CONTROL SYSTEM FOR A D.C. MOTOR

Abstract: A d.c. motor control system, intended primarily for use in battery powered vehicles, includes a control switch (RLa, RLb) for varying the connections of the motor field winding (24) to determine the mode of operation of the motor and speed interlock means for preventing said control switch being operated to change the motor mode whilst the motor is actually running. Instead of using a mechanical speed transducer the speed interlock means includes means for supplying current to the motor field winding (24) when current would not otherwise be flowing therethrough and means (BR1, T<u1>u, P<u16>u etc.) connected to detect the voltage across the armature which occurs in these circumstances if the motor is in motion.

Description: Technical field This invention relates to a control system Tor a d. c . motor . Background Art Control systems nave previously been proposed for electric vehicle motors in which there are interlocks "bet-ween various driver operable controls and control devices in the control system to prevent damage to the system. For example , one such interlock may be provided to prevent a control device in the form of a contactor controlling' the connection of the motor for reverse ajad forvard motoring from being operated except when the vehicle is at rest . Such, an interlock, which vill be re erred to hereina ter as " a speed interlock" , necessitates the provision of means determining whether or not the vehicle is in motion. In the previous proposals the control system has in¬ cluded a speed transducer which is used both for providing logic signals for use in the speed interlock functions and for providing analog signals for the control system. T e speed transducer was a mechanical device driven by the • traction motor and producing a puls e train, the frequency of which was proportional to speed. Such a transducer added considerably to the cost and complication of the con¬ trol system and, being a mechanical device , required main¬ tenance . It is an object of the invention to provide a control system for an electric vehicle traction motor incorporating at leas t one spe ed interlock function but no me chanical speed transducer . Dis cloaurs of invention In accordance with the invention there is provided a control system for d. c sαtor comprising control means for varying the connections of the motor armature winding and/ field winding so as to enable the motor to operate in a pi rality of different modes, and speed interlock means for preventing operation of said control means to change the motor connections from at least one mode to at least one other mode whilst the motor is running characterised in th said interlock means is sensitive to the voltage across th armature winding of the motor and means are provided for supplying a field current to the field winding when said interlock means is required to be operative. Thus, when a change over from forward drive to revers- drive is demanded, for example, the speed interlock means will be required to be operative at a time when the field and armature currents would otherwise be zero. " When it is required for the speed interlock means to be operative a current pulse is applied to the field winding. If the moti is at rest the voltage across the armature winding will be zero. If, on the other hand, the motor is running, a voltai signal will be generated by the motor which signal is detβc ted and used to prevent the change-over. Alternatively it can be arranged for the field curreni to have a predetermined minimum level below which it is never allowed to fall. Brief Description of Drawings An example of the invention as applied to an electric¬ al vehicle d.c. traction motor control system is shown in the accompanying drawings in which, ' Figure 1 is a block diagram of the armature and field winding current controls, Figure 2 is a block diagram of a logic circuit associated with the circuit of Figure 1, Figure 3 is an electrical circuit diagram of the armature current control portion of Figure 1, Figure k is an electrical circuit diagram of an armature chopper circuit and current signal generator circuit forming pa-rt of Figure 1, Figure 5 s an electricaJ circuit diagram of a field current conrrol circuit portion of Figure 1, Figure 6 is an electrical circuit diagram of_ a field chopper circuit fprming part of Figure 1, and Figures z. f 7TD and c_ together make up the electrical circuit diagram of the logic circuit of Figure 2. Best Mode of Carrying Out The Invention Referring firstly to Figure 1, the armature current control makes use of an armature chopper circuit 10 which is shown in detail in Figure k . Three thyristor drive circuits 11, 12, 13 control the chopper circuit 10, these drive circuits deriving their input rom 1 a Schmitt bistable circuit 1^. This Schmitt bistable circuit 1 receives its input from a difference amplifier 15 which receives at one input terminal a signal representing the instantaneous armature current demand and at its other input terminal a current feedback signal from a current signal generating circuit l6. ' The current demand signal is generated by selecting the larger of two voltages on the sliders of two potentiometers 17, 18 operated respectively by accelerator and brake pedals controlled by the driver of the vehicle in which the system is installed. Such selection is carried out by a comparator circuit 19 • The. potentiometers 17, 18 are connected between respective output terminals of a demand shaping circuit 20 intended to reduce the maximum possible demand . signals with increasing vehicle speed. In fact such shaping is carried out without the use of a mechanical speed transducer, signals already present in the system being used instead. This arrangement is described in detail in copending application number 51066/77 Q £ even date to which reference may be had for a full description of the circuit 20. A demand detector circuit 21 is also connected to the sliders of the potentiometers 17, 18 and provides a control signal to the field current control as well as an input to the logic circuit of Figure 2. The field current control includes a field chopper circuit 22 which supplies current via a forcing and reverε ing relay network 23 to the motor field winding 2k , The field chopper circuit 22 is connected to a Schmitt bistabl circuit 25 "via an βpto-isolator 26. The Schmitt bistable circuit receives a linear aignal from a field current com- paratβr 27 which receives one input from a field current demand signal generator 28 and another from, a field curren signal generator l6ει. The field current demand signal generator 28 has inputs from the output of the comparator from the demand detector 21, from various points in the logic circuit of Figure 2 and also from a field weakening difference amplifier 9. This latter amplifier 2$ receive inputs from a start up circuit 2 > 0 and from a further amplifier A5 which is operative during braking to compare the output of amplifier 15 with a fixed reference value. Before turning to the detailed circuit diagrams the block logic diagram of Figure 2 will be briefly explained. Basically the function of the logic circuit is tβ control relays RL1 axd RL2 in the field current control and a contactor RL3 in the armature current control, and also tα control the sequence of events when the system is switched on and off, so that a properly controlled power up and pow eff sequence occurs. An ignitiβn switch circuit kO contra the supply of power to a power relay k-3. which controls the supply of current to regulators 2 for supplying regulated voltages to the electronic control and logic circuits and also to a main isolator & (shown in- Figure h ) . There are also various interlocks controlling the energising and de- energising of the power relay k-1. such interlocks ensuring that the system cannot be brought into operation when the vehicle battery is on charge , when the 12V auxiliary battery is discharged, when the vehicle is moving or when there is sin armature current demand signal present. - - A block 43 in Figure 2 represents a logic circuit (controlling the relay RLl) which is effective during change over between the various motor operating modes i.e. forward motoring, reverse motoring and braking, and serves tβ ensure that the field current is rapidly reduced to zero, before the newly selected mode is brought into opera¬ tion. A logic circuit 44 controls the relay KL2 and the contactor RL3 and also provides inputs to phase check gates • k-5 which are responsible for bringing the field forcing logic into operation. These gates detect the