GB1603933A - Regenerative braking systems - Google Patents

Description

(54) REGENERATIVE BRAKING SYSTEMS (71) We, THE GARRETT CORPORA TION, a Corporation organised under the laws of the State of California, United States of America, of 9851-9951 Sepulveda Boulevard, P.O. Box 92248, Los Angeles, California 90009, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to regenerative braking systems of the rype in which a d.c.

motor acts as a generator to feed power back into the power supply.

According to the present invention, a motor arrangement comprises a d.c. motor, positive and negative supply lines, and a bridge circuit connecting the motor armature to the supply lines, the bridge circuit having first and second legs connecting one side of the motor armature to the positive and negative lines respectively, and third and fourth legs con necting the other side of the motor armature to the positive and negative lines respectively, the first and fourth legs containing means arranged to pass a regenerative current from the motor to the supply lines, and the second and third legs containing means arranged to pass a motoring current from the supply lines to the motor, and the motor arrangement also including means, separate from the bridge circuit, for energising the field of the motor, and controlling means arranged to control the bridge circuit and the field energising means in such a way that the motor can be selectively operated in a motor mode, and in a regenerative mode in which the direction of current in the armature of the motor is the same as in the motor mode, while the direction of the field energisation is reversed by the field energising means on a change between the motor mode and the regenerative mode.

Thus, with the invention, at least part of the circuit which is responsible for controlling the motor armature current can, like the armature, be arranged to carry current in one direction only, and can therefore be a solid state chopper using only one, unidirectional, main thyristor to control the armature current in both the motor mode and the regenerative mode.

One of the second and third legs may include a switching device serving to switch the arrangement between its motor mode (when the switching device is closed) and its regenerative mode (when the switching device is open). The other of the second and third legs may then indude a thyristor chopper circuit controlling the current through the motor armature during operation in the motor mode.

The arrangement may also include a short circuit path connected across the motor armature, and means controlling the short circuit path to short-circuit the motor armature intermittently during operation of the motor in the regenerative mode. If the motor e.m.f.

is insufficient to return energy continuously to the power supply, the intermittent closing of the short-circuit path will nonetheless make regenerative operation possible, by building up the armature current during the periods when the armature is short-circuited. The chopper circuit, together with one of the first and fourth legs of the bridge, may constitute the short-circuit path.

The motor arrangement may also include means, such as a resistive braking path selectively connected across the armature, during operation in the regenerative mode, to limit the transfer of energy to the supply lines if a parameter which is dependent on the voltage across the supply lines should exceed a predetermined value. For example the resistive braking path may comprise those parts which constitute the short-circuit path, with the exception of the chopper circuit, and may also comprise a resistive path connected in parallel with the chopper circuit.

One specific embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of a solid state blended regenerative/resistive brake system; and Figure 2 is a series of graphs which depict typical wave forms associated with the blended brake mode.

The blended regenerative/resistive braking system, shown schematically in Figure 1, comprises a DC source 10, a single reactor force commutated chopper 14, a DC motor having an armature 12 and a field winding 36, and related circuitry. The chopper circuit includes a main thyristor 18, a commutation thyristor 30 and capacitor 26, a commutation inductor 32, a charging diode 28, a freewheeling diode 34, a filter capacitor 20 and inductor 16, and a smoothing inductor 22, which are also used in blending regenerative and dynamic braking. The DC source 10 is connected to the armature 12 of the DC motor through the single reactor force commutated chopper 14; the path for the flow of armature current is from the positive terminal of the DC source 10, through the filter inductor 16, the main thyristor 18, and the smoothing inductor to the armature of the motor 12.From the armature, the current then returns to the negative terminal of the DC source, through a switch 24, which could in practice be a thyristor. The main thyristor 18 and the commutation thyristor 30 are triggered as necessary by control logic 50.

The system of Figure 1 may be operated in either a motor mode or a braking mode.

In the motor mode, the field winding 36 is energised with the polarity shown in Figure 1. The operation of chopper 14 in the motoring mode can be described briefly as follows. Before the main thyristor 18 is triggered on, the DC source charges the commutation capacitor 26 through the charging diode 28 and the armature 12, so that the side of the capacitor 26 which is closest to the anode of the main thyristor 18 is positive and the side of the capacitor closest to the charging diode 28 is negative. To energize the armature 12, the main thyristor 18 is gated on, causing the voltage at the junction of the smoothing inductor 22 and the commutation inductor 32 to rise.This rise in voltage tends to produce a flow of current through the smoothing inductor 22 and the armature 12, as indicated by an arrow iL; if a current was already freewheeling through the armature 12 and through the freewheel diode 34 and the commutation inductor 32, the rise in voltage will tend to increase the current iL through the armature, but will tend to decrease the current in the inductor 32. The charging diode 28 ensures that the rise in voltage is prevented from reaching the negative side of the commutation capacitor 26, so that no discharging of the capacitor occurs.

