signal causes the current to drop in accordance with the jerk limiter operation and when a reasonable level of field current is reached after maybe a one second time interval at time 68 the brake thyristor 34 is fired to be conductive . the armature current goes to zero as shown by curve 62 when the brake thyristor 34 becomes conductive and the field current is regulated at a suitable value by the chopper main thyristor 14 as shown by curve 64 for a predetermined time period as may be desired , for example 25 milliseconds , between time 68 and time 70 . when the field current is stabilized and when desired for the brake mode of operation , at time 70 the power thyristor 26 is commutated to be not conductive by the commutation circuit 46 and the armature current now reverses as shown by curve 62 . the field current shown by curve 64 and the armature current shown by curve 62 can now be increased , after a predetermined time period such as 25 milliseconds to settle the armature current , by the chopper main thyristor 14 and in accordance with the jerk limiter to provide the desired amount of brake effort . the time duration from time 68 to time 70 and the time duration from 70 to 72 can be empirically established for the particular motor circuit of a given vehicle , with the above value of 25 milliseconds being for purpose of example . at time 74 when brake effort is no longer desired , if the chopper main thyristor 14 stops conducting the respective armature and field currents will fall to zero as shown by the curves 62 and 64 . if desired a return to power mode operation as previously described can then be made . in fig7 b , there are shown current waveforms to illustrate the coast mode operation of the apparatus shown in fig1 . assuming the vehicle propelled by the motor 12 is moving along a roadway track at time 80 after a previous power mode of operation and with the chopper main thyristor 14 not conducting , the armature current as shown by curve 82 and the field current as shown by curve 84 are both zero . to initiate the brake mode of operation at time 86 , both the power thyristor 26 and the brake thyristor 34 are fired to be conductive and the field current builds up to a value as regulated by the main thyristor 14 . when the field current has stabilized , at time 88 the power thyristor 26 is commutated and the armature current as shown by curve 82 goes to a controlled reverse value corresponding with the field current shown by curve 84 and as determined by the chopper main thyristor 14 controlling these current values in the brake mode of operation . when the brake mode is no longer desired by the p signal , the main thyristor 14 stops conducting and the armature current goes to zero as does the field current . it is readily apparent and known by persons skilled in this art how to provide the required firing and commutation control signals from the conduction control apparatus 46 to effect each of the power mode , brake mode and coast mode operations as illustrated in fig7 a and 7b . it was the practice with the prior art power mode and brake mode motor control apparatus , such as shown in fig6 and described in the above - referenced published article , for the mechanical change - over switch operation between these modes of operation to require from a one second to a three second time delay . for a vehicle traveling at 80 mph along a roadway track , this time delay would result in a considerable vehicle travel distance before the brake operation was started after the request was made for the brake mode . the present motor control apparatus shown in fig1 in one operable embodiment provided a power mode to brake mode change - over in a time interval of about 50 milliseconds or less , which would give a faster speed of response to a change - over request for a vehicle propelled by motor 12 and would increase the vehicle throughput along a given track since the required safety clearances are reduced . it is within the teachings of the present invention to utilize power transistor switch devices for one or more of the main thyristor 14 , the power thyristor 26 and the brake thyristor 34 switch devices , depending upon the involved current and voltage levels being low enough for available power transistor devices . for example , depending upon the horsepower requirements for the motor 12 , a power transistor might be suitable for one or more of these switch devices . this could make unnecessary the provision of the commutation circuit 46 in the apparatus of fig1 if a power transistor were substituted for the power thyristor 26 as well known to persons skilled in this art .