GB2220313A

GB2220313A - The control of a series wound electrical machine in an electrically operated vehicle

Info

Publication number: GB2220313A
Application number: GB8911758A
Authority: GB
Inventors: Siegfried Puschel
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1988-05-26
Filing date: 1989-05-22
Publication date: 1990-01-04
Anticipated expiration: 2009-05-22
Also published as: FR2633788B1; DE3817936A1; GB8911758D0; GB2220313B; FR2633788A1

Description

1 2220313 APPARATUS FOR AND METHOD OF CONTROLLING A SERIES WOUND

ELECTRICAL MACHINE IN AN ELECTRICALLY OPERATED VEHICLE The invention concerns an apparatus for and method of controlling a series wound electrical machine in an electrically operated vehicle.

It is generally known how to achieve an increase in speed in d.c. motors by weakening the magnetic field of the field coils. This is achieved by means of a resistance in the exciting circuit, which reduces the exciting current and thus the magnetic flux and the torque, while the speed increases due to the Induced source voltage that remains the same.

One possible application of this principle is in electrically operated vehicles, e.g. in electric fork lifts that are driven by one or more series wound motors and receive their energy from a battery. The series wound or inverse-speed motor and the electric plant are designed so that there is a high torque and a high efficiency at low speeds of the electric fork lift. If the fork lift moves to another working location, for example, it is desirable to cut down the torque of the fork lift. In this state, an r.p.m. increase is obtained in the motor and thus an increase in the speed of the fork lift due to the operator switching a resistance into the exciting circuit. A field weakening is particularly desirable in electric fork lifts in such situations.

-2 According to one aspect of this invention there is provided apparatus for controlling a series wound electrical machine in an electrically operated vehicle, said apparatus having at least one resistance connected in parallel with an exciting winding of the machine, the resistance being arranged to be connected into and out of circuit with the exciting winding by means of a summing switch which has inputs connected to a speed set-point pickup, a speed actual-value pickup, means for detecting the operating-mode of the series wound machine, and an on-off switch., such that said summing switch causes said resistance to be connected into circuit with the exciting winding to provide field weakening and so an enhanced speed characteristic of the machine when said inputs receive a predetermined set of signals.

According to another aspect of this invention there is provided a method of controlling a series wound electrical machine in an electrically operated vehicle, said method including the steps of detecting speed demand via a speed set-point pickup so as to supply a first signal, detecting actual speed value via a speed actual value pickup so as to supply a second signal, detecting the operating mode of the series wound machine so as to supply a third signal and detecting a field weakening request via an on-off switch so as to supply a fourth signal, summing said first to fourth signals in a summing switch and using the latter to switch a resistance into and out of an exciting circuit of the machine whereby field weakening is provided in response to a predetermined combination of said first to fourth signals.

Through a simple logic switching of the parameters, a field weakening can thus be instituted for a special operating state. The detection of the r. p.m. actual-value can take place here through an inductive pulse generator or a sensor designed as a tachodynamo or as an incremental pickup in the drive unit or directly on the series wound motor, in which case the r.p.m. proportional frequency detected is converted to a speedproportional voltage. The setting of the travel command pickup can be ascertained through the control voltage produced by it.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing in which the sole figure is a block circuit diagram of a motor arrangement of an electrically operated vehicle embodying the present invention.

Referring to the drawing, a series-wound d.c. motor supplied by a battery 1 is designed as an inverse-speed motor 3 through an appropriate arrangement of the exciting winding 2. The current circuit can be closed by means of a switch 4. The figure provides only essential details for the purpose of this description. Control and regulating elements, as they are required, for example, for the electrical circuit of an electric fork lift, are not shown. A resistance 5, which can be switched by means of a contactor 6, is present in the exciting circuit, in parallel with the exciting winding 2. A field weakening and thus an r.p.m. increase in the inverse-speed motor 3 can thus be achieved. Control of the contactor 6 is shown only symbolically.

