26 Sep New technology, new blessing? - induction cookers

Induction hobs combine the advantages of gas and electric hobs and are now widespread in private homes and professional kitchens. For good reason, we think.

The technology is not really new. But although induction stoves have been around for over 100 years, they have only really caught on in the last few years. While they were still too expensive for the general public until the 1990s, they are now much cheaper to buy thanks to new manufacturing processes.

We show the advantages of induction technology and explain why the slightly higher acquisition costs on average are worthwhile.

Gas, ceramic, induction

Induction hobs can be precisely regulated

In terms of appearance, non-functional hobs are similar to electric hobs, which we colloquially refer to as ceramic stoves . In fact, they are also equipped with the same type of glass-ceramic plate but differ in their function.

With electric hobs, the heat is generated by heating coils. These heat up relatively slowly and stay hot for a while after cooking, so the reaction time is comparatively long, energy is lost and there is an increased risk of burns.

How induction technology works are quickly explained: Current generates an alternating magnetic field in the coil located under the glass-ceramic plate. This passes to the bottom of the cookware and is converted into heat.

This is why special, ferromagnetic cookware is necessary. This is available in all price ranges. It is particularly important to ensure that the bases of the pots and pans are completely flat so that the heat is evenly distributed.

Ideally, not only is the base suitable for induction thanks to a magnetic coating, but also the walls of the cookware, so that the heat can spread even more optimally.

This can be regulated very precisely through many levels and both very high and very low temperatures are possible. Like a gas stove, the water boils very quickly and meat can be seared, but unlike on an open gas flame, chocolate, for example, can also be melted without a cumbersome water bath.

The question of security

An induction cooker is just right for those who cook with a lot of heat but prefer not to have an open flame in their kitchen. Ultimately, this has some disadvantages, such as an increased risk of fire from potholders & Co. and time-consuming cleaning.

In contrast to the electric stove, the plate is not heated when cooking with induction technology. Although it can heat up indirectly through contact with the hot bottom of the pan, it cools down again quickly. The stove is also switched off automatically if there is no cookware on it.

Turning back halfway on the way on vacation to make sure that the stove is really off and that the house does not burn down during the long absence is no longer required.

If you are still not convinced by induction hobs, you may be convinced by the energy balance, which is quite impressive: Induction is significantly more economical in terms of power consumption than electric hobs. This is due to the short response time and the low energy loss. For those who cook a lot, this can make up for the higher acquisition costs.

Although gas stoves are also quite economical due to cheap gas prices, a lot of heat is given off into the environment. In small rooms or when cooking for a long time, this can lead to a very warm kitchen.

In restaurant kitchens, in which gas stoves are actually standard, temperatures often rise to 50 ° C. For this reason, many professionals now also rely on induction.

Induction hob

An induction hob is a kitchen appliance in which the metallic cookware is heated by inductively generated eddy currents.

Mode of action

Energy in the form of an alternating magnetic field is transferred to the bottom of the cookware, where it is converted into heat.

Underneath the glass-ceramic cooking surface, there is a current-carrying coil that generates an alternating magnetic field. This is induced in a metal pot placed above it by induction eddy currents, which heat the metal of the pot and from there the contents through heat conduction. The frequencies usually used for this area are in the range of about 25 to 50 kHz.

Although induction heating works in principle with all-metal (electrically conductive) pots (e.g. also made of aluminum), special cookware with bases made of ferromagnetic material is necessary for a sufficiently good function with commercially available induction hobs.

The reasons are:

①The magnetic alternating field of the induction coil spreads in all directions without a pot and is even repelled by highly conductive, non-ferromagnetic pots. The use of ferromagnetic material in the bottom of the pot causes this field to be bundled, which means that the emitted electromagnetic energy is specifically transferred to the electrically conductive underside of the pot. On the one hand, this reduces the required field and, on the other hand, minimizes radiation to the environment.

②The magnetic reversal losses in the bottom of the pot provide additional heat beyond the eddy current heat. They contribute about 1/3 to the heating output.

The (ferromagnetic) material must also have a sufficiently higher specific electrical resistance than the copper of the induction coils. That is the case with iron.

