IL112191A - Mobile power supply - Google Patents

Description

Τ»3 Π-ϊ QO Mobile Power Supply S. Karnieli & Sons (1985) Ltd. tt"i»3 (1985) » tt*)3 .V The inventor: Alexander Isurin C. 93176 FIELD OF THE INVENTION This invention relates to electrical power supplies, in particular to portable or transportable generators for producing substantially constant frequency a.c. voltages.

BACKGROUND OF THE INVENTION The national electricity grid distributes electrical power even to the remotest parts of the country. In Europe, the voltage supplied by the -national grid is approximately 220 V at a frequency of 50 Hz, whilst in the U.S.A. and countries which adopt a similar standard, the voltage is 110 V at a frequency of 60 Hz. However, there are many developing countries which still do not have a national grid. Furthermore, even in developed countries, easy access to the national grid is not always available. To this end, there clearly exists a need for a portable or transportable generator capable of producing similar voltages as the national grid and capable of maintaining a similar, constant frequency.

Attempts have been made in the prior art to address this need. Such attempts almost invariably employ some sort of alternator driven by an engine, typically a motor vehicle engine, the alternator output being controlled so as to compensate for inevitable fluctuations in engine speed. The principal disadvantages of such alternators are: 1. Their angular speed must be maintained at 50/60 Hz to within an accuracy of 2%; and 2. They are large and bulky owing to their low frequency operation.

One such device marketed under the name "Energator" produces 220 V d.c. at a power rating of 1 kW. The unit is particularly adapted to function as a power supply for a portable welding unit but, seeing that its output is d.c, it does not emulate the national grid supply which is a.c. A further disadvantage of supplying d.c. voltage is, of course, that the electrical supply is not amenable to transformation and it is therefore of limited application, as well as being inherently more dangerous than corresponding a.c. voltages. Furthermore, no provision is made for charging the vehicle battery at the same time as power is drawn from the power supply and, consequently, prolonged use of the power supply can result in total discharge of the battery.

Some of these problems have been overcome in a belt driven high voltage generator and control unit marketed under the name "DynaWatt" which produces 220 V/50 Hz a.c. at an output of 3000 VA. Whilst, in this case, the output voltage is indeed amenable to electrical transformation, the power rating of the alternator is exceeded beyond its rated value and there is, in any case, insufficient power for operating an intermediate size welding unit. Furthermore, in order to produce maximum power output, the alternator speed must be increased to such an extent that damage to the engine ensues.

U.S. Patent No. 3,916,284 (Hilgendorf) describes an alternating The generator proposed by Hilgendorf is suitably primarily for resistive loads only and suffers from a number of other drawbacks, some of which will be discussed in greater detail below when describing the present invention. Even at this stage, however, it may be noted that Hilgendorf employs a commutation period of 120° during which the rectifier output is zero. However, the alternator rotates continuously and, consequently, during the 120° commutation period output power is generated by the alternator but no output voltage is produced. This detracts greatly from the efficiency of the system. Of course, efficiency would be increased if the commutation period were reduced but, as Hilgendorf explains in column 3 lines 11 ff of his patent, the relatively large commutation period of 30° at the beginning and end of each half-cycle, produces a wave shape having substantially all of the third harmonic removed and thus achieves the object of purifying the square wave and eliminating, as far as possible, all harmonics thereby leaving only the fundamental frequency. Thus, were Hilgendorf merely to decrease the commutation angle, he would not be able to achieve this objective.

The above-described situation is also undesirable because, during the commutation period when the alternator is producing power but there is no load, the alternator produces high voltage spikes which may even result in dielectric breakdown. For this reason, it is undesirable to run generator sets at zero load but no provision against this is provided by Hilgendorf in his system.

Yet a further drawback inherent in the Hilgendorf system results in the use of SCRs for rectifying the alternator output voltage. SCRs conduct when their anode voltage isj >sitive with respect to their cathode voltage and a suitable trigger voltage is applied to their gate terminal. However, once the SCR starts to conduct, it will remain conducting even in the absence of a gate voltage providing that the anode voltage is positive with respect to the cathode voltage by more than a fixed holding voltage which is characteristic of the SCR. If SCR rectifiers are used with inductive loads, then when the SCR stops conducting, there will be an abrupt cessation of the current flowing through the inductor and, consequently, there will be produced a large back EMF in accordance with Lenz's Law. The back EMF produced by the inductor will be applied across the SCR and will maintain it in a conducting state since it is greater than the holding voltage of the SCR. For this reason, SCRs are unsuitable for use with inductive loads and, as a corollary, inductive loads are not suitable for use with the Hilgendorf system.

