Rapid uniformly heating, highly efficient griddle.

The present invention relates essentially to commercial griddles and more particularly to griddles employing magnetically permeable griddle plates heated by induction coils to their Curie temperatures which may vary from plate to plate or from one surface of the griddle plate to the other.

Griddles are one of the major cooking appliances in commercial kitchens, particularly in a majority of the fast food and full service chain restaurants. The prior art griddles are either gas or electric powered and typically have one heating element, one thermocouple and one thermostat per 30 cm (linear foot) thus permitting wide temperature variations between heating elements. Literally power is pushed into the food.

The problems with the prior art griddles are numerous. Such griddles typically have a 20° (70°F). temperature variation across their surfaces, they cannot deliver power to specific incremental areas, thus cold areas may call for heat and hot areas as a result are overheated or vice versa. The heat-up time from turn-on is typically 20 minutes and the griddles have a slow response time to changes in temperatures caused by change of load. Such griddles are difficult to clean; the relatively large surface areas cannot be removed for cleaning. And further, the conventional griddle does not provide cooking temperatures withirn 7½ cm (three inches) of its periphery.

There are also prior art consumer oriented induction cooking stoves with special pots. The pots have ferromagnetic bottoms and the stove has induction coils. These devices have coils that do not provide uniform temperature across the bottom of the pot or pan and provide minimum shielding of R.F. radiation. These coils are not designed for use in plate technology or large size pots.

Such a device is found in French Patent 2,527,916. This patent discloses a pot or pan with a ferromagnetic bottom. Several pots are provided each with a ferromagnetic bottom of different Curie temperatures whereby different cooking temperatures are provided.

It is an object of the present invention to provide a hot griddle system wherein griddle plates are heated to a uniform essentially constant temperature across their surfaces and which are heated quickly to cooking temperatures.

It is another object of the present invention to provide interchangeable griddle plates which provide different fixed cooking temperatures.

It is yet another object of the present invention to provide induction coils and related structure for heating magnetically permeable griddle plates to uniform temperatures substantially across their entire surfaces.

It is still another object of the present invention to provide griddle plates which shield the environment from excessive magnetic fields.

Yet another object of the present invention is to provide a griddle plate or plates each of which can provide at least two different cooking temperatures.

Still another object of the present invention is to provide a highly efficient griddle structure employing heat insulation for under the griddle plates and a cover that may be used during non-cooking intervals so that the plates are maintained essentially at about effective Curie temperature with the expenditure of little energy.

A system for providing a cooking surface which is heatable to a predetermined temperature for cooking food, in accordance with the invention, comprises a plate including a first layer of magnetically permeable material having a first Curie temperature and a second layer, and an induction coil for generating a magnetic field, the magnetically permeable material being selected to produce the predetermined temperature on the cooking surface when the magnetically permeable material is heated to its Curie temperature upon exposure to the magnetic field characterised in that the plate is formed as a griddle plate with the cooking surface formed as a surface of the griddle plate, the second layer of the griddle plate being for use as the cooking surface and in that the induction coil is, in use in cooking food, positioned adjacent the griddle plate, whereby a substantially uniform temperature is maintained across the cooking surface of the griddle plate.

Preferably, a plurality of interchangeable griddle plates incorporating magnetically permeable, that is, high mu material such as Alloy 34, Alloy 36 and the like, are subjected to an essentially uniform alternating magnetic field by induction coils to heat the griddle plates to their effective Curie temperatures at which temperature the plates become essentially non-magnetic and heating is materially reduced. The temperature falls and the plates reacquire their magnetic properties and start heating again whereby the plates maintain a quite constant temperature at about their effective Curie temperatures.

Uniformity of temperature across the plates is ensured by a specific design of the induction coils and related structure that maintains a magnetic field across the griddle plates such that each region of the plates responds uniformly to a change in load at that region. Measurements of temperature across the plates showed variations of about ±5°C (±10°F). These small variations result from the fact that the plates respond incrementally to changes in load. Thus if batter is placed on an area of a plate, that area has its temperature reduced, it becomes highly magnetic and generates heat at a fast rate whereas immediately adjacent areas unaffected by changes in load continue to idle; that is, cycle over a short range about the effective Curie temperature.

