GB2056725A

GB2056725A - Generator output regulator

Info

Publication number: GB2056725A
Application number: GB7928144A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-08-13
Filing date: 1979-08-13
Publication date: 1981-03-18

Abstract

Rectified direct current at constant voltage is obtained from an alternator 1, driven over a wide range of speeds, by moving a permanent magnet rotor 5 axially in relation to the stator 3 to decouple the magnet flux from the stator windings 2 when the speed rises. The rotor 5 is moved by means such as an hydraulic piston 16 controlled by a bi-directional valve 21 according to an error signal proportional to the difference between the actual generator voltage and the required constant voltage. The rotor is moved in a direction to reduce this error signal. <IMAGE>

Description

SPECIFICATION Electrical generator producing constant direct current over a wide speed range This invention is concerned with electrical alternators required to produce a constant voltage or current when driven over a wide range of speeds. A particular application is to alternators used on heavy duty vehicles which are driven from the vehicle engine at speeds varying over a wide range and which are required to supply direct current at a substantially constant voltage. For convenience, the invention will be described in relation to this use, though it will be understood that it is not restricted to such use.

Alternators on heavy duty vehicles are required to need little or no maintenance attention during some years of operation when working in hot, dirty and vibrating surroundings. They should be light in weight, of minimum volume and yet have a sufficiently low temperature rise that the life of insulating materials, electronic components and lubricants is not unreasonably diminished. More particularly, they are driven from an engine of which the speed may vary over a wide range of four or five or more to one which results in unacceptable losses and consequent temperature rise in the alternators when running at other than the designed optimum speed.

From one aspect, the invention provides an alternator comprising a wound stator, a high coercivity permanent magnet rotor coaxial with the stator and means to move the rotor axially into and out of the stator core in accordance with the speed of the rotor to decouple the permament flux from the generating coils of the stator to maintain constant the output (voltage or current) of the stator winding.

From another aspect, the invention provides an alternator comprising a wound stator, a rotor excited by permanent magnets of high coercivity and rotatable over a wide range of speeds within the stator, means to compare the output of the stator winding with a predetermined reference output to derive an error signal proportional to the difference between the actual output and the reference output and means operated in accordance with the error signal to move the rotor relatively to the stator core in a direction to reduce said difference.

From another aspect, the invention provides an electrical generator producing direct current at constant voltage or curent from a power source operating over a wide range of speeds comprising an alternator having a wound stator and a permanent magnet rotor driven by the power source the magnets of said rotor being of high coercivity, and said rotor being axially movable in relation to said stator, rectifying means connected to the output of said stator winding, a regulating system to compare the actual voltage from said rectifying means with a reference voltage to produce an error signal proportional to the difference between said actual voltage or current, and the desired constant voltage or current and means controlled in accordance with said error signal to move said rotor axially into or out of the stator core in a direction such as to reduce said error.

The permanent magnets of the rotor operate on a straight line portion of their B-H curve so that the rotor may move into and out of the stator core without appreciable demagnetisation of the magnets.

The reference voltage may be a zener reference voltage and the error signal may be amplified to produce a signal sufficient to control-the movement of the rotor.

The means for axially moving the rotor may be of any suitable form, mechanical, electrical or hydraulic. In one preferred form, the amplified error signal is fed to the operating coil of an hydraulic bi-directional valve which controls movement of an hydraulic piston moving the rotor. In another form, the rotor is moved by a linear step-by-step electricmotor controlled by the amplified error signal. In another form a servo motor, gearing and a lead screw is used.

Other parts of the invention are embodied in the preferred form which will now be described in some detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view partly in section on the line A-A of Fig. 2 of an electrical generator according to the invention; and Figure 2 is an end view, in the direction of the arrow B in Fig. 1.

The electrical generator shown in Figs. 1 and 2 comprises an alternator 1 with for example, twelve poles. The stator 4 has a winding 2 wound in the conventional way on a laminated, slotted stator core 3. The rotor 5 has twelve poles produced by twelve ferrite permanent magnets mounted between twelve laminated soft iron poles from which the magnetic flux passes radially into the stator. Ferrite permanent magnets have high coercivity as is necessary to avoid demagnetisation during operation of the machine. The winding on the stator is arranged to produce three phase alternating current at a frequency which is six times the revolutions per second of the rotor rotational speed.

The rotor 5 is mounted on a drive shaft 6 comprising an outer tubular portion 7 surrounding an inner shaft 8 which is axially movable therein. Radial pins 9 extend from the shaft 8 through longitudinal slots 10 in the outer tube 7 and engage recesses 11 in the rotor 5. The rotor 5 is thereby rotated by the drive shaft whilst being axially movable therealong from a position shown in full lines, in which it is aligned with the stator core to a position, indicated by dotted lines, in which it lies partially outside the stator core.

