GB2097205A - Control system for a DC motor - Google Patents

Abstract

The system includes a thyristor chopper circuit including a main thyristor (11) connecting one end of the armature winding (10) to one rail (12) of a supply and means for turning the thyristor (11) on and off to regulate armature current and a mode selection thyristor (15) which connects the other end of the armature winding to the other rail (16) of the supply, said mode selection thyristor being rendered conductive when motoring operation is required, but left non-conductive when braking operation is required. <IMAGE>

Description

SPECIFICATION Control system for a D.C. motor This invention relates to a control system for a d.c. motor and has particular reference to a motor control system with separate controls for the armature and field windings.

It has already been proposed, (see, for example, PCT Application No. GB78/00046) to connect a single switch contact in series with the motor armature winding and the output element of a chopper circuit across the power supply, such contact being closed during normal motoring, but opened during braking, a braking current path through the armature winding and the output element then being established, for example via a diode.

Various switching elements are also associated with the field winding, but since this carries a smaller current than the armature winding, these swiching elements do not present the same problems of contact life as does the armature contact.

It is accordingly an object of the present invention to provide a d.c. motor control system for separate armature and field control, but without any switch contacts in series with the armature winding.

A d.c. motor control in accordance with the invention comprises a field current control, a chopper circuit comprising a main semiconductor switch device connecting one end of the motor armature winding to one of a pair of supply conductors and means for turning said main switch device on and off to regulate the magnitude of the current flow in the armature winding, a mode selection semiconductor device connecting the other end of the armature winding to the other supply conductor, and control means for said mode selection switch device for causing the mode selection switch device to be maintained conductive during motoring operation, but not during braking operation.

The mode selection switch device is preferably a thyristor, since such a device will, once fired, remain conductive as long as motoring current is flowing through it.

However, where the chopper is a thyristor chopper, it is convenient for the main thyristor and the mode selection thyristor to be fired simultaneously during motoring operation, but for the main thyristor to be fired alone during braking operation. in this case firing circuits for the main thyristor and the mode selection thyristor are interconnected so that the main and mode selection thyristors are fired simultaneously for motoring and an inhibiting circuit is provided to inhibit operation of the mode selection thyristor firing circuit when braking operation is required.

In the accompanying drawings: Figure 1 is a circuit diagram showing an armature current controlling thyristor chopper circuit included in one example of a control system in accordance with the invention and Figure 2 is a circuit diagram of a part of a circuit for firing the thyristors of the chopper circuit.

The control system described herein is identical in most respect to that described in detail in PCT Application No. GB78/00046 and circuit details omitted from the following description may be found in that application.

The circuit shown in Fig. 1 includes the motor armature winding 10 one end of which is connected by the main output thyristor 11 of the chopper circuit to the main battery negative conductor 1 2. A fuse 1 3 is included in this connection. The other end of the armature is connected via a current sensor 1 4 and a mode selection thyristor 1 5 to the positive conductor 16.A freewheel diode 1 7 is connected between the first-mentioned end of the armature winding and the conductor 1 6 to conduct motor current when thyristor 11 is non-conductive and a "brake" diode 1 8 is connected between the conductor 1 2 and the current sensor 14, to provide a current loop to carry current generated by the motor during braking i.e. when thyristor 1 5 is non-conductive. A resistor 20 and a capacitor 21 are connected in series with one another across the anode-cathode of the main thyristor 11.

For turning off the thyristor 11 there is a commutating thyristor 22 which, when fired, diverts the motor current through an inductor 23 into a commutating capacitor 24 connected to the conductor 1 2. A further thyristor 25 in series with a further inductor 26 is connected across the capacitor 24. A resistor 27 and a diode 28 provide a charging path for the capacitor 24 to maintain the voltage on the capacitor 24 when the control is operating in "d.c." mode (to be explained herei nafter).

The field winding 29 of the motor is connected by a field control circuit 30 to the supply conductors, which field control circuit includes both reversing relays for determining the direction of field current and also analog current control circuits which are as described in the aforesaid PCT Application No.

GB78/00046.

In operation, the thyristor 11 is fired whenever it is desired to increase the armature current. When decreased current is required thyristor 25 is fired and then thyristor 22 is fired after a fixed delay. Firing of thyristor 25 causes current to flow from capacitor 24 through inductor 26, such current flow continuing for one half-cycle of oscillation of the tuned circuit constituted by capacitor 24 and inductor 26. The capacitor 24 then carries a reverse charge so that when thyristor 22 is fired all the motor current is diverted into capacitor 24 and the thyristor 11 therefore becomes non-conductive. When capacitor 24 is fully forwardly charged again thyristor 22 ceases to conduct and the motor current flows through diode 1 7 until thyristor 11 is fired again.

