TECHNICAL FIELD OF THE INVENTION

The present invention relates to a magneto matching system for bicycles and bicycles including that system. BACKGROUND TO THE INVENTION

Commonly bicycles have dynamos to power their various components including lighting. These dynamos are generators that normally generate AC so are more properly called alternators or magnetos. They generate electricity by the relative motion of wires, normally in the form of a coil, and a magnetic field. The magnetic field may be produced by a permanent magnet or other coils. This invention can be applied to magnetos or other AC generators.

When any AC power source, for example the mains or a magneto is connected to a load the power transferred from source to load depends on the voltage the current and the power factor.

The power transferred to a load is the product of the voltage the current and the power factor. The power factor is the cosine of the angle between the AC voltage and current. Thus, the maximum value of the power factor is 1 and this occurs when the load is purely resistive.

It is well understood that when connecting a load to a fixed AC voltage supply such as the mains the maximum power is transferred when the power factor of the load is 1. Commonly the power factor of a load may be improved by power factor correction. The impedance of the load to be corrected has both resistive and reactive components. Power factor correction is achieved by cancelling the reactive component of the load by adding a reactive component of opposite sign such as a capacitor or an inductor.

However, smaller magnetos and, in particular, those used on bicycles do not generate a fixed AC voltage and do not operate at a fixed frequency. Voltage and frequency vary with bicycle speed. Also in these cases, the source impedance of the magneto has a large reactive component. Peak power transfer does not occur when the power factor of the load is 1. Peak power transfer occurs when the impedance of the load has a reactive component that is equal and opposite in sign to that of the magneto. For a fixed frequency, the peak power point can be achieved by adding a reactive component to the load. Such a

component, typically a capacitor, would be of equal and opposite reactance to that of the magneto. The reactance of such components varies with frequency. Thus, to match the load to the magneto the added reactive component would need to vary with bicycle speed and hence magneto frequency.

As the skilled person is aware variable reactive components of suitable value are not practical for these low frequencies. Use of one or more fixed value components is also cumbersome and is a compromise.

A method of peak power matching the magneto to the load over a suitable frequency range without needing a variable reactance is sought. This forms the subject of the present application.

SUMMARY OF THE INVENTION

The present invention aims to provide solutions to the problems identified in the previous section.

A typical magneto as used on a bicycle has 10 or more poles to generate a convenient frequency from the relatively slow rotation of the bicycle wheels. The frequency generated at a minimum stable cycling speed of 8KPH will typically be around 15 Hz and a typical touring speed will be 16KPH giving 30Hz.

The typical bike magneto will have an internal leakage inductance of about 0.12 Henry and a resistance of about 4 Ohms. It can thus be seen that even at 15Hz the reactance of the inductor is about 3 times the resistance. Thus, at all practical bike speeds the power output of the magneto is limited by its internal reactance by a factor of 3 or more. The objective of this invention is to null out all or part of this internal reactance by presenting to the magneto a suitably matched load. This increases the power output significantly.

Since the magneto is highly inductive a conventional approach would be to add a series capacitor to the load that resonates with the internal inductance and cancels it out.

Such a capacitor would need to be AC rated and hence bulky. It would also need to vary with frequency to keep the resonance it forms with the internal inductance of the magneto tuned to the varying frequency of the magneto as the bike varies in speed.

This application takes a different approach. If one examines what a capacitor is doing in this situation it is absorbing power at some points in each cycle of the magneto, storing that energy, and then returning that power to the magneto at other points in the cycle. If, in the above situation, one looks at the load and the capacitor in combination they form a system that absorbs power at some points in the cycle and returns a portion of that power at other points in the cycle, the balance being the output power. Thus, if one designs an active load that mimics the situation one would have if one fitted a capacitor one can achieve the same effect without the capacitor.

There are several ways of achieving this mimicking. A key aspect of this is that energy from the magneto must be stored in some way and returned to the magneto at specific times. By adjusting the timing and amount of energy transferred each way one can emulate the effects of a variable capacitor and adjust that emulated variable capacitance to optimise the power transfer at varying frequencies and bike speeds.

In a real situation in addition to the bike speed varying the bike's power needs vary. It may or may not be running lighting, charging a battery, or have any other varying power demands. A sophisticated control system would not adjust the simulated capacitor to maximise the power generated. It would adjust the simulated capacitor to match the amount of power generated to the needs of the bike at that time. This avoids generating heat due to surplus generating capacity at higher bike speeds. It also reduces the load on the pedals by not wasting power produced by the cyclist. Embodiments of the present invention have shown that it is possible to at least double the power output of the magneto at low bike speeds where otherwise there would be insufficient power to maintain lighting. Even greater gains are possible at higher speeds.

An embodiment of this invention may consist of a synchronous DC to AC converter where the DC side is connected to a battery or other energy storage device such as an electrolytic capacitor or supercapacitor. The AC side is connected to the magneto.

The amplitude, frequency, and phase of the AC output of the synchronous converter are adjusted to present at all times the same instantaneous voltage to the magneto as would be present if it was loaded by an ideal value capacitor and resistive load. Synchronous DC to AC converters can be made using modern components with very high efficiencies and power can flow in both directions. Power would thus flow into the AC side of the converter as it would a conventional load.

