Carbon a Nonmetal ! But why it is used in Dc motor brushes ?

           One reason is the high melting temperature of carbon. 

The inductances of the energized windings, and the mechanics of a brush type motor or generator make arcing and sparking an unavoidable characteristic  of the machine at the brush interface. Electrical arcs produce very high temperatures. High enough to melt metals. 

Carbon has a very high melting point (~3500degC) compared with other conductors. Brushes made with copper or steel would wear out faster because of factors including:
1) The melted metal will break away from, or even be vaporized, leaving the brush with less material to do its job.
2) If the melted material doesn't vaporize or otherwise move away from the interface, it creates a mess for the brush. It can weld together bristles of the brush. It can build up material that interferes with the motion of the rotor.
3) As the metal melts in the presence of an arc, conductive material can be liberated, therby prolonging the arc, exasperating the wearing action.

Carbon is less prone to those high temperature effects and associated impacts to the generator operation.

Source : Dan Church

Why 50 hertz frequency in India ?

IT STARTS FROM THE BEGINNING OF ELECTRICITY-

Early in the history or electricity, Thomas Edison's General Electric 
company was distributing DC electricity at 110 volts in the United States. 
Then Nikola Tesla devised a system of three-phase AC electricity at  240 volts. Three-phase meant that three alternating currents slightly out of phase were combined in order to even out the great variations in voltage occurring in AC electricity. He had calculated that 60 cycles per second or 60Hz was the most effective frequency. 
Tesla later compromised to reduce the voltage to 120 volts for safety reasons.
With the backing of the Westinghouse Company, Tesla's AC system 
became the standard in the United States. 
Westinghouse chose 60 Hz because the arc light carbons(arc lamp) that were popular at that time worked better at 60 Hz than at 50 Hz.

Europe goes to 50Hz and 230V

Meanwhile, the German company AEG started generating electricity and became a virtual monopoly in Europe. 
They decided to use 50Hz instead of 60Hz to better fit their metric standards, but they stayed with 120V.

Europe stayed at 120V AC until the 1950s, just after World War II. 
They then switched over to 220V for better efficiency in 
electrical transmission. Great Britain not only switched to 220V, but 
they also changed from 60Hz to 50Hz to follow the European lead. 
Since many people did not yet have electrical appliances in Europe after the 
war, the change-over was not that expensive for them. 

U.S. stays at 120V, 60Hz

The United States also considered converting to 220V for home use but felt it would be too costly, due to all the 120V electrical appliances people had. 
A compromise was made in the U.S. in that 240V would come into the house where it would be split to 120V to power most appliances. 
Certain household appliances such as the electric stove and electric clothes dryer would be powered at 240V.

India got 50Hz, because it was colonized by England, which when they developed their electrical systems, choose 50 Hz.
From technical point of view operating 50 Hz versus 60 Hz would not make much difference but, to achieve it, either the prime movers - for example steam turbines, gas turbines and diesel engines  would need to be able to tolerate a 20% increase in speed or the alternators they drive - which produce the electricity  would need to be completely rebuilt with extra poles and windings so that they could continue to run at the same rotational speed. 
The costs of doing such re-engineering would be enormous and could not be justified as "economically worthwhile" from the point of view of actual necessity.

We  could have chosen any other frequency other than 50/60 hz but
50/60 HZ is an optimum frequency which keeps the transmission losses to tolerable limits. 
The higher will be the frequency, the more will be the losses.
and lower frequencies would causes the size, weight & hence the cost to increase. Also, more flickers are noticed in lesser frequencies than higher frequencies.

The voltage and frequency of AC electricity varies from country to country throughout the world.
 Most use 220V and 50Hz. About 20% of the countries use 110V and/or 60Hz to power their homes.220V and 60Hz are the most efficient values, but only a few countries use that combination. 

Effects of electric shock on human body...

