Induction cooking

An induction cooktop is a cooker, in which the metallic cookware is heated inductively generated by eddy currents.

Operation

Energy is transferred in the form of an electromagnetic alternating field on the bottom of the cooking vessel where it is converted into heat.

Below the cooking surface made ​​of glass ceramic, there is a current-carrying coil which produces an alternating magnetic field. This induced in a metal pot placed over it by inducing eddy currents that heat the metal of the pot by the ohmic resistance. While the correct frequency is in the range of about 20 to 50 kHz.

Although induction heating for all metal (and hence electrically conducting ) pots ( for example, copper bottom ) works in principle, is a high efficiency with a commercial induction hob cookware with a bottom made ​​of ferromagnetic material (from the Latin ferrum, iron ' ) is recommended. The reasons for this are:

• Ferromagnetic material in the pot bottom concentrates the electromagnetic alternating field in the same. The coupled electromagnetic energy generates eddy currents in the outer surface of the pot. Due to the ohmic resistance of the ferromagnetic material, a large part of the electric energy ( about 2/3 of the heating power ) converted into thermal energy.
• Another portion of the introduced into the bottom of the pot energy is converted by the core loss (hysteresis) into thermal energy. This corresponds to about 1 /3 of the heating power.
• In the non-ferromagnetic pot bottom the magnetic field is less bundled and can thus spread to a greater extent in space. This can lead to undesirable interactions with the environment of the current-carrying coil: Electromagnetic Environmental

Ferromagnetic material is shown by the fact that a magnet will stick to it.

The material has a good electro-magnetic conversion efficiency into heat energy have a significantly higher electrical resistance than the highly conductive copper RF induction coil. This is usually the case in ferrous alloys. Induction -suitable cookware meets these conditions normally and can be the symbol showing the wire coil of a coil in a square, seen on the pot or pan bottom. However, this symbol has no functional guarantee. You can check a pot only in practical tests on induction suitability. This thicker soils are beneficial for better heat distribution.

Most induction hobs switch the exciter field from automatically when there is no, too small, or if a pot on the hob. However, the electronics can be a resting metallic object it holds for a pot, fool.

Construction

A large, flat, single-layer coil of Litz generates the alternating magnetic field beneath the cooking surface. To form capacitors with a resonant circuit which is driven by the IGBT switching transistors. There are several circuit designs. The switching transistors are controlled by, for example, located in the resonant circuit converters and fed by a controllable direct- voltage intermediate circuit. The DC voltage is obtained by means of controllable rectifier from the AC mains voltage. The amount of DC voltage in the DC intermediate circuit determines the heat output. A further possibility is the pulse width control of the excitation of the resonant circuit.

Benefits

Since induction cooktops because of their functioning not only the bottom of the cooking vessel, but also the side walls are heated, heat is distributed better, and the food to be cooked is heated faster. The reaction time is very short, and the food to be cooked very quickly heated. Due to this short reaction time, the cooking process can be more accurately dosed than in a stove with electric hotplates.

The cooking surface remains cool next to the pot, since it does not heat itself and the heat does not conduct. The area below is only by conduction, that is, only secondarily by the contact with the pot heated ( thermal reverse conduction and radiation of the pot ); Food residue can not burn onto the cooktop next to the bottom of the pot, which allows easier cleaning thus.

After a longer cooking time and the cooking surface is so hot that burns.

• Only a very small part of the electromagnetic field remains unused if the hob is completely covered by the pot or pan bottom. The amount of unused energy by convection, radiation and conduction is very low for the hotplate.
• In comparison to the gas cooker less energy is dissipated through convection ( by the rising hot gases next to the cup ) to the environment.
• Compared to conventional electric stove with heating coil or infrared heating the cooking process is much less energy withdrawn by radiation and convection. The cooking device is not heated above the first to be heated hob: Boil water, although the cooking surface is cooler than 100 ° C.

Disadvantages

• Cooking vessels are required with a ferromagnetic base for cooking on an induction hob. Thus, any of the cast or deep drawing steel pots made ​​good. Even just magnetizable stainless steel pots can be used.

• In contrast, aluminum pots and copper vessels and non-magnetic stainless steel vessels can not be used. All non-metallic vessels, especially those made ​​of glass and ceramics are already completely unsuitable.

• For cooling, less robust control electronics of the hob, a fan is usually installed, which - depending on the design and age-related wear of axle bearing - works more or less noisy. However, the noise found in cooking drown ( extractor, hissing when frying, etc.) the fan noise, so it is not usually detectable.

• Depending on the ratio between the size of the pots and the induction coil may lead to a (usually very low ) hum due to magnetostriction.