The firing instants of the four thyristors are determined by the circuit shown in Fig. 2. In Fig. 2 a block 31 represents an armature current signal demand generator which is sensitive to the position of the vehicle accelerator and brake pedals and also to the speed of the motor as described in PCT Application No.

GB78/00046. A current feedback circuit 32 is associated with the current sensor 14 of Fig. 1 and provides an output signal representing the instantaneous value of the current.

The signals from blocks 31 and 32 are applied to the non-inverting and inverting inputs respectively of an operational amplifier A1 via two resistors R, and R2 respectively. Bias resistors R3 and R4 connect the respective non-inverting and inverting inputs of amplifier A, to a + 8V rail 33 and a feedback resistor R5 connects the output of amplifier A, to its inverting input so that amplifier A1 operates as a difference amplifier. A capacitor C, connects the output of amplifier A, to a ground rail 34.

An operational amplifier A2 has its inverting input connected by a resistor Rs to the output of amplifier A,. A resistor R7 connects the non-inverting input of amplifier A2 to the rail 33 and a resistor R8 connects the output of amplifier A2 to is non-inverting input so that amplifier A2 operates as a Schmidt trigger circuit, the output of which goes high when the current demand signal from circuit 31 is less than the current feedback signal from circuit 32 by more than a predetermined margin and goes low when the signal from circuit 31 is more than that from circuit 32 by more than this margin.

The circuits of Fig. 2 are arranged, inter alia, to fire thyristor 11 when the output of the amplifier A2 goes low, to fire thyristor 25 when the output of amplifier A2 goes high and to fire thyristor 22 a fixed delay after the output of amplifier A2 goes high. A "minimum off-time" circuit is provided to ensure that the output of amplifier A2 does not go low too soon after it went high for the thyristor 22 to have stopped conducting again. This circuit includes a resistor R9 and capacitor C2 in series between the output of amplifier A2 and the base of a pnp transitor Q,.The emitter of transistor Q, is connected to the rail 33, its base is connected by a resistor R,o to the rail 34 and its collector is connected to the base of a pnp transistor Q2 and also connected by a resistor R" to the rail 34. The emitter of transitor Q2 is connected to the rail 33 and its collector is connected by a resistor R,2 to the rail 34. A resistor R,3 connects the collector of the transistor Q2 to the anode of a diode D1, the cathode of which is connected to the non-inverting input of amplifier A2.

Thus, when the output of amplifier A2 goes high the transistor Q, turns off and transistor Q2 turns on for a time determined by the values of resistors Rg and R10 and of capacitor C2. During this time sufficient current is supplied to the non-inverting input of amplifier A2 to ensure that its output cannot go low, whatever happens to the signal at the inverting input thereof.

Each of the thyristors 11, 15, 22 and 25 has an associated firing circuit 35, 36, 37 and 38 respectively. Each of these firing circuits is identical and only circuit 35 will be described. The circuit 35 includes an npn input transistor Q3 which has its base connected to a first input terminal 35a and also connected to the ground rail 34 by a resistor R,3. The collector of the transistor Q3 is connected by two resistors R,4 and R15 in series to a + 1 2V supply rail 39. The junction of these resistors R,4 and R,s is connected to the base of a pnp transistor Q4 which has its emitter connected to rail 39 and its collector connected by two resistors R,6 and R,7 in series to the rail 34.An output transistor Q5 has its base connected to the junction of resistors R,6 and R,7 and its emitter connected to the rail 34. The collector of transistor Q5 is connected by a resistor Ra8 in series with the primary winding of a pulse transformer 40 to the rail 39. A capacitor C3 is connected in parallel with the resistor R,8 and a recirculating diode D2 in series with a resistor R,g is connected across the primary winding. The secondary winding of the transformer 40 is connected to the associated thyristor gate and cathode terminals via a diode D3. A firing pulse is supplied to the thyristor whenever the signal at the input terminal goes high.In the case of circuit 35 only, there is another input terminal 35b connected to the base of the pnp transitor Q4 and firing of the associated thyristor thus also occurs when the signal at this terminal goes low.

The terminal 35a is connected by a resistor R20 and a capacitor C4 in series to the collector of a pnp transistor Q6 which has its emitter connected to the rail 33 and its base connected by a resistor R2, to the output of amplifier A2. A resistor R22 connects the base of the transitor Q6 to the rail 33. Transistor Q8 turns on whenever the output of amplifier A2 goes low and thereby provides a positivegoing input pulse to terminal 35a.