Griddle plates of different Curie temperatures may be used concurrently. Thus a griddle having several different closely controlled temperatures at the same time is provided. Each griddle plate is fabricated so as to prevent unnecessary stray radiation and by covering different regions of a griddle plate with different high mu materials one plate can provide multiple temperatures. Thus with only a few griddle plates many different cooking temperatures may be provided. In fact by using half size plates it is possible to provide six different highly regulated cooking temperatures at the same time or by using different high mu materials on both surfaces of the plate, three full size plates can provide six different temperatures.

If the griddle plate employs different Curie temperatures on opposite surfaces the operation is disclosed in U.S. Patent No. 4,695,713. If a non-magnetic, low-resistance layer such as copper is disposed between the two ferromagnetic surfaces, the operation is described in U.S. Patent No. 4,752,673. Shielding may be and is provided by following the teachings of U.S. Patent No. 4,701,587. The teachings of these patents are incorporated by reference.

The griddle plates take about five minutes to reach maximum temperature from a cold start. This maximum temperature can be maintained at little cost of energy by placing a cover over all areas not being used. Heat loss is minimized particularly if a poor heat conductive material is employed for the cover or covers. Further an insulating pad may be disposed between the coils and the griddle plates to, among other things, reduce heat loss from the griddle plates. The combination of these two features provide an unusually efficient system, the insulating pad reducing heat loss at all times and the cover during non-cooking intervals.

The induction coils are supplied with a constant alternating current to maintain uniform response to the energy input. The more uniform the current the better the temperature regulation. For the sake of analysis the griddle plates are considered to reflect resistance into the coils and under these circumstances the regulation is governed by the equation<maths id="math0001" num=""><math display="block"><mrow><mfrac><mrow><mtext>Δ|I|</mtext></mrow><mrow><mtext>|I|</mtext></mrow></mfrac><mtext> &lt; - 1/2 </mtext><mfrac><mrow><mtext>R</mtext></mrow><mrow><mtext>R</mtext></mrow></mfrac></mrow></math><img file="EP0442219B1_D0001.tif" /></maths> where |I| is current and R is the reflected resistance If the current is held constant regulation is good. If the current is permitted to rise, the regulation becomes poorer as the value of the left side of the equation approaches the right side and the system fails if the value of the left side of the equation exceeds the value of the right side.

The term "effective Curie temperature" is the temperature at which a material becomes, for purposes of this invention, essentially non-magnetic. Such temperature may be as little as 1°C. or as much as 100°C. less than absolute Curie temperature depending upon the material employed.

Reference is made to high mu materials or magnetically permeable materials or the magnetic permeability of materials. These materials provide for a high degree of concentration of magnetic flux in them as long as they are below their effective Curie temperatures. The flux produces eddy current and hysteresis losses as well as resistive losses. Such materials may be principally ferromagnetic or ferrimagnetic but other materials whose degree of magnetic permeability varies with temperature are also available. <ul id="ul0001" list-style="none"><li>Fig. 1 is an exploded view of the griddle system of the present invention;</li><li>Fig. 2 is a perspective view of one embodiment of a griddle plate;</li><li>Fig. 3 is a perspective view of a tri-clad griddle plate;</li><li>Fig. 4 is a detailed view of the coil configuration of the present invention;</li><li>Fig. 5 is a view illustrating the arrangement of various ferrite bars under the coil structure;</li><li>Fig. 6 is a view in cross-section of a cover for the griddle plates; and</li><li>Fig. 7 is a diagram of the control circuit employed with the present invention.</li></ul>

Referring specifically to Fig. 1 of the accompanying drawings, there is illustrated an exploded view of a griddle system comprising a stand 2 which in the embodiment illustrated supports three induction coils 4 identical in construction. The stand has a grease trap 6 along its front horizontal surface and on the vertical front surface has controls 8, one for each coil 4.

Removably disposed over the coils 4 is a coil insulator 11 that serves several functions; it insulates the coil from extreme temperatures, it minimizes heat loss from the griddle plates to be described, and it has sound absorption properties that helps reduce noise often associated with magnetic induction systems. The insulator 11 is a refractory fiber blanket formed from very pure alumina, silica or other refractory oxides. The material presently being employed is a non-woven glass.

Disposed above and in contact with the insulator 11 is a griddle surface 10 fabricated from glass or a light weight, high temperature polymer. The griddle surface 10 serves to deflect grease into the grease trap 6 and prevent it from falling on the coils 4.