After gating on, the main thyristor 18 will continue to conduct current until it is reverse biased. This reverse biasing is accomplished by gating the commutation thyristor 30 on. A current will therefore flow around the L-C circuit comprising the capacitor 26, the main thyristor 18, the commutation inductor 32 and the commutating thyristor 30, until the charge on the capacitor 26 has been reversed. At this point, the current in the circuit will reverse, so that the commutating thyristor 30 turns off, and the current is instead carried by the charging diode 28. This current will attempt to flow into the cathode of the main thyristor 18, since the smoothing inductor 22 will resist any rapid change in current. This will in turn reverse bias the main thyristor 18, which will turn off. The capacitor 26 is then completely recharged to its original polarity by the DC source 10.A more detailed description of the actual workings of the chopper circuit 14 can be found by referring to U.S. Patent No. 3,763,418.

When the braking mode is desired, it can take the form of resistive or regenerative braking. For normal regenerative braking, the control logic 50 opens the switch 24 between the armature 12 and the negative terminal of the DC source 10 and also changes the position of electro-mechanical contacts 38 to reverse the energisation of the field winding 36.

When the circuit is opened between the armature 12 and the negative terminal of the DC source 10 the motor current iL ceases to flow. Reversing the polarity of the field coil 36 means that the e.m.f. of the motor will tend to cause a current to flow in the same direction as iL, as indicated by an arrow iR; this is the braking current iR. The magnitude of iR is representative of the amount of braking which is being applied to the armature 12. The current iR follows a path from the armature 12, through a diode 40 to the anode of the main thyristor 18, which is identified as a node 46. From the node 46 the current will flow to the positive terminal of the DC source 10 through the filter inductor 16. The return path for the braking current is from the negative terminal of the DC source 10, through the free-wheeling diode 34, the commutation in ductor 32, and the smoothing inductor 22 back to the armature 12.

The braking system as so far described avoids the need for large amounts of additional circuitry by utilizing the components of the chopper circuit 14 in a dual capacity. For example, the filtering components, inductor 16 and capacitor 20, which smooth the chopper wave forms in the motor mode also act to filter switching actions in the braking mode. The dual use of other components such as thyristors 18 and 30 and associated circuitry will be described subsequently.

While the current iR is flowing into the DC source 10 its magnitude is decreasing.

A current sensor 52 monitors iR and relays this information to the control logic 50.

When the value of iR decreases below the value necessary for the required braking effect, the control logic 50 will turn on the thyristor 18. With the thyristor 18 gated on, only the impedance of the inductor 22 is connected across the motor armature 12, and this essentially short circuits the current flow to the inductor 16, so that the current will flow from the armature 12, through the diode 40, to the node 46 and through the thyristor 18 to the smoothing inductor 22 and back to the motor armature 12. When this happens the braking current will increase, so that it soon returns to the necessary magnitude. If, for example, the desired braking is represented by a current iR of 1,000 amps, the short circuit will be applied at a current of about 950 amps and will cause the current to rise in value.When the current returns to a value above 1,000 amps, the current sensor 52, which monitors the current iR, will relay this information to the control logic 50. The control logic 50 will then issue a gating pulse to the thyristor 30, which will initiate a reverse bias action to the main thyristor 18, in the same manner as previously described for the motoring mode of operation, causing the main thyristor 18 to turn off so that current will no longer flow from the node 46 through the thyristor 18. Once again iR will then flow from the armature 12 through the diode 40 to the node 46 and through the filter inductor 16 to the positive terminal of the DC source 10 and from the negative terminal of the DC source 10 through the diode 34, the commutation inductor 32 and the smoothing inductor 22 to the armature 12.