• In case of failure of a single cooktop can with economical design of the electronics a miniature fuse ( 5x20mm, see photo ) of the controller to be blown. Modern designed electronics are protected against overload by inherently better design.

• Contrary to the instructions for use electric hobs and ceramic hobs can also be used unconventionally, for example, for the baking of pizza, sandwiches, Tortillawraps in the infrared radiant heat directly on the glass ceramic cooktop. This is generally not possible with induction cooktops.

Electromagnetic Environmental Compatibility ( EMEC )

The Federal Office of Public Health of Switzerland was 2006/ 07 induction cookers for compliance with the ICNIRP reference levels for investigating the magnetic field exposure. Basis of the determination of the reference values ​​of the external fields are basic restrictions for field strengths inside the body, which in the scientific literature to occur secured adverse health effects has been published. From the lowest inner box size at which the respective frequency range such an occurrence is described, under the condition of maximum coupling of the external field to the exposed person, the reference values ​​of the external fields, where the magnetic flux density is derived. In this case, a safety factor flows that takes into account the data quality and individual differences in sensitivity. Even in the worst case, the reference value can not lead to a violation of the basic restriction order. With the coming in induction cooking appliances for use frequencies in the human body caused neuronal excitation (tingling, muscle twitching, etc.) is crucial. The ICNIRP guidelines of 1998 determined from the studies published up to that point with a safety factor of 50 a reference value of the magnetic flux density of 6.25 μT. The guidelines of 2010 were due to the improved through interim releases available data reduce the safety factor to 10 and put the reference value brings to 27 μT fixed.

The tested equipment held under conditions of normal use in 30 inches horizontal distance from the unit, in accordance with the applicable test specifications, the reference value of 6.25 μT in force on that date ICNIRP guidelines a. In realistic lower horizontal distances, this value was partly exceeded, at a distance of 1 centimeter in front of the device, it reached a maximum of 10 μT, sides and back up to 26 μT. As a result, highly recommend the Swiss Federal Office of Public Health and the German Federal Office for Radiation Protection, a minimum distance of 5 to 10 cm from the front edge of the hearth.

Above the hob immediately adjacent to the cookware significantly higher flux densities up to 84 μT were measurable. When not used as intended by the use of too small cookware or not centered on the cooking zone positioning, making the cooking zone is not fully covered, or by using cookware with uneven surfaces or those that are not ferromagnetic, as more widely even stronger stray fields. Since the reference values ​​, the assumption of maximum coupling, ie maximum exposed body cross sections, underlies what is not given here, but the basic restrictions are not necessarily exceeded. To answer this question, the end of 2011, a further study on the effects in the human body conducted on behalf of the Swiss Federal Office of Public Health.

Due to the alternating magnetic fields can be influenced in principle pacemaker in their function. Even though modern devices are protected against such interference, it is recommended by the manufacturers to maintain a minimum distance of 40 cm from an induction hob with a pacemaker.

The induction coil and the pan thereon form an electrical capacitor. With activated induction coil, the pan is electrically recharged slightly and continuously. If the pan is touched, a low leakage current flowing through the body. In order to avoid such leakage during cooking, the Swiss Federal Office of Public Health recommends the use of non-metallic cooking utensils. Modern inductors are shielded from such streams. Here, a graphite layer is applied to the topsheet of the inductor. This graphite layer is in turn connected to a ground wire. In older inducers may occur in the cookware to voltages of over 200 volts. This is felt by people in sensitive areas such as back of the hand ( blood vessels ) as a light, to mediocre " tingling ". After the above ground, the voltage must not be higher than 30 V ( or ground the graphite layer over 5 k ).

Electromagnetic compatibility ( EMC)

Induction hobs work in lower long-wave range (50 kHz) and emit electromagnetic waves at these frequencies. The operating frequency is below but civilian long-wave transmitter (> 100 kHz) and also below the lower measuring limit agreed to test the Electromagnetic Emissions ( 150 kHz).

However, there may be interaction with radio controlled watches, as these on a frequency of 77.5 kHz, in Germany. During synchronization with DCF77 transmitter this communication might be disturbed. After switching off the hob this error should no longer exist, however.

Other disorders with higher frequencies are generated by the power semiconductors (IGBTs, thyristors ); they must be, as with other electrical appliances with regard to system perturbation ( conducted interference ) and radiation so low.

Induction cookers contain complex electronic assemblies and are therefore potentially sensitive to transient surges in the power grid than other electric cookers. The protection of the electronics is at the meter board by high- frequency barriers that limit the spread of high-frequency disturbances from the distribution of the power utility over the home network.

Market development

The table shows the sales in some European countries with stand 2004. 2005 were sold in Germany around 80,000 pieces.