Disposed above and resting on the griddle surface 10 are removable griddle plates 12, 14, 16 and 18 which are fabricated from high mu materials. All four of the plates may have the same Curie temperature, different temperatures or any combination in between depending upon what foods are to be cooked on the plates. To complete the structure, a removable splash guard 20 is disposed about the back and two sides of the top surface of the stand 2. In the particular embodiment discussed herein a full size griddle plate is about 30-60 cm (1 foot by 2 feet).

The power supply (not illustrated) is held in the base of the stand 2 under the coils. It supplies 1300 watts per square foot of constant alternating current at 22 KHz or more but preferably under 50 KHz. The power work unit employed was a power supply taken from a Sanyo induction cooker Model No. 564.4498511. The Sanyo control board is replaced by the circuitry described below relative to Fig. 7.

As previously indicated, the plates 12, 14, 16 and 18 are removable and each one can have a different Curie temperature. Referring to Fig. 2, each plate 22 is quite thin and is made of a sheet of stainless steel 24 that, in this instance, is 0.3175 cm (0.125 inch) thick with a bottom layer of high mu material 26 about 0.089 cm (0.035 inch) thick. The plates are easily cleaned. The use of an alloy layer that is 0.089 cm (0.035 inch) thick (about 5 skin depths) at 26 MHz permits the plate to accept all available power of the power unit and causes the plate to achieve its effective Curie temperature. Since the plates and for that matter the griddle surface 10 carry no electrical components they may be immersed in water for cleaning.

The thickness of the magnetic material on the griddle plates should be such as to take all of the energy from its coil that is available and essentially isolate the surrounding area from the magnetic flux. With the aforesaid power supply frequency the magnetic material on the plate as indicated above, is 0.089 cm (35 thousandths of an inch), this being approximately five skin depths at the frequency employed. See U.S. Patents No. 4,256,945 and 4,701,587, the subjects matter thereof relating to skin depth and copper clad ferromagnetic materials being incorporated herein by reference.

Referring to Fig. 3 a tri-clad plate 28 is employed. Here the upper surface 30 is 304 stainless steel at 0.12 cm (0.048 inch) thickness, a bottom surface 32 is a nickel-iron, high mu material at a thickness of 0.089 cm (0.035 inch) and sandwiched between is a copper 101 plate 34 at a thickness of 0.191 cm (0.075 inch). The total plate thickness is 0.389 cm (0.153 inch) thus presenting little thermal inertia but providing adequate magnetic shielding.

The plate of Fig. 3 may have both lower and upper surfaces of high mu materials of different Curie temperatures. Thus each of the plates of Fig. 1 may be a tri-clad plate with two high mu surfaces to provide as many as eight cooking temperatures with four plates. The lower surface shields the upper surface from appreciable flux so that the lower magnetic surface controls temperature. The number of griddle plates is indefinite being restricted only by the number of different cooking temperatures desired. Although there are magnetic stainless steels their Curie temperatures are quite high and therefor when a two sided magnetic griddle plate is employed both surfaces usually are iron alloys as follows: <tables id="tabl0001" num="0001"><img file="EP0442219B1_D0002.tif" /></tables> These temperatures are approximate and should not be considered to be precise.

Examples of tri-clad construction are Stainless 304, cu 101 and selected alloy and Stainless 400 series, cu 101 and selected alloy. The 400 series stainless provides increased shielding over the 304 stainless. Also two temperatures in one plate can be alloy, cu 101 and alloy. A bi-clad plate can be stainless and alloy. Referring now to Fig. 4 of the accompanying drawings there is illustrated the coil structure.

Coil 42 is fabricated from Litz wire 36 and non-conductive rubber or plastic spacing material 38. In order to obtain a magnetic field that maintains a uniform temperature across the griddle plate surface the spacing between the turns of the Litz wire are non-uniform going from a spacing of 0.64 cm (0.25 inch) in the center of the coil to no spacing at the edges.

To be specific the Litz wire is 0.25 cm (0.10 inch) in diameter. The coil length is 1.15 m (37 feet 10 inches) with the first 60 cm (two feet) having a spacing of 0.64 cm (0.25 inch) between turns, the next 0.82 m (27 feet) having 0.32 cm (0.125 inch) between turns, the next 0.15 m (five feet) having 0.26 cm (0.103 inch) between turns and the last 0.12 m (three feet 10 inches) having no spacing between turns except for the insulation over the wire.

