WO2003043174A1

WO2003043174A1 - A switch regulated rectifier generator system

Info

Publication number: WO2003043174A1
Application number: PCT/SE2002/002063
Authority: WO
Inventors: Lennart Stridsberg
Original assignee: Stridsberg Innovation Ab
Priority date: 2001-11-12
Filing date: 2002-11-12
Publication date: 2003-05-22

Abstract

A switch controlled rectifier generator system comprises phase windings (U1-6, V1-6, W1-6) in which phase emfs are induced. The phase windings are connected in H-bridges having legs in which switch sets (301- 312) are connected and which are connected between the poles (117, 118) of a DC rail. The switch sets include switches and diodes connected in parallel, the switches controlled by a control unit (119). By the full H-bridge configuration an efficient control of the phase currents in the phase windings for varying operating conditions is obtained. Thus, the switches of suitable switch sets connected to a phase winding can be enabled to permit adding the phase emf and the voltage of the DC rail to rapidly establish a suitable initial current in the phase winding in the beginning of each half period of the phase emf. The switches can then disabled, later in the same half period, and the current in the phase winding will then, during the rest of the half period, continue as in a conventional diode rectifier, passing through diodes of the switch sets, the switches of which were not enabled/disabled. The times of enabling/disabling can be selected to provide a desired, average DC current in the DC rail. This control method can be used when the peak value of the phase emf is lower than, equal to or larger than the DC rail voltage.

Description

A SWITCH REGULATED RECTIFIER GENERATOR SYSTEM RELATED APPLICATIONS

This application claims priority and benefit from Swedish patent applications No. 0103800- 9, filed November 12, 2001, No. 0200203-8, filed January 22, 2002 and No. 0200941-3, filed March 24, 2002, the entire teachings of which are incorporated herein by reference. TECHNICAL FIELD

The present invention is concerned with electrical generators, especially generators used to supply electric energy to DC rails connected to batteries or relatively large capacitors. BACKGROUND OF THE INVENTION AND PRIOR ART Generators that supply energy to DC systems often have to supply a rather constant voltage even though the generator speed varies considerably. A common example is the generator in a conventional car, which has to feed a battery with some 14 V when driven by an engine running from 1000 to 4000 rpm. In other cases generators running at constant speeds must be able to regulate the current supplied to a load with varying voltage. An example is serial hybrids. Generators in such hybrids are often intended to permit some kind of heat engine to work at some optimal load point with small variations in speed, torque and power. This constant power feeds an electric drive train and an energy storage such as a battery.. The voltage of this storage depends on the load from the drive train and on the state of the storage. For example, if the load is high and the depth of discharge is high, the system voltage will be low. Prior art solutions often use generators designed in a way that permits a variation of the emf obtained at a given rotational speed. In the common case of a "claw pole" rotor with DC excitation through slip rings, the change of the voltage constant, for example expressed as V/rpm, can be achieved by adjustment of the rotor excitation current.

Several prior art solutions combine a permanent magnet rotor with control coils where a DC control current affects the voltage constant of the generator.

U.S. patent 5,663,605 describes a generator rotor having 12 rotor poles, 6 of which has permanent magnets and 6 of which have coils. The voltage constant obtained when the wound rotor poles have no current can be increased by a coil current that gives the six wound poles polarities that create an emf that assists the emf of the permanent magnet poles. A control current in the opposite direction will cause the emf of the wound poles to counteract the emf of the permanent magnet poles. U.S. patent 5,656,922 describes another solution where windings on the rotor can increase the flux from permanent magnets on the rotor. Such arrangements require DC current to be supplied to the rotor, for example through slip rings. U.S. patent 4,656,379 shows a generator consisting of two stator and rotor parts where half of the rotor poles have permanent magnets and the remaining half have non-wound iron poles. The voltage constant of the generator can be adjusted by a control DC current flowing in a coil around the center of the stator and coaxial with the generator shaft. This arrangement does not require any slip rings. The published Japanese patent application 8140214 shows a similar arrangement.

To increase efficiency and to reduce weight, it is however preferable to only have permanent magnet poles in the rotor. Such generators do not permit a variation of the voltage constant directly. The use of a three phase inverter can solve this problem but adds switching losses and requires expensive electronic power switches.

Both for conventional and hybrid vehicles and for emergency power units, it is convenient to be able to use a generator as a starter motor for the heat engine that power the vehicle. For this purpose it is advantageous if the same electronic circuits can be used both for the starter motor function and for the rectifier function. SUMMARY

It is an object of the invention is to provide a generator system having a high efficiency.

Another object of the invention is to provide a generator system capable of supplying a desired current into a power bus with a voltage range corresponding to that found in normal battery systems. Another object of the invention is to provide a generator system having a low weight.

