GB2463917A

GB2463917A - Limiting the power generated by an electric generator

Info

Publication number: GB2463917A
Application number: GB0817883A
Authority: GB
Inventors: Ian Forster
Original assignee: Visteon Global Technologies Inc
Current assignee: Visteon Global Technologies Inc
Priority date: 2008-09-30
Filing date: 2008-09-30
Publication date: 2010-03-31
Anticipated expiration: 2028-09-30
Also published as: GB2463917B; GB0817883D0

Abstract

An electric generator system 1 comprises a generator 2 and a control system 10 that generates a power signal 12 representing the generated current, an electronic controller 14. The controller generates a digitised output demand signal 18 for the generator 2 which extends across a range of values including a lower and a higher range. The controller determines for each range of values a rate of change in the power signal 12 with changes in the digitised demand signal 18) and stores them in a memory 15. The controller compares the rates of change and restricts the operation of the generator 2 at higher demanded electrical power when the ratio of the higher demand rate of change (fig 2, 22) to the lower demand rate of change (fig 2, 20) is below a threshold. The threshold may be stored in memory and vary depending on generator usage time and the demand may depend upon an input 16 from a consumer such as a battery to be charged. The control system may be used to control a vehicle alternator which can thereby produce maximum current efficiently at or just below the saturation point without accurate measurement of the output current.

Description

Limiting the Power Generated by an Electric Generator

BACKGROUND

a. Field of the Invention

The invention relates to electric generators and more specifically relates to the control of electric generators during operation.

b. Related Art Generators work on the principle that an electric current is generated in a wire when it is cut by moving magnetic flux lines. The two main parts of the generator are the magnetic field component and the coils in which the current is produced.

in general, the magnetic field is produced by electromagnets on the rotor and the coils where the current is produced are located on the stator of the generator. In many cases, the stator consists of copper coils around a laminated iron core.

During use, as the rotor rotates, magnetic flux lines cut through the stator coils producing an electromotive force that drives a current around a circuit or device connected to the coils. * ** * *.. * S *SS

The amount of electromotive force or current that can be generated is limited by the saturation flux of the laminated iron. Once the iron is saturated, it can carry no more magnetic flux and the generator is not able to generate any more electric current. Any attempt to increase the output of the generator, past the saturation a...

point, results in heating of the electromagnets which produce the magnetic field, a.,...

* with no increase in the output current.

Therefore, to produce the maximum current most efficiently, the generator should be operated at or just below the saturation point.

Often, alternators are designed so that the rectifiers can handle the maximum current, and the field coils and driver transistors are designed to handle any attempt to demand more than the maximum current. However, this comes at a slight on-cost. Additionally, dynamos and switched reluctance generators are often designed to measure the current being generated and actively limit the demand so that it does not exceed the maximum. However, this has the disadvantage that the current must be measured very accurately to achieve the best output from the generator.

SUMMARY OF THE INVENTION

According to the invention, there is provided an electric generator system, comprising an electric generator and a control system for controlling the operation of the generator, the control system comprising, a means for generating a power signal representative of the electrical current generated within the electric generator, and an electronic controller including a memory, wherein the controller is arranged to: -generate a digitised output demand signal for demanding electrical power from the generator, the output demand signal extending across a range of values that includes a first range of values and a second range of values corresponding with, respectively, relatively lower and higher ranges of demanded electrical *...

power; -determine for the first range of values a first rate of change in the power *:: signal with changes in the digitised output demand signal and to store the first rate *...25 of change in the memory; S...

-determine for the second range of values a second rate of change in the S.....

* power signal with changes in the digitised output demand signal and to store the second rate of change in the memory; -compare the second rate of change with the first rate of change and to restrict the operation of the generator at said higher ranges of demanded electrical power when the ratio of the second rate of change to the first rate of change is be!ow a threshold.

It may be preferable that the operation of the generator is precluded at the higher ranges of demanded electrical power when the ratio of the second rate of change to the first rate of change has fallen below the threshold. In other circumstances it may be preferable to restrict the operation of the generator by lowering the output demand signal below the second range of values after the ratio of the second rate of change to the first rate of change has been determined by the controller to be below the threshold.

The threshold may be a predetermined threshold stored in the memory of the electronic controller. Preferably, the controller is arranged to monitor the time over which the electric generator has been used and to vary the threshold depending on the time over which the electric generator has been used.

