Microwave oven design and functionality

Microwave Ovens -- Construction and Operation

Microwave ovens use a wide variety of electrical circuits and mechanical components to produce microwave energy for heating and cooking [3]. There are two primary sections within a microwave: the control section and the high-voltage section. The control section consists of a timer (electronic or electromechanical), a system to control the major power output, and diverse interlock and protection devices. The components in the high-voltage section serve to increase the voltage. Higher and higher voltages are then converted to microwave energy.

When plugged into an electrical circuit, electricity enters the microwave oven through a series of fuse and safety protection circuits. These circuits include various thermal protectors that can also deactivate the oven in the event of overheating or a short [2]. Under normal circumstances, electricity passes through to the interlock and timer circuits. When an oven door is closed, an electrical path is created through a network of safety interlock switches. Once the operator sets the oven timer to begin cooking, the voltage path is extended.

Generally, the control system includes either an electromechanical relay or an electronic switch called a triac [3]. The control circuit generates a signal that causes the relay or triac to activate, thereby producing a voltage path to the high-voltage transformer. This high-voltage transformer, along with a special diode and capacitor arrangement, has the capacity to increase the typical household voltage of about 115 volts to as much as 3000 volts [1]. Such high voltage is required for the magnetron tube to convert the high voltage into undulating waves of electromagnetic cooking energy [3].

The microwave energy is transmitted into a metal channel called a waveguide, which feeds the energy into the cooking cavity of the microwave where it encounters the slowly revolving metal blades of the stirrer blade. Some models use a rotating antenna while others rotate the food through the waves of energy on a revolving carousel. Either way the goal is to evenly disperse the microwave energy throughout all areas of the cooking compartment. Some waves go directly toward the food, others bounce off the metal walls and flooring; and, still others may reflect off the microwave door [2]. In this way, microwave energy reaches all surfaces of the food from every direction.

More than 60% of American households now have microwave ovens due to the speed and efficiency they offer [1]. Total cooking times are much shorter than traditional methods of food preparation and a greater number of food nutrients are retained. Microwaves do most of their work on the water held within food. Water molecules constitute dipoles which are akin to a bar magnet with both a positive pole and a negative pole. The microwave ovens electromagnetic field oscillates as it passes through the water molecules in the food, changing the polarity of the field and causing the dipole/water molecules to flip themselves in order to be aligned with the new polarity. Heat is created by the resulting friction of the water molecules reversing direction millions of times a second.