LECTURE-4

Microwave Oven

WHAT ARE MICROWAVES?

Microwaves are a form of electromagnetic energy,

like light waves or radio waves.

Its frequency range is from 300 MHz (0.3 GHz) to

300 GHz.

Microwaves are used extensively in

communications.

Good for transmitting information because it can

penetrate haze, light rain and snow, clouds, and

smoke.

Also used in radars and in detecting speeding cars.

Microwave has become most familiar as the energy

source for cooking food.

ELECTROMAGNETIC SPECTRUM

Electromagnetic radiation exists in a range of

frequencies called the electromagnetic spectrum.

Each frequency has a specific wavelength and as

the frequency decreases, the actual length of the

wave gets longer.

c = fl

The frequency used in microwave oven is 2.45 GHz

HISTORY OF MICROWAVE OVEN

In 1945, Percy Spencer, working for Raytheon to

develop magnetrons for active radar signals,

noticed a chocolate bar in pocket melted while

standing in front of an operating magnetron.

He then tested popcorn in front of the magnetron

and it quickly popped all over the room.

In 1947, Raytheon made the first microwave oven –

Radarange (6 ft tall, 3000 W power $3K) but did not

sell well. In 1965, a countertop version was

developed for $495.

CONTD…

HOW DOES A MICROWAVE OVEN COOK

FOOD?

Microwave ovens selectively make liquid (water)

hot. Most food, even "dry" foods, have water in

them.

Water (H2O) is a polar molecule with 2 hydrogen

atoms being more positive than the single oxygen

atom.

CONTD…

In liquid water, the molecules are in constant motion and are normally randomly oriented.

Glass, paper, ceramic, or plastic containers are used in microwave cooking because the microwaves pass through them.

Metal reflects microwaves– Unsafe to have metal pans/aluminum foil in

oven, may damage oven.

WHAT HAPPENS IF WATER MOLECULES ARE

IN AN ELECTRIC FIELD?

The molecules will experience a torque from the

electric field, and become aligned with direction of

the field.

Therefore, water molecules are oriented by the

electric field.

WHAT HAPPENS TO WATER MOLECULES IN

AN OSCILLATING ELECTRIC FIELD?

The water molecules follow the oscillation of the

electric field.

They collide more frequently with the molecules

(water and other) around them.

The molecules move faster and faster, and the

temperature increases (heating).

WHY DO THEY HAVE TURNTABLES?

Microwaves are reflected by the oven wall, forming

“standing waves” in the cooking chamber.

Standing waves

– waves that oscillate in place.

Nodes – zero amplitude (no oscillation) at all times

Anti-nodes – locations with maximum amplitude

CONTD…

In side a microwave oven, the electromagnetic

waves also form standing waves from reflections at

the walls.

Without a turntable, the food will not be cooked

uniformly.

INTERACTION OF MICROWAVES WITH METALS

Microwaves, incident on the metal walls of the oven,

behave similarly to visible light hitting a mirror.

The microwaves are absorbed very effectively, since

the electric fields of the waves interact very strongly

with the nearly free electrons of the metal. These

accelerated electrons re-radiate electromagnetic

waves at the same frequency and in phase, hence

the microwaves are perfectly reflected.

MAGNETRON

Magnetron is an electronic device which generates

high energy microwaves. It is the core element in a

microwave oven.

Working principle:

When a charge/charge particle accelerates in

space, it generates electromagnetic waves.

This statement is the derivation of Maxwell’s law

which says that a classical electromagnetic

radiation is ultimately generated when a charged

particle is accelerated through space.

PARTS OF MAGNETRON

Cathode

Anode

Permanent magnet

CATHODE

Cathode in magnetron is a tungsten filament which

emits electrons on the principle of thermionic

emission, when an optimum voltage is applied.

ANODE

Anode in magnetron is a copper cup like structure

which is a little bit modified to perform more

functions than a simple anode does.

PERMANENT MAGNET

The purpose of the permanent magnet is to apply a

continuous magnetic field across the cathode area.

WORKING OF MAGNETRON

RESONATOR

THEORY

In a magnetron, the source of electrons is a heated

cathode located on the axis of an anode structure

containing a number of microwave resonators.

Electrons leave the cathode and are accelerated

toward the anode, due to the dc field established by

the voltage source E. The presence of a strong

magnetic field B in the region between cathode and

anode produces a force on each electron which is

mutually perpendicular to the dc field and the electron

velocity vectors, thereby causing the electrons to spiral

away from the cathode in paths of varying curvature,

depending upon the initial electron velocity a the time

it leaves the cathode.

CONTD…

Inside the anode there are a number of cavities

designed to resonate at 2.45 GHz. A voltage of

several kV is applied between the electrodes and a

magnetic field is applied parallel to the axis such

that electric and magnetic fields are perpendicular

to each other.

THEORY OF OPERATION OF MICROWAVE OVEN

In the high-voltage section of a microwave oven,

the diode and the capacitor function together to

effectively double the already-high voltage. This is

called a voltage-doubler circuit.

Voltage-doubler circuits are fed with the stepped-up

AC voltage from the high-voltage transformer's

secondary (or output) winding. Typically, a

transformer would step up 220 volts to about 2000

volts, which would have an approximate peak value

of 2800 volts.

THE HALF-WAVE VOLTAGE DOUBLER

During the first positive half-cycle, which is

designated on the sine wave graph as T1 , the

voltage from the transformer increases accordingly

with the polarity shown.

