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Guide: How an induction hob is made

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Cristiano Passeri View Drop Down
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    Postato: 06 Lug 2015 alle 13:44

How an induction hob is made

First issue: 5 June 2015

Last update: 23 February 2017

Here we can see how an induction hob is built and how it works. The model examined here is an AEG 68001 K-MN, produced in year 2009.

The most of what is written here is valid for all the current models (2015) produced by the 2 major German manufacturers groups (AEG/Electrolux, Bosch/Siemens/Neff). The assembling layout and the electronic boards are very similar. The operation principles related to the inverters and induction coils are generally valid for all manufacturers. The small and cheap portable induction cookers differ in several parts from these devices.

The induction hobs by AEG/Electrolux and Bosch/Siemens/Neff, at least the new models presented from the year 2009, use electronic boards of the same generation of these shown here. This generation is more reliable then the previously.


The induction coils:

 - After removing the stainless steel frame and the ceramic glass, the induction coils and the control panel are naturally placed on the upper level.

The induction coils are covered with a thin sheet made with a material containing Glimmer, a mineral material very resistant to high temperatures. The upper side of the sheet is reflective, to reject away from the coil the heat coming down through the ceramic glass from the bottom of the pot.

Photo: Ceramic glass removed

Under the reflective sheet is placed a disc made of a heath insulating material, about 2-3 mm thick. This insulating disc has the function to protect the induction coil from the heat coming down from the pot and to avoid that too much heat comes inside of the device. When cooking by high temperatures using oils or without fats, the glass reaches over 200°C (392°F).

Photo: Induction coils

Once removed the thermal insulating material, the induction coils are visible. In this model the coils are fixed with glue on a sheet made with Glimmer. Under of it, 4 rectangular ferrite bars are fixed, oriented in sunburst way. These bars push up towards the pot the part of magnetic field that would tend to spread in the direction below the coil. This helps also to overcome the actual distance between the coil and the bottom of the pot.

The coils8cm and 21cm cooking zones are made with 24 and 26 whorls with copper wire of about 3 mm diameter. The coil of the 14,5cm zone is made with 26 whorls with copper wire of about 2 mm diameter.

The diameters of the coils actually match with the nominal diameters of the cooking zones here, differently than in the portable induction cookers which I have seen.

The induction coil of the 21 cm zone has a space among the whorls in the middle part. This is to improve the spreading of the heat in along the whole diameter. Otherwise the magnetic field (and the generated heat) would trend to concentrate in the middle diameter of the coil. This happens with all coils in general, but the effect is more important in bigger ones. There could be also circuital reason for this solution, like to keep a similar number of whorls for all the coils, but I don't know. 

The induction coils are mounted on aluminium supports, which keep the coils pushed against the ceramic glass. The parallelism between the surface of the coil and the bottom of the pot is also important for the uniformity of the heat spreading over the pot bottom.

In the centre of the coils is mounted a temperature sensor, usually a PTC or NTC resistor. It is used to switch off the cooking whet it reaches a too high temperature, when for example an empty top has been forgotten on a zone switched on. This protection acts however only in emergency cases, when the temperature is very high. Otherwise, it could interfere when cooking by high temperatures.
On some models this sensor is use also to set and approximately maintain the temperature of the pot, as in many portable induction hobs.

Photo: Induction coils of a flexy zone, composed by 4 sub-zones. Below in the page, see also the part concerning their electronic boards. (Bosch PIP875N17E. Photo by our forum member Bass79)

- The electronic boards:

The induction hobs of the current generation are composed by only 3 electronic boards. 2 Boards contains the 'inverter' devices which provide the power to the induction coils and 1 board for the command panel.

Each one of the inverter boards has a built-in power supply, completely independent from the other board. It is no more present a board with a common power supply section for all the boards.

Each inverter board is independently from the other and it is directly connected to the connection terminal of the hob. One inverter board is connected to the L1 terminal and the other one to the L2. However for electric hobs is not important which phase you use for the connection.

One inverter board manages the 2 cooking zones of the left side of the hob and the other board manages the 2 zones of the right half. 

The command panel is connected to only one of the 2 inverter boards, from which it gets also the power supply. The board which is connected to the control panel has the role of 'Master' and the the other is 'Slave'. A 'bridge' cable between the 2 boards brings the commands to the other inverter board.

Notes: generally all the induction hobs with max. total power of 7,2-7,4 KW are composed by two inverter boards, where each one manages a couple of 2 cooking zones, usually arranged in left and right half of the hob. Each inverter board is designed to don't exceed the maximum power of 3700W (16A/230V). This is because they are deigned to operate with a 3-Phase power supply connection, which has the allowed limit of 16A on each phase. This is the basic limit per the 3-Phase power connections for homes in Europe (or at least in Germany).

This is the reason why it is not possible to activate the booster level on both zones of the same half of the hob. And you can't set the max. level for both a 18cm zone (1800W) and a 21cm zone (2200W) when they are on the same half of the hob (or they are powered by the same inverter board).

Induction hobs with a total power higher than 7,4kW, like the hobs with 6 cooking zones with total power of 10-11kW, contain 3 inverter boards, which use all the 3 phases. Electric hobs for home use usually doesn't exceed this limit, because they would require a 3-phase power connection with more than 16Ax3 (on 230V) = 3700W x3 = 11KW (when all cocking zones are switched on at the max. power).

