J. Schedel, I. Khan( and E Uken(
Naval Engineering Services, Cape Technikon(
This paper entails the development of an industrial induction cooker. The induction cookers output power is to be controllable over the full desired operating range. The cooker is also to function with all ferrous vessels. The cooker consists of three main components, namely the load, the inverter stage and the control stage. This paper also briefly discusses the efficiency of the cooker. It also looks at the difference between the simulated output power versus the actual output power.
Cookers can broadly be divided into two types, namely gas cookers and electric cookers. The main function of the cooker is to heat the vessel in which the food is. This vessel in turn distributes the heat to the food and cooks it. Such cooking can amount to as much as 10% of all energy used in a household. [1,2] By utilizing this energy efficiently the energy used for cooking can be substantially reduced. Most cookers expend wasted energy in the form of heat.
Cookers have always been designed to generate heat in a manner that will most efficiently transfer this heat to the cooking vessel. Induction cookers are a relatively new application of induction heating. Induction cookers do not work on the same principal as other cookers. They do not generate heat, which is then transferred to the cooking vessel. Induction cookers work on the principal that they generate the heat in the cooking vessel itself. This improves the transfer of energy and reduces the amount of wasted energy normally associated with conventional cookers.
The main disadvantage of induction cookers is that their initial cost is more than that of a conventional cooker. Another disadvantage of induction cookers is that only ferrous vessels will operate on them.
2. INDUCTION HEATING
Induction heating involves the varying of the magnetic fields in the primary, thus inducing circulating eddy currents, caused by electromagnetic induction, in the secondary. In the case of the induction cooker the cooking vessel acts as a short-circuited secondary, and the induction coil acts as the primary.
Induction heating can be defined in the following manner: By varying the current in the primary circuit an alternating magnetic field is setup. When a conductive material (the cooking vessel in the case of the induction cooker) is placed within this alternating magnetic field, eddy currents are induced in the conductive material. Due to the resistivity of the material, the eddy currents will cause conduction losses in the material, leading to heat being generated within the material [3,4,5].
Induction heating employs three main effects, namely electromagnetic induction, skin effect and heat transfer. Induction heating allows a defined section of the vessel to be heated accurately. The skin effect allows the penetration depth of the magnetic field to be controlled thus ensuring maximum heat transfer .
3. INDUCTION COOKING
Conventional cookers such as gas and ‘electric resistance’ cookers loose a lot of energy to the environment in the form of heat, as the heat must be transferred form the ‘element’ to the cooking vessel. This heat loss results in poor thermal efficiency. Due to the nature of induction heating all the heat that is generated by the induction cooker is generated within the cooking vessel, thus the thermal efficiency of the induction cooker is far greater than conventional cookers.
The power of an induction cooker is instantly controllable. This results in quick rise in temperature, which in turn results in reduced cooking times. If the cooking vessel is removed from the cooker the power is instantly reduced to a minimum. As the heat is generated in the cooking vessel and not the element, the surface of the cooker remains relatively cool. The only heat is that which is...