The outer tubular portion 7 of the shaft 5 is supported in bearings 12, 1 3 and extends away from the driven end to provide a stub axle 14 on which is mounted a gear pump 15.

The inner shaft 8 is also extended and carries on its end a piston 1 6 housed in a hydraulic cylinder 1 7. The piston 1 6 is mounted on the shaft 8 for axial movement therewith without rotation thereon.

The alternating current generated in the stator winding 2 is fed to a three phase full wave bridge rectifier 1 8 which converts the alternator output into direct current with a superimposed ripple of about four percent of the direct voltage and of six times the frequency of the alternating current. The rectifier 1 8 is conveniently mounted on an heat sinks 1 9 supported from the casing of the alternator.

The rectified voltage from the rectifier 18, which is proportional to the speed of the driving shaft 5, is compared with a reference voltage provided by a zener device 20 to derive an error signal which is amplified as necessary. This error signal will be proportional to any departure from the stipulated voltage and is used to control the axial. movement of the rotor 5 to reduce this error.

The error signal voltage, amplified, is fed to the operating coil of an hydraulic bi-directional valve 26. When the error signal shows that the actual voltage or current magnitude is higher than the desired magnitude, the bidirectional valve 26 causes fluid from the pump 1 5 to displace the piston 1 6 in the cylinder 1 7 in such a direction as to reduce the voltage error. Excess fluid from the pump returns through the constant pressure relief valve 21. The bi-directional valve 26 is connected to the inlet and outlet of the pump 1 5 by passages 22, 23 and to opposite ends of the cylinder 1 7 by passages 24, 25.

When the drive shaft 5 is rotating at minimum speed, the rotor 5 is aligned with the stator core 3 and the output from the rectifier 1 8 is of the desired voltage or current. As the shaft 6 speeds up, the alternator voltage rises and an error signal is obtained which shifts the rotor 5 along the shaft 6 to the left as shown in Fig. 1 and thereby reduces the coupling between the rotor and the stator winding and the alternator voltage is thereby reduced until the required direct current voltage is obtained. When the driving power source is rotating at its maximum speed, the regulating system will have commanded the piston to decouple the rotor from the stator to its maximum extent. The m.m.f. of the permanent magnet and of the stator winding carrying full load current remain substantially unchanged, but the area of the flux path from the rotor into the stator is reduced, so generating a proportionally lower e.m.f. per revolution per second, but a constant output voltage because the input drive shaft speed is proportionally higher. Whilst the m.m.f. round the magnetic circuit remains unchanged, the iron loss in the stator core is reduced in proportion to the extent of the decoupling.

The ferrite permanent magnets operate on a straight line portion of their B-H curve so that they do not become demagnetised by the movement of the rotor.

It will be understood that other means may be used to move the rotor axially in response to variation in rotor speed, as for example a linear electric motor operating on the rotor and moving it step-by-step.

Claims

1. An alternator comprising a wound stator, a high coercivity permanent magnet rotor coaxial with the stator and means to move the rotor axially into and out of the stator core in accordance with the speed of the rotor to decouple the permanent flux from the generating coils of the stator to maintain constant the output (voltage or current) of the stator winding.

2. An alternator comprising a wound stator, a rotor excited by permament magnets of high coercivity, and rotatable over a wide range of speeds within the stator, means to compare the output of the stator winding with a predetermined reference output to derive an error signal proportional to the difference between the actual output and the reference output and means operated in accordance with the error signal to move the rotor relatively to the stator core in a direction to reduce said difference.

3. An electrical generator producing direct current at constant voltage or current from a power source operating over a wide range of speeds comprising an alternator having a wound stator and a permanent magnet rotor driven by the power source the magnets of said rotor being of high coercivity, and said rotor being axially movable in relation to said stator, rectifying means connected to the output of said stator winding, a regulating system to compare the actual voltage from said rectifying means with a reference voltage to produce an error signal proportional to the difference between said actual voltage or current and the desired constant voltage or current, and means controlled in accordance with said error signal to move said rotor axially into or out of the stator core in a direction such as to reduce said error.

4. An electrical generator according to either Claim 2 or Claim 3, in which the reference voltage is a zener reference voltage.

5. Apparatus as claimed in any of the preceding claims in which the permanent magnets of the rotor operate on a straight line portion of their B-H curve.

6. Apparatus as claimed in any of the preceding claims, in which the rotor is moved axially by an hydraulic piston controlled by a bidirectional valve in accordance with the departure of the actual voltage or current from the required voltage or current.

7. An alternator substantially as described with reference to the accompanying drawings.

8. An electrical generator producing direct current at constant voltage or current from a power source operating over a wide range of speeds substantially as described with reference to the accompanying drawings.