condition vhich occurs when the driver has selected a new mode of operation but the various conditions required for change-over have not yet been met. For providing a speed logic input to the interlock circuit' 3a and the logic circuit 44 there is an armature voltage detector 46 (shewn in Figure 7£.) vhich is used instead of a speed transducer. Finally Figure 2 shows a demand clamping circuit 7 which receives inputs from the field force logic 43, the logic circuit 44 and from "proving" contacts on the contac¬ tor RL3» to control demand clamping signals applied to the comparator circuit 19» and the field current demand signal generator 28, during changes in mode of operation of prevent demand signals being generated at these times. Turning now to Figure 3, the armature, current control circuit includes the demand shaping circuit 20 which applies a maximum voltage appropriate to the existing vehicle speed to the non-earthy ends of the potentiometers 17 and 18. The comparator circuit 19 includes an operational amplifier Al (vhich is a current differencing type operational amplifier such as ■ £ • National semiconductors type LM 3 00) • The sliders of the two potentiometers 17, 18 are connected by respective resistors R , R^ to the inverting and non- inverting input terminals ' of ampli ier A... Hysteresis is provided by a feedback resistor R_ to the non-inverting input terminal. The output terminal of amplifier A. is connected to a teππ±nal marked MTR/BK which is connected to the logic circuit 44 and is at high voltage when the voltag on the slider of the brake potentiometer is higher than tha on the slider of the accelerator potentiometer. The demand detector 21 of Figure 1 is represented by a npn transistor N. with its base connected to the common point of two resistors R and R- connected in series betwee the sliders of the potentiometers 17 and 18. The emitter of the transistor N_ is connected to the earth rail of the supply and it is biased so as to be just non-conductive whe: there is a zero voltage at the sliders of both potentiomete: 17, 18. The bias circuit for the transistor N., consists of a pair of resistors R^, R_ in series between the base-, of the transistor N_ and the +8v supply rail, a resistor E„ connecting the base of the transistor N_ to the earth rail and a diode D. with its anode connected to the common point of resistors R,-, R,^ and its cathode connected to the earth rail. A capacitor C. is connected between the base of the transistor N. and the earth rail. A very small voltage on the slider of either potentiometer 17 or 18 will suffice to turn on transistor N which has its collector connected to the +8v rail by two resistors R q , R.. Q in series. The common point of these resistors R_, R_ _ is connected to the base of a pnp transistor P. which has its emitter connected te the +8v rail and its collector connected to a terminal marked D>0 which is connected to interlock circuit 4l. A diode D„ connects the collector of the transistor N_ to a terminal marked b_ (see Figure $ ) . The motor/brake comparator circuit 19 also includes an arrangement whereby only the larger of the two voltages at the sliders of the two potentiometers 17, 18 is passed on to the amplifier 15. This arrangement includes a pair of npn transistors N^, N_ with their bases connected by respective resistors R , R.. „ to the sliders of the poten¬ tiometers 18 and 17 respectively. The emitters of these transistors N_, N are connected together and a common resistor R_ _ connects them to earth. A pair of capacitors C_, C_ connect the bases of the transistors ϊl„ , N to the earth rail and the collectors of these transistors are connected together and via a common resistor R_ to the cathode of a diode D , the anode of which is connected to the +8 rail. It will be appreciated that only the transistor N_ or N_ which has its base at the higher voltage will conduct at any given time and that transistor will then act as an emitter follower, so that the voltage on the resistor R_ „ will be just one V below the voltage at the slider of the appropriate potentiometer. The resistors R _ and R_ ? and the capacitors C 2 , C„ act to limit the rate of change of the output voltage of the circuit, each R-C circuit having a time constant of some "JOuiS . These also act as noise filters . Two terminals M and B of the demand clamp circuit 7 (see also Figure 7 " b) are connected directly to the bases of the transistors N„ and N so that when the signals at these terminals are both low, the transistors are turned off. There are two output terminals c_ and e ^ shown in Figure 3» Terminal c is connected to the emitters of the transistors N p and N_ and also to the field current control (Figure 5) * Terminal _ is connected to the collector of a pnp transistor P« which has its base connected to the collectors of the transistors N ≥ and N„, its collector connected to the +8v rail by a resistor R.. r- and its emitter connected to the same rail by a resistor ^--, 7 ' Terminal _e_ provides an input to the field current demand signal generator 28 (see also Figure 5) anc * acts as a current source providing a current with a minimum level determined by R and increasing lin¬ early with the voltage at the emitter of the transistors N 2 and N . The amplifier 15 which is another integrated .circuit current differencing operational amplifier (eg -J- LM 390°) is connected to operate as a linear difference amplifier. To this end the non-inverting input terminal of the amplif 15 is connected by a resistor R_ g to the output of an R.C. filter circuit R- < QJ C. across the resistor R η _ , and is als. connected by a resistor R,,-. and a variable resistor R ? in to the +8v rail (to provide bias current). The inverti: input terminal of amplifier 1 is connected by a bias resistor R„ 2 to the +8v rail and by a resistor R„_ to a terminal marked I . from the current signal generator SZ g circuit l6 (see also Figure 4). The output terminal of th' amplifier 15 is connected by a capacitor C to the earth rail and also to the anode of a diode D.. The cathode of the diode D, is connected by a feedback resistor R_. to th< inverting input terminal of the amplifier 15. The amplifier 15 produces an output signal which is linearly related to the error between the demanded armaturt current and the actual armature current as measured by circuit 16. This error signal is applied via a resistor R 2 to thi inverting input terminal of an operational amplifier A_ on which the Schmitt bistable circuit l4 is based. The non- inverting input terminal of the amplifier A„ is connected • the +8v rail by a bias resistor „, and normal feedback is provided via a resistor R„_ between the output terminal of the amplifier A_ and its non-inverting input terminal to provide hysteresis (ie to set the switching threshold voltages above and below that determined by the biasing resistor R„g). The Schmitt bistable circuit has outputs to the three thyristor drive circuits 11, 12 and 13. The output to the thyristor drive circuit 11 is taken from the common point of the two resistors R π and Q in series bβtween the earth rail and one side αf a capacitor C >- . The other side of capacitor C^ is connected to the collector of a pnp transistor P vhich has its emitter connected to the +8v rail, its collector connected by a load resistor R_ Q to the earth rail and its base connected to the common point of two resistors R__ , R~ 2 connected in series between the +8v rail and the output terminal of the amplifier A . The transistor P turns on when the output of the amplifier A 2 is low so that there is a positive going output pulse delivered to drive circuit 11 when the output of amplifier A2 goes low. This occurs when the actual armature current falls below the demand current by more than the ' margin established by the hysteresis of the Schmitt bistable. The input to the drive circuit 13 is taken from the common point of two resistors R,,~ Ξnd R„ (ι connected in series between the earth rail and one side of a canacitor C„. The ( other side of the capacitor C is connected to the collector of a pnp transistor P> , the emitter of which is connected to the +8v rail and the collector of which is connected by a load resistor R to the earth rail. The base of the transistor P. is connected to the common point of two resistors tween the +8v rail and the collector of an npn transistor ^, the emitter of which is connected to the ground rail. The base of the transistor N. is connected to the common point of two resistors R 0 , _ 0 connected in series between the earth . ° y rail and one side of a capacitor C„, the other side of whicα is connected