Another object of the invention is to provide a generator system having low component costs.

The above objects are achieved by the invention, the characteristics of which appear from the appended claims. One basic principle of the invention is to use switches to permit adding of the voltages from the generator phase emf and the DC rail to rapidly establish a suitable initial current in the generator phase winding in the beginning of each half period of the generator phase emf. The switches are then disabled and the current in the phase winding will then continue as in a conventional diode rectifier. This principle can be used when the peak value of the phase emf is lower than, equal to or larger than the DC rail voltage.

Thus generally, a switch controlled rectifier generator system comprises phase windings in which phase emfs are induced. The phase windings are connected in H-bridges having legs in which switch sets are connected and which are connected between the poles of a DC rail. The switch sets include switches and diodes connected in parallel, and the switches are controlled by a control unit such as an electronic processor. By the full H-bridge configuration an efficient control of the phase currents in the phase windings for varying operating conditions can be obtained. In the principle method described above, thus the switches of suitable switch sets connected to a phase winding can be enabled to permit adding the phase emf and the voltage of the DC rail to rapidly establish a suitable initial current in the phase winding in the beginning of each half period of the phase emf. The switches can then disabled, later in the same half period, and the current in the phase winding will then, during the rest of the half period, continue as in a conventional diode rectifier, passing through diodes of the switch sets, the switches of which were not previously enabled/disabled. These switches can now, if required, be enabled, i.e. to an on-state, to reduce the voltage drop over the switch sets. The times of enabling/disabling the first mentioned switches can be selected to provide a desired, average DC current in the DC rail. This control method can be used when the peak value of the phase emf is lower than, equal to or larger than the DC rail voltage but preferably in the last or latter cases. In particular, a DC generator system can comprise at least one phase winding in which a phase emf is induced, in many cases at least two phase windings and typically three phase windings. The system further comprises a DC rail including two poles into which DC current is fed from the one or more phase windings. A DC voltage, the rail voltage, exists between the two poles. Switch sets are provided connected to the DC rail and the one or more phase windings. Each switch set includes a switch and a rectifier diode connected in parallel with each other.

The system has a multitude of structural details which can be used alone or in combination with each other.

Thus, each of the two ends of a phase winding can be connected to the two poles of the DC rail through switch sets, and the phase winding can be arranged, such as by having a sufficient number of winding turns, so that the voltage of the DC rail for at least some operating conditions of the DC generator system is higher than the peak of the absolute value of the phase emf induced in the at least one phase winding.

A control unit connected to and controlling the switches of the switch sets can be arranged to control, for each of the phase windings the switches of selected ones of the switch sets, at a first time in the beginning of a half period of the phase emf, to connect the phase winding to the DC rail in such a way that the DC rail voltage is added to the phase emf. By this connecting a relatively large voltage over the phase winding is produced, giving a relatively rapidly or steeply or even very rapidly increasing current through the phase winding. The same switches can be controlled to force, at a later, second time in the same half period, the phase current in the phase winding to enter the DC rail through other switch sets, in the direction of and passing at least partly through the diodes of these other switch sets, i.e. not through said first mentioned switches and not through the diodes connected in parallel with said first mentioned switches.

The first time can be selected to be in the beginning of a half period of the phase emf, in which beginning a phase emf having a relatively low absolute value is induced.

The first and second times can be selected so that the average net current to the DC rail from the phase winding during the half period achieves a desired value. For operating conditions of the DC generator system where the peak value of the absolute value of the phase emf induced in the phase winding is lower than the voltage of the DC rail, the system can work, after said second time in the half period, as a periodically switched bridge, controlling at at least one third time the switches of the selected switch sets to connect the phase winding for a relatively short time period in the same way as at and directly after said first time.

For operating conditions of the DC generator system where the peak value of the absolute value of the phase emf induced in the phase winding is lower than the voltage of the DC rail, the switches of selected ones of the switch sets can be controlled to be in on-states for only periodically repeated, relatively short time periods to connect the phase winding to the DC rail through the selected switch sets to create a substantially sinusoidal current in the phase winding.

The switch sets can generally be connected to the phase winding to permit adding of the phase emf and the voltage on the DC rail. Then for a phase winding, selected first ones of the switch sets that are connected to the phase winding can be controlled to have their switches to be in on-states, i.e. conducting states, to rapidly establish a suitable initial current in the phase winding in the beginning of each half period of the phase emf of the phase winding. Then, after a controlled time period, the switches of the first switch sets can be controlled to be in off-states, i.e. non-conducting states, allowing the current in the phase winding to pass or continue through the rectifiers of second ones of the switch sets connected to the at least one phase winding. The second switch sets are then different from the first switch sets. A phase winding and the switch sets connected thereto are preferably connected in an H- configuration, also called as an H-bridge. The phase winding is then connected in the web portion of the H-configuration and two switch sets are connected in series with each other in each leg of the H-configuration. The ends of the phase winding is connected to the connection points of the switch units in the legs. The ends of the legs of the H-configuration are preferably connected to the poles of the DC rail.