In some embodiments it may be advantageous to provide additionally a user control. In use, the control provides an input demand signal, and the output demand signal is based on the input demand signal.

It may be preferable that the electronic controller generates a maximum output demand signal based on the second range of values, indicating the operating limit of the generator.

Also according to the invention, there is provided a method of controlling an electric generator, using a control system comprising an electronic controller : including a memory, in which the method comprises the steps of: -generating a digitised output demand signal to demand electrical power I.....

* from said generator, the output demand signal extending across a range of values that includes a first range of values and a second range of values corresponding with, respectively, relatively lower and higher ranges of demanded electrical power; -determining a first rate of change in the power signal with changes in the digitised output demand signal for the first range of values and storing the first rate of change in the memory; -determining a second rate of change in the power signal with changes in the digitised output demand signal for the second range of values and storing the second rate of change in the memory; -comparing the second rate of change with the first rate of change; and -restricting the operation of the generator at said higher ranges of demanded electrical power when the ratio of the second rate of change to the first rate of change is below a threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an electric generator system; Figure 2 is a graph showing the relationship between the demanded electrical power from a generator and the resulting output power; and * S. * * Figure 3 is a flow diagram representing the steps in the method for *S..

controlling an electric generator. *5II * . S. 0.

S **... * .

DETAILED DESCRIPTION S...

S

*S.SSS * A preferred embodiment of the electric generator system 1 is shown in Figure 1. A generator 2, for example a motor vehicle alternator, receives an input 4 in the form of mechanical power. This mechanical power may be from a drive system, for example a drive belt or wheels, or similar. The generator works to provide an electrical power output 6. The electrical power is drawn from the generator 2 by some form of electrical power consumer 8. The electrical power consumer may be in the form of a battery or the electrical components of a vehicle.

A control system 10 receives a power signal 12 based on the electrical power output 6 of the generator 2. The power signal 12 may be a direct measurement of the current generated by the generator 2. The control system 10 includes an electronic controller 14 and a memory 15. The electronic controller 14 may be a microprocessor or may be a circuit board including at least one microprocessor.

The memory 15 may be of any suitable type and may be physically part of the controller 14.

In a preferred embodiment, the electronic controller 14 receives an input demand signal 16. This input demand signal may be provided from a user control (not shown). In one embodiment, the input demand signal 16 is based on an error signal. For example, if the electrical power consumer 8 is a battery then an error signal may be generated due to the difference in the actual voltage level and the desired voltage level of the battery. This error signal may then be amplified to produce the input demand signal 16.

The electronic controller 14 generates a digitised output demand signal 18, which **,*0 may be based on the input demand signal 16. The digitised output demand signal 18 is used to demand an increase in electrical power generated by the generator 2 and is in the form of discrete step increases in demand. These discrete steps b extend across a range of values from an initial or current electrical power output level up to the desired or final output level. It is therefore possible to define a first : range of values and a second range of values in which at least the highest value of the second range of values is greater than the highest value of the first range of * *. * ** * S values.

In a preferred embodiment, the digitised output demand signal 18 is in the form of step increases in a current passing through a power transistor(s) (not shown) connected to the electromagnets (not shown) which produce the magnetic field of the generator 2.

For each step increase in demanded electrical power, there is a corresponding step increase in electrical power output 6 of the generator 2, greater than or equal to zero. This is shown by the representative power output curve shown in figure 2.

The decrease in the gradient of the curve at higher power output values is due to saturation of the iron carrying the magnetic flux in the generator. Often there is a sharp change in gradient at the saturation point, with very little increase in output power with further increases in the demand signal above this point.

The electronic controller 14 uses the power signal 12 and digitised demand signal 18 to calculate a first rate of change in the power signal with demand signal at relatively lower values 26 of the digitised output demand signal 18. For example the first rate of change may correspond to the slope of a first region 20 of the power output curve in figure 2. The value of this first rate of change is stored in the memory 15 of the control system 10.

The electronic controller 14 then calculates a second rate of change in the power signal with demand signal at subsequent and relatively higher values of the digitised output signal 18. This second rate of change may correspond to the 2O slope of a second region 22 of the power output curve in figure 2. The value of this subsequent and second rate of change is also stored in the memory 15 of the control system 10. * . S..

The electronic controller 14 then calculates the ratio of the second rate of change to the first rate of change. This ratio is compared to a threshold value. The *.** threshold value may be predetermined and stored in the memory 15. In other S....