to the output terminal of the amplifier ~ . Said one side of the capacitor C„ is also connected to the cathode of a diode D_, The anode of which is connected to _> the earth rail. The transistors . ' and P, turn on for a time dependent on the time constant of the cauacitor C~ with the resistor 8 R o , R (typically about 0.7ms) when the output of the amplifier A 2 goes high. At the same time a short duration positive going pulse is passed to the drive circuit 13. The capacitor Cg and resistors R g, R_ provide part of a ττHτHτπ-nm off-time circuit which is associated with the Schmitt bistable circuit l4. The remainder of this minimun off-time circuit is provided by a resistor Rr n and a diode I. J- in series between the collector of the transistor P. and the non-inverting input terminal of the amplifier 2 . These components ensure that, however the input to the Schmitt bistable circuit behaves immediately following the output of amplifier - going high, the output of amplifier A will not go low agai for a preset time, i.e. until transistors P^ and N switch off when capacitor C„is charged up, because of the additional heavy position feed¬ back to the amplifier 2# The input to the drive circuit 12 is derived from the signal at the collector of the transistor P^ via a mono- stable delay circuit based on an operational amplifier A_. The collector of the transistor P, is connected via a capacitor C Q and a resistor R^, in series to the inverting input terminal of the amplifier A_. The non-inverting inp terminal of this amplifier is connected by a bias resistor R. 2 to the +8v rail and by a positive feedback resistor R^, to the output terminal of amplifier A-. A diode D_ has its anode connected to the output terminal of the amplifier A_ and its cathode connected by a resistor R, . to the inverting input terminal of the amplifier A_ to provide negative feedback when the output of amplifier A is high, a capacitor C η _ connecting the cathode of the diode D to the earth rail. When transistor P^ turns on the output of the ampli ie A goes low until capacitor C has discharged through the resistor R. , . The output of amplifier A_ then goes high again the capacitor C having meanwhile become fully charge and remains high until transistor P,, turns on again. The BU KE A output terminal of amplifier is connected to the earth rail via a capacitor C _ and two resistors and the input to the drive circuit 12 is taden from the common point of these resistors. The armature chopper circuit shown in Figure 4 includes a mai thyristor THI, a coπnπutating thyristor TH2 and a "ring-round" thyristor TH3, which are connected to be triggered by the dr±ve circuits 11, 12 and 13 respectively. The main thyristor TΞl has its cathode connected via a main fuse 50 to an earth conductor and its anode is connected to one end of the armature winding 51 of the motor. The other end of the armature winding $1 is connected via a contact RI 33- of the contactor RI 3 to a high voltage positive supply conductor, the positive and negative supply conductors being connected by the isolator contacts 48 to the terminals of a high voltage (e.g. 200-300 volts) battery. A power diode Dg has its cathode connected to said other end of the armature winding 51 and its anode connected by an auxiliary fuse 2 to the earth conductor. This diode is operative during braking, when the contact Rl 3 a i s open. A further power diode D q has its anode connected to the anode of the thyristor THI and its cathode connected to the supply rail. This further diode is operative to conduct decaying armature current each time the main thyristor THI is turned off. A resistor R_ n and a capacitor C- 2 are connected in series between the anode and cathode ef the thyristor THI. The commutating thyristor TH2 has its anode connected to the anode of the main thyristor THI and its cathode connected by a fuse 53 and an inductor 4 to one side of a commutating capacitor C „, the other side of vhich is connected to the earth conductor. Said one side of the canacitor C,_ is also connected via a resistor R_„ and a 1 _sl diode D _ in series to the supply rail. The "ring-round" thyristor TH3 has its cathode connected to the earth con- ductor and its anode connected via an inductor _05 to said one side of the capacitor C _. Vhen thyristor THI is fired current flows through the armature " 1 and the relay contact L3 (assuming this to b« closed). Vhen the armature current reaches a sufficiently high level for the output of the amplifier A 2 (Figure ) tc be driven high, the thyristor TH3 is fired immediately and the thyristor TΞ2 is fired after the delay mentioned above. The " capacitor C_- is positively ' charged at this time, charg having been maintained if the thyristor TΞl has been conducting for a long period by current trickling into the capacitor C _ via the resistor R .. Vhen thyristor TH3 fires, the capacitor C _ commences discharging through the inductor 55 , peak current being reached as the capacitβr C. becomes completely discharged. Current continues to flew in the inductor 55 > however, charging capacitor C_ _ to a peak reverse voltage at which thyristor TE3 ceases to con¬ duct. The delay set by the delay circuit constituted by the monostable circuit A is longer than the time taken by this "ring-round" βperation. Vhen thyristor TH2 is fired, however, the armature current is diverted in the now rever¬ sed charged capacitor C_„, allowing thyristor THI to turn off. This diverted armature current continues to flow unti the capacitor C_ „ again becomes fully charged in the origin sense whereupon thyristor TH2 turns off and the continuing armature current (new decaying) flows through the diode D Q . Vhen the armature current has fallen low enough to cause the output of amplifier A 2 to go low again thyristor THI is fired. In this way the armature current is kept between predetermined limits relative to the demanded armature current. Figure 4 also shows the current signal generator 16 of Figure 1, which includes an operational amplifier A, with its inverting and non-inverting input terminals connected by resistors i_ ~ nd R ;_? 2 ? aand R_ „ 3_j„ to the output terminals of a Hall plate device 56 energised by two resistors R.^, R_^ connecting it to the +8v and earth rails respectively. A resistor R ς g connects the non-inverting input terminal of amplifier A. to the earth rail and an npn transistor N is connected as an emitter follower to the output terminal of the amplifier A.. A resistor R and a diode D in series connect the collector of the transistor N_ t* the +8v rail and the emitter of this transistor is connected by two resistors R g and R - in series to the inverting input terminal of the amplifier i. so that this operates as a difference amplifier. The emitter of the transistor N- is connected by a temperature compensation network to the ground- rail, such network consisting of two resistors Rg 0 , Rg, in series and a thermistor RT. connected across the resistor g n « The output to the difference amplifier 15 is taken from the junction of the resistors Rg Q » Rg-i • To provide a logic output when there is no difference input to the amplifier ^ (i.e. when the armature current is zero), a -DUΌ transistor P_ has its base connected to the collector of the transistor N_,its emitter connected to the +8v rail and its collector connec¬ ted to the motor/brake logic circuit 44 (see Figure 2 and Figure 7b) . Turning now to Figure * the field current control circuit includes the start up circuit 30 based on an npn transistor __!>_■ which has its emitter connected to the ground rail and its base connected to the common point of two resistors Rg^, "'h. across a capacitor C. - connected at one side to the ground rail and at the other side by a resistor ^ _ to the output terminal of the amplifier A (Figure 3). A capacitor C. ,- connects the collector of the transistor -. r to the ground rail such collector being also connected by a resistor R D,-O,- to the outnut terminal of the difference a nlifier 15 (see Figure 3)« The transistor ^ is D normally on, capacitor C . _ charging up whenever the output of amplifier A is high and discharging only slowly vhen the. BUR A iT - 1k- output of amplifier A„ ±s lov (i.e. vhen thyristor THI is on) » Should the output of amplifier A_ remain lov for an extended period (indicating- that the demanded current cannot be achieved) transistor N,- turns off, permitting it collector to follov the signal at the output of the difference amplifier 15• The collector of the transistor Ng is connected to th anode of a diode E,-,j the cathode of vhich is connected by a resistor R_ 0 to the inverting input terminal of the difference amplifier 9 (Figure l). The non-inverting input terminal of amplifier 2 is connected by a bias resistor R__ to the +8v rail and a feedback resistor Re ¬ connects the output terminal of the amplifier 2 to its inverting input terminal. The ripple rejection circuit shorn in Figure 1 is constituted by a resistor -. ^ .