Mechanical switches can be connected between portions of a phase winding for allowing, when being in a first state, all of the phase emf induced in the phase winding to be provided to the DC rail, and in a second state, only the emf induced in a portion of the phase winding to be provided to the DC rail.

In another configuration, mechanical switches can be connected between portions of the phase winding for allowing, when being in a first state, all of the emf induced in the phase winding to be provided to the DC rail, and in a second state, the emfs induced in the portions of the phase winding to be provided in parallel to the DC rail.

A phase winding can be arranged so that the phase emf induced therein varies periodically and takes both polarities. In particular, the phase emf can have a waveform that is nearly or substantially sinusoidal or resembles a sinusoid.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which: - Fig. la is a principle circuit diagram of a conventional brushless generator using a conventional inverter,

- Fig. lb is a principle circuit diagram of a conventional brushless generator using a rectifier bridge.

- Fig. 2 is a schematic cross sectional view of a generator having a stator with windings of three separated phases,

- Fig. 3a is a schematic circuit diagram of a switched system comprising three H-bridges and a generator with three separate phases,

- Fig. 3b is a schematic circuit diagram of an unswitched system comprising twelve diodes and a generator with three separate phases,

- Fig. 4 is a diagram of emf and current for a switch controlled rectifier generator as a function of time,

- Fig. 5 is a diagram of emf and current for a generator with a diodes only rectifier as a function of time,

- Fig. 6 is a diagram of emf and phase and output currents for a conventional switch controlled generator as a function of time,

- Fig. 7 is a diagram of emf and current of the system of Fig. 4 at high speed as a function of time, - Fig. 8 is a diagram of emf and current of the system of Fig. 4 at lower speed as a function of time,

- Fig. 9 is a diagram of emf and current of the system of Fig. 4 at even lower speed as a function of time,

- Fig. 10 is a circuit diagram of a relay connection to permit regulation over a larger speed range, Fig. 1 1 is a circuit diagram of a relay connection to permit constant power regulation over a larger speed range, and

- Fig 12 is a diagram showing an alternative method of driving the switches for a condition similar to that shown in Fig. 9.

DETAILED DESCRIPTION In the embodiments to be described, the three phases of the generator are not in direct galvanic contact with each other; i.e. if the generator is disconnected from the electronic circuits, the three phases of the generators of the embodiments shown will be insulated from each other. This gives advantages in reliability and control compared to the conventional Y-connection of the three phases as shown in Figs, la and lb where the three phases are in galvanic contact with each other in the center point of the Y connection..

However, the principle to be described below can be used also with other phase connections, for example the Y-connected system shown in Fig. la.

Fig. la is a schematic circuit diagram of an electric DC generator having a three phase winding with phase windings 108, 109 and 110 connected in a Y-arrangement. The stator phases are in the conventional manner connected to a six switch bridge including switch sets 111 - 116. The bridge includes three legs connected between the positive and negative rails 117, 1 18. Each leg contains two switch sets connected in series with each other, one of the terminals of a phase winding connected to the connection point between the two switch sets in a leg. The switch sets are controlled by a control circuit 119. Each switch set can comprise a single semiconductor switch of a suitable kind or a switch such as an FET and a rectifying element such as a diode connected in parallel to each other, the rectifying element or part of the switch set connected to allow current flow from the negative rail 118 to the positive rail 117. In some cases, the semiconductor switch and the rectifying diode are integrated in the same chip. The function of the switch sets should thus be to always permit current to flow in one direction and, when the switch part of the switch sets is enabled or in an on-state, flow is also permitted to flow in the opposite direction. The generator output is delivered on the rails as a DC voltage U_ra,ι and a current I_out flowing through a load L connected between the rails. The load can include an accumulator, e.g. for additional driving of a vehicle.

Fig. lb is a schematic circuit diagram of an electric generator having a three phase winding with phase windings 108, 109 and 110 connected in a Y-arrangement as in Fig. la. The stator phases are in the conventional manner connected to a six diode rectifier bridge including three legs in which diodes 121 - 126 are connected in the same way as in Fig. la. Fig. 2 is a schematic cross sectional view of an electric generator of a type suitable for the invention. The generator has permanent magnets 199 in its rotor 200, and a three phase winding in which each phase winding includes six separate windings Ul - U6, VI - V6 and Wl - W6, not shown in Fig. 2, for the phases U, V and W respectively, one separate winding of which are wound around poles of the stator of the generator. In the figure, the reference signs of the separate winding coils are drawn on the stator poles around which they are wound. The emf of the three phases are 120 electrical degrees apart, as is common for three phase electrical machines.