To check whether cookware is suitable for induction hobs, a magnet can be held on the floor. If this sticks, the floor is (Ferro) magnetic and therefore suitable.

Most induction hobs switch off automatically as soon as no or no suitable, ie ferromagnetic, pot is on the hotplate.

electrical construction

A large, flat, single-layer coil made of high-frequency stranded wire generates the alternating magnetic field under the cooking surface. Together with capacitors, it forms an oscillating circuit that is driven by IGBT switching transistors. There are various circuit concepts for this. The switching transistors are controlled, for example, with current transformers located in the resonance circuit and fed from a controllable DC voltage intermediate circuit. The DC voltage is obtained from the AC mains voltage via controllable rectifiers - its level determines the heating output. Another possibility is the pulse width control of the excitation of the resonance circuit.

advantages

①Similar to the gas stove, very short reaction time when changing the setting (very short preheating time; fast, precisely dosed searing)

②In some cases, sensors prevent overheating after all the water has boiled

③rapid parboiling of large quantities, even with heavy pans, as the heat in the bottom of the pan is generated directly by induction and the hotplate does not have to be heated first,

④the cooking surface remains cool next to the pot, as it does not heat itself, but is only warmed up secondarily through contact with the pot (so-called rewarming)

⑤Much easier cleaning as food residues hardly burn-in: the glass-ceramic surface only heats up underneath the pan, but not on the critical surfaces around the pan

⑥Improved safety, as energy is only given to metal in a targeted manner: induction cooking surfaces switched on accidentally (or by small children) only heat when a saucepan is on them

⑦Energy saving:

⑧with short cooking times, as only a relatively small amount, is heated and neither, as with gas stoves, large parts of the energy is lost through radiation and convection , nor is the energy deposited in the hot plate (therefore, for example, particularly suitable for cooking with a pressure cooker)

⑨Heat generation only in the pot, even with different pot diameters: if the pot is too small, only this heats up; the surrounding area of the hob remains cold.

disadvantage

Since a ferromagnetic base of the cookware is recommended or even necessary for the induction hob to function, some types of stainless steel pots and aluminum pots cannot be used. All glass, ceramic, etc. non-conductive pots are generally unsuitable. Therefore it is sometimes necessary to buy new pots and pans.

As an alternative to expensive special dishes, simple enameled steel pots and iron frying pans are recommended. Commercially available stainless steel pots with a multi-layer pot base are also suitable for induction cooking. These are on the packaging, e.g. Sometimes the underside is also marked with a special symbol.

In principle, induction and all-electric hobs have a significantly higher primary energy consumption than gas hobs, especially with medium and long cooking times and not too high power, unless the electricity is generated from renewable sources.

There seems to be the problem, at least with inexpensive single induction hobs, that they cannot be operated with low power, which limits their practical value somewhat. You shouldn't be fooled by the number of performance levels.

Another disadvantage is the noise development, which is noticeable on the one hand as a low-frequency operating noise on the fan and on the other hand as a high-frequency noise on the pot.

People with a pacemaker should note that these devices work with a strong magnetic field that could block or otherwise interfere with the pacemaker and therefore be dangerous.

Electromagnetic environmental compatibility (EMC)

The Swiss Federal Office of Public Health examined the 'magnetic field exposure from induction cookers' in an exposure study to ensure that the ICNIRP limit value recommendations were adhered to.

The fields can slightly heat human tissue, but they are much less than z. B. Microwaves absorbed. It would therefore even be safe to put your hand on the hob if it were to work without a pot.

Electromagnetic Compatibility (EMC)

Interference emission

Induction hobs work in the lower long-wave range and emit electromagnetic waves at these frequencies. However, the operating frequency is below civil long-wave transmitters and also below the lower measurement limit for testing electromagnetic interference (150 kHz).

Further interference at higher frequencies is generated by the power semiconductors ( IGBT, thyristors ); in terms of network feedback (line-related interference) and radiation, they must be as low as with other electrical devices.

Interference immunity

Induction cookers contain complex electronic assemblies and are therefore potentially more sensitive to transient overvoltages in the power grid than other electric cookers.