Induction heating circuits forcooking appliances

Abstract

 A pan heating coil for the induction heating circuit of a cooking appliance comprises a coil of Litz wire of flat pancake form. The conductors of the wire are divided into two groups (L1, L2) which are electrically connected together to form a three terminal inductor. The mutual inductance between the groups is substantially unity.

Description

[0001] This invention relates to induction heating circuits for cooking appliances.

[0002] Such circuits may comprise a rectifier for converting A.C. mains supply to direct current which is then converted by an inverter to an alternating supply at a suitable frequency, usually in the range of from 20-35 kc/s. That supply energises a coil which induces currents in a suitable utensil placed over the coil thereby heating the utensil and its contents.

[0003] The present invention is particularly concerned with the coil and its construction and has the object of providing a coil having a requisite inductance but which is of a smaller physical size than has been possible hitherto.

[0004] The present invention also provides a method of energising such a coil.

[0005] According to the present invention, a pan heating coil for an induction heating appliance of a cooking appliance comprises a coil of Litz wire of flat pancake form with the conductors of the Litz wire divided into two groups electrically connected together to form a three terminal inductor, the connection between the groups being of opposing ends of the groups and in which the mutual inductance between the two groups is substantially unity.

[0006] According to another aspect of the present invention, an induction heating circuit for a cooking appliance comprises a pan heating coil powered from a power source via a switching device and in which the pan heating coil is a coil of Litz wire of flat pancake form with the conductors of the Litz wire divided into two groups electrically connected together to form a three terminal inductor, the connection between the groups being of opposing ends of the groups and in which the mutual inductance between the two groups is substantially unity.

[0007] The switching device may be in series connection with one group across the power supply, the other group being in series connection with a diode across the power supply, the diode being poled to allow current flow through the other group only when the switching device is non-conducting.

[0008] Means may be provided for switching the switching device into conducting and non-conducting states of equal duration.

[0009] By way of example only a heating coil embodying the invention and a method of energising the coil will now be described in greater detail with reference to the accompanying drawing which is a circuit diagram of the embodiment.

[0010] A rectified and smoothed d.c. supply VS is applied across a capacitor CS in parallel connection with which is a branched circuit containing an inductor LM and a switching device S1 in one branch and a second inductor LA and a diode D1 in the other branch.

[0011] Inductors LM and LA constitute the pan heating coil which has the physical form of a single pancake winding of Litz wire. The two inductors are produced by dividing the conductors of the Litz wire into two groups. The two inductors thus formed are electrically connected to produce a three terminal inductor, the joint between the two groups being a connection of opposing ends of the groups. The mutual coupling between the two groups is as near unity as possible.

[0012] When switch S1 is closed, current will flow through inductor LM only and will increase linearly at a rate defined by VS/LM amps/sec.. Whilst that current flow is increasing, the voltage at the cathode of diode D1 will be increased to 2VS because of the coupling between the inductors.

[0013] If switch S1 is now opened, the inductor terminal voltage at the junction S1-C2 will increase rapidly to a value of at least 2VS. At that voltage, diode D1 will be forward biassed and will conduct allowing energy stored in the inductor to flow into capacitor CS.

[0014] As switch S1 is opened, any leakage inductance between inductors LM and LA will produce a voltage at the junction additional to the 2VS excursion and therefore leakage inductance must be kept to a minimum.

[0015] The switching cycle just described is repeated at a frequency within the range 20 kHz to 35 kHz, the ON-OFF periods being of equal duration so avoiding any net d.c. component.

[0016] In practice, the switch device S1 will be a semi-conductor switch for example a thyristor which may be a gate turn-off thyristor or a bipolar high voltage device. ON-OFF switching of the semi-conductor switch may be controlled by a timing circuit in a manner similar to that described in co-pending Patent Application No. (Case 218). The timing circuit will be under the control of the user.

[0017] Also included will be an interrogation circuit and a pan sensing circuit as described in the Patent Application just referred to as well as circuit monitoring devices.

[0018] The cooking appliance may be a hob unit in case one or more of the pan heating units may be of the form described above. Other pan heating units may be gas burners and/or electric heating units.

[0019] The invention may also be embodied in a cooker which may be free-standing. One or more of the top or pan heating units may be of the form described above. Other top heating units may he gas burners and/or electric heating units.

Claims

1. A pan heating coil for an induction heating circuit of a cooking appliance comprising a coil of Litz wire of flat pancake form with the conductors of the Litz wire divided into two groups electrically connected together to form a three terminal inductor, the connection between the groups being of opposing ends of the groups and in which the mutual inductance between the two groups is substantially unity.

2. An induction heating circuit for a cooking appliance comprising a pan heating coil powered from a power source via a switching device and in which the pan heating coil is a coil of Litz wire of flat pancake form with the conductors of the Litz wire divided into two groups electrically connected together to form a three terminal inductor, the connection between the groups being of opposing ends of the groups and in which the mutual inductance between the two groups is substantially unity.

3. An induction heating circuit as claimed in claim 2 in which the switching device is in series connection with one group across the power supply, the other group being in series connection with a diode across the power supply, the diode being poled to allow current flow through the other group only when the switching device is non-conducting.

4. An inducting heating circuit as claimed in claim 2 or 3 in which means are provided for switching the switching device into conducting and non-conducting states of equal duration.

5. A pan heating coil for an induction heating circuit of a cooking appliance substantially as herein described with reference to and as illustrated by the accompanying drawings.

Drawing