Terminal 35b is connected by a resistor R23 and a capacitor C5 in series to the output of an operational amplifier A3 connected to operate as an oscillator. Amplifier A3 has its inverting input connected by a resistor R24 and a capacitor C8 in series to the rail 34 and a resistor R25 connects the output of amplifier A3 to the junction between resistor R24 and capacitor C6. The non-inverting input of amplifier A3 is connected by a resistor R26 to its output terminal and by a resistor R27 to the rail 33.The amplifier A3 operates in known manner as an oscillator producing a square wave output of frequency approximately 1OHz. An npn transistor Q7 has its emitter connected to rail 34 and its collector connected to the junction of the resistor R24 and the capacitor C6 so as, when conductive, to hold the capacitor C9 discharged and thereby prevent operation of the oscillator.

Transistor Q7 has its base connected to the junction of two resistors R38 and R39 connected in series between the output of amplifier A2 and the rail 34, so that the oscillator can operate only when the output of the amplifier A2 is low.

As described in the aforementioned PCT Application No. GB78/00046 the field current is held at a fixed level at low speeds. As speed increases, a point will be reached where it becomes impossible for the demand armature current to be reached so that the output of amplifier A2 will remain low for extended periods. When this situation exists the field current is weakened to reduce the back e.m.f. generated by the motor and to allow increased armature current to flow. Control of armature current in these circumstances is effected by varying the field current, the armature then operating in so-called d.c.

mode. The onset of this condition is recognised by a timer connected to the output of the Schmitt trigger amplifier A2 and included in the field control. The frequency of the oscillator based on amplifier A3 is chosen to be so low that the first output pulse is only produced after this timer has run its time and field current weakening is occurring. The pulses from amplifier A3 serve to refire the thyristor 11, if for any reason, it has ceased to conduct. This could occur, for example, as a result of the fluctuations in the field current caused by noise spikes etc. In the d.c. mode the armature voltage is almost equal to the battery voltage, and consequently even a brief duration increase in field current could have the effect of making the armature voltage largerthan the battery voltage, thereby reverse biasing the thyristor 11 and casuing it to turn off.The oscillator based on amplifier A3 will turn the thyristor 11 on again should this occur.

The input to firing circuit 38, which controls thyristor 25 is connected by a resistor R28 and a capacitor C7 in series to the collector of the transistor Q2 Thus thyristor 25 is fired once each time the output of the Schmidt trigger amplifier A2 goes high.

The fixed delay between firing of thyristor 25 and 22 is achieved by a monostable circuit based on an operational amplifier A4.

The inverting input of this amplifier A4 is connected by a capacitor C5 and a resistor R29 in series to the collector of the transistor Q24 A capacitor C9 connects the junction of resistor R29 and capacitor C8 to the rail 34. The noninverting input of amplifier A4 is connected by a bias resistor R30 to the rail 33. A diode D4 has its anode connected to the output of amplifier A4 and is cathode connected by a resistor R31 to the inverting input of the amplifier A4. A capacitor C,O is connected between the cathode of diode D4 and the rail 34.

Another diode D5 has its anode connected to the output of amplifier A4 and its cathode connected by a resistor R32 to the non-inverting input of amplifier A4. The output of amplifier A4 is connected by a resistor R33 and a capacitor C11 in series to the input of firing circuit 37.

In its stable state the output of amplifier A4 is high, thereby maintain capacitors C8, C9 and C10 charged. When transistor Q2 turns on the output of amplifier A4 is driven low as capacitor C8 is charging. Capacitor C10 discharges relatively slowly through resistor R into the inverting input of amplifier A4 until this discharge current is no longer sufficient to hold the amplifier A4 output low. The amplifier output therefore goes high and this transition causes a positive-going pulse to be applied to the input of firing circuit 37 to fire thyristor 22.

An oscillator based on an operational amplifier A5 is also employed for triggering the firing circuit 37. Amplifier A5 has its noninverting input connected by a bias resistor R34 to the rail 33 and by a feedback resistor R35 to the output of amplifier As. The inverting input of amplifier A5 is connected by a resistor R36 and a capacitor C12 in series to the rail 34, a resistor R37 being connected between the output of amplifier A8 and the junction of the resistor R36 and the capacitor C12.A resistor R40 connects this junction to the collector of an npn transistor Q8 which has its emitter connected to rail 34 and its base connected to the junction of two resistors R41 and R42 which are in series between the collector of transistor Q6 and the rail 34. The output of amplifier A9 is connected by two resistors R43 and R44 in series to the rail 33, the junction of these resistors being connected to the base of a pnp transitor Qg which has its emitter connected to rail 33 and its collector connected by a resistor R45 to the rail 34. A resistor R46 and a capacitor C13 in series connect the collector of transistor Qg to the input terminal of firing circuit 37.

