This invention relates to a power transfer circuit capable of transferring electrical power from a source to a load across an air gap.

It is known to transfer low powers across gaps

5. by electro-magnetic induction but an object of the present invention is to enable powers of perhaps 100 watts to be efficiently transferred across a gap of perhaps 1-3 mm. One application of such a device is to the supply of power to a missile on a launching

10. pad, where, until now, it has been necessary to have a plug and socket connection, which is pulled apart when the missile is fixed, and has to be manually connected when a new missile arrives on the pad. There are however, many other applications for example to

15. replace slip rings in a rotating machine. or in a battery-charger circuit where there is no physical connection between the charger and the battery.

According to the present invention, a power transfer circuit includes a ferromagnetic or other

20. transformer core having an air gap, input and output windings on parts of the core on either side of the air gap, and a d.c./a.c. converter circuit including the input winding in a resonant circuit determining the frequency of d.c./a.c. conversion, which frequency

25. is determined automatically in accordance with the input winding inductance at the particular air. gap obtaining. The input winding may be in parallel with a capacitor.

The use of the resonant circuit having as one

30. component the inductance of the input winding, which

O PI

automatically varies with the air gap, means that the transformer action between the input and output windings by way of the ferromagnetic core is always at resonance frequency at which the reactive current

5. in the primary winding is at minimum so that the efficiency of the a.c./d.c. conversion is as high as possible.

In a preferred form of the invention, the resonant circuit is included as part of an oscillator

10. including a pair of switched transistors in a push pull arrangement switched alternately by means of an auxiliary winding coupled with the resonant winding on the ferromagnetic core. The resonant circuit is preferably supplied with d.c.through a large enough

15. choke to ensure substantially constant current operation. The transformer comprises effectively primary and secondary assemblies, each with a core and a winding, the assemblies being separated by the air gap across which the power is to be transferred.

20. An operating frequency of the order of 10 KH __ is suitable.

The gap may vary in length between say 0.5 mm and

3.0 mm while still having efficient power transfer across it because of the resonance operation of 25. oscillator. The gap can vary in length and also laterally, so that precise relative positioning of the two core parts is not necessary.

The invention may be carried into practice in various ways, and one embodiment will now be 30. described by way of example, with reference to the

accompanying drawing, of which the single figure is a circuit diagram of a power transfer circuit. It is required to transfer electrical power of perhaps 100 watts, from a d.c. source 11 to

5. a load 12, across a gap 13._

The gap 13 is defined between two halves 14 and 15, of a pot-type transformer core, one half of which carries the primary power winding 16 and the other of which carries a secondary winding 17

10. connected to the load 12. The positive side of the.-source 11 is connected through a large choke 18 to the centre tap of the primary winding 16 and the negative side of the source is connected to the common emitter connection 19 of a pair of switching

15. transistors T and T_. The bases of the two transistors and T_ are connected to opposite ^' ends of an auxiliary winding 21 closely coupled to the winding 16 for sharply turning the transistors T ₁ and ₂ ON and OFF alternately when the circuit

20. including the winding 16 is in oscillation. A capacitor 22 is connected across the primary winding 16 so that the frequency of oscillation is determined by the value of that capacitor and the value of the inductance of the primary winding 16

25. That inductance decreases with an increase in the core- air gap 13, so that the frequency of the oscillatory circuit is set automatically by the length of air gap actually obtaining. This means that the transformer including the windings 16 and 17 always operates

30. at the resonance frequency of the circuit 16, 22, so

O PI WIPO .

that the reactive current in the transformer is at a minimum and the conversion from d.c. to a.c. power in the winding 16 is very efficient. It is true that the efficiency of power transfer between the

5. primary and secondary windings 16 and 17 falls off with an increasing air gap 13, and the gap 13 must not be excessive.

During one half oscillation cycle, one transistor is in a saturated state and the other transistor

10. is cut off.

During the next half cycle, the states of the transistors are interchanged as is the polarity of voltage across the tuned circuit. Hence., the tuned circuit is permitted to execute a continuous sinusoidal

15. oscillation but in a manner wherein alternate ends of th circuit are grounded by the switching action of the respective transistors.

Saturation of the transistors is achieved by base current which is provided by a single resistor 'R ¹

20. from the positive supply to the base circuit. Cut off is achieved by the appropriately phased voltage provided by the base winding 21 that is coupled to the tuned circuit.

The d.c. supply to the oscillator via the choke

25. 18 to the centre tap of the tuned circuit offers a sufficiently high impedance at the operating frequency to constitute a virtually constant current source.

It follows therefore that the centre tap of the tuned circuit will follow a positive half sine wave

30. of voltage during every alternate half cycle of the

O

tuned circuit oscillation.

It also follows that this voltage waveform at the tuned circuit centre tap will be doubled in- value at the collector of the cut off transistor.

5. In order to achieve correct operation of the oscillator circuit, it is essential that the transistors should be capable of fast turn off, otherwise at the crossover between half cycles, both transistors will be on during a small part of the cycle and this

10. will result in poor circuit efficiency.

A similar effect will occur if the loaded Q,

(Q=R /L ) is so low as to cause the otherwise sinusoidal P P voltage to be distorted. In practice, a 'Q' less than unity is seen to cause the collector voltage 15. waveform to approach zero prematurely and this again causes poor circuit efficiency.

The *Q' may be very low when the core gap is zero, but with a gap of 0.5mm, the 'Q' has improved to a value of 2. 20. The frequency of oscillation increases by only

50% from the minimum core gap of 0.5 mm up to about

3.00 mm

A sheet of protective material can be bonded over the core faces within the gap. 25. In one example with 100 watts dissipated in the load 12, and a gap 13 of 3 millimetres, in a transformer with an overall diameter of about 2 centimetres, the improvement in efficiency of transfer of power from the d.c. input 11 to the load 12 is from 30. about 25 percent by previous means, to 70 percent by

O PI

use of the method according to the invention, according to which the transformer operates at the resonance frequency of the capacitor 22 and primary winding 16. Reasonably efficient transfer of power can be effected

5. in spite of gap changes from about 0.5 mm to 3.0 mm, so that the mechanical tolerances of the transformer core construction are not very critical, and an expensive transformer is not needed. Also changes due

10. to changes in temperature or other environmental conditions are not very critical. The coupling by way of the air gap 13 incidentally provides electrical isolation between the windings 16 and 17, so that the coupling can be used in an area where there is a danger

15. of fire, can be used under water, and can be used safely where there is a possibility of inadvertent human contact with the conductors in the load circuit. However, the main advantage is that it is not necessary to having a physical electrical or other connection

20. between the source and the load.

A single transformer could be used for a number of different loads, such as the load 12, by including a triac 24 in series with each load and a corresponding secondary winding 17; one such additional load and

25. winding arrangement is shown at 25, and includes another series triac 26. The triacs can be triggered in turn by use of control logic so that power from the primary winding 16 can be transferred across the same gap 13 to any of the loads at a time. A low power oscillator

30. and small transformer can be used in a similar transform

OM

arranged for providing a triggering drive via the gap to the triacs. In another arrangement there is a ^' single winding 17 and each of the loads is connected across it in series with its own triac such as is 5. shown at 27 and 28.