It is well known that regenerative braking should be terminated if the source voltage increases greatly over its nominal value. Should the DC source 10 have, for example, a nominal value of 700 volts, it may be desired that the braking current should cease to flow into the DC source 10 when the voltage exceeds 750 volts. When this occurs, resistive braking can be initiated through a further thyristor 42 to limit charging of the DC source above 750 volts. A voltage sensor 48 determines the voltage of the DC source 10 by sensing the voltage at the junction of the filter inductor 16 and the filter capacitor 20, which is identified as node 45. The voltage at the node 45 differs from the voltage of the source 10 only by the small voltage drop across the inductor 16.When the voltage reaches 750 volts, the voltage sensor 48 will relay this information to the control logic 50, which will then supply a gating pulse to the thyristor 42. This thyristor 42 is connected, in series with a dissipation resistor 44, across the main thyristor 18, so that, when the thyristor 42 is conducting, but the main thyristor 18 is blocking, current will flow in a circuit comprising the diode 40, the thyristor 42, the resistor 44, and the inductor 22, so that the braking power can be dissipated in the resistor 44.Should the voltage sensed by the voltage sensor 48 at the node 45 drop below the predetermined level, a corresponding signal will be relayed to the control logic 50, which will then supply a gating pulse to the commutation thyristor 30 which, in conjunction with the commutation capacitor 26, the commutation inductor 32 and the charging diode 28, will reverse bias the thyristor 42 in the same manner as when commutating the main thyristor 18, thus opening the circuit to disconnect the dissipation resistor 44 from the operative braking circuit.

It should be noted that, if for any reason it is desired that none of the braking energy should be returned to the source 10, the dissipation resistor 44 may be used to dissipate the whole of this energy. In this case, at all times during braking, either the thyristor 18 or the thyristor 42 would be on.

Referring now to Figure 2, typical wave forms of various currents and voltage are depicted, to illustrate the operation of the components of the circuit of Figure 1 during an exemplary braking cycle; the times of the various events in the cycle are identified as times t1 to t. At time tl, the circuit is switched into its regenerative braking mode.

At time t2, resistive braking begins. At time t3, the armature is shorted to increase the armature current. At time t the circuit returns to the regenerative braking mode and the cycle is repeated, with times t5 and tfi corresponding to times t2 and t2.

Figure 2 shows, as Vis, V42 and V,,, the voltages across the components 18, 42 and 20 of the circuit of Figure 1, and, as I,,, 142, I,, 122 and IIB, the currents flowing in the components 18, 42, 34, 22 and 16 of Figure 1.

Between times tl and t, the main thyristor 18 is off, as can be seen by the voltage drop depicted on graph 2A, of VIR. At the same time, since the thyristor 18 is not conducting, its current is zero, as can be seen on graph 2B of I . During this period, as can be seen on graph 2F of 122, the current flowing through the inductor 22 is decreasing.

At time t2 the thyristor 42 is gated on, as shown by zero voltage on graph 2C of V42, between time t, and t3. When the thyristor 42 is triggered on, its current goes from zero to maximum value as can be seen on graph 2D, while the current through the diode 34, as seen on graph 2E, goes to zero since the triggering of the thyristor 42 now provides a path for the current iR through the resistor 44 to the motor armature 12. The triggering of the thyristor 42 is a result of the voltage sensor 48 having sensed that the voltage across the capacitor 20, representing the voltage of the source 10, is excessive (see graph 2G) and regenerative braking terminates until the source voltage has been re duced to an acceptable level, as shown at time t4 in graph 2G.

During the time period t, to tithe current through the thyristor 42, 142, and the current through the inductor 22, 12,, which are the same as the armature current, continue to decrease as shown on graphs 2D and 2F respectively. At time t3, the main thyristor 18 is triggered on, because the armature current has become too small for effective braking. This reduces the voltage drop across the thyristor 18 essentially to zero. At this time the current through the thyristor 18 and the inductor 22 increases, as can be seen on graphs 2B and 2F, since an effective short is placed across the terminals of the armature 12 through the inductor 22.The commutating thyristor 30 is triggered on, to reverse bias the main thyristor 18 and the thyristor 42 when the armature current has reached its required level, or when the source voltage is adequately reduced. If the armature current has reached its required level, but the source voltage has not yet fallen low enough to accept further braking energy, the effect of gating off the main thyristor 18 without gating off thyristor 42 can be achieved by gating off both thyristors by gating on the commutating thyristor 30, and then gating the thyristor 42 on again.In the present example, however, the source voltage has fallen low enough to accept further energy at t*, so that both thyristors 18 and 42 are shown as being tumed off at time 4. When this happens the voltages V, of the graph 2A and Vj2 of the graph 2C will increase again to their maximum values and the current through the diode 34 will go to its maximum value, as shown on the graph 2E. Current will again flow into the DC source and the current I,, will again start to decrease as the voltage across the capacitor 20 increases as regenerative braking resumes and the cycle repeats.