- The inverters:

The heart of the induction hob are the inverters, the devices which generate the power for the inductor coils. On the inverter boards are mounted the crucial and most interesting components, but also the more subjected to failures.
The inverter transforms the input voltage of 230V 50-60Hz in this case, into another alternate voltage but with waveform, amplitude and frequency different from the original and variables (up to 40-70 kHz, depending on models).

the upper side of the board are mounted the power components, which are the ones subjected to higher voltages and currents, and the larger size components.

general view

Photo: left inverter board

The crucial components are the ones fixed on the big heather sink of aluminium. As short explanation, the first 'black rectangle' on the left is a rectifier diode bridge which provides power to all the 4 power transistors (IGBT), which provide power to 2 induction coils. The power transistors are the other 4 'back squares' mounted on the heather sink. To generate a complete wave are required 2 transistors, thus each couple of 2 transistors supplies one induction coil.

in hobs which are mounted in furniture with insufficient air circulation, the power transistors and some electrolytic capacitors are more subjected to failures over the time. But meanwhile to individually replace these components costs from some cents up to 5-6each piece, the repair services of the manufacturers offer only the replacement of the whole inverter board, for a cost of about 400and more. The price of a new induction hob which contains 2 pieces of the same board. 

As comparison let's take a look to a Bosch/Siemens inverter board of the same generation. As you can see, they look very similar, so that they may appear to be a result of a common developing project. One of the few differences visible by sight is the choice to use blocks of 4 capacitors connected together to make a bigger one, instead to use a single bigger one, but it has no effects on the operation of the circuit. The choice may have montage reasons.

Photo: inverter board Bosch/Siemens

Photo: inverter board Bosch/Siemens (bottom side)

On the bottom side of the board is mounted the 'smart' part of the inverter. I didn't removed the board of my hob, but the substance is the same. The several chips manage the generation of the signal which will power the induction coils. The software loaded on these chips manages the frequency, amplitude and, partially, the waveform of the signal which will be converted into power by the components mounted on the other side. The modulation of the power depending on the cooking levels happens here. More rarely the failures occur on this side of the board, where the components are more difficult to be removed and soldered for an electronic hobbyist. But sometime it happens.

In the years, the boards are keep updated producing different revisions. Between a revision and the successive normally are implemented minor changes and improvements, e.g. the value of a component is changed or a small circuital modify is made.

 Inverter boards of these types are currently (2015) mounted as power modules for couples of 2 circular cooking zones (for the groups Bosch/Siemens and AEG/Electrolux). Hobs with a flexy zone subdivided into 4 smaller sub-zones should use a different board, for that half of the hob. Instead, the inverter boards of bridge zones should be basically similar, if not equal, to these shown here.

 Curiosity: manufacturers of induction hobs don't produce the inverter boards currently by themselves, but these are assembled by third parts companies. Inverter boards of the Bosch/Siemens group are assembled by the Spanish company Electronica Cerler, and these of the AEG/Electrolux by the German company E.G.O., but with production factories worldwide located. I don't know whether just the assembling of the boards is commissioned to these companies, or also the complete designing and developing (based on customer's specifications). These companies produce and develop electronic boards for many manufacturers of household appliances worldwide.

The Flexy zones:

Induction hobs with one or more Flexy zone, composed by 4 (or 5) sub-zones, are in the fact very similar to those with only round zones, despite a much higher selling price.
The part which differs more is just the coils, which they are 4 smaller ones than 2 bigger.

Differently than expected, the inverter board is structurally the same as a board for two traditional round zones, or bridge zones. It is provided with 4 power transistors, which allow to manage only 2 independent cooking zones.

Photo: board for flexy zone (Bosch PIP875N17E. Photo by our forum member Bass79)

On a small additional board (on the right in the photo) on which are mounted 4 relays for a worth of few € (the 4 small orange boxes), the 4 coils are combined among them, as needed for the use. Numbering the coils with 1,2,3,4 starting from the first on the top, the combinations are for example:

(1) and (2+3+4) when the flexy zone is divided in a small and a bigger '3 stripes' zone.
(1) and (3) when two small pots are used, one independently from the other.
(1+2) and (3+4) when two bigger pots are used, as it they were on 2 round zones.
(1+2) + (3+4) when the flexy zone is used as single big zone, it works actually like two standard bridged zones, only that each coil is composed by 2 smaller coils connected in series.

Obviously, the board is provided with some additional components to command the relay board, and to manage 2 additional temperature sensors. But this few additional hardware does not justify in any way the selling price majored by 250-350, compared to an hob with standard round zones (up to +600 for some models with double extendable Flexy zone).
The 'extendable' Flexy zone, recently introduced on the market, will use the same concept. They will use simply 5 coils and 5 relays, but the structure doesn't change.

The control panel:

It doesn't contain elements of particular interest, but its main chip contains important software functions, like the power management and other settings. It is seldom subjected to failures. 

Under the glass surface basically two types of touch sensors exist: capacitive, like in this case, and optic. Capacitive sensors are usually better than optic, because they are not affected by the environmental light and they are less sensitive to dirty residues on the glass and to small objects.

Shortly described, capacitive sensors are composed by a metallic surface to which is applied a very weak electric field. The field is altered by the proximity of an object with consistent mass, better if also conductive, like a finger.

Optical sensors are composed by an infrared LED and a photocell placed beside of it. The photocell receives more or less of the light coming from the LED when something is placed, or not, over both them. They are more simply and lower cost, but they suffer of the mentioned disadvantages. On the other hand, optic sensors are more suitable when the touch buttons must be smaller or more near to each other. Capacitive sensors require a not too small surface area and a larger distance between buttons.

Photo: capacitive buttons. The springs keep the buttons pushed on the glass.

Modificato da Cristiano Passeri - 28 Feb 2019 alle 11:41
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