For the reasons explained above, most prior art generator sets limit the permissible reactive load so that, for example, a portable generator may be rated at 3 kW for a resistive load but only 1.5 kW for reactive loads. Thus, for reactive loads, only a fraction of the alternator power may be exploited.

Yet another problem associated with prior art systems such as Hilgendorf, relates to the control of the output voltage. Generally, this is achieved by means of controlling the rotor current: the higher the rotor current, the higher is the alternator voltage, and vice versa. However, when the rotor current is reduced to zero, there still exists residual magnetism in the rotor magnetic circuit and so the alternator will still generate power. No provision is made for this by Hilgendorf with the result that the only partial voltage regulation is possible.

SUMMARY OF THE INVENTION It is an object of the invention to provide a generator of the kind described in which the drawbacks associated with hitherto such generators are substantially reduced or eliminated.

According to the invention there is provided an electrical power supply for supplying to any kind of electrical load an a.c. voltage of substantially fixed magnitude and frequency, comprising: an alternator having a rotor and stator winding, a drive means for rotating the rotor winding so as to generate an a.c. output voltage across the stator winding, a reference signal having a frequency equal to said fixed frequency, a modulator coupled to the rotor winding and responsive to said reference signal for supplying modulated current thereto so as to modulate the a.c. output voltage, a phase shifter coupled to the stator winding and responsive to said reference signal for demodulating the a.c. output voltage, and a controllable filter coupled to the phase shifter and responsive to said reference signal for filtering the demodulated a.c. output voltage.

Thus, in the electrical power supply according to the invention, the rotor current is modulated with a correction signal representative of a difference between magnitude and frequency from the desired alternator a.c. output, whereby the desired a.c. output is achieved.

Preferably, a battery charging unit is provided and is automatically actuated when the power rating of the load fed by the power supply unit is less than the power rating of the power supply, the surplus being used by the battery charging unit for recharging the battery.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how the same may be carried out in practice, a portable generator adapted for coupling to a vehicle alternator will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a power supply according to the invention; Fig. 2a is a circuit diagram showing in more detail the power supply shown in Fig. 1; Fig. 2b is a detail of the power switch amplifier shown in Fig. 2a; Fig. 3a is a schematic representation of a SCR rectifier connected across an inductive load in a typical prior art system; Fig. 3b shows schematically a modification to the circuit shown in Fig. 3a as employed in the invention; Fig. 4a is a graphical representation of a SCR waveform in a typical prior art system; Fig. 4b is a graphical representation of the SCR voltage waveform achieved with the invention; and Figs. 5a - 5e and 6a - 6h are graphical representations of waveforms associated with the power supply shown in Fig. 2a.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT .„ Referring to Fig. 1 there is shown a power supply designated generally as 10 comprising an alternator 11 having rotor and stator coils 12 and 13, respectively. The rotor coil 12 of the alternator 11 is rotated by means of a drive means 14 which preferably is a motor vehicle engine.

A signal generator 15 produces a signal having a specified frequency, typically 50 Hz which is fed to a modulator 16 whose output is coupled to the rotor coil 12 so as to modulate the rotor current fed thereto.

Connected to the stator coil is a phase shifter 17 whose output is connected to a controllable filter 18, an output of which is fed back to the modulator 16 for modulating the rotor current. Both the phase shifter 17 and the controllable filter 18 are responsively coupled to the signal generator 15, their respective operation being controlled by the 50 Hz signal produced thereby.

A single-phase, a.c. voltage is derived from the output of the controllable filter 18 although the alternator 11 itself is a three-phase device. That is to say, the alternator actually has three stator coils, only one of which is shown in Fig. 1. However, if desired, the circuit described above with reference to Fig. 1 can be duplicated in the other two stator coils so that each phase of the three-phase alternator current can be controlled.

Referring now to Figs. 2a, 2b, 3a, 3b, 4a and 4a, the various components shown functionally in Fig. 1 will now be described in more detail. The three stator coils 13 are connected to the phase shifter 17 directly in Fig. 2a, although for some applications it is connected by a three-phase transformer (not shown). The phase shifter 17 comprises a controllable rectifier having two parallel banks 19 and 20 connected in antiphase, each comprising three pairs of series-connected SCRs 21, each pair of SCRs being connected in parallel. Firing of the SCRs 21 is controlled via the signal generator 15 whose output is coupled to each of the two banks 19 and 20 by respective opto-couplers 23 and 24 which electrically isolate the high voltage alternator output from the low voltage signal generator 15.

