How does an Induction Motor work ?

We all know that World War II was one of the most traumatic events in human history, but on the other hand it also resulted in several inventions that have completely changed the world. One of the key inventions of this era was the cavity magnetron, a device which made radars super efficient. Cavity magnetrons are also used in microwave ovens, where they are responsible for producing high powered microwaves.

Working of magnetrons

Cavity magnetrons work on the principle of LC oscillation. In this article I am going to explain in precise details about magnetrons.

1. When cathode and filament are present

Let's consider a cathode and a filament. When I give the power to the filament, the filament heats up the cathode, and due to this, electrons are emitted from it. This phenomenon is known as thermionic emission (refer fig 1). In this case, however, the electrons will return to the cathode.

Fig 1 : Thermionic emission

2. When anode is present

Now I am going to place an anode with positive potential (refer fig 2). The emitted electrons accelerate and move towards the anode. The theory of radiation states that the charges produce radiation when they accelerate. However, in this arrangement, the electrons radiate inefficiently as they spend very little time in the interaction space.

Fig 2 : Movement of electron when anode is present

3. When magnetic field is present

In order to increase the time spent by the electrons in this space, I will introduce permanent magnets into the structure. The magnetic field forces the electrons to take a curved path (refer fig 3). Since the path of the electrons is now curved, the time that the electrons spend in the interaction space is increased. The final structure thus formed is known as a hull magnetron.

Fig 3 : Movement of electrons during magnetic field

Hull magnetrons can be further improved with help of the LC oscillations. Now in the section below, I will explain to you how to achieve oscillation in a magnetron.

4. When cavities are introduced : Cavity magnetron

To achieve oscillation, design the anode with cavities. These cavities cause huge differences in the physics of magnetrons. To understand the effect of cavities, I will use the following example. Let's take a metal bar with a cavity and pass a negative charge near to it. The negative charge will obviously repel the free electrons of metal. Similarly, when the negative charge passes near to the cavity, the electrons around the cavity’s surface are disturbed (refer fig 4a).

Fig 4a : Positive and negative charges occur across the cavity surfaces

An accumulation of positive and negative charges occurs across the cavity surfaces due to this disturbance. In short the cavity surfaces act like capacitor plates. If we connect an inductor across the cavity surface, the charges will start oscillating(refer fig 4b).

Fig 4b : An inductor connecting across the cavity surface for oscillating

This simple physics is the basis of the cavity magnetron. A magnetron has many such cavities. Many electrons are ejected from the cathode by thermionic emission (refer fig 5). Now I will track the effect of the very first electron ejected onto these cavities.

Fig 5 : Movement of electrons when cavities are present

As I have explained above, this electron will induce positive and negative charges on the cavity surfaces. Here the cavities are arranged in a circular manner. This means the charged cavity surface pair cannot stay in isolation. To keep the electric field zero in the metal, all the cavity pairs have to be charged with the opposite polarity(refer fig 6).

Fig 6 : Electron will induce positive and negative charges on the cavity surfaces

The charges placed on the opposite ends separated by a gap between them act as a capacitor and the curved shape of the cavity acts as an inductor. This means that the charges accumulated will go for a simultaneous LC oscillation.

Connect antenna to the cavity

Now I will connect an antenna to one of the cavities (refer fig 7). With the help of this antenna and a metal loop, this oscillation is extracted and converted into EM waves. These oscillations will be sustained in the magnetron since the electrons continually flow from cathode to anode and transfer their energy.

Fig 7 : Oscillation is extracted and converted into EM waves

Spoke wheel

Now let's see what happens to the remaining electrons in the interaction space. Here the very first electron that reached the cavity surface has already created a charge pattern on the cavities. This means the remaining electrons have attracted to the positive charge regions and they have formed an interesting spoke wheel pattern (refer fig 8). Since the charges on the cavities are oscillating, the spoke wheel has to spin.

Fig 8 : Electrons form a spoke wheel pattern

What is mutual coupling?

As you must have noticed that the antenna connected only to a single cavity, but a curious question here is why it is connected only to a single cavity, not all the cavities? Well this is the magic of mutual coupling. As we know that due to the phenomenon of mutual coupling, if two coils are placed near each other and a current is passed through one of the coils will result in varying magnetic flux. These magnetic flux generated by the first coil will interact with the second coil, inducing relatively EMF across it. This phenomenon would also function when several coils are placed close to each other. Hence in the magnetron, the extraction of oscillating energy from one cavity would be the same as the extraction of all of the cavities combined.

Applications of magnetron

1. Microwave oven

2. Radar

3. Sulphur lamp

4. Microwave generator

Advantages of cavity magnetron

1. It has a compact size which makes the radar size smaller.

2. Cavity magnetrons are able to produce high powered pulses at a shorter wavelength, and this led to the detection of smaller objects being possible.

That’s all about magnetrons. I hope you understood and enjoyed this explanation of one of the most complicated engineering technologies : cavity magnetron.

Thanks for reading!


Prerna Gupta

Prerna Gupta, a postgraduate in control and instrumentation. Currently, she is a product manager at Lesics Engineers Pvt. Ltd. Her areas of interest are telecommunication, semiconductor materials and devices, embedded systems, and design. Prerna has done projects such as MOSFET, optical fibre cable, routing, GPS systems, satellites, etc. Check out this link for more information about the author.