Figs. 3a and 3b are schematic circuit diagrams showing the phase windings Ul - U6, VI - V6 and Wl - W6 of an electric generator, such as that illustrated in Fig. 2, connected to three separate H-bridges having legs which are connected between the positive rail 1 17 and the negative rail 118. For example, the windings of phase W are connected as the web of to the H- bridge 309-310-311-312. In the example of Fig. 3a, switch sets are connected in the two legs of this H-bridge, the switch sets being of the general kind illustrated in and described with reference to Fig. la. In a first leg of the H-bridge switch sets 309 and 310 are connected in series, one end terminal of the W phase winding connected to the connection point of these two switch sets. The other end terminal of the W phase winding is connected to the connection point of the two switches 311 and 312 connected in series with each other in the other leg of the same H-bridge. The current in each of the phase windings can be detected by sensors 313-315 connected in series with the windings. The control unit 119 will in most applications also have a position sensor like item 120 of Fig. 1.

Figs. 4, 5 and 6 show waveforms for a typical case of driving the same ferromagnetic stator and rotor of an electric DC generator to give the same average current I_out of 675 A, corresponding to 225 A from each phase, flowing out of and into the DC-rails having a voltage of 220 V, but using three different electronic circuits/switching and rectifying arrangements connected between the generator phase windings and the DC rails. Thus, the diagram Fig. 4 illustrates the operation of a generator having an extended switching arrangement connected as illustrated in Fig. 3a, the diagram of Fig. 5 illustrates the operation of a generator having only diode rectifiers connected in the conventional way as illustrated in Fig. 3b and the diagram of Fig. 6 illustrates the operation of a generator that is switch controlled in the conventional way as illustrated in Fig. 1 a.

To get the same current when using different rectifying/switching electronic circuits, the number of turns in the stator windings are adjusted to fit the electronic circuits used. The number of turns will affect the generator emf constant, which increases linearly with the number of turns, the generator inductance, which increases with the square of the number of turns, and the generator resistance, which approximately increases with the square of the number of turns.

The number of winding turns for the two systems in which switches are used have been selected so that the limit, i.e. the highest, rotational speed for the rotor is 1.6 times the speed used in the characteristic operation diagrams shown in Figs. 4 - 6. In the case of the system operated as illustrated by the diagram of Fig. 4, the speed limit is reached when the target current is reached with a switch enable time that approaches zero, as will be described below with reference to Fig. 7. In the case of the conventional switched system operated as shown by the diagram of Fig. 6, the speed limit is selected so that the peak value U_pp of the phase to phase emf voltage U_pp is equal to the DC rail voltage U_rail, see Fig. la for the definition thereof. In the system shown in Fig. 3b with simple diode rectifiers, there is not much choice for the winding. With the given generator speed and DC rail voltage, the properties of the winding are narrowly defined.

The diagram of Fig. 4 shows the switching waveform for one phase of the generator of Fig. 3 a using three separate H bridges. As can be seen in the figure, the generator phase emf, the bell shaped curve E, has a peak value of 262 V that is higher than the nominal DC rail voltage, shown as a constant voltage U of 220 V. The current fed to the DC rails is shown as "I charge U". In the leftmost part of the figure during time from 0 to 10, the current is rapidly falling towards zero, which is the end of the current pulse show in the right part of the figure. The current is then zero until the switches are enabled at the time t = 127. During the period time from t = 10 to 127, the emf E is smaller than the rail voltage U, so the diodes in switch sets 301 - 304 of Fig. 3a will not be open, and furthermore the switch parts of the switch sets are disabled. No current will therefore flow through the phase winding.

At time t = 127, selected switches, for example 301 and 304 of Fig. 3a, are enabled, i.e. set to their conducting states or on-states, so that the battery voltage U is added to the emf. E of the generator phase winding. This causes a high voltage of some 370 to 440 V over the phase winding, causing the winding current to rapidly reach 251 A. This current is negative, i.e. energy is taken from the DC rails. At time t = 164, the same switches are disabled, i.e. set to their nonconducting states or off-states, forcing the same current to flow in the reverse direction through the diodes included in the other two switch sets 302 and 303 and to the DC rails, for example, to a battery. For some types of semiconductor switches, it may be advantageous to enable switches parallel to these diodes, included in the same switch sets, to reduce the voltage drop over the switch sets. For each half cycle of the generator phase period, the switches are therefore enabled only once with no current passing through them and therefore causing no switching losses, and they are disabled only once as commanded by the control circuit, in the case illustrated in Fig. 4 when the stator winding emf reaches the voltage of the DC rail. During the time period t = 164 to 463, the emf E is larger than the rail voltage U. During this period, there is a positive net voltage over the phase inductance, causing the phase current to increase to a peak value of 417 A. Thereafter, the emf is lower than the rail voltage, causing the current to decrease, and at t = 10 in the following half period, it reaches 0 and remains so until suitable switches, such as the switches of those switch sets 303 and 302 which were not enabled in the former half-period, are enabled. Thus, for each half period with positive emf, as shown in Fig. 4, switches 401 and 404 are enabled, at time t = 164, and for each half period with negative emf, not shown in Fig. 4, switches 402 and 403 are enabled, at time t = 164.

