GB2062380A - Regenerative braking systems for D.C. motors - Google Patents

Abstract

A regenerative braking system for a D.C. motor 1, 2 comprises an electronic switch 5 in series with the motor, means for cyclically turning the switch on and off to control the flow of current from a battery 3 to the motor during motor driving and to control the flow of current from the motor to the battery during regenerative braking, and means 4, 10, 11 for selectively interconnecting the motor and the battery in driving and regenerative braking modes. Means are provided for initiating self- excitation of the motor when the system is in the regenerative braking mode, the self-excitation initiating means comprising at least one capacitor 14 connected such that when the system is in the driving mode the capacitor is charged by the battery and when the system is in the regenerative braking mode the capacitor is discharged through the motor. The capacitor may be connected in series with a resistor (15) and diode (17) across the battery and in a closed circuit with the motor, the electronic switch and a further diode (16), Figure 3 (not shown). <IMAGE>

Description

SPECIFICATION Regenerative braking systems for d.c. motors The present invention relates to regenerative braking systems for D.C. motors, and in particular to pulse controlled regenerative braking systems.

Pulse controllers for controlling the energisation and regenerative braking of D.C.

traction motors are known. The use of such pulse controllers with their low power losses and smooth operating characteristics in both energisation and regenerative braking modes, can significantly increase the range of batterypowered electric vehicles.

Known pulse controllers comprise an electronic switch such as a thyristor, which controls the connection of the motor to a power source such as a battery. During normal motor driving and regenerative braking the electronic switch is turned on and off cylically. In the normal motor driving mode of operation the ratio of "on" to "off" periods (the mark/space ratio) determines the mean power supplied to the motor. In the regenerative braking mode of operation, where the motor acting as a generator is used to recharge the battery, the electronic switch is turned "on" and "off" to prevent the motor current failing too low or rising too high.

In a regenerative braking system, when an attempt is made to achieve regenerative braking by switching to the regenerative braking mode, the motor current must rise to a minimum current before self-excitation can occur. In order to ensure that this minimum current is reached means are provided for building up the field current.

One previously proposed means for building up the field current to initiate self-excitation comprises a switch connected in series with the field winding. The switch may be closed for a predetermined period such that the field winding is connected across the power source for the said predetermined period. In order to protect the field winding a resistance is provided in series with the switch.

During the build up of a current in the field winding the power dissipation in the resistance is very high. This results in low efficiency and reliability and, because of the power dissipation requirements of the resistance, the use of extra heatsinking materials.

An object of the present invention is to obviate or mitigate the above problems.

According to the present invention there is provided a regenerative braking system for a D.C.

motor comprising an electronic switch in series with the motor, means for cyclically turning the switch on and off to control the flow of current from the battery to the motor during motor driving and to control the flow of current from the motor to the battery during regenerative braking, means for selectively interconnecting the motor and the battery in driving and regenerative braking modes, and means for initiating self-excitation of the motor when the system is in the regenerative braking mode, characterised in that the selfexcitation initiating means comprise at least one capacitor connected such that when the system is in the driving mode the capacitor is charged by the battery and when the system is in the regenerative braking mode the capacitor is discharged through the motor.

Preferably the capacitor is connected in series with a resistor and diode across the battery and in a closed circuit with the motor, the electronic switch and a further diode, the further diode preventing charging of the capacitor except via said resistor.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings wherein: Fig. 1 is a circuit diagram of a know regenerative breaking system; Fig. 2 is a circuit diagram of a first embodiment of the present invention; and Fig. 3 is a circuit diagram of a further embodiment of the present invention.

Referring to Fig. 1, a D.C. series motor comprising an armature 1 and field winding 2 is shown connected across a battery 3 in series with line contacts 4 and an electronic switch 5 controlled by pulse generator 6. Forward and reverse contacts 7, 8 control the connection of the field 2 in series with armature 1.

The electronic switch 5 may comprise a thyristor arranged to be turned on and off cyclically according to pulses supplied by the pulse generator 6. A plugging diode 9 is connected across the armature and a flywheel diode 10 is connected across the armature and field. The circuitry described above enables the motor to be driven in either direction.

A further diode 11 is provided to enable the regenerative braking of the motor. When the motor is rotating, the contacts 4 are opened, and the switch 5 is closed, the motor acts as a generator and current builds up in the path defined by the armature 1, field 2, switch 5 and diode 11.