CONTD…

During the capacitor charging time there is no voltage to the magnetron because the current takes the course of least resistance.

The voltage across the capacitor will rise to the transformer secondary voltage to the maximum 2800 volts.

As the transformer secondary voltage begins to decrease from its maximum positive value (at time increment T2 on the sine wave graph), the capacitor will attempt to discharge back through the diode. The diode is like a one-way street in that it will not conduct in this direction. Thus, the discharge path is blocked, and the capacitor remains charged to the 2800 volts.

CONTD…

Refer to Figure 2. At time T3, the transformer

secondary (output) voltage swings into the negative

half-cycle and increases in a negative direction to a

negative 2800 volts, with polarities as shown.

CONTD…

The transformer secondary and the charged capacitor are now essentially two energy sources in series. The 2800 volts across the transformer winding adds to the 2800 volts stored in the capacitor and the sum voltage of 5600 volts is applied to the magnetron cathode.

There are two fundamental characteristics of this 5600-volt output:

First, because a voltage doubler is also a rectifier, the output is a DC voltage.

Second, the resulting output voltage that is applied to the magnetron tube is actually a pulsating DC voltage. This is because the doubler generates an output only during the negative half-cycle of the transformer's output (secondary) voltage.

CONTD…

So, the magnetron tube is, in fact, pulsed on and off

at a rate of 50 or 60 times per second, depending

on the frequency of the line voltage.

RADIATION AND ITS TYPES

Radiation is the release of energy from any source and travel

through space in the form of EM waves. There are many

different sources of energy around us. For example, our

bodies give off heat, which is a form of energy. Energy is also

released from everyday things such as:

Household electrical appliances

Heaters

The sun

X-ray machines

Not all radiations are harmful. It depends on the type of

radiation and how much exposure to it you have. There are

several types of radiation. All of which can be grouped under

either:

Ionizing radiation

Non ionizing radiation

IONIZING RADIATION

Radiation that has enough energy to remove tightly

bound electrons from atoms, thus creating ions.

This is the type of radiation that people usually think

of as 'radiation’. For example:

Alpha particles

Beta particles

Gamma rays

X-rays

NON IONIZING RADIATION

Radiation that has enough energy to move atoms in a

molecule around or cause them to vibrate, but not enough

to remove electrons, is referred to as "non-ionizing

radiation”.

Non ionizing radiation has enough energy to move things

around inside a cell but not enough to change cells

chemically. The radiation from a microwave oven is non

ionizing. Other examples include:

Ultraviolet rays from the sun.

Sound waves.

Radio waves.

Radiation waves given off from household electrical

appliances, heaters, mobile phones with or without

headsets, and computers and their screens.

SPECTRUM OF IONIZING AND NON-IONIZING

RADIATIONS

BIOLOGICAL EFFECTS OF MICROWAVES

Non-ionizing radiation can produce non-mutagenic effects such as inciting thermal energy in biological tissue that can lead to burns. Recently, the International Agency for Research on Cancer (IARC) from the WHO (World Health Organization) released a statement indicating that radiofrequency electromagnetic fields (including microwave) are possibly carcinogenic to humans.

Certain body organs are particularly sensitive to the thermal effect of microwaves. For example, if the lens of the eye were exposed to excessive heat from microwaves, its circulatory system would be unable to provide sufficient cooling, and it would cook like the white of an egg. Exposure to high levels of microwaves can cause cataracts. Also, the stomach, intestines and bladder are especially sensitive to thermal damage from high levels of microwaves.

SAFETY STANDARDS

Safety Code 6 - Part III (Microwave Ovens) of the

Radiation Emitting Devices Regulations (C.R.C., C.

1370) specifies the limit for the leakage radiation at 5 cm

from the surface of the microwave oven to be 5.0

mW/cm2

ANSI/IEEE-C95.1-1991 - the power density should not

exceed 1.6 mW/cm2 at 2450 MHz (microwave oven

frequency) for human exposure in uncontrolled

environments.

IRPA (International Radiation Protection Association)

recommends exposure limit of 5 mW/cm2 for RF workers

and 1 mW/cm2 for the general public. These exposure

limits are averaged over 6 minutes.

HOW FAR AWAY IS SAFE?

One characteristic of microwaves is that they

disperse and dissipate very quickly in the

atmosphere.

For example, If you are standing 2 inches away from

a microwave oven, and are being zapped by

5mw/cm2 of microwave energy, then you wisely step

back to a distance of 20 inches or roughly an arm’s

length. Your level of exposure would drop by a factor

of 100, (the square of the distance) to 0.05mw/cm2.

CONTD…

Stay at least an arm’s length away from the

front of an operating oven.

SAFETY TIPS FOR OPERATION OF

MICROWAVE OVENS

Do not operate oven when empty.

Exercise extreme caution if you have a pacemaker implant.

Microwave radiation may cause pacemaker interference.

Persons with pacemaker implants should not be near a

microwave oven unless they are sure that it is in good

operating condition and there is no leakage of microwave

radiation.

Check to see that door seal and inside surfaces of door and

oven cavity are clean after each use.

Keep out of the reach of children. Do not permit young

children to operate the oven.

Do not put face close to door window when oven is

operating.

TROUBLESHOOTING AND MAINTENANCE

SAFETY

Will be discussed during practical.