- and a caυacitor C,,„ in series betveen the cathode of the diode " . 11/ D _ and the non-inverting input terminal of the amplifier . Current can flov into the inverting input terminal of the amplifier 29 via the diode D__ vhenever the signal at the output terminal of the amplifier 15 is high and the transistor ^ has turned off. These conditions occur only during forvard motoring. Alternatively current can flov into the inverting input terminal of the amplifier 29 via a diode E... from the output terminal of an amplifier A_ connected- as an inverting amplifier producing an output dependent on the difference betveen the signal at the of the amplifier 15 and a reference value (set by a resistor R connecting the non-inverting input of the amplifier A., to the +Sv rail) . The a olifier A_ has its inverting terminal connected bv a resistor R I.„ f 4, to the terminal A and by a resis tor R„ „ _ to its out-out terminal . ■i- i This current flov- o ccurs only during braking . In e ither ^ R EA event, vhen the c-urrent loving into the inverting input terminal of amplifier 29 rises above that floving into the non-inverting input terminal, the output of the amplifier 2 vill fall linearly belov a normal high value. The output terminal of the amplifier 29 is connected by tvo resistors R__, R_.g to the +8v rail. Three further resistors -y Ryg aod R__ in series connect the junction of the resistors R~,-» ^- £_ *° "kke ground rail. The junction of the resistors ^ t ^rp. ^- s connected to the cathode of a diode E__, the anode of vhich is connected to the output terminal of an operational amplifier Ag. The inverting input of this amplifier A,- is connected to its output terminal by a resistor ao and to the +8v rail by a resistor .. The non-inverting input of the amplifier Ag is connected by a resistor R R2 » ^° *^- e emitter of the transistor N_, N„ (Figure ) * Amplifier Ag acts as a non-inverting amplifier of the voltage on the resistor R (Figure 3) to boost field current at high armature current demand levels. The junction of the resistors R„ ( oo» ∑ Q y is connected bv a resistor R Q _ to the base of an nun transistor N_ the emitter of vhich is connected to the eart rail viά. a resistor and the collector of vhich is connected to the collector of the transistor P (Figure 3) vhich is on vhenever a demand for motoring or braking is present. The transistor N can be turned off either by the signal t a terminal F (see Figure 7b) going lcrv or an npn transistor N„ being turned on. Transistor „ has its emitter connected to the ground rail, its collector connec¬ ted to the base of the transistor X_ (and to terminal F) and its base connected to the junction of tvo resistors ts 0 - 8;-? and Ro(- vhich are in series betveen a terminal F' (Figure 7JD) and the ground rail. Terminal F 1 is also connected to the anode of the diode D (Figure 3) so that transistor N o can turn on only when the signal at the terminal F' is high and transistor N (Figure ) is off (indicating that neither brake nor accelerator pedal is depressed). The transistor N_ acts (vhen transistor P„ is on) as an emitter follower and its emitter is connected by a resistor R θP _ to the non-inverting incut terminal of the field current difference amplifier 27, vhich terminal is also connected to the +8v rail by a resistor R fi „. A resistor R Q . connects the inverting input terminal of amplifier 27 to the +8v rail and another resistor R Qn connects this terminal to the output of the field current signal generator 29 vhich is similar to the armature current signal generator l6 (shown in detail in Figure k ) _ Feedback around the amplifier 27 is provided by a resistor R Q _ and a capacitor C " in parallel vith each other between the output and inverting input terminals of the amplifier 27. The Schmitt trigger bistable circuit 25 of Figure 1, i constituted by an operational amplifier A vith a resistor R „ connecting the output term n l of amplifier 7 to the inverting input of amplifier A_« The non-inverting input o amplifier A_ is connected by a resistor R_- to the +8v rail and by two resistors R Q L » R Q - in series to the output- terminal of amplifier A_, a capacitor C.g being connected across the resistor Rq t -» The output of amplifier 27 rises and falls linearly vith the error between the demanded field current and the actual field current. ∑n steady state conditions R Q . , R QE - ^ r> _z> provide a small positive - feedback current which establishes hysteresis in the oneration of amtilifier „ so that the i output of amplifier A goes low when the output of amplifier 27 rises above one set level and goes high when the output aurolifier 27 t falls below a lower s et leve The capacitor C-o introduces additional positive feedback for a short period immediately following each change in level of the output of amplifier A_, thereby inhibiting a further change in level for this period, irrespective of how the output of amplifier 2 behaves. The output terminal of amplifier A^, is connected by a resistor _ to the base of an npn transistor Ng which has its emitter connected to the earth rail and its collector connected by a resistor R q _ to the +8v rail. A further npn transistor N q has its base connected to the collector of the transistor Nr > , its emitter connected to the earth rail and its collector connected by a resistor R Qo and the light-emitting diode of the opto-isolator 26. , Turning now to Figure 6 it will be seen that the photo-transistor of the opto-isolator 26 has its base connected by a resistor R.. OQ to the earth rail and its emitter connected directly to the same rail. The collector of the photo-transistor is connected by a resistor R-, Q -. to a +12v rail connected to a tapping on the traction battery and is also connected to the base of .an npn transistor N_ _ which has its emitter connected to the negative supply rail and its collector connected by a resistor R, n2 to the +12v rail. The collector of the transistor N. _. is connected to the base of an npn transistor N_ . the emitter of which is connected to the earth rail. An npn transistor N. ? has its collector connected to the base of the transistor N. - and its emitter connected to the earth rail. The base of the transistor N_ 2 is connected to the collector of an npn transistor N. „ which has its emitter connected to the earth rail and its collector connected by a resistor R ±-, π Jj~ to the ÷12v rail. The base of the tran- sistor N is connected by a resistor R ιn ^, to the earth rail and by two resistors , R g to the anode of a zener diode ZΩ. , the cathode of which is connected to the +12v rail. The collector of the transistor N__ by a resistor R-_,- is connected to the junction of the resistors R 105 aDd R 106' The collector of the transistor N._ is connected to the cathode of a diode D 2 » ^ B anoc β 0 " vhich is connected by a resistor R_ __, to the cathode of a diode D 2 _ , the anode of which is connected to the +12v rail. The cathode of the diode D 21 is connected by a capacitor C to the earth rail The cathode of the diode D 20 is connected by two resistors R- 0 Q, K-i rj q i 21 series to the +12v rail. The anode of the diode D_ 0 is connected by two resistors R _ and R_ _ in series to the earth rail. An npn transistor N. has its base connected to the junction of the resistors R.. Q and R_ _ , and a τmτι transistor T? _- has its base connected to the J. JL " ~ o junction of the resistors R. _ and R-__. The transistors N-i and Pg have their emitters connected respectively to the negative supply rail and the +12v and their collectors interconnected by a resistor R- „. The collector of the transistor P,- is connected by a resistor R... _ and a capacitor C_. in parallel to the base of an npn transistor N the emitter of which is connected to the base of an npn transistor N.g the emitter of which i connected to the negative supply rail. The collectors of the transistors N_ and N.. - are connected together and a diode D 22 has its cathode connected to the base of the transistor N. _ and its anode connected to the base of the transistor -N-g. The collectors of the transistors N.._, N.g are connected to the normally closed contact of a change¬ over contact set R 2a of the relay RL2 and also to the normally open contact of a change-over contact set RL2b of the relay R 2. The other contacts of these two contact set are connected together and via a normally closed contact RLla of relay RL1 to a high voltage supply via a filter „. The field winding 2k is connected between the common terminals of the contact sets RL2a, R 2b. The collectors o transistors N. ς , N.. r are also connected by a diode D 2 to the output of the filter F , a further diode D ■ connecting the said other contacts of the contact sets R 2a, RL2b to the negative supply rail. In addition, a capacitor C 22 connects the collectors of the transistors N.