Graph 2H, showing I,,, shows that the current through the filter inductor 16 is constantly flowing. It is shown as a slightly varying direct current which results from the LC combination of the inductor 16 and the capacitor 20 and the pulsating current from the switching of the thyristors 18 and 42. While thyristors 18 and 42 are nonconducting, between times t1 and t2, the current through the inductor 16 is decreasing.

When the thyristor 42 is switched on at time t,, the current through the inductor 16 begins a slight increase which is due to the fact that the filter capacitor 20 has been charged to a voltage higher than the input power source. At t3, when the thyristor 18 is triggered on and the armature current is increasing through the thyristor 18, owing to the short circuit, the current through the filter inductor 16 is increasing to a maximum value. This maximum value occurs about midway between time t3 and time t4 when the filter capacitor 20 is charged to a voltage equal to the voltage of the input power source 10.At that point it begins to decrease and this decrease continues from time t4 to time 4, during which time both the thyristors 18 and 42 are turned off, so that the voltage on the capacitor 20 is increasing.

At time t4 the thyristor 42 is gated on and the current I,, through the filter inductor 16 will begin to increase as it did at time 4.

Thus, the cycle is repeated.

If the thyristor 42 remains on, with the braking current flowing through the resistor 44, for any appreciable period, the current Il6 through the filter inductor 16 will decrease until it becomes zero. However, during the brief periods of resistive braking used in the preferred embodiment the current I,, will not reach zero but will decrease in average value the longer or more often the thyristor 42 is conducting.

WHAT WE CLAIM IS:- 1. A motor arrangement comprising a d.c.

motor, positive and negative supply lines, and a bridge circuit connecting the motor armature to the supply lines, the bridge circuit having first and second legs connecting one side of the motor armature to the positive and negative lines respectively, and third and fourth legs connecting the other side of the motor armature to the positive and negative lines respectively, the first and fourth legs containing means arranged to pass a regenerative current from the motor to the supply lines, and the second and third legs containing means arranged to pass a motoring current from the supply lines to the motor, and the motor arrangement also including means, separate from the bridge circuit, for energising the field of the motor, and controlling means arranged to control the brdige circuit and the field energising means in such a way that the motor can be selectively operated in a motor mode, and in a regenerative mode in which the direction of current in the armature of the motor is the same as in the motor mode, while the direction of the field energisation is reversed by the field energising means on a change between the motor mode and the regenerative mode.

2. An arrangement as claimed in Claim 1 in which one of the second and third legs includes

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. since the triggering of the thyristor 42 now provides a path for the current iR through the resistor 44 to the motor armature 12. The triggering of the thyristor 42 is a result of the voltage sensor 48 having sensed that the voltage across the capacitor 20, representing the voltage of the source 10, is excessive (see graph 2G) and regenerative braking terminates until the source voltage has been re duced to an acceptable level, as shown at time t4 in graph 2G. During the time period t, to tithe current through the thyristor 42, 142, and the current through the inductor 22, 12,, which are the same as the armature current, continue to decrease as shown on graphs 2D and 2F respectively. At time t3, the main thyristor 18 is triggered on, because the armature current has become too small for effective braking. This reduces the voltage drop across the thyristor 18 essentially to zero. At this time the current through the thyristor 18 and the inductor 22 increases, as can be seen on graphs 2B and 2F, since an effective short is placed across the terminals of the armature 12 through the inductor 22.The commutating thyristor 30 is triggered on, to reverse bias the main thyristor 18 and the thyristor 42 when the armature current has reached its required level, or when the source voltage is adequately reduced. If the armature current has reached its required level, but the source voltage has not yet fallen low enough to accept further braking energy, the effect of gating off the main thyristor 18 without gating off thyristor 42 can be achieved by gating off both thyristors by gating on the commutating thyristor 30, and then gating the thyristor 42 on again.In the present example, however, the source voltage has fallen low enough to accept further energy at t*, so that both thyristors 18 and 42 are shown as being tumed off at time 4. When this happens the voltages V, of the graph 2A and Vj2 of the graph 2C will increase again to their maximum values and the current through the diode 34 will go to its maximum value, as shown on the graph 2E. Current will again flow into the DC source and the current I,, will again start to decrease as the voltage across the capacitor 20 increases as regenerative braking resumes and the cycle repeats. Graph 2H, showing I,,, shows that the current through the filter inductor 16 is constantly flowing. It is shown as a slightly varying direct current which results from the LC combination of the inductor 16 and the capacitor 20 and the pulsating current from the switching of the thyristors 18 and 42. While thyristors 18 and 42 are nonconducting, between times t1 and t2, the current through the inductor 16 is decreasing. When the thyristor 42 is switched on at time t,, the current through the inductor 16 begins a slight increase which is due to the fact that the filter capacitor 20 has been charged to a voltage higher than the input power source. At t3, when the thyristor 18 is triggered on and the armature current is increasing through the thyristor 18, owing to the short circuit, the current through the filter inductor 16 is increasing to a maximum value. This maximum value occurs about midway between time t3 and time t4 when the filter capacitor 20 is charged to a voltage equal to the voltage of the input power source 10.At that point it begins to decrease and this decrease continues from time t4 to time 4, during which time both the thyristors 18 and 42 are turned off, so that the voltage on the capacitor 20 is increasing. At time t4 the thyristor 42 is gated on and the current I,, through the filter inductor 16 will begin to increase as it did at time 4. Thus, the cycle is repeated. If the thyristor 42 remains on, with the braking current flowing through the resistor 44, for any appreciable period, the current Il6 through the filter inductor 16 will decrease until it becomes zero. However, during the brief periods of resistive braking used in the preferred embodiment the current I,, will not reach zero but will decrease in average value the longer or more often the thyristor 42 is conducting. WHAT WE CLAIM IS:-