The DC power output passed through the synchronous converter is taken from the energy storage device. Thus, this would mimic the actions of a large variable capacitor without requiring one. Another embodiment of this invention would be to replace the synchronous DC to AC converter with an H-bridge. The magneto is connected to the centre of the H-bridge. The top and bottom rails of the H-bridge are connected to the energy storage device which may be some type of capacitor or a battery. An H-bridge is the same structure as a bridge rectifier with the diodes of the bridge rectifier replaced with switches. These switches may be transistors. The transistors should have a suitable switching speed and low on resistance. Commonly the transistors may be Field Effect Transistors (FETS). Such FETS typically have body diodes which function even when the transistors are off and provide a fall-back bridge rectifier action. Thus, even if the transistors are all off the magneto will provide some power to the energy storage device. This enables the system to start from a fully discharged state.

A control system should monitor the voltage and current flowing through the H-bridge. If this is done while the transistors in the H-bridge are off then the frequency and phase of the magneto can be determined. The control system can then turn on the transistors with appropriate timings so as to modify the phase of the currents flowing through the magneto. The transistors could use a variable mark space ratio to approximate a sine wave current in the magneto. However, it is possible to switch the H-bridge transistors at the same frequency as the magneto and thereby run the system using a square wave rather than a sine wave. Since the magneto has a large inductive component the harmonics of the square wave have negligible effect and can be ignored. One does not need to exactly emulate a capacitor to achieve the desired effect.

By monitoring the current in the H-bridge continuously one can phase lock to the magneto and maintain the correct frequency and phase to achieve the desired power transfer benefits.

If for whatever reason phase lock is lost the system can turn off the transistors to allow the phase lock to be reacquired using voltage measurements.

Phase locking can be achieved by several means including by measuring the real and quadrature components of the magneto current and thus determining the relative phase angle of the magneto and the H-bridge. Current can alternatively be measured in the DC output of the H-bridge rather than at the magneto.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the invention will now be described with reference to the accompanying drawing, Fig. 1 , which shows a circuit diagram for a magneto matching system according to an embodiment of the present invention using an H-bridge.

DETAILED DESCRIPTION OF THE DRAWING

Fig. 1 shows the circuit diagram of an embodiment using an H-bridge. The magneto is shown as the box containing V1 , L1 , and R1. L1 and R1 represent the internal resistance and leakage inductance of the magneto. V1 represents a perfect magneto which generates the same open circuit AC voltage as the real magneto would do at the same bike speeds but has no internal resistance or leakage inductance. Thus, the box represents the key electrical characteristics of the real-world magneto.

D1 through D4 form a Bridge rectifier. If switches S1 through S4 remain off then the illustrated system would function as a conventional generator and bridge rectifier providing DC power at the output, shown relative to ground. The output need not be grounded and would typically be the ground plane of the control system.

C1 represents an energy storage device. It could be an electrolytic capacitor, a super capacitor a rechargeable battery or any other energy storage mechanism. The output may be connected to other energy storage systems and voltage converters such that C1 need not store all the energy and could be omitted.

R2 is a small value resistor which the control system uses to continuously measure the instantaneous current flow. The control system also continuously monitors the instantaneous voltage across the output of the physical magneto. The control system controls the timing of the switching of switches S1 through S4 to approximately emulate the equivalent effects of placing a suitable value resonant AC capacitor in series with the leakage reactance L1. This cancels out some or all the impedance of L1 increasing the power delivered to the output.

The values of components shown are typical but not critical to the functioning.

Initially the system is run with the switches turned off and the control system monitors the voltage across the magneto. From this the control system derives the frequency and phase of the magneto. The control system phase locks to the waveform from the magneto. It will be noted that when the switches are all off there is a pattern every cycle of times when the diodes D1 through D4 are in a forward conducting state. Consider the case where the controller turns on each switch S1 through S4 only during the moments in the cycle that D1 through D4 would have been forward conducting. The result is that the energy losses in the diodes D1 through D4 are eliminated but otherwise the situation is largely unchanged.

Now consider the case where a physical AC capacitor of the correct value is fitted in series with L1 such that it is at resonance with L1 but the switches are all off. Again, there will a pattern every cycle of times when the diodes D1 through D4 are in a forward conducting state. However, in this new pattern the timings of the forwards conducting states of diodes D1 through D4 have advanced in phase. The power output will be greatly increased as it is now only limited by R1. We later refer to the connection between L1 and this physical capacitor as Node P.

If the controller now turns on switches S1 through S4 in this new pattern then the energy losses in the diodes D1 through D4 are eliminated but otherwise the situation is again largely unchanged.

Now if we remove the physical AC capacitor that we had fitted in series with L1 so that the circuit is as it is in the above diagram but we maintain an advanced phase of the timing of the switches S1 through S4 then with the correct phasing of the switching we can emulate a capacitor that is in resonance with L1. The correct timings needed to achieve this must match the timings of the waveform that would have existed at Node P. This is because the emulated capacitor is now effectively inside the H-bridge not outside it. Also, the voltage generated at the output will be higher as it is now a rectification of the voltage that would have existed at Node P.

As the speed of the bike varies the timings of the switches need to be adjusted to maintain the correct power flow. Due to the emulated resonance effect the voltage generated will vary with frequency. Ideally a switching regulator will convert this voltage to that needed by the systems of the bike. Ideally the switching regulator will be controlled to harvest the power from the output of the diagram at the optimum voltage so as to match the impedance of the virtual Node P. Note that in effect the virtual Node P is realised as the physical connections to the magneto since the physical capacitor is emulated within the H-bridge.

Note that if the switching regulator supports bi-directional power flow then the requirement for C1 can be eliminated. Thus at the times in each cycle when the switches S1 through S4 are returning power to the magneto this power comes back via the switching regulator rather than C1 needing to be large enough to store that energy. In the following claims, features which are defined in separate claims that may preferably or advantageously be used in combination with each other, are to be construed as being claimed as such as well as in the form of the independent claims set out herein.