Microwave Ovens

Microwave Ovens

Microwave ovens are one of a few electric methods of cooking. The microwave oven evolved from research on Doppler radar technology by accident when a researcher (Percy Spencer at Raytheon) found a chocolate bar in his pocket had melted when he was exposed to microwave radiation.

1.) How it Works
1.a) The Magnetron
2.) High Frequency Power Supply
3.) Notes on the History

1.) How it Works:

The microwave oven uses a magnetron to generate microwave energy. The microwave energy travels through a waveguide and is distributed into a metal cavity where the food is cooked. The waves are absorbed by the food and excite water molecules in the food. This heat then transfers throughout the food cooking it.

The microwave oven is faster than a conventional oven because the microwaves can penetrate into the food and heat the food from the inside out. In a conventional oven heat comes from the outside and slowly works its way into the food through standard slow heat transfer processes.

1.a) The Magnetron

The cavity magnetron is the main component of a microwave. The magnetron is an oscillator where electrons are emitted from a hot cathode and whirl past resonant cavities which are part of the anode at speeds that generate microwave energy. This microwave energy is coupled out of the vacuum via a probe which is connected to a waveguide to deliver the energy to the oven (see our video below for a motion graphic illustrating this whole procedure). The magnetron is a high powered vacuum tube.

The concept of the magnetron was invented by Dr. Albert W. Hull in Schenectady in the 1930s. His designs were for more power control and a few oscillators were also devised for generating energy in the 100s of MHz.

Left: Albert Wallace Hull developed the magnetron which is the basic component of the microwave oven, radar and other technologies. Photo: The Schenectady Museum (miSci)

Video below: Mr. Rudy Dehn was one of the original developers of the microwave oven at General Electric in the 1960s and 70s.  He tells us how the microwave works and shows us older magnetrons vs the current magnetron:

Above: older magnetrons were more fragile and larger than newer magnetrons. Invention of better ceramics helped make it more robust.  Magnetrons are a type of oscillator which eject electrons from a coated filament.  This is similar to how a vacuum tube operates.

2.) High Frequency Power Supplies

Power supplies are needed to power magnetrons as they require a high voltage (3 kV) dc supply. The magnetron converts the high voltage DC to the required 2.45 GHz which is an engineering challenge. Large industrial/commercial ovens may use 915 megahertz magnetrons to excite the larger oven cavities.

Below: video: W.Kornrumpf from the Edison Tech Center explains the evolution of high frequency power supplies from huge devices with tubes, to handheld solid state devices of today.

Right: One of the first freestanding ranges with a microwave oven for sale to the public was developed by General Electric. The microwave energy was transmitted from the magnetron under the oven to a glass dome in the back top of the oven through a waveguide (915 MHz). The energy was then emitted into the cavity heating the food. Modern microwave ovens still use waveguides to deliver the microwave energy from the magnetron to the cavity.

Below is the 1960s era power supply used with the oven on the right.

3.) Notes on the History of the Microwave Oven

The microwave oven began as the product shown above – a three level appliance with standard oven on top, range in the middle and large microwave assisted by Calrod elements below. It was believed that the main use of the microwave would be to speed up roasting turkeys and other large items, and thus the Calrod element was needed. Later on the device was changed into the counter-top appliance we know today.

Below Mr. Rudy Dehn recalls the early days of the 915 MHz microwave oven at Louisville at General Electric:

“I was part of a GE delegation at a hearing before the FCC to urge retaining 915 MHz designation to continue because of greater `skin depth’ at the lower frequency. Of course, as things developed, users were much more interested in quickly cooking or reheating much smaller items.

The GE oven depended on an elastomer-cored metal mesh gasket for sealing the door. It took only a few food spills to gunk up that gasket whereupon the shielding deteriorated and significant microwave power escaped. That resulted in quite a recall and hastened the demise of that product. Percy Spenser is regarded as the person who came up with the oven concept but since Raytheon had no experience in appliances, they negotiated a deal with Amana and the Radar Range ensued. As the Japanese developed countertop units, the concept of a combined thermal/microwave product became history.”
-Rudy Dehn

Further Reading on Microwave History:
The Cavity Magnetron >
More on the history of “Radar and the Microwave Oven” >

Related Topics:

Vacuum Tubes

Electric Ranges and Calrods

Induction Cooktops

12 Major Forms of Electric Light

Radio

More Stuff

Article by M.Whelan, R. Dehn and W.Kornrumpf
Sources:
Interview with Rudy Dehn. Edison Tech Center. 2012
Interview with Bill Kornrumpf. Edison Tech Center. 2012
Albert Hull – Engineering Hall of Fame. Edison Tech Center. 2007

Photos:
Edison Tech Center
Whelan Communications