Induction cookers are quick to cook and highly energy efficient and safer due to the lack of an open flame. Here's how you design one.
The induction cooking craze has been getting lots of attention even though the [enabling] technology has been around for about a century. The induction cooktop is quite popular, both in domestic and commercial usage. Induction cooktops are, however, relatively new to many regions in emerging Asia-Pacific economies and are expected to see rapid growth as a market. This is due to growing awareness and rapid urbanisation in Asia-Pacific and Eastern Europe regions.
Induction cooking uses magnetic coils to heat cookware. The cooking surface remains cool, while heat is generated within the cookware. Cooking by means of induction cooktop is quick and highly energy efficient compared to other conventional methods; on top of it the lack of an open flame makes them safer.
Induction cooking is the process of generating heat directly in a ferromagnetic utensil/pot/cookware by means of electromagnetic induction and subsequent generation of eddy currents. The electromagnetic induction principle was discovered long back in 1831 by Michael Faraday. Electromagnetic induction refers to a phenomenon by which electric current is generated in a closed circuit by the fluctuation of current in another circuit placed next to it.
The ferromagnetic cookware is to be placed on top of an induction cooktop. Below a ceramic or glass surface lies a resonant coil as shown in Figure 1 below. The induction cooktop and cookware can be considered as a transformer in which cookware act as shorted secondary (load). An alternating is made to flow through the resonant coil, which leads to generation of oscillating magnetic field. The magnetic field induces an electric current inside the cookware.
The induction cooktop only works with cookware made of certain materials that have specific properties. To be heated by the magnetic field, the cookware must be made of a ferromagnetic material, such as stainless steel or iron.
Figure 1: Induction cooktop working principle: The changing magnetic field interacts with the ferromagnetic cookware and induces an eddy current in the cookware, which in turn produces the heat for cooking.
The induction cooktop consumes power as long as the cookware is present on top of it. Unlike a gas burner or electrical stove the induction cooktop, it is incapable of producing heat on its own. In case the induction cooker is made to operate while no cookware present on top of it or the cookware is removed while induction cooktop is in operation, then the resonant coil sees it as if there is no load (open circuit) and there is be no energy transfer. In case of no cookware the induction cooktop enters sleep mode consuming minimum (standby) power i.e. <1W.
Generally semiconductor devices are used as switching element in various power converters. Insulated gated bipolar transistor (IGBT) are used in induction cooking systems. For power conversion, “soft switching” techniques are preferred over “hard switching” to have minimum switching losses.
In soft switching, the voltage or current is manipulated to become zero across the resonant switch at the moment of switching. Soft switching can be further categorised in two methods: Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS).
ZVS and ZCS switching technique has its pros and cons and their use-case is application specific. The voltage or current administered to the switching circuit can be made zero by using the resonance created by an L-C resonant circuit. This type of converter has a “resonant converter” topology. The two main resonant power conversion topologies used in induction cooktop are:
Table 1: Comparison of quasi resonant and half bridge resonant converter.
The induction cooktop works on the principle of a LC resonant converter. The resonant frequency not only depends on resonant tank circuit but also on the size and material of the cookware/utensil. This causes the system to have an oscillating resonant frequency. To control the power delivery of the system, which is in turn transferred to cookware, the input mains voltage and the current through IGBT are monitored by the microcontroller and the system switching frequency is suitably adjusted.