„, N_g to the anode of a diode D ? _ the cathode of vhich is connected to the negative supply rail. A resistor R . r is in parallel vith the diode B 2 ς * A resistor R- j .77 in series with a capacitor C ? _ connects the cathode of diode D 2 to the cathode of diode D ? } , * a. diode D 2 g bridging the resistor 1/ / • Referring now to Figure z__ t the +12v supply is connectec to one side of an ignition switch kθ . The other side of the switch 0 is connected to the common pole of a three way direction selector switch 6θ having reverse, neutral and forward contacts. The reverse and forward contacts are connected to the anode of two diodes -D-, 0 J D_. which have their cathodes connected together and, via a resistor R- nπ to the cathode of a zener diode ZD„ the anode of which is connected to the earth rail. A t>air of resistors R, 201 and R^o ? connect the reverse contact of the selector switch 6θ to the earth rail, the junction of these resistors being connected to the base of a npn transistor N. _ which has its emitter connected to the earth rail and its collec¬ tor connected to a terminal marked g (see Figure 7^ , ) • The neutral contact is connected to the anode of a diode D„ 2 the cathode of which is connected via the power relay winding 0 to the collector of an npn transistor N „• The. relay kl has normally open contacts k±z which control the supply of power to all the control circuits. For energising the relay θ there is a further diode D _ which has its anode connected to the +12v supply rail controlled by the contacts 1^ and its cathode connected to the cathode of the diode D 2* "^ reew el diode D , is connected across the relay winding *+l. The base of the transistor N _ s connected by a resistor R- 0 „ to the earth rail and by two resistors R 2θ ' ^20^ "^ 3 er ^ e s ^° th.e cathodes of two diodes D _ and zVi c . The anode of the - diode D__ is connected to the 3° 35 neutral contact of switch 60, and the anode of the diode D_g is connected to the collector of a pnp transistor P . The emitter of the transistor P_ is connected to the +12v and its base is connected to this same rail by a resistor 20 g and by two resistors R 207 > & 2 08 ^ n ser es ^° ii β collector of an npn transistor N . A capacitor C__ connet the common point of resistors ea t rail and a capacitor C__ connects the junction of the resistors R 207 and 20o to the +12v rail. A diode D has its anode connected to the collector c the transistor P and its cathode connected by two resistoi R 2θq , R 210 i 21 series to the earth rail and by two resistors R 2 ^ _ , R 2 - ι2 in series to the +12v rail. A capacitor C_ 2 is connected between the earth rail and the common point of tl resistors R 2Qq , R 21Q which point is also connected to the cathode of a diode D 3_° 0 the anode of which is connected to the cathode of the zener diode ZD 2< Two resistors R 2To anc R_ . a 1 * 8 connected in series across the capacitor C 2 and their junction is connected to the base of the transistor N q . A pnp transistor P^ has its base connected to the common point of the two resistors ^ 211 ^212' *^ e ecι i " k ', ' er c the transistor P„ being connected to the +I2v rail and its collector being connected via a resistor R- 21 g to a terminal f (see figure 7b). A further diode D has its anode connected to the common point of the resistors ^ ?0 q» ^ 2 10 axLC ^~ ts ca'<; ^ l - 0 i e connected to the collector of an npn transistor „ and also connected to the anodes of two diodes D^_ and D.. the cathodes of which are connected to two interlock functions, one being a switch contact in a charger plug and the other being a contact which is normally held open when the " battei-y voltage is not too low for satisfactory operation. The emitter of the transistor N„ n is connected to the earth rail and its base is connected to the common point of two resistors R„ . „ f R 2 ,o i series between the collector of an npn transistor N 2 - and the earth rail. The emitter of the transistor N 2 _ is connected to the earth rail and its base is connected to the common point of two resistors „ Q and H._ 20 in series between the switch 0 and the earth rail, The collector of the transistor N 2 _. is connected by a resistor R 221 to the +5 supply rail. A resistor R Z2 and a capacitor C__ are connected in series between the base of the transistor N 2n and the +5v rail. The collector of the transistor N 20 is connected by a resistor R 22 - ι o *ke cathode of a diode D. 2 the anode of which is connected to a terminal h"(see Figure 7^ , ) • An npn transistor __ has its emitter connected to the earth rail and its collector connected to the cathode of the zener diode ZD„ . The base of the transistor N 2 _ is connected by a resistor R 22 £, to the earth rail and is also connected to the cathode of a diode D> > • The anode of the diode D.. is connected by two resistors 22ς and 22f to two terminals marked SP>0 (see Figure 7_c) and D 0 (see Figure 3). A resistor R„„ and a capacitor C_. in series connect the anode of the diode D.. to the +5 rail. A diode D. has its anode connected to the anode of the diode D.. and its cathode connected to the collector of the transistor N Q . Turning now to Figure 7b_, the terminal h is connected to the .base of a pnp transistor P q which has its emitter connected to the +5 V r il The base of the transistor P. 9 is connected by a resistor 2 Λ 0 to the +5v rail and by two resistors R ^ ^ and 2 ^ 2 in series to the output terminal of an exclusive OR gate Gl ( ■ - of a TT integrated circuit type 7^86). The junction of resistors f,-, » R 2£ιo is connected by a caDacitor C^_ to the earth rail. The anode of a diode D, r- is connected to the collector of the transistor P Q and its cathode is connected by three resistors R- 2Λ -,> ^2kk and R_ι _ in series to the rail, a capacitor C ,- being connected between the junction of resistors R_. _, . anc the earth rail. The junction of the resistors R „ L. > ^- k * . ^" 3 coπ necte to the base of an npn transistor N 2 _ which has its emitter connected to the earth rail and its collector connected to the terminal F (See Figure 5). The collector of the transistor P q is connected by two resistors A ' ^2*-7 ^ seI "i ΘS o the earth rail, the commo point of these resistors being connected to the base of a transistor N„^, which has its emitter connected to the eart rail and its collector connected by a resistor R O to the +5v rail. An npn transistor N 2 _ has its base connected to the collector of the transistor N 2^ , its emitter connected to the earth rail and its collecter connected by the windin of the relay RL1 to the +12v rail, a freewheel diode D 2 _ being connected across this winding. The collector of the transistor N, is also connected to the cathβde of a diode B.. 7 ' the anode of which is connec ted by three resistors R_. , R 2e . Q and 2 -, in series to the +5 V rail. A capacitor C__ is connected between the common point of resistors ^ 2 ,q» 2ςn and the +5"v rail and the base of a pnp transistor P, Q is connected to the common point of the resistors R 2c0 and R 2c - t • T^ e emitter of the transistor P is connected to the +5 rail and its collector is connected by two resistors 2 c 2 » R 2=;? ^" n series to the earth rail. The base of an npn transistor N 2 g is connected to the common point of the resistors R_ _ , R ? ,„ and its emitter is connected to the ea_rth rail. The collectβr of t transistor N 2 g is connected by a resistor R p - j , to the +5v rail and also by a capacitor C_ 0 and a resistor R_ _ „ in 3« 25 series to the base of the transistor P, n . The base of the transistor N 2 g is connected by two resistors R ς and R 2r7 in series to the terminal marked ϋ>0 (see Figure k ) and a capacitor 0 39 is connected between the junction of these resistors and the earth rail. The collector of the transistor P _ is connected by a resistor R 2 g Q to the cathode of a diode D O * the anode of vhich is connected to the collector of a pnp transistor P which has its emitter connected te the +5v rail. A resistor R 2 £ ι connects the base of the transistor P.. to the +5v