Signals are fed to a summing switch 7 by sensors that detect the instantaneous states of various parameters, among other things. Inputs to the summing switch 7 are provided by an on-off switch 8 that can be manually actuated by the operator, an r.p.m. set-point pickup 9, to which a prescribed travel speed is assigned, an r.p.m. actual-value pickup 10, which detects the instantaneous travel speed, an operating mode pickup 11 that recognises whether the series-wound machine is operating in the generator or motor mode and a sensor 12 for detecting the direction setting of the travel command pickup (by detecting the control voltage determined by the setting of the travel command pickup). If a signal is present at each of the sensors, the summing switch 7 also transmits a signal to a final amplifier 13, which, as a result, actuates the contactor 6 and thus institutes the field weakening. The field weakening is kept switched on until any one of the sensors no longer transmits a signal to the switch 7.

Both units 9 and 10 are sending binary signals to switch 7. Unit 10 detects the r.p.m. actual value by using for example an inductive pulse generator. The r.P.m. proportional frequency detected is converted to a speed-proportional voltage. The voltage is supplied to a threshold switch in the unit 10 which gives a binary signal to unit 7 when the voltage is higher than a certain voltage level.

Unit 9 works in a similar manner. The signal representing the desired travel speed is produced by a voltage source which is able to generate a voltage proportional to the desired speed. The voltage is supplied to a second threshold switch which gives a binary signal to switch 7 when voltage is higher than a certain voltage level.

Unit 9 is arranged so that the field weakening is actuated only when a further acceleration is desired. If the vehicle's speed is at that moment below a certain value as detected by unit 10, the field weakening is not provided because that would cost too much energy.

Thus the pickup 9 is responsible for detecting a control voltage which is proportional to the desired vehicle speed as determined by the travel command pickup. Output of a signal from the pickup 9 to the switch 7 then depends on the setting of the r.p.m. set-point pickup 9.

If the vehicle is decelerated by reducing the control voltage down to zero volts the machine works as an electrical generator and produces a current flow which is detected by pickup 11 as a change in operating mode.

If the driver wants to decelerate the vehicle in a more positive way, he reverses the electrical machine. For this purpose he has to change the preselected travel direction by operating the travel command pickup and then to increase the control voltage measured by pickup 9 for providing the machine with a current flow. As a result, when braking the vehicle by reversing the machinel the latter works as a motor but tends to accelerate the vehicle (i.e. deceleration) in the direction opposite to the vehicle direction at the beginning of the braking operation. This operating mode (braking by reversing the electrical machine) is detected by means of sensor 12 which operates in parallel with the pickup 11 so that fiel weakening is only possible when the machine is not working in the braking or generator modes.

For field weakening operation, a prescribed travel speed must already be exceeded and there must be a command on the part of the operator to further Increase the travel speed. For this purpose, the travel speed (pickup 10) and the setting of the travel command pickup, e.g. the pedal setting (pickup 9), are monitored. At the same time, the vehicle drive must be in quadrant I or III of its operational mode cycle, i.e. forward or backward acceleration in the motor operation, which necessitates a supervision (pickups 11 and 12) of the operating mode of the series wound machine. Only if all these conditions are met and the operator also actuates the switch 8 does the field weakening occur. Whether these parameters for initiating the field weakening are present is detected by the summing switch, which then forwards a signal if necessary. The use of such a field weakening enables a higher travel speed and thus a higher turnover efficiency at a lower energy consumption in comparison with a fork lift design not having the possibility of field weakening.

It is preferred that some of the parameters have switching thresholds of the two-point regulation type such as to provide hysteresis between the switching in and out of the field weakening, which induces definite switching states for at least a minimum period of time, so that the drive can be adjusted to it. Fluttering of the contactor is also prevented.

Thus, the travel speed must fall below the ON switching value by a certain amount for a switching out or OFF of the field weakening to occur.

The travel speed will then be reduced if there is a load on the drive due to ascending a rise.