Two main types of effects of Electric shock :

Stimulation of muscles and nerves

• We've all felt that buzzing or tingling sensation without experiencing injury. A current as low as 0.25 milliamperes (mA) can cause this feeling.
• Starting at 10 mA, most people cannot let go of the shock source because their muscles contract.
• Above 50 mA, an electric current can trigger cardiac arrest if it passes through the heart.

Electric burns to tissue and organs

• Above 100 mA, electrical marks appear on the body at the points of contact.
• Above 10,000 mA (10 amperes), burns are severe and amputation is required.

Specific effects :

On the heart :

With every contraction, the heart pumps blood carrying oxygen throughout the body. The rhythm of the heartbeat is controlled by electrical impulses, which can be seen on an electrocardiogram. Current passing through the heart can cause an irregular heartbeat called arrhythmia, or even total disorganization of the rhythm, called ventricular fibrillation.

When ventricular fibrillation occurs, the heart stops pumping. The victim rapidly loses consciousness and dies if a healthy heartbeat is not restored by applying a second electric shock with a device called a defibrillator.

Heart rhythm disturbances can occur at the time of the shock or in the 24 hours following the accident.

On the muscles :

Muscles are stimulated by electricity. The effect of an electric shock depends on which muscles the current goes through. A current of more than 10 mA causes sustained contraction (tetanus) of the flexors, that is, the muscles that close the fingers and draw the limbs towards the body. The victim thus cannot let go of the source of current.

If the extensors (the muscles that open the figures and extend the limbs away from the body) are tetanized, the victim is propelled away from the current source, sometimes as much as ten metres!

Muscles, ligaments and tendons may tear as a result of the sudden contraction caused by an electric shock. Tissue can also be burned if the shock is lasting and the current is high.

On the nervous system : 

Nerves are the tissue that offers the least resistance to the passage of an electric current. Some nerve damage caused by shock clears up with time, but some is permanent. The victim may feel pain, tingling, numbness, weakness or difficulty moving a limb.

When a shock occurs, the victim may be simply dazed or may experience amnesia, seizure or respiratory arrest.

Ultimate damage to the nerves and the brain will depend on the extent of the injuries caused by the heat along the path of the electric current and may develop up to three years after the shock. Nerve damage can also cause psychiatric disorders.

Electrical burns :

Electrical burns are not like burns caused by fire or by touching something hot. Electrical burns result from the heat generated by an electric current passing through the body, which literally cooks the tissue from within. Outward signs of electrical burns may be microscopic or nonexistent, and internal damage may be much more serious than the external injuries suggest. That's the iceberg effect.

Electrical marks appear at the body's point of contact with the current. They are typically tiny charred or hard craters that do not hurt because the nerves have been destroyed.

If a lot of tissue is destroyed, the waste generated can cause serious kidney or blood circulation disorders.

Electrical burns often have serious consequences: scarring, amputation, loss of function, loss of sensation and even death.

Elsewhere in the body :

Electric shock can also affect the eyes, causing cataracts to develop over time. Other disorders can appear in the weeks or months following the accident, depending on which organ the current passed through the  body.

Source : Hydroquebec

What is the diffrence between an insulator and dielectric ???

What is the difference between an insulator and a dielectric?

This is a question that can be confusing to many engineers. This is because the terms "insulators" and "di-electrics" are often used interchangeably. Insulators are substances which permit very less current flow through them. Substance such as porcelain, wood are examples.

Dielectrics are also insulators. But, more specifically, they are materials which can be polarized. In dielectric materials, the electrons are bound to the nucleus and have limited movement. When an external voltage is applied to the dielectric, the nucleus of the atoms is attracted to the negative side and the electros are attracted to the positive side. Hence, the material gets polarized. This is a key feature of a dielectric.

Thus a dielectric can be defined as an insulator that can be polarized.  Thus all dielectrics are insulators, but all insulators are not dielectrics. A dielectric can thus store charge.  This characteristic makes it very useful in the form of capacitors.

Dielectric substances conduct very little electricity but are good supporters of electric fields. They also dissipate very less energy, i.e. have low dielectric loss.

Source : electrotechnik