rail and a resistor R- 2 g 2 connects the same base to the output terminal of another exclusive OR gate G2. The anode βf the diode D. « is also connected to the anode of a diode Ω^ q , the cathode «f vhich is connected by three resistors R 2 g^» R 6 and R 2 *5 in series o the earth rail* A capacitor C. _ is connected between the earth rail and the junction of the resistors R 2 g~ and ^ 2 g.. A diode D _ nas its cathode connected to the cathode of the diode D, q and its anode connected to the N terminal of the switch 60 (Figure 7^ ) * -A. further diode D-, has its cathode connected to the junction of the resistors R ^ ,, and R^^- j , and its anode connected via a resistor R- 2 gg to the anode of the diode D ji fi * The junction of the resistors R^c and R 2 g c is connected to the base of an npn transistor N 2 _ vhich has its emitter connected to the earth rail and its collector connected to the terminal M (see Figure _ > ) . __ pair of resistors & 2 g 7 and R- 2 gg are connected in series between the earth rail and the junction of the resistor R 2 g and .R 2 ι * -An npn transistor N 2 „ has its base connected to the junction of these resistors R 2 £ 7 and R 2 68' : ^ s e itter connected to the earth rail and its collector connected to the terminal BK (see Figure 3)« the cathode of a diβde D.„, the anode of which is connected to the collector of a pnp transistor P, ? . The emitter of the transistor P 2 is connected to the +5v rail and its base is connected by a resistor R - Q to the +5v and by a resistor R„„~ to the collector of a tinp transistor P ^ ( 0 - ^ 13 The emitter of the transistor P is connected to the +5v rail and its base is connected by a resistor ^ to the +5 rail and by a resistor R 2 „ 2 to the output terminal of a further e clusive OR gate G3. The collector of the -2k- transistor P._ is connected by a resistor R 27 -, to the earth rail and also by a resistor 27 to the terminal F' (see Figure 5). An npn transistor N 2q has its emitter connected to the earth rail and its collector connected by a resistor R 27ς tβ the +5v rail and also connected to one input terminal of the gate G2. The base of the transistor N 2q is connected by a resistor R 27 g to the earth rail and by two resistors R and R 27o i series to the +5v rail. The junction of tϊ resistors 277 and R 27 g s connected by a contact RI 3b, of the contactor RL3 to the earth rail and also via a resistor R 2 _ q to the anede of a diode D _ whose cathode is connected to the base of the transistor N 2 _. The collector of the transistor N_ q is connected to the anode of a diode D-^, the cathode of which is connected by a resistor R R to the base of the transistor N_„. The relay winding RL2 is connected between the +12v rail and the collector of an npn transistor N__., the emittε of which is connected to the earth rail, a diode D 2Q being connected across this winding. The base of the transistor N_ n is connected by a resistor R 2 o- ι to the earth rail and by a resistor R 2o2 to the collector of a pnp transistor P_2,» the emitter of which is connected to +5v rail. The base of the transistor P.κ is connected by a resistor R.„, to the +5v rail and by a resistor R 2o , to the Q output terminal of a bistable latch circuit L (which may be of a TTL integrated circuit type 7^75).. The DATA input of the latch ..is connected to the output terminal of another exclusive OR gate G , and its CLOCK input terminal is con¬ nected to the collector of the transistor N 2f -. The DATA input terminal of the latch L and its Q output terminal arε connected to the two input terminals of the gate Gi. BU tA The gate G has one input terminal connected by a resistor R_g to the +5v rail and also connected to the collector of an npn transistor N 1 , the emitter of which is connected te the earth rail. The base of the transistor U is connected by a resistor R„gg to the earth rail, by a resistor 2 g 7 to the terminal MTR/BK (see Figure 3) and by a resistor R 2 o to the cathode of a diode D_„, the anode of vhich is connected to the earth rail. The cathode of the diode p m is also connected by a capacitor C. _ to the cathode of a diode D, τ which cathode is connected to the ground rail by a resistor R 2 g « " ^ ca P ac it°r C^ 2 connects the cathode of the diode D_^ to the cathode of a diode D__ ° 51 which is also connected to the earth rail by two resistors R_ qo , and R 2Q -, in series, the anode of the diode D _ being connected to the earth rail. An npn transistor N_ 2 has its base connected to the junction of the resistors R 2qf ., R 2q _ and also to the resistor R 2l ( see Fi ure 7a.) via terminal f. The emitter of the transistor N _ is connected to the ground rail and its collector is connected to the cathodes of two diodes D_ 0 and _„,, the anode of the diode D_ 0 being connected to the terminal F' and the anode DO of the diode D_ q being connected via two resistors R^a * 2q ^ in series to the earth rail. The anode of the diode D is also connected by a resistor R 2 9 *° tlιe ajαode ° the diode Ω m r - , vhich is also connected to the collector of a pnp transistor P. _. The emitter of the transistor P-_ is connected to the + rail, and its base is connected to this rail by a resistor R 2q _. A resistor R 2 q connects the base of the transistor P.. _ te the Q. output terminal of a bistable latch circuit L 2 which has its CLOCK input connected to the collector of the transistor _, and its DATA input connected to the collector of the transistor N-... The Q, output termin¬ al of the latch 2 is also connected to an input terminal of the gate G2, via terminal m. The other input terminal of the gate G is connected to the Q output terminal of a third bistable latch circuit which has its DATA input terminal connected by a resistor R 97 to the +5v rail and also connected to the collector of thβ transistor N (see Figure a . The CLOCK input tβrmi al of the latch L_ is connected by a resistor ~ 0 to the +5"V rail and is also connected to a terminal n_ (see Figure 7£.). The junction of the resistors ∑ Q? and R 2 T is cormβc ed to the base of an npn transistor N„_ vhich has its emitter connected to the earth rail and its collector con¬ nected by a relay vinding RlΛ to the +12v rail, the relay RlA having a normally open contact controlling the contact RI 3. The relay vinding RlΛ is bridged by a free-wheel diod D ?Q and a capacitor G _k bridg ng i* s normally open contact Turning finally to Figure 7c, a bridge rectifier BR1 has its input terminals connected to opposite ends of the motor armature vinding. The negative output terminal of the bridge rectifier-is connected by a rail 70 to one end a transformer primary T_ . The positive output terminal of the bridge rectifier is connected by a resistor R., nn ^° the cathode of a zener diode ZD_, the anode of vhich is connected to the rail 70. A capacitor C n is connected acπ the zener diode ZD . The unijunction transistor TJ. , has its base 1 connected to the rail 70» i^ 3 base 2 connected by a resistor R„ π - to the cathode of the zener diode ZD_ a: its emitter connected by a resistor R„ n? to the cathode of the zener diode ZD_ . A capacitor C__ is connected betveen the emitter of the unijunction transistor TJ. and the other end of the primary winding T_ , a diode D^ n being connectec across the vinding T.. The secondary vinding T_ of the transformer is conneci at one end to the +5^ rail. Its other end is connected by resistor -,Q-, to the anode of a diode D^- 1 and the cathode c a diode Dg,» the cathode of the diode D ^ being connected to the +5^" rail. The anode of the diode D-„ is connected o«c ιy a capacitor C to t 5 tne + 3 rail and by a resistor R„ Ok to the base of a ΌΠD transistor P. _, . A resistor R„__ lb 305 connects the. base of the transistor P ,- to the +5v rail and - the emitter .of this transistor .is connected to the same rail. The collector of the transistor P,/- is connected to the terminal marked SKXD and is also connected by a resistor - r to the base of an npn transistor N„. » the emitter of vhich is connected to the earth rail. A resistor R connects the base of the transistor N.,^, to the earth rail and the collector of the transistor N_ is connected by a resistor R o8 to the base of the transistor P g to provide latching action. The collector of the transistor N . is connected to the terminal n and is also connected by a capacitor C _ and a resistor ~ 0 q in. series to .the base of a pnp tran¬ sistor R-i rγ * The emitter of the transistor P _ is connected to the +5"v rail and its collector is connected by a resistor R_ 10 to the base of the transistor . • The base of the transistor P._ is connected by a resistor R_.. to the +5v rail and by tvo resistors R^ 12 » R τ-π ^ n seτ±es to the anode of a diode Dg„, the cathode of vhich is connected (via a terminal p) to the output terminal of the gate G3. A capacitor C_, connects the junction betveen the resistors R,.., 2 , ϊ ., to the earth rail. Having, thus, described the various components shown in Figures a. to 7c and the connections betveen them, the operation of the logic circuits vill nov be explained in detail. At start up, vhen the ignition svitch kO is closed and the svitch SO is in neutral position the transistor N „ can turn on, being supplied vith base current through the diode D and the resistors ^ Q k nd ■ R 20 "' * Current can then flow through the diode D„. and the relay coil kl . The contacts of this relay provide power to the circuits and also relay ' latching current via the diode D . Initially, the tran¬ sistor P is turned on via resistors R„. _ , R n , R so that its collector is high. The ca D acitor C, _ charges up via the diode D_ Q and the resistor R 2 g 3 from the neu t ral contact of the svitch 6θ. Capacitor C /ιn holds on both 'kO ttrraannssiissttoorrss NN„„__,, aanndd NN 22gg vvhhiicchh aacctt ttoo clamp the motoring and braking demand signals to zero. "When the svitch 6θ is moved to, say, the forward position, the transistor N__ receives base current via diodes D„ Q and D_ g and resistors R 200 a - nd R 2 l V T an.sisto N_ Q conducts and causes transistor P to turn on via the resistors R 20 o a & ^217 * r ^ 3 -^- s i- 3 a latching arrangement and transistor N q is held on via diode E 7 » and resistors o „ however, loses its supply of base current because the transistor P_ collector has gone high and P Q therefore tur 7 o off, the function of which will be explained hereinafter. Also transistors N„_ and N 2 Q turn off after a delay whilst the capacitor C. _ discharges, since base current via the neutral svitch position has been lost, ' so that an armature current demand can now be made. There are certain restrictions and conditions govern± the selection of forward and reverse drive for safety reasons. Firstly, the transistor 2 _ is turned on by a +12v . supply from the vehicle "ignition" switch kO through the resistor R 21 _. This keeps the junction of the resisto: R 2 .._ and R 221 normally low (and transistor therefore of. except when first powering up the circuit when the +5v supply rises rapidly and transistor N 2Q is turned on via tJ capacitor C_^ and resistor R., 22 w ^--^- s't capacitor C is charging up. Transistor -N.. turns on the transistor P through the resistor R., 2 ~ and the diode D, „ during this pe: od (approximately 25 S) for reasons explained hereinafter. At the same time another capacitor/resistor combination C_. and R - ?7 kold the transistor N on for about 150π3. Transistor N_ prevents transistor ιq from turning on during this time because of the connection of the collector ' Rt of transistor N 2 to the junction- of resistor R on ajac ^ diode D_Q. These delays ensure -correct starting up of the circuit when switching on and immediately selecting forward or reverse drive. There are also four interlock signals which prevent forward or reverse being selected, these being the charger plug interlock and auxiliary battery low cut-out vhich when low (Ov) hold the cathode of diode D low via diode D. or D. and prevent transistor N from turning on. The other two interlocks are provided by the D>0 and SP>0 signals, vhich, when high, turn transistor N 22 on and also prevent transistor N „ turning on, so that if transistor N- Q is on these two signals can have no effect because transistor N 22 is held off by the diode D- _ and the transistor N. q . Vhen the "ignition" switch θ is opened transistor Η„ loses its base drive and turns off, so that transistor N 2 _ turns on and removes the base current from the transistor N q via diode D « After a delay of about 200mS, while capacitor C-.. discharges, the transistor P turns off and transistor P„ turns on. This makes the collector of transistor P g go high vhich is used, as explained hereinafter to make the contactor RI } open. The transistor P has previously turned on and clamped all the demand signals by turning on transistors N__, N 2 _ and N 2g so that no current will be floving in the armature and field windings. After a further delay of about 150mS vhilst capacitor C_ Q is discharging .the transistor N lg turns off causing the relay kl to drop out and remove pover from the entire circuit. The vehicle can also be shut down by taking either of the interlock diodes D. Q , D^. to Ov, the same sequence than taking place. Vhen the vehicle is actually being driven forward (or is in the fox-ward drive connection mode and at rest) and reverse drive is required, svitch 6θ is operated to select reverse drive. In this reverse position base current for the transistor N._ is provided by resistor S- 201 and transistor N._ therefore holds the DATA input of the latch L low. The latches can only change state vhen the signal at their CLOCK inputs are high and in the case of the late L , this can only occur if the vehicle is not moving, movement being detected by sensing the voltage across the motor armature vith a small field current flowing. If a voltage is present the motor must be rotating, the bridg rectifier BR1 detecting such voltage (the polarity βf whic will depend on whether motoring or braking mode is in operation) . The output of the bridge rectifier BR1 is dropped by the resistor R, QO and the zener diode ZD to +15v. Capacitor C_ 0 provides smoothing during chopping when the armature voltage varies. The voltage across the caoacitor drives an oscillator based on the unijunctio transistor T , and the pulses generated by the oscillator transferred to the remainder of the circuit through the pulse transformer T_, T,,. The diodes -Dg- < » D >r 2 , the re¬ sistor R«,„„ and the canacitor C_„ form a diode punro circui 303 D 2 - which acts to turn on the transistor P g via resistor R 0Λ The transistor R,g turns on transistor N„. and also holds the CLOCK input of the latch L_ low to prevent change over of the latch. Yhen a change from forward to reverse is demanded the following sequence occurs:- The demanded change is detected and a small field current is applied to the motor After a delay, if no armature voltage has been detected, t change over is allowed. Normally the output of gate G3 is lov because both of its inputs are the same. Transistor and resistor enting the change over of latch 3. ' Vhen the direction change is demanded the DATA input to latch L„ goes high, so that one input of gate G3 alse goes high as does the output of gate G3. Transistor P.„ turns off and transistor P _ turns on thereby clamping the armature demand via trainsistors ? _ and N 2g « Transistor P__ turning off removes the supply of the base of transistor N„ (Figure 5) v a resistors 27L and R g _ (Figure 5)« This releases the base of transistor N (Figure 5) so that a τm ' τHπmτn field current demand is applied to the field chopper. A back emf is therefore generated if the motor is rotating. During this time the diode g_ has become reverse biased and capacitor C . discharges through the resistor R„ 2 into the base of transistor P__ and after a delay of about lOOmS transistor P__ turns off and transistor N_, also turns off, provided that transistor P..g is not on. Turning off of transistors P__ and N- is speeded up by the capacitor C__ and resistor R„ oq providing positive feedback, thereby ensuring ά. fast edge to the signal at the CLOCK input of the latch L which is desirable for inter erence-free operation. The latch L_ can now change to the required state and its Q output is compared by gate G with the motor/bra-ke demand signal from the collector βf the transistor N„ . The output of the gate G goes to latch L_ whose output drives the field reversing relay RL2 via resistor i resistor R 2 g 2 ωd transistor N__. This circuit arrangement gives the required field current directions for forward and reverse, motoring and braking as shown, in the following table:- • BU 1 " ' -32- The relay RL1 is arranged to be operated for a suffici¬ ent time when a changeover is demanded to ensure reduction of the field current to zero. Thus the reversing relay RL2 is never required to break any current which could damage it: contacts. The change-over sequence is as follows :- The output of latch L_ changes as described above and the change passes through the gate Gk to the. latch L. DATA input. However, latch L.. cannot change immediately because transistor N 26 is held on by transistor P 10 ■ ' and transistor N 2 _. The demanded change is detected by gate