By selecting suitable times for enabling, for example as shown at t = 127 and disabling, for example as shown at t = 164, the switches, in particular the time period between enabling and disabling (???), , the charge current can be regulated over a wide interval even if the rail voltage and generator speed are kept constant.

The diagram of Fig. 5 shows the switching waveform for the generator system illustrated in Fig. 3b using three separate H rectifier bridges having no switches, for example one such H bridge including the diodes Dl, D2, D7 and D8. The curve "I charge U" with a peak at 403 A is the current fed to the battery from one of the phases. The diodes start to conduct when the emf of the generator phase winding, shown as the continuous bell shaped curve E with a peak at 427 V, becomes larger than the DC rail voltage, shown as the horizontal line U at 220 V. As the generator emf is higher than in the system the characteristic diagram of which is shown in Fig. 4, the inductance is higher, i.e. more winding turns are required to get the higher emf, and as the voltage difference between the battery and the winding emf initially is very low, the current will rise slowly. If the rail voltage and generator speed are kept constant the charge current will be fixed.

Fig. 6 shows the switching waveform for the system illustrated in Fig. 3 a using three separate H bridges and a high frequency inverter, not shown. The circuit diagram of this is identical with that of the system of Fig. 4; but the difference is the manner in which the switches are used. The curve "I char U" with a peak at 469 A is the current for each half switching period fed to the battery from phase U. The curve "I wind U" with a peak at 745 A is the current in the winding. The switches are switched on and off at a high frequency. To simplify the figure, the ripple in the currents caused by the switching frequency is ignored. The currents shown have the values that the average current should have if the real current values for some hundred cycles were averaged. The rail voltage is shown as a horizontal line U at 220 V and the emf of the generator phase winding is shown as a continuous bell shaped curve "E phase U" with a peak at 131 V. For each half cycle of the generator period, the switches are closed and opened some 12 times with a peak current of 745 A.

Some key data for the three systems the electric characteristics of which are shown in Figs. 4 - 6 are shown in Table 1 below:

Table 1

The system the characteristics of which are illustrated by the diagram of Fig. 4 seems to offer better price/performance than the systems the characteristics of which are illustrated by the diagrams of Figs. 5 and 6.

The system the characteristics of which are illustrated by the diagram of Fig. 5, using simple diode rectifiers as in Fig. 3b, has no costs for switches such as IGBT transistors, but has twice the stator copper loss. The generator must therefore be made much larger and heavier and will use at least twice as much expensive magnet material. Added to this is that the current will depend only on generator rpm and battery voltage whereas the current from the system the characteristics of which are illustrated by the diagram of Fig. 4 can be adjusted by changing the turn on or turn off times of the switch.

The system the characteristics of which are illustrated by the diagram of Fig. 6, a conventional high frequency inverter, has slightly lower stator copper losses but will have higher iron losses due to the added iron loss in the stator iron when the stator poles act as high frequency inductances. As a first approximation, the generators the characteristics of which are illustrated by the diagrams of Figs. 4 and 6 will be approximately equal in size and cost.

However, the semiconductor costs for the system the characteristics of which are illustrated by the diagram of Fig. 6 are much higher than for the system the characteristics of which are illustrated by the diagram of Fig. 4. The maximum current is almost three times higher. The conduction losses can be estimated from the total charge passing through switches for each generator period, and is some 10 times larger for the system the characteristics of which are illustrated by the diagram of Fig. 6. The switching losses can be estimated by adding the current being switched for each switching occurrence during one generator period. In the case of the system the characteristics of which are illustrated by the diagram of Fig. 4, there are only two occurrences, one of which has zero current. The switching losses in the system the characteristics of which are illustrated by the diagram of Fig. 6 are therefore some 40 times larger than those of the system the characteristics of which are illustrated by the diagram of Fig. 4.