To assist in producing the uniform heating in one specific embodiment, ferrite bars 40 are located as illustrated in Fig. 5 below the coils 42. The coils are rectangular, 28 cm by 25 cm (eleven inches by ten inches). Each coil has associated with it 8 ferrite bars, 6.4 cm (2-1/2 inches) long 1.9 cm (3/4 inch) wide and 0.048 cm (.0187 inch) thick. These sizes are approximate. The bars 40 are located at each end of the horizontal and vertical centerlines of the rectangle and at each end of two lines at approximately 35° on both sides of the vertical centerline, all as illustrated in Fig. 5.

The purpose of these bars 40 is two-fold. First, the bars reduce the flux concentration under the coil 42 and increases it over the coil so as to increase the flux coupling to the magnetic material. By reducing the flux under the coils the bars assist in preventing leakage of electromagnetic radiation. A second use of the bars is to assist the coil structure in providing uniformity of temperature across a griddle plate. In most prior art griddles it is assumed that application of heat must be heavily concentrated about the edges because there is no heat source outside of the edges of the griddle whereas the center of the griddle is surrounded by heated material.

It has been found, however, that the above statement is not totally correct and that the center of the griddle is cooler than the rest of the surface if that philosophy is followed. Thus in the specific embodiment, a small part of the coil is devoted to heating the center section of the griddle and the increased magnetic flux produced at the edges as a result of use of the ferrite bars compensates for the loss of flux diverted to the lateral central region of the griddle plate. As previously indicated, the variation in temperature across the griddle is ±5°C (± 10°F). to within 1.25 cm (1/2˝) of its edges.

Referring now to Fig. 6 of the accompanying drawings, there is illustrated in partial cross section, a cover 44 that may be used with the griddle system of the present invention. The cover 44 has inner and outer layers 46 and 48 fabricated from a heat resistant, poor heat conducting material and is a hollow structure. The outer materials used may be a liquid crystal polymer or stainless steel. Central region 50 of the cover 44 may be empty or may have a poor heat conducting fibrous filler 46 of glasswool or ceramic.

When placed on a griddle plate such as griddle plate 30 of Fig. 3, the surface of the plate-is reasonably isolated from the ambient air and loss of heat is maintained quite low. The plate does not overheat because its temperature will not rise above its effective Curie temperature and thus the temperature of the griddle plate 30 cycles slowly about its effective Curie temperature and little energy is expended. A further feature that enhances the efficiency of the apparatus is the insulator 11. The insulator 11 as described above reduces heat loss at all times. When the cover 44 is used in conjunction with the insulator, the covered griddle plate is greatly isolated from the ambient and effective Curie temperature is maintained with relatively little expenditure of energy.

Referring specifically to Fig. 7 of the accompanying drawings, there is illustrated in block diagram form a control circuit for the Sanyo unit. The unit includes the Sanyo inverter 60 feeding the griddle coil 62. The inverter 60 feeds a low power signal over lead 64 to a control circuit 66 which in turn supplies an on/off signal to the inverter 60.

The purpose of the control is two fold, first to make available full power to the coil if a griddle plate is present and to provide low level pulses to the coil if the griddle plate is not present or a light load is present in the form of a pan or other small cooking vessel or utensil. Specifically, if the power being supplied to the load by inverter 60 indicates a griddle plate is present the inverter 60 delivers some power all the time, the level depending upon the load. If the griddle plate is not present, a low power indication is supplied to control circuit 66 which goes into a low duty cycle pulsing mode causing the delivery of only small amounts of power to the coil 62. If now a griddle plate is replaced the power absorbed rises above a predetermined threshold and the full power is again available.

In addition a thermal switch (not illustrated) is located adjacent a griddle plate to sense coil temperature. Such over temperature can occur if the power supply allows the current to rise as effective Curie temperature is achieved. In the event the value of the left side of the equation on page 4 hereof exceeds the value of the right side thereof, a run-away condition can prevail. The temperature sensing switch in such event would shut the system down.

Although the present invention is disclosed as a griddle for cooking food such a system has many applications outside of the food industry such as in laboratories, particularly chemical and plastics laboratories, driving off moisture and solvents from various articles, heat treatment of metals, curing of resins and in any application that requires carefully controlled temperatures of a heated surface.