The controllable filter 18 includes a conventional LC filter comprising an inductor coil 25 in series with a pair of snubbers comprising, respectively, a diode 26 connected in series with a parallel connection of a capacitor 27 and a resistor 28, and a diode 29 connected in series with a parallel connection of a capacitor 30 and a resistor 31. The two diodes 26 and 29 are connected in anti-phase so that each snubber passes current during a corresponding half of the a.c. cycle. Connected across the supply rails 40 and 41 are a pair of rectifier diodes 33 and 34 connected in antiphase and each being switchable by means of a switch contact 35 and 36, respectively, whose operation is controlled by the signal generator 15 via respective opto-couplers 37 and 38.

The single-phase a.c. output is derived from the high voltage supply rail 40 of the controllable filter 18 whose low voltage supply rail 41 is at zero potential.

The voltage produced by the alternator 11 across its stator coils 13 depends on several factors, including the rotor speed and the rotor current. Increasing the rotor speed increases the frequency with which the rotor magnetic circuit cuts the stator magnetic circuit and thereby increases the alternator voltage output. Likewise, increasing the rotor current increases the field intensity of the rotor magnetic circuit, with the same effect. There are also other factors, such as the number of magnetic poles in the stator (and rotor) magnetic circuits but these are invariable quantities determined by the particular design of alternator and are therefore fixed by the manufacturer and are not amenable to subsequent modification. be employed in order to adjust this is most easily subject to — ■ — — — variation_by__the. end-user.

A transformer 45 has a primary winding 46 connected across the supply rails 40 and 41 of the controllable filter 18 and has a secondary winding 47 connected across a full-wave bridge rectifier 48 so as to produce an unsmoothed d.c. waveform at an output 50 thereof as shown in Fig. 6f of the drawings. The unsmoothed d.c. output 50 is thus a function of the a.c. voltage output across the controllable filter 18. The signal generator 15 is connected to a wave shaper 51 which is responsive to the 50 Hz signal produced by the signal generator 15 for generating a synthetic unsmoothed d.c. waveform 53 at a first output 54 thereof. A so-called "dead-time pulse" 55 is produced at a second output 56 of the wave shaper 51. The output 50 of the full wave bridge rectifier 48 as well as the output 54 of the wave shaper 51 are fed to an error amplifier 57 whose output is a function of the difference in both magnitude and frequency between the actual unsmoothed d.c. waveform derived from the controllable filter 18 and the synthetic waveform 53 produced by the wave shaper 51. An output 58 of the error amplifier 57 is coupled to a Pulse Width Modulator 59 which is also responsively coupled to the second output 56 of the wave shaper 51 and whose output is fed to a power switch amplifier designated generally as 60 for supplying rotor current to the rotor 12. The error amplifier 57 together with the Pulse Width Modulator 59 and the power switch amplifier 60 constitute the modulator 16 shown functionally in Fig. 1.

Also provided is a battery charging unit 61 for supplying charging current to a storage battery 62 and being coupled to the wave shaper 51 so as to be responsive to the dead-time pulses 55 produced thereby. The battery charging unit is connected to an output of the alternator 11 which is rectified by a rectifier 63 and smoothed by a large capacitor 64. The width of the dead-time pulses 55 is a function of the difference between the available alternator power output and the actual power consumed by a load (not shown) connected across the supply rails 40 and 41 of the controllable filter 18. The actual load power is, in turn, a function of the load voltage and the load current, the load voltage being monitored via the transformer 45 and the signal (shown in Fig. 6f) derived at the output 50 of the bridge rectifier 48. The load current is monitored by means of a current transformer 65 disposed on the low voltage supply rail 41 and having an output which is shown schematically coupled to the signal generator 15 to which the wave shaper 51 is responsively coupled.

Fig. 2b shows a detail of the power switch amplifier 60 which comprises a first bank of series connected diodes 66 and 67 connected in parallel with a second bank of series connected diodes 68 and 69, and across which are two bank of switches 70a, 71a and 70b, 71b. The switches 70a, 70b and 71a, 71b are controlled by the Pulse Width Modulator 59 so that the switches 70a, 70b are both open when the switches 71a, 71b are closed and vice versa. The rotor coil 12 of the alternator 11 is connected to the respective junctions of the switches 70a, 71a and 70b, 71b so that when the switches 70a and 70b are closed, current is fed to the rotor 12 in a first direction, whilst when the switches 71a and 71b are closed, current is fed to the rotor 12 in a second direction opposite to the first direction.