The oscillator based on amplifier A5 operates whenever the output of amplifier A2 is high. Its components are chosen so that it runs at a frequency of about 50Hz, which is significantly lower than the normal running frequency of the armature current control loop when the chopper is operating (i.e. when not in the d.c. mode). This oscillator does not, therefore, take part in the normal current control operation. Its purpose is to ensure that the capacitor 24 of Fig. 1 is charged up when the circuit is connected to the battery, but no demand signal is applied. This occurs as part of the normal start up routine of the control, i.e. a delay is inserted between power being connected and a demand signal being generated. During this delay the oscillator causes thyristor 22 to be fired so that capacitor 24 is charged up by current flowing through the armature.It should be noted in this respect that the value of resistor 27 in Fig. 1 is too large for this purpose and resistor 27 can only supply sufficient current to prevent leakage from capacitor 24 when the latter is already charged.

It is necessary to ensure that the thyristor 1 5 is fired whenever thyristor 11 is fired during motoring, but not during braking. To this end a diode D8 has its anode connected to the collector of transistor Q4 and its cathode connected by a capacitor C14 and resistor R47 to the rail 34. Two resistors R48 and R49 in series connect the junction of capacitor C14 and resistor R47 to the input of circuit 36. In order to prevent triggering of circuit 36 during braking an npn transistor Q10 has its emitter connected to rail 34 and its collector connected to the junction of resistors R48 and R49.The base of transistor Q,0 is connected to the collector of transistor Q,1 which is the same as transistor 33 shown in Fig. 7b of the drawings of PCT Application No.

GB78/00046. Since the thyristor 1 5 in the present case replaces the relay RL4 and the contactor RL3 of the prior circuit, a load resistor R50 is connected between the collector of transistor Q,1 and the + 5V rail. Furthermore the interlock function provided by relay contact RL3b in the prior construction is now replaced by a direct connection between the collector of transistor Q11 and the junction of resistors R277 and R278 in Fig. 7b.

An additional diode D7 has its anode connected to the collector of the transistor Qg and its cathode connected to the cathode of diode D6. By virtue of this connection circuit 36 is also triggered each time the oscillator based on amplifier A8 operates, thereby providing a path for charging current for capacitor 24 during power up.

It will be appreciated that the chopper circuit of Fig. 1 need not necessarily be a thyristor chopper circuit ther types of semiconductor switching device can equally be used. Similarly the device 1 5 need not be a thyristor, although this is preferable, particularly when a thyristor chopper circuit is used.

Claims (6)

1. A d.c. motor control comprising a field current control, a chopper circuit comprising a main semiconductor switch device connecting one end of the motor armature winding to one of a pair of supply conductors and means for turning said main switch device on and off to regulate the magnitude of the current flow in the armature winding, a mode selection semiconductor switch device connecting the other end of the armature winding to the other supply conductor, and control means for said mode selection switch device for causing the mode selection switch device to be maintained conductive during motoring operation, but not during braking operation.

2. A d.c. motor control as claimed in Claim 1 in which said mode selection semiconductor switch device is a thyristor said control means acting to fire said thyristor when motoring operation is required.

3. A d.c. motor control as claimed in Claim 2, in which the chopper circuit is a thyristor chopper circuit, said main semiconductor switch device being a thyristor.

4. A d.c. motor control as claimed in Claim 3, in which firing circuits for said main and mode selection thyristors are interconnected so as to operate simultaneously, an inhibiting circuit being arranged to inhibit operation of the mode selection thryristor when braking is required.

5. A d.c. motor control as claimed in Claim 4, in which the chopper includes a commuating capacitor and a commutating thyristor in series with the commutating capacitor across the main thyristor, a firing circuit for the commutating thyristor, a circuit for triggering the commutating thyristor when commutation of the main thyristor is required, and a further circuit for repeatedly triggering the commutating thyristor firing circuit during a start-up cycle for initially charging the commutating capacitor, the mode selection thyristor firing circuit also being connected to be triggered by said further circuit.

6. A d.c. motor control substantially as hereinbefore described with reference to the accompanying drawings.