The Most Efficient Cooking Experience
Induction cooking is more than magic, it’s energy efficiency in the kitchen.
Cooking food hadn’t changed much since the beginning of time, until the invention of the stove. In the last century gas and electric cooktops became popular, but now, with modernization of cooking, induction cooktops provide for the most efficient cooking experience to date. Not only are induction cooktops extremely energy efficient, but they are the safest of all cooking options. Amazingly, you cannot get burned from touching the ceramic top due to the design of the cooktop. While this seems impossible, the design of induction cooktops utilizes simple electromagnetic technology to accomplish this.
How they work
Induction cooktops use an electromagnetic field to heat a ferromagnetic base within iron and steel cookware, which then uniformly heats food. Induction cooktops consist of an electromagnetic coil under a ceramic plate that generates an alternating magnetic field which when it comes into contact with ferromagnetic materials produces heat. In this way, only the cookware is heated, and the ceramic plate stays at room temperature. This is not only safer as the ceramic plate doesn’t heat, but it is more efficient and reaches cooking temperatures in a shorter time and distributes heat better, resulting in more uniform cooking.
The high efficiency of induction cooktops sets them apart them from their gas and electric counterparts. In fact, studies show that they have an efficiency rate of close to 90 percent, which is significantly better than the 40 percent efficiency of gas cooktops.
Induction cooktops reach high efficiency because it heats up only the cookware that has a ferromagnetic base. The induction cooktop’s alternating magnetic field operates at a frequency of 20 to 50 kHz and does not produce heat. Rather, this field generates eddy currents in the ferromagnetic base of the cookware producing heat. Acting like a short-circuited secondary winding of a transformer, hysteresis losses produce more heat in cookware. The power circuit of induction cooktops is the secret that creates the efficiency of induction cooktops.
Figure 1: The basic configuration of the power circuit of an induction cooktop, which includes an EMC/EMI input filter, rectifier with intermediate-circuit capacitor, inverter with snubber and resonance capacitors.
Because this magnetic field functions at high frequencies, IGBTs that can operate at high frequencies and steep switching edges are needed. As a result, EMC filtering is necessary in order to maintain a high efficiency in the induction cooktop.
Since an EMC filter is required, two household-class X2 capacitors with a capacitance of 1 µF are ideal for the application. Consideration should be taken on whether or not a low insertion height is needed in the application. Further, industrial-class Y1 capacitors with between with 2.2 nF and 15 nF help to suppress symmetrical interference and transients, providing EMC filtering required. Power chokes that operate with power compensation variant may also be needed.
Figure 2:Basic circuit diagram of the EMC input filter which ensures that no reactions occur in the power line due to the high frequencies generated in the power circuit.
The input section also contains the key protective functions of inrush current limiting and overvoltage protection. Two important functions are performed by the current limiter, which include:
- Reducing the inrush current to a tolerable level
- Protect electronics from overvoltages
Because the intermediate circuit capacitors are uncharged when the appliance is started, direct application of the power can cause high-charging currents to flow momentarily endangering the rectifier. In order to reduce the inrush current to a tolerable level, an inrush current limiter (ICL) that is based on a leaded disk NTC thermistor is utilized. Thermistors which are cold at the moment of turn-on have a relatively high resistance. As the current flows through them, they heat up and their resistance drops.
Figure 3: Protective components include capacitors, current-compensated power choke, NTC inrush current limiter and disk varistor.
With these characteristics, these thermistors protect the rectifier and prevent a surge to the power fuse keeping it from blowing.
Varistors may be used to protect the electronics in the system from overvoltages. Such protective components should be able to inhibit all typical power voltages.
Temperature protection and control
Because of the very nature of an induction cooktop, proper temperature monitoring is needed. Temperature monitoring helps to prevent thermal overload of the IGBT power switches. Surface mount temperature sensor that are fast and heat resistant should be used in induction cooktops. Such sensors can be housed in ceramic or aluminum, depending on where they are placed in the system. This would minimize influence by electromagnetic energy in the induction cooktop.
For example, a NTC thermistor potted into aluminum/ceramic case with cable outlet that has a temperature range of -10°C to 250°C with a short-term peak temperature up to 280°C can be used. Such thermistors should have high insulation voltage, flange housing for good thermal coupling to hot surfaces.
Figure 4: The power circuit of induction cooktop needs snubber, intermediate circuit and resonance capacitors.
Snubber, Intermediate circuit and resonance capacitors
In addition to the IGBT power switches, this subassembly contains capacitors with various functions. The switching of the induction coil produces high induction-voltage peaks. As a result, the IGBT path of the collector-emitter must be safeguarded. This protective function is made possible by a variety of snubber capacitors. Leaded snubber capacitors and others with strap terminals that can be screwed directly onto the power semiconductors of frequency converters should be used.
These film capacitors should have very low ESR and ESL values and should be self-healing due to the environment of the circuit, thereby improving reliability. And, in order to induction coils to operate properly, resonant capacitors must be connected to them in series.
Intermediate circuit (DC link) capacitors are needed to minimize ripple voltages and level out the rectified AC voltage. Ordinarily, aluminum electrolytic capacitors with snap-in or screw terminals are used for this purpose in industrial electronics applications, such as rectifiers and power supply systems. However, film capacitors with low parasitic ESR and ESL values are often preferred for space reasons in induction cooktop.
It is fascinating to learn how the power circuit makes the energy efficiency of induction cooktops possible, and understanding all the integrated components. The technology has fundamentally helped create a marvelous step forward the modernization of cooking. It is no wonder that professional chefs, like Gordon Ramsay, are adopting induction more and more.