When it is desired to reduce the power output of the system 10, mere reduction of the rotor current is not sufficient owing to hysteresis in the rotor magnetic circuit whereby some residual magnetism remains even when the rotor current is reduced to zero. However, by means of the power switch amplifier 60, current may be fed through the rotor 12 in an opposite sense to the original rotor current, so as to counteract the effect of hysteresis.

Having described in detail the system according to the invention, it is instructive to emphasize those points which distinguish the invention from typical prior art systems. Thus, referring to Fig. 3a, there is shown a schematic diagram of an alternator shown schematically as 75 for supplying electrical energy to an inductive load 76 via a conventional rectifier comprising SCRs 77 and 78. As is known, each of the SCRs 77 and 78 conducts during one half-cycle of the alternator a.c. voltage output for so long as its anode voltage exceeds its cathode voltage by an amount greater than the holding voltage for the SCR and providing that a suitable trigger is applied to the SCR gate terminal. Once the SCR is conducting, however, the gate voltage may be removed and the SCR will continue to conduct for substantially the remainder of the half-cycle. Thus, consider the specific case that the SCR 77 is conducting whilst the SCR 78 is non-conducting. The output voltage is controlled by adjusting the time during the a.c. half cycle during which the SCR is closed and, as has been explained above with particular reference to U.S. Patent No. 3,916,284 (Hilgendorf), there is a 60° commutation period subsequent to the SCR 77 switching off and the SCR 78 being switched on. Thus, at the transition where the SCR 77 suddenly becomes non-conducting, there will be an abrupt interruption in the current through the inductive load 76, thereby resulting in a high back EMF which will cause the SCR 77 to remain conducting. In this case, both the alternator 75 and the inductive load 76 are connected in parallel across the SCR 77 and, if the back EMF is sufficiently large, could cause irreparable damage thereto. It is for this reason that prior art systems are normally severely limited with respect to the maximum reactive load which may be connected.

Fig. 3b shows schematically the effect of the controllable filter 18 which is connected in the invention between the bridge rectifier 17 and the load 76. A normally open switch 79 is effectively connected across the bridge rectifier 17 and closes, under control of the controllable filter 18 so that when the SCR 77 stops conducting and prior to applying the gate voltage to the SCR 78, the switch 79 closes thereby conducting the high back EMF generated by the inductive load 76 to ground and preventing damage to the bridge rectifier 48. It will be understood that exactly the same thing happens during the other half-cycle when the SCR 78 conducts and the SCR 77 is in the off state.

The controllable filter 18 decreases non-linear distortions, particularly those of the staircase type which are the principal type under continuous operation of the SCRs 19 and 20 at 154°. The choke 25 together with the snubbers 26, 27, 28 and 29, 30, 31 integrate and smooth the output voltage.

Fig. 4a relates to the prior art configuration depicted in Fig. 3a and shows the output voltage waveform. In Fig. 4a there is shown a large commutation period during which neither of the SCRs 72 and 73 conducts. This large commutation period is required in order to minimize third harmonics and ensure that the resulting a.c. output voltage approximates more closely to a sinusoid.

Fig. 4b shows the a.c. output voltage waveform for the present invention wherein the actual SCR voltage waveforms are depicted in dotted line, superimposed on which is the actual a.c. output voltage produced by the controllable filter 18. It will clearly be seen that during the period that the SCRs are non-conducting, the controllable filter smooths the a.c. output voltage so as to produce a smooth, symmetrical zero crossing transition which proximates to a sinusoidal waveform and avoids the problem of third harmonics in the a.c. output voltage.

With particular reference to Figs. 5a to 5e and 6a to 6h of the drawings, the operation of the circuit shown in Fig. 2 will now be described.

Fig. 5a shows the voltage waveform across the stator winding 13 of the alternator 11. It will be seen that the voltage waveform comprises two components: a high frequency component representing the modulated alternator output; and a low frequency 100 Hz envelope (shown in dotted outline) for modulating the high frequency component.

The waveforms shown in Figs. 5b and 5c represent the voltages across the banks 19 and 20 of SCRs within the phase shifter 17. The firing angle of the respective SCRs is adjusted in accordance with the signal generated by the signal generator 15 so that when one bank 19 of SCRs conducts the other bank 20 of SCRs 21 is reverse biased, and vice versa. In this way, the voltage across the rectifier diodes 33 and 34 shown in Fig. 5d is a 50 Hz sinusoidal waveform, superimposed on which there is a high frequency ripple corresponding to the high frequency component of the alternator voltage shown in Fig. 5a. After filtering by the controllable filter 18, the high frequency component is filtered out, resulting in a substantially pure 50 Hz sinusoidal waveform across the supply rails 40 and 41, as shown in Fig. 5e.