The parameter settings of the travel command pickup and the operating mode also have such switching thresholds.

CLATMS:

Apparatus for controlling a series wound electrical machine in an electrically operated vehicle, said apparatus having at least one resistance connected in parallel with an exciting winding f the machine, the resistance being arranged to be connected into and out of circuit with the exciting winding by means of a summing switch which has inputs connected to a speed set-point pickup, a speed actual-value pickup, means for detecting the operating-mode of the series wound machine, and an on-off switch, such that said summing switch causes said resistance to be connected into circuit with the exciting winding to provide field weakening and so an enhanced speed characteristic of the machine when said inputs receive a predetermined set of signals.

2. Apparatus for controlling an electrical series wound machine for use in an electrically operated vehicle, said apparatus comprising an exciting circuit including:

a) b) a summing switch having a plurality of inputs; at least one resistance which can be selectively switched into and out of the exciting circuit in dependence on a signal from an output of said summing switch which is arranged to provide a field weakening selection signal when predetermined respective signals are received at said Inputs; c) d) e) f) actual speed value pickup means connected to the summing switch so as to supply to a first of the inputs of the latter a first signal indicating fulfilment of a required condition of the instantaneous travel speed of the vehicle; travel command signal pickup means connected to the summing switch so as to supply to a second of the inputs of the latter a second signal indicating fulfilment of a prescribed condition of the travel speed demanded via the travel command pickup; means for detecting one or more prescribed modes of operation of the machine and for supplying a third signal to a third Input of the summing switch during the or each said prescribed mode; a field weakening command means connected to a fourth input of the summing switch whereby the presence of a field weakening command as fourth signal is detected and sent to the summing switch so that when said first to fourth signals are present at the inputs of the summing switch said at least one resistance is switched into the exciting circuit to provide field weakening.

3. Apparatus according to claim 1 or claim 2, wherein the field weakening is switched off when the actual travel speed of the vehicle is less than a prescribed travel speed.

4. Apparatus according to any one of the preceding claims, wherein the field weakening is switched off when a prescribed setting of the travel command pickup is not reached. 5. Apparatus according to any one of the preceding claims, wherein the operating mode detecting means is arranged to switch off the field weakening when the series wound machine is providing generator operation. 6. Apparatus according to any one of the preceding claims, wherein the operating mode detecting means is arranged to switch off the field weakening when the travel command pickup is arranged to provide deceleration of the machine. 7. Apparatus according to any one of the preceding claims, further comprising at least one contactor electrically connected in series with the or each said resistance and a final amplifier electrically connected between said summing switch and the contactor. 8. Apparatus for controlling a series wound electrical machine in an electrically operated vehicle, said apparatus being substantially as described herein with reference to the accompanying drawing. 9. A method of controlling a series wound electrical machine in an electrically operated vehicle, said method including the steps of detecting speed demand via a speed set-point pickup so as to supply first signal, detecting actual speed value via a speed actual value pickup so as to supply a second signal, detecting the operating mode of the series wound machine so as to supply a third signal and detecting a field weakening request via an on-off switch so as to supply a fourth signal, summing said first to fourth signals in a summing switch and using the latter to switch a resistance intd and out of an exciting circuit of the machine whereby field weakening is provided in response to a predetermined combination of said first to fourth signals.

10. A method according to claim 9, wherein during field weakening, when the actual travel speed does not reach a specific value. the field weakening is switched off.

11. A method according to claim 9 or claim 10, wherein during field weakening, when a prescribed setting of speed demand is not reached, the field weakening is switched off.

12. A method according to any one of the preceding claims, wherein during field weakening, when a change occurs in the operating mode of the series wound machine from motor operation to generator operation, the field weakening is switched off.

13. A method of controlling a series wound electrical machine in an electrically operated vehicle, said being being substantially as described herewith with reference to the accompanying drawing.

Published 1989 atThe Patent Office. State House, 6671 HighRolborn. London WClR4TP. Further copies maybe obtained from The Patent Office. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87