For applications as flywheel starter/generator, the system the characteristics of which are illustrated by the diagram of Fig. 5 cannot be used as it does not permit operation as a starter motor. The system the characteristics of which are illustrated by the diagram of Fig. 4 gives twice as much torque for a given current, the torque per ampere in motor operation varying linearly with the emf at a given speed in generator operation.

The diagrams of Figs. 7 - 9 show electric characteristics of the generator-electronic circuit combination the characteristics of which are illustrated by the diagram of Fig. 4 at other driving conditions. Relative to Fig. 4, the speed and charge currents are increased 1.6 times in Fig. 7, decreased by 0.75 times in Fig. 8 and decreased by 0.56 times in Fig. 9. The load conditions are selected so that the generator torque is approximately constant.

The diagram of Fig. 7 shows the characteristic curves of an operating case for the maximum speed possible to maintain the selected speed (???) independent constant torque at the generator shaft. The switches are not enabled at all in the case shown in Fig. 7. If the generator shaft speed decreases, the generator torque can be maintained by having a switch enable time longer than zero. If the speed should increase over the case shown, the current will increase in the same way as for a system with simple diode rectifiers and the torque taken up will increase over the constant value shown in Figs. 4, 7, 8 and 9. The case illustrated in Fig. 7 thus generally corresponds to the case including only diode rectifiers in the H-bridges, compare also the diagram of Fig. 5. The diagrams of Figs. 8 and 9 show characteristic curves for lower speed cases. As the speed decreases, the current waveform becomes less even and the maximum current in the switches increases. It should be noted that in both cases, the generator emf, denoted by "E" in the figures, is lower than the battery voltage, "U" in the figures.

The cases illustrated by the characteristic curves of Figs. 7 to 9 represent a speed variation of almost a factor 4. This is far above the economically relevant speed range of 2:1 for a truck diesel.

The possible regulation range for higher speeds are limited; for each speed where the generator peak EMF is larger than the voltage of the battery, there will be a minimum current even if the semiconductor switches are not activated at all. This could overcharge a battery if a vehicle is run for a long time at high engine speeds.

Fig. 10 is a schematic circuit diagram showing a system in which mechanical switches 1001 - 1003 having three positions are inserted in series with three of the phase windings. As shown, the switches can permit

- half of each phase disabled, switch movable member in the top position as shown. This will give half the EMF and half the inductance of the case when all stator phase coils are connected in series

- all phase windings disabled, switch movable member in the horizontal position, - all phase windings connected in series, switch movable member in the lower position; this will give full EMF and full inductance.

A less costly arrangement would be to replace the three-way switches of Fig. 10 with simple two-way switches. This simplifies the switch hardware and gives two alternatives:

- all phase windings disabled, switch movable member in the horizontal position; this gives no charge and can be used to avoid battery overcharge.

- all phase windings connected in series, switch movable member in the lower position; this will give full EMF and full inductance.

The schematic circuit diagram of Fig. 11 shows a more complex system where nine mechanical two-way or on-off switches 1101 - 1109 are used, i.e. three switches per phase. As shown, such an arrangement of switches can permit

- all phase windings of a phase connected in series with each other. For phase U, this requires switches 1101 and 1103 to be disabled and switch 1102 to be enabled. This will give full EMF and full inductance. This case permits a good regulation over high speeds.

- the phase windings of a phase divided in two equal groups connected in parallel with each other. For phase U, this requires switches 1101 and 1103 to be enabled and switch 1102 to be disabled. This will give half of full EMF and a quarter of full inductance. This case permit good regulation over low speeds and an optimum copper loss in the generator for a given charge current.

- the phase windings of a phase divided in two equal groups, only one of which is used. For phase U, this requires switches 1101 and 1102 to be disabled and switch 1101 to be enabled. This will give half of full EMF and half of full inductance. This case can be relevant if low currents are to be supplied at lower speeds.

- all phase windings disabled, all switches 1101 - 1109 disabled. This case protects against battery overcharging at very high speeds.

The mechanical switches do not have to switch DC currents as they are inserted before the rectifiers. The switching loads on the mechanical switches can be reduced by regulating the charge currents down to a minimum before changing the state of the mechanical switches. In applications where the system is used as a starter motor generator, the ability to change the connections between windings as shown in Figs. 10 and 11 permits a limited current from the electronic power circuits to give a higher starting motor torque at low speeds.

In the diagram of Fig 12 characteristic curves are shown for an alternative way of controlling the switches for cases where the generator emf E is lower than the rail voltage U_rail. The generator speed is the same as for the case, the characteristics of which are shown in Fig. 9, but the switch state is changed 8 times during the generator half period while it is changed only twice when controlled as illustrated by the characteristic curves in Fig. 9. This permits lower peak currents in the switches and diodes.