Figs. 6a and 6b show, respectively, the SCR drive signals fed by the opto-couplers 23 and 24 to the two banks 19 and 20 of SCRs 21. As will be seen, these are substantially square wave pulses having a variable width. If the width of these pulses were exactly equal to the width of a 50 Hz half cycle (i.e. 10 ms), then one of the two banks 19 and 20 of SCRs would be conducting throughout the whole of the duty cycle. This is necessary only in the case that the power rating of the load connected across the supply rails 40 and 41 of the controllable filter 18 exactly equals the power output of the alternator 11. Generally, however, the power rating of the load is somewhat less than the actual power which is supplied by the alternator 11 and the phase shifter 17 does not need to output voltage for the complete duty cycle. The extent to which the phase shifter 17 can be switched off for part of the duty cycle depends on the actual load and is controlled by the feedback loop provided by the current transformer 65.

The duration of the pulses shown in Figs. 6a and 6b depend on the power load and varies from 154° in the idle mode to approximately 165° at 30-40% of the maximum available power. At 165° all power produced by the alternator will be used by an externally connected load, thereby resulting in very high efficiency. The remaining 15° of the alternator power is used to charge the storage battery 62 via the battery charging unit 61. The pulse duration is varied via the feedback circuit comprising the current transformer 65 coupled to the signal generator 15 and provides a compromise between the desire, on the one hand, to maximize the output power and the desire, on the other hand, to minimize the distortions in the output voltage.

At 154° it is easier to achieve undistorted sinusoidal voltage at the output of the controllable filter 18 but at 165° the efficiency and the output power are maximized. As soon as 30-40% of the maximum available power is achieved, monitoring is stopped and the pulse width remains at approximately 165°. It is only when the pulse width is equal to or greater than 165°, that maximum efficiency and output power can be realized.

Figs. 6c and 6d show the switching signals fed by the opto-couplers 37 and 38 to the diode switches 35 and 36, respectively. These signals are in 180° anti-phase so that when the rectifier diode 33 is forward biased, the rectifier diode 34 is reverse biased, and vice versa. It will be understood that the diode switches 35 and 36 correspond to the switch shown schematically as 79 in Fig. 3b for short-circuiting the rectifier 17 during the commutation period thereof, in order that the high back EMF generated by an inductive load 76 coupled across the output rails will be short-circuited to ground.

Fig. 6e shows the variable width dead-time pulses 55 produced at the second output 56 of the wave shaper 51 for controlling the pulse width modulator 59 and the battery charging unit 61.

Fig. 6f represents the synthetic unsmoothed d.c. waveform 53 produced at the first output 54 of the wave shaper 51, as explained in detail above with reference to Fig. 2a of the drawings.

Figs. 6g and 6h represent, respectively, the rotor current and rotor voltage waveforms produced at the output of the modulator 16. The voltage waveform shown in Fig. 6h is an unsmoothed, rectified 50 Hz half-sinusoidal waveform. The current waveform shown in Fig. 6g is substantially sinusoidal in shape but intersects the time axis at varying locations dependent on the correction current produced by the pulse width modulator 59.

The circuit shown in Figs. 1 and 2a may be connected in various configurations so as to produce a.c. voltage (as described) or controlled d.c. in cases where this is required. It will be appreciated that other modifications can be made to the particular configuration described above without departing from the spirit of the invention.

Although in the preferred embodiment, the phase shifter is coupled directly to the stator winding of the alternator, the connection can, if desired, be effected via a transformer. Such a connection is particularly common if the alternator is driven by a motor vehicle engine.

Although the description of the preferred embodiment relates only to single phase alternators, there may be employed alternators producing multi-phase power and the stator winding may then be coupled to the phase shifter via a multi-phase transformer. In such case, a single modulator, phase-shifter and controllable filter may be connected to a single phase only of the transformer; or, alternatively, a respective modulator, phase shifter and controllable filter may be connected to each phase of the transformer.