Gl whose outpui goes low and turns on transistor P q which turns on transistor 2 . The demand clamping transistors 2 _, 2 _ and N 2 „ are also turned on by the gate Gl output, thereby ensuring that there are no field or armature current demands , The tran¬ sistor 2j , clamps the base of transistor 2 _ to Ov and therefore de-energises the relay RL1 so that its contacts open and the field current decays rapidly to zero. After a delay somewhat longer than that required to reduce the field current to zero (about 75∑πS), the capacitor C37 dis¬ charges and transistor P 10 iurns off. Thus transistor N 26 loses its base current from resistors R and also turns 252 Oil so that the latch CLOCK input goes high and latch L χ ca change te its new state. Gate Gl then changes back to it normal high output state, the relay RLl is re-energisβd and the clamping transistors are turned off after short delays produced by capacitor C g with resistor R 2j ,£, and capacitor C^ 0 with resistors R (7,k εmd ^-2.6" ^° a ^-^- ow ^k θ relays to attain their correct conditions. The vehicle is then drivable in the required direction. The same sequence of events occurs in the opposite direction of change over, except that the latches and gates end up in the required state as shown in the table above. Field forcing also occurs on switch on by the turning on of the transistor P q via transistor 2n to ensure that nβ field or armature current demand is made before the relays have settled in their normal operating positions. At shut down a similar sequence occurs when the ignition svitch is opened or one of the interlock diodes D^ Q , D,, . is taken low. The change over from motoring to braking is very similar to the forward-reverse change over. In motoring the MΓ /E selector (i.e. the output of amplifier A_ Figure 3) i- 3 high so that transistor N__ is on and the output of latch 2 is low. Vhen the MTR/BK signal goes low, the DATA input of latch L 2 and one input of gate G change state,latch L. therefore changes when transistor ^26 ^ varΩ - s °££ as described above, also latch 12 changes at the same time. Thus transistors P. _ and N turn off, gate G2 detects the change (because of the delay in contactor RL3 operation). Transistor N„ Q turns off and causes N 2 _ to turn on via resistors R-„ o t 270 and - diode D (in braking) or transistor 2g to turn on via resistor R 275 and R 280 a Ld diode D c 2 - 11 motoring. This ensures that the braking demand is clamped in motoring a.nd vice versa. Vhile the Q output signal of larch L and the signal at the collector of transistor N„ Q are out of phase, the output of the gate G2 is lov and the transistor P holds he transistors N „ and N„ g on. These demand clamps are - t releas ed after the contactor RL3 has reache d its new position, that is transistor P.. _ has turned off and removed drive to transistor N__ and the relay RL , so that the brake contact RL3 is in its de-energised state which is the normally open (braking) position. There are two further safety interlocks to protect the contactor:- namely on I 0 signal from the current transducer to tran¬ sistor N 2 g to prevent the latch changing if an armature current is flowing and transistor P.. conducts during chang ' over to hold transistor N 2 g on so that the latch cannot change back to its original state until braking has been obtained and transistor P _ turns off. These interlocks ensure that there is no possibility of breaking a fault current. Because of residual magnetism in the motor it is desirable to ensure that the field current is reversed be¬ fore the contactor RL3 is closed into the motoring position to prevent large uncontrolled currents being generating in the armature 51 2-nd recirculating diode D_ . The I>0 interlock then prevents changing back into braking unless the motoring field is applied to reset the magnetic circuit in the motor. By resetting the field before closing the contactor, the problem is avoided. During braking, when change over to motoring is demanded the MTR/ BK signal goes high so that the collector of transistor N_ 1 goes low, the field reversal process takes place exactly as described above through gate G and latch L. after a delay for field forcing. The latch L 2 also changes at the same time and transistor P turns on so that its collector goes high. This positive-going edge is transmitted through diode D ,- , capacitor C. 2 επd resistor R 2Qn to turn on transistor N„„ which removes the base drive to tran- sistor N_„ through diode D so that transistor N„_ cannot turn on. The relay RlA therefore stays de-energised and the contactor RL3 stays in the open (braking) condition until capacitor , ^ charges up and transistor N turns off. During this delay, .the Q output of the latch L,, and the contact RL3b are out of phase so that gate G2 output is low and the armature demand is clamped by transistor P.. supplying base current to transistor N 2 and N 2g . The field reversing relay has already changed and a reset signal is τjrovided by holding off the field claπro via diode D_ 0 5o and transistor N__ so that transistor 0 turns off and 2 o allows a small field demand to be made at the emitter of transistor N_ (the demand is for a current of approximately 2 amps). Because the transistor N_ 2 serves the dual role of holding the contactor in braking and allowing a field - current to flow in the correct (motoring) direction, the field resetting pulse must always occur at the right instant, i.e. after the field has reversed, but before the braking contact RL3 closes. Vhen the transistor N_ 2 goes off, the brake contactor RL3 closes, the output of gate G2 goes high and the armature demand clamps are released after capacitor CL. has discharged. The vehicle is then in the motoring mode with the magnetic circuit set in the right direction so that the recirculating diode D Q is reverse biased. To ensure that the field pulse always occurs correctly, the duration of the motoring signal at transistor N_. should be longer than the time required to change from braking to motoring. This is achieved by another capacitor-resistor network C^, , R-„o connected to the base of transistor N_. to hold that transistor on when the collector of transistor P. goes high until the capaci¬ tor C.. is charged. This delay is made slightly longer than the contactor hold-off delay produced by capacitor C. 2 and resistor R 2q _, so that rapid changes from motoring to braking and back again do not interfere with the oueration of the field reset circuits. The diodes D_„ and D D D__ and the resistor R 2 gg also contribute by allowing capacitors C t , . and C^„ to discharge quickly after selecting braking ready for the next change into motoring. An extra function of these components occurs on start up. In neutral transistor P g is on and holds transistor N 2 on via the resistor R 21 g . This holds the contactor in its braking condition and produces a small field current. If the vehicle is moving transistor P ,. will turn on transistor 2 „ and prevent either forward or reverse being selected. U R E

Claims: CLAIMS 1. A control system for a d.c. motor comprising control means for varying the connections of the motor armature winding and/or field winding so as to enable the motor to operate in a plurality of different modes, and speed interlock means for preventing operation of said control means to change the motor connections . from at least one mode to at least one other mode whilst the motor is running characterised in that said interlock means is sensitive to the voltage across the armature winding of the motor and means are provided for supplying a field current to the field winding when said interlock means is required to be operative. 2. A control system as claimed in claim 1 further charac¬ terised in that said interlock means includes a rectifier BRl connected to the armature and a means sensitive to the d.c. output of the rectifier. 3» A control system as claimed in claim 2 .in .which said means sensitive to the d.c. output of the rectifier includes an oscillator TJ. , Rm. Q t c -- ι a isolating trans¬ former T_ , T 2 having its primary vinding connected to the oscillator and a detector circuit ^ η , D,- 2 , C_ 2 , P f - connected to the secondary winding T„ of the isolating transformer. k m A control system as claimed in any preceding claim characterised in that the means for supplying current is required to be operative supplies a fixed length current ■ Dulse of predetermined magnitude thereto. BURE4

Inventors: CAMPBELL G
Applicants: CAMPBELL G, LUCAS INDUSTRIES LTD, LUCAS IND LTD
Application year: 1978
Main classification: H02P7
IPC codes: B60L15, H02P7