For cases where the generator emf E is lower than the rail voltage U_rajl there is also the alternative way of controlling the switches in the conventional way as illustrated in Fig. 6

Some key data for the four load conditions of the systems the characteristics of which are illustrated by the diagrams of Figs. 4, 7, 8, 9 and 12 are shown in the table 2 below: Table 2

The embodiments shown and described above have generators with three phases. The same principle of operation of a generator is however possible to apply to any number of phases. While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.

Claims

1. A DC generator system comprising

- at least one phase winding in which a phase emf is induced,

- a DC rail including two poles into which DC current is fed from the at least one phase winding, a DC voltage existing between the two poles,

- switch sets, each switch set including a switch and a rectifier diode connected in parallel with each other, characterized in

- that each of the two ends of the at least one phase winding is connected to the two poles of the DC rail through switch sets, and

- that the at least one phase winding is arranged so that the voltage of the DC rail for at least some operating conditions of the DC generator system is higher than the peak of the absolute value of the phase emf. induced in the at least one phase winding.

2. A DC generator system according to claim 1, characterized by a control unit connected to and controlling the switches of the switch sets, the control unit arranged to control, for the at least one phase winding the switches of selected ones of the switch sets, at a first time in the beginning of a half period of the phase emf, to connect the at least one phase winding to the DC rail in such a way that the DC rail voltage is added to the phase emf, thereby creating a relatively large voltage over the phase winding giving a relatively rapidly increasing current through the at least one phase winding, and to control, at a later, second time in the same half period, said switches to force the phase current in the at least one phase winding to enter the DC rail through diodes of the switch sets.

3. A DC generator system according to claim 2, characterized in that the control unit is arranged to use a first time occurring in the beginning of a half period of the phase emf, when a phase emf having a relatively low absolute value is induced.

4. A DC generator system according to claim 2, characterized in that the control unit is arranged to use first and second times selected so that the average net current to the DC rail from the at least one phase winding during the half period achieves a desired value.

5. A DC generator system according to claim 2, characterized in that the control unit is arranged to control the switches of the switch sets, for operating conditions of the DC generator system where the peak value of the absolute value of the phase emf induced in the at least one phase winding is lower than the voltage of the DC rail, to work, after the second time in the half period, as a periodically switched bridge, controlling at at least one third time, the selected switches to connect the phase winding for a relatively short time period in the same way as at and directly after said first time.

6. A DC generator system according to claim 1, characterized by a control unit connected to and controlling the switches of the switch sets, the control unit arranged to control, for operating conditions of the DC generator system where the peak value of the absolute value of the phase emf induced in the at least one phase winding is lower than the voltage of the DC rail, the switches of selected ones of the switch sets to be in on-states for only periodically repeated, relatively short time periods to connect the at least one phase winding to the DC rail through the selected switch sets to create a substantially sinusoidal current in the at least one phase winding.

7. A DC generator system according to claim 1, characterized by a control unit connected to and controlling the switches of the switch sets, the switch sets connected to the at least one phase winding to permit adding of the phase emf and the voltage on the DC rail, the control unit arranged to control for the at least one phase winding selected first ones of the switch sets that are connected to the at least one phase winding to have the switches of the first ones of the switch sets be in on-states to relatively rapidly establish a suitable initial current in the at least one phase winding in the beginning of each half period of the phase emf of the phase winding and then, after a controlled time period, be in off-states allowing the current in the at least one phase winding to pass or continue through the diodes of second ones of the switch sets connected to the at least one phase winding, the second ones being different from the first ones.

8. A DC generator system according to claim 7, characterized in that the control unit is arranged to control the switches of the second ones of the switch sets to be in on-states when the current in the at least one phase winding is allowed to pass or continue through the rectifiers of the second ones of the switch sets to reduce the voltage drop over the second ones of the switch sets.

9. A DC generator system according to claim 1, characterized in that the at least one phase winding and the switch sets connected thereto are connected in an H-configuration, the at least one phase winding in the web portion the H-configuration and two of the switch sets connected in series with each other in each leg of the H-configuration, one end of the at leas one phase winding connected to a connection point of the switch units in one leg and another end to a connection point of the switch units in the other leg.

10. A DC generator system according to claim 9, characterized in that the ends of the legs of the H-configuration are connected to the poles of the DC rail.

11. A DC generator system according to claim 1 , characterized by mechanical switches connected between portions of the at least one phase winding for allowing, when being in a first state, all of the emf induced in the at least one phase winding to be provided to the DC rail, and in a second state, only the emf induced in a portion of the at least one phase winding to be provided to the DC rail.

