How a Microwave Works

In this article we will highlight some of the principles governing how a microwave oven works.

For information on how to tackle and resolve faults and problems on a wide range of appliances see our other articles »


Firstly though, please observe the following warning:

To produce the microwave ovens radiation, the mains electricity is 'stepped up' by either a transformer or an inverter to over three thousand volts, which is particularly dangerous as it also has a high current supply.

Unlike other electrical appliances in the home, the combination of high voltage and current can easily kill (rather than just shock) if accidental contact occurs. For this reason, when servicing the appliance repair agents are always advised to observe 'the three D rule': disconnect from the power and leave the door open (to prevent accidentally starting the appliance if the plug was not disconnected) before discharging the high voltage side prior to any repairs. This final point is especially important as it can easily cause accidental shock for an untrained person, and therefore we do not recommend that anyone who is not a professional repair engineer remove the cover from a microwave.

Microwave ovens were originally developed from radar technology, following the discovery that radio waves (with a frequency of approximately 2.5 GHz) could be absorbed by substances containing water, fat or sugars and heat them up, and that exposing the food for long enough to such wave energy would cook it.

Radio waves of this frequency pass through container materials such as china, glass and plastic with little effect, but cannot pass through the metal walls or the metal grille behind the door window. If the clay that the crockery was made from has any iron content it will also heat when exposed to microwaves, while food and liquids conduct heat to plates and containers.

The microwave energy in the appliance is produced by a component called a magnetron, which is a relatively old-fashioned technology that was replaced in most other contexts by the transistor in the 1960s.

The magnetron is essentially a thermionic valve and (as with any such valve) requires a heater, voltage supply and a cooling fan to prevent damage from excess heat. When the microwave oven is turned on to cook, the cooling fan can be heard along with an immediate 'thump' from the transformer, followed by a quiet fizzing as the magnetron starts to emit wave energy.

To reduce the cooking power in most microwave ovens, the power to the magnetron is simply cycled on and off.

For example, a microwave oven on half power may have its magnetron turned on for 10 seconds, then turned off for ten seconds. Therefore, if an extra ten seconds of cooking on half power is selected, the magnetron may not emit any cooking energy as it may be in the 'off' part of the cycle.

In a microwave oven, the radio waves penetrate the surface of the food and cook it quickly, while in a standard oven cooker heat has to be absorbed by the surface of the food and then conduct through to the inside.

The radio waves are quite narrow and disperse in a straight line until they are reflected by contact with the metal walls or are absorbed by the food/liquid within the cavity. In early microwave ovens this created hot and cold cooking spots, so to prevent this occurring the energy is helped in distributing by using either a turntable or a wave guide stirrer.

Typically, microwave energy will penetrate and cook food that is up to two centimetres in thickness. If food is thicker than four centimetres (such as a potato) the outside will cook quickly but the inner centre will rely on the conduction of heat, and therefore best results with microwave cooking are achieved in food less than four centimetres in diameter.

Standard microwave ovens don't tend to brown or crisp the food's surface due to the low air temperature inside, but some ready meals such as pies and pizzas come in packaging designed to absorb some of the microwave energy and create hot air for browning or crisping the food. Some microwave ovens also incorporate grill elements to achieve the same result.

Microwave Spares and accessories

* All information provided is a guide only. BuySpares accepts no liability for any problems occurred while attempting any advice shown. If in any doubt contact a qualified repair service.

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