Claims (22)

CLAIMS:

1. An electrical power supply for supplying to any kind of electrical load an a.c. voltage of substantially fixed magnitude and frequency, comprising: an alternator having a . rotor and j^tojLwinding, output voltage across the stator winding, the a.c. output voltage, a phase shifter coupled to the stator winding and responsive to said reference signal for demodulating the a.c. output voltage, and a controllable filter coupled to the phase shifter and responsive to said reference signal for filtering the demodulated a.c. output voltage.

2. The power supply according to Claim 1, wherein the a.c. output voltage is multi-phase and the phase shifter comprises a multi-phase controllable rectifier.

3. The power supply according to Claim 2, wherein the multi-phase controllable rectifier includes SCRs whose firing angle is adjustable as a function of the frequency of said reference signal.

4. The power supply according to Claim 3, wherein the controllable filter smooths any discontinuity in the demodulated a.c. output voltage appearing across the controllable rectifier at zero-point crossings of the a.c. output voltage and reduces high frequency components from the a.c. output voltage.

5. The power supply according to Claim 3 or 4, wherein during a non-conductive part of the a.c. half-cycle, power is fed to a fixed external load.

6. The power supply according to Claim 5, wherein the fixed external load is a battery charging unit.

7. The power supply according to any one of the preceding claims, wherein an external inductive load is connected across the controllable filter and a normally open switch is connected across the inductive load and is responsive to the controllable filter for closing when the phase shifter is switched from a conducting state to a non-conducting state, thereby short-circuiting to ground a back EMF produced by the inductive load.

8. The power supply according to any one of the preceding Claims, wherein current is fed to the rotor through a power switch amplifier permitting rotor current to fed to the rotor in either direction, thereby counteracting the effect of hysteresis in the rotor magnetic circuit.

9. The power supply according to any one of the preceding Claims, wherein the reference signal is fed to the phase shifter and to the controllable filter by means of opto-couplers.

10. The power supply according to any one of the preceding Claims, further including: a sampling means for sampling the a.c. output voltage, and comparator means for comparing a magnitude and frequency of the sampled output voltage with said fixed magnitude and frequency, respectively, so as to produce respective error signals; the modulator being responsive to the respective error signals for modulating the amplitude and frequency of the rotor current.

11. The power supply according to Claim 10, wherein the sampling means includes: a voltage transformer having the a.c. output voltage connected across a primary winding thereof, and a rectifier connected across a secondary winding of the voltage transformer for producing an unsmoothed rectified signal.

12. The power supply according to Claim 11, wherein: there is further provided a wave shaping means responsive to the reference signal for generating a synthetic unsmoothed rectified signal, and the comparator means compares the rectifier signal with the synthetic signal.

13. The power supply according to Claim 12, wherein: the modulator includes a pulse width modulator coupled to a power amplifier and responsive to said synthetic signal for generating a correction pulse for feeding to the power amplifier, and an output of the power amplifier is connected across the rotor windings.

14. The power supply according to Claim 13, further comprising: a battery charging unit coupled to the pulse width modulator and responsive to said correction pulse for generating a battery storage signal, and a storage battery coupled to the battery charging unit for receiving current in response to said battery storage signal.

15. The power supply according to Claim 14, wherein: there is further included a power output determination means for determining an output power demand of a load across the alternator, and the correction pulse has a width which is a function of the difference between a nominal alternator power output and the output power of the load.

16. The power supply according to Claim 15, wherein the power output determination means includes: voltage control means across the load for maintaining the magnitude of the a.c. output voltage at a predetermined value, current means coupled to the stator winding for determining an output current of the power supply, and multiplication means coupled to the voltage control means and to the current means for determining a product of said predetermined value and the output current.

17. The power supply according to Claim 1, wherein the controllable filter comprises: an LC network, a pair of switchable rectifier diodes connected across said LC network, said rectifier diodes being connected in anti-phase and being responsive to respective anti-phase actuation signals derived from the reference signal for conducting.

18. The power supply according to any one of the preceding Claims, wherein the phase shifter is coupled to the stator winding of the alternator via a transformer.

19. The power supply according to Claim 18, wherein the alternator ' produces multi-phase power and the transformer is a multi-phase transformer.

20. The power supply according to Claim 19, wherein the modulator, the phase-shifter and the controllable filter are connected to a single phase only of the transformer.

21. The power supply according to Claim 20, wherein a respective one of the modulator, the phase shifter and the controllable filter are connected to each phase of the transformer.

22. The power supply according to any one of the preceding Claims, wherein the drive means is a motor vehicle engine. For the Applicants, DR. REINHOLD COHN AND PARTNERS 93176spc. J JT/prg/2S.12.1994