* S embodiments the threshold may be set dependent on the time for which the generator has been operating. The electronic controller may monitor the time over which the generator has been used during a single use and may monitor the number of times the generator 2 has been used, or the total time. This time value may then be used to vary the threshold value.

When the ratio of the second rate of change to the first rate of change is below the required threshold value, the electronic controller 14 restricts the operation of the generator 2. This restriction may involve precluding the operation of the generator 2 at higher demanded electrical power, for example above a value of the digitised output demand signal 18 in the second range of values. It may be preferable in some circumstances to restrict the operation of the generator 2 by lowering the digitised output demand signal 18 to a value below the second range of values.

This may be of particular advantage when the change in slope of the power curve is very sharp or if the generator 2 is to be used for prolonged periods of time.

In some embodiments it may also be advantageous for the electronic controller 14 to generate a maximum output demand signal 24 based on the second range of values. This maximum output demand signal 24 may be output to a display, to inform a user of the electric generator system I of the operating limit of the generator 2. The maximum output demand signal 24 may also be stored in the memory 15, or output to a suitable device. Furthermore, the maximum output demand signal 24 may be a value based on the first range of values of the digitised output demand signal 18, or a similar suitable value.

*O Therefore, the invention offers a means of controlling an electric generator and restricting its operation to a value of generated current below the saturation point of the generator. Furthermore, this control does not require accurate measurements of the output current of the generator as it only requires changes in the demand signal and output power to be measured. I...

S S *

Claims

CLAIMS1. An electric generator system, comprising an electric generator and a control system for controlling the operation of the generator, the control system comprising, a means for generating a power signal representative of the electrical current generated within the electric generator, and an electronic controller including a memory, wherein the controller is arranged to: -generate a digitised output demand signal for demanding electrical power from the generator, the output demand signal extending across a range of values that includes a first range of values and a second range of values corresponding with, respectively, relatively lower and higher ranges of demanded electrical power; -determine for the first range of values a first rate of change in the power signal with changes in the digitised output demand signal and to store the first rate of change in the memory; -determine for the second range of values a second rate of change in the power signal with changes in the digitised output demand signal and to store the second rate of change in the memory; -compare the second rate of change with the first rate of change and to *.*ZO restrict the operation of the generator at said higher ranges of demanded electrical * *e ***. power when the ratio of the second rate of change to the first rate of change is * *..below a threshold. * .
2. An electric generator as claims in Claim 1, in which the operation of the :a5 generator is precluded at said higher ranges of demanded electrical power when the ratio of the second rate of change to the first rate of change is below a * threshold.
3. An electric generator as claims in Claim 1, in which the operation of the generator is restricted by lowering the output demand signal below the second range of values after the ratio of the second rate of change to the first rate of change has been determined by the controller to be below said threshold.
4. An electric generator system as claimed in any preceding Glaim I claim, in which the threshold is a predetermined threshold stored in said memory.
5. An electric generator system as claimed in any of Claims I to 3, in which the controller is arranged to monitor the time over which the electric generator has been used and to vary the threshold depending on the time over which the electric generator has been used.
6. An electric generator system as claimed in any preceding claim, comprising additionally a user control, in use the control providing an input demand signal, in which the output demand signal is based on the input demand signal.
7. An electric generator system as claimed in any preceding claim, in which the electronic controller generates a maximum output demand signal based on the second range of values indicating the operating limit of the generator.
8. A method of controlling an electric generator, using a control system comprising an electronic controller including a memory, in which the method comprises the steps of: -generating a digitised output demand signal to demand electrical power *:.::, from said generator, the output demand signal extending across a range of values ***.that includes a first range of values and a second range of values corresponding with, respectively, relatively lower and higher ranges of demanded electrical power; -determining a first rate of change in the power signal with changes in the digitised output demand signal for the first range of values and storing the first rate 0*SS** * of change in the memory; -determining a second rate of change in the power signal with changes in the digitised output demand signal for the second range of values and storing the second rate of change in the memory; -comparing the second rate of change with the first rate of change; and -restricting the operation of the generator at said higher ranges of demanded electrical power when the ratio of the second rate of change to the first rate of change is below a threshold.
9. An electric generator system, substantially as herein described, with reference to or as shown in the accompanying drawings.
10. A method of controlling an electric generator, substantially as herein described, with reference to or as shown in the accompanying drawings. * I. * * * * 0* **** * . **** I. * * **********. * * * *****.*... * *