1. A motor arrangement comprising a d.c.

motor, positive and negative supply lines, and a bridge circuit connecting the motor armature to the supply lines, the bridge circuit having first and second legs connecting one side of the motor armature to the positive and negative lines respectively, and third and fourth legs connecting the other side of the motor armature to the positive and negative lines respectively, the first and fourth legs containing means arranged to pass a regenerative current from the motor to the supply lines, and the second and third legs containing means arranged to pass a motoring current from the supply lines to the motor, and the motor arrangement also including means, separate from the bridge circuit, for energising the field of the motor, and controlling means arranged to control the brdige circuit and the field energising means in such a way that the motor can be selectively operated in a motor mode, and in a regenerative mode in which the direction of current in the armature of the motor is the same as in the motor mode, while the direction of the field energisation is reversed by the field energising means on a change between the motor mode and the regenerative mode.

2. An arrangement as claimed in Claim 1 in which one of the second and third legs includes

a switching device serving to switch the arrangement between its motor mode (when the switching device is closed) and its regenerative mode (when the switching device is open).

3. An arrangement as claimed in Claim 2, in which the other of the second and third legs includes a thyristor chopper circuit controlling the current through the motor armature during operation in the motor mode.

4. An arrangement as claimed in Claim 1 or Claim 2 or Claim 3, which also includes a short-circuit path connected across the motor armature, and means controlling the short-circuit path to short-circuit the motor armtaure intermittently during operaion of the motor in the regenerative mode.

5. An arrangement as claimed in Claims 3 and 4 in which the chopper circuit, together with one of the first and fourth legs of the bridge, constitutes the short-circuit path.

6. An arrangement as claimed in Claim 4 or Claim 5 in which the controlling means is arranged to control the short-circuit path in dependence on motor armature current, the short-circuit path being closed when the armature current falls below a predetermined value, and being opened when the armature current rises above a predetermined value during operation in the regenerative mode.

7. An arrangement as claimed in any of the preceding claims, which also includes means arranged, during operation in the regenerative mode, to limit the transfer of energy to the supply lines if a parameter which is dependent on the voltage across the supply lines should exceed a predetermined value.

8. An arrangement as claimed in Claim 7 in which the energy transfer limiting means includes a resistive braking path which is selectively connected across the motor armature.

9. An arrangement as claimed in Claim 8 in which the resistive braking path is connected across the motor armature when the voltage across the supply lines exceeds a predetermined value during operation in the regenerative mode, and is disconnected again when the voltage across the supply lines falls below a predetermined value.

10. An arrangement as claimed in Claim 8 or Claim 9 in which the resistive braking path includes one of the first and second legs, and one of the third and fourth legs.

11. An arrangement as claimed in Claim 10 when appendant to Claim 5 in which the resistive braking path comprises those parts which constitute the short-circuit path, with the exception of the chopper circuit, and also comprises a resistive path connected in parallel with the chopper circuit.

12. An arrangement as claimed in Claim 11 in which the resistive path in parallel with the chopper circuit is controlled by a separate thyristor.

13. A motor arrangement substantially as herein described with reference to the accompanying drawings.