12. A DC generator system according to claim 1, characterized by mechanical switches connected between portions of the at least one phase winding for allowing, when being in a first state, all of the emf induced in the at least one phase winding to be provided to the DC rail, and in a second state, the emfs induced in the portions of the at least one phase winding to be provided in parallel to the DC rail.

13. A DC generator system according to claim 1, characterized in that the at least one phase winding is arranged so that the phase emf induced in the at least one phase winding varies periodically and takes both polarities.

14. A DC generator system according to claim 1, characterized in that the at least one phase winding is arranged so that the phase emf induced in the at least on phase winding varies periodically and has a waveform that is substantially sinusoidal.

15. A method of controlling a DC generator system according to claim 1, characterized in that the switches of selected ones of the switch sets at a first time, in the beginning of a half period of the phase emf when a relatively low phase emf. is induced, are controlled to connect the phase winding to the DC rail in such a way that the DC rail voltage is added to the phase emf, thereby creating a large voltage over the phase winding giving a relatively rapidly increasing current through the phase winding, and that at a later, second time in the same half period the switches are controlled to force the phase current in the phase winding to enter the DC rail, passing to and from the DC rail only through diodes of the switch sets, the first and second times selected so that the average net current to the DC rail from the at least one phase winding during the half period achieves a desired value.

16. A method according to claim 15, characterized in that the switch sets, for operating conditions where the peak value of the absolute value of the phase emf induced in the at least one phase winding is lower than the voltage of the DC rail, are operated as a switched bridge, the selected switches are controlled at at least one third time to connect the phase winding for a relatively short time period in the same way as at and directly after said first time.

17. A method according to claim 16, characterized in that for operating conditions where the peak value of the absolute value of the phase emf of the at least one phase winding is higher than the rail voltage the switch sets are controlled to take, during the rest of the half period after the second time, the same states as directly before the first time.

18. A DC generator system comprising

- at least two phase windings in which phase emfs are induced, - a DC rail including two poles into which DC current is fed from the at least two phase windings, a DC voltage existing between the two poles,

- switch sets, each switch set including a switch and a rectifier diode connected in parallel with each other, characterized in that each of the two ends of each of the at least two phase windings is connected to the two poles of the DC rail through switch sets.

19. A DC generator system comprising

- at least one phase winding in which a phase emf is induced,

- a DC rail including two poles into which DC current is fed from the at least one phase winding, a DC voltage existing between the two poles, - switch sets, each switch set including a switch and a rectifier diode connected in parallel with each other, characterized by a control unit connected to and controlling the switches of the switch sets, the control unit arranged to control, for the at least one phase winding the switches of selected ones of the switch sets, at a first time in the beginning of a half period of the phase emf, to connect the at least one phase winding to the DC rail in such a way that the DC rail voltage is added to the phase emf, thereby creating a relatively large voltage over the phase winding giving a relatively rapidly increasing current through the at least one phase winding, and to control, at a later, second time in the same half period, said switches to force the phase current in the at least one phase winding to enter the DC rail through diodes of the switch sets.

20. A DC generator system according to claim 19, characterized in that the control unit is arranged to use first and second times selected so that the average net current to the DC rail from the at least one phase winding during the half period achieves a desired value.

21. A DC generator system comprising

- at least two phase windings in which a phase emf is induced, - a DC rail including two poles into which DC current is fed from the at least one phase winding, a DC voltage existing between the two poles,

- switch sets, each switch set including a switch and a rectifier diode connected in parallel with each other, characterized by a control unit connected to and controlling the switches of the switch sets, the control unit arranged to control, for operating conditions of the DC generator system where the peak value of the absolute value of the phase emfs induced in the at least two phase windings is lower than the voltage of the DC rail and for each of the at least two phase windings, the switches of selected ones of the switch sets to be in on-states for only periodically repeated, relatively short time periods to connect the at least one phase winding to the DC rail through the selected switch sets to create a substantially sinusoidal current in the at least one phase winding.

22. A DC generator system comprising

- at least one phase winding in which a phase emf is induced, - a DC rail including two poles into which DC current is fed from the at least one phase winding, a DC voltage existing between the two poles,

- switch sets, each switch set including a switch and a rectifier diode connected in parallel with each other, characterized by a control unit connected to and controlling the switches of the switch sets, the switch sets connected to the phase winding to permit adding of the phase emf and the voltage on the DC rail, the control unit arranged to control for the at least one phase winding selected first ones of the switch sets that are connected to the at least one phase winding to have the switches of the first ones of the switch sets be in on-states to rapidly establish a suitable initial current in the at least one phase winding in the beginning of each half period of the phase emf of the at least one phase winding and then, after a controlled time period, be in off-states allowing the current in the phase winding to continue through the diodes of second ones of the switch sets connected to the at least one phase winding, the second ones being different from the first ones.