Once the armature voltage exceeds the battery supply voltage, the switch 5 may be opened and current is then driven through the battery 3 via diodes 10 and 11. Regenerative braking is thus achieved and by careful control of the switch 5 pulses of current can be passed through the battery for as long as the armature voltage can be maintained above the supply voltage.

If regenerative braking is to start it is necessary for the field current to be such that a sufficiently high voltage is generated in the armature to render the diode 11 and switch 5 conductive.

Once this has occurred the motor will be selfexciting.

As is known from example German Patent Publication P2806373, self-excitation of the motor can be achieved by closing a switch 12 in series with a resistor 13 for a predetermined period sufficient to initiate self-excitation. Current is supplied to the field winding 2 through the switch 1 2 and resistor 1 3. The current supplied to the field winding increases the generated voltage in the armature which increases the field current stiil further and self-excitation takes place.

Because of the large current supplied to the field winding to initiate self-excitation it is necessary for the resistor 1 3 to be capable of dissipating large amounts of power. In addition, where the switch 12 is an electronic switch such as a transistor, it must be capable of carrying large currents. Furthermore, the increase of current in the field winding when the switch 12 is closed is relatively slow, and therefore the start of regenenrative braking is delayed.

An embodiment of the present invention will now be described with reference to Fig. 2, components common to Figs. 1 and 2 bearing the same reference numerals.

A capacitor 14 is provided across the armature 1, field 2 and switch 5. The switch 12 and resistor 13 of Fig. 1 are omitted.

During the normal driving mode of operation, switch 4 is closed. This allows the battery 3 to supply current to the motor and also to capacitor 14, thus charging the capacitor to battery voltage.

On switching to the regenerative braking mode, switch 4 is opened and capacitor 14 discharges through armature 1, field winding 2 and electrode switch 5, thereby initiating self-excitation of the motor.

In the embodiments of Fig. 2, if the capacitor 14 is particularly large the capacitor charging current drawn through the switch 4 when it is closed after the termination of regenerating braking may be excessive. This may result in sparking across the contacts of switch 4 thus reducing its useful life.

A further embodiment of the invention will now be described with reference to Fig. 3. The embodiment of Fig. 3 avoids the risk of damage to the switch 4 as described above.

With reference to Fig. 3 a resistor 1 5 is provided in series with capacitor 14. The resistor 1 5 provides a trickle charge to the capacitor 14 from battery 3, thus bypassing switch 4. To prevent the flow of current via switch 4 to capacitor 14 a diode 1 6 is provided between the switch 4 and the capacitor 14. To prevent the flow of current from the capacitor when in the driving mode diode 17 is provided in series with resistor 1 6 between the battery 3 and the capacitor 14.

Because the voltage available at the capacitor 14 of the embodiments of Figs. 2 and 3 is relatively large, motor self-excitation is initiated more quickly than in the case of the known system described with reference to Fig 1 where voltage is dropped across resistor 13.

In heavy duty systems such as are installed in large vehicles, it is known to provide filter capacitors across the battery to smooth the battery voltage during normal drive. The capacitor 14 of Figs. 2 and 3 can also perform this function.

No filter capacitor is needed during regenerative braking.

In the embodiments of Figs. 2 and 3, the initiation of self-excitation is achieved without using a switch such as switch 12 of Fig. 1, without having to provide a switch control circuit to control the time for which the switch 12 is closed, and without passing current through a resistor such as resistor 13 of Fig. 1. Greater energy efficiency and reliability result.

Claims (4)

1. A regenerative braking system for a D.C.

motor comprising an electronic switch in series with the motor, means for cyclically turning the switch on and off to control the flow of current from the battery to the motor during motor driving and to control the flow of current from the motor to the battery during regenerative braking, means for selectively interconnecting the motor and the battery in driving and regenerative braking modes, and means for initiating self-excitation of the motor when the system is in the regenerative braking mode, characterised in that the selfexcitation initiating means comprise at least one capacitor connected such that when the system is in the driving mode the capacitor is charged by the battery and when the system is in the regenerative braking mode the capacitor is discharged through the motor.

2. A regenerative braking system according to claim 1, characterised in that the capacitor is connected in series with a resistor and diode across the battery and in a closed circuit with the motor, the electronic switch and a further diode, the further diode preventing charging of the capacitor except via said resistor.

3. A regenerative braking system according to claim 1, characterised in that the capacitor is connected in a closed circuit with the motor and the electronic switch.

4. A regenerative braking system for a D.C.

motor substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.