## Friday, January 7, 2011

### Electromagnetic Induction

Electromagnetic Induction

Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. It underlies the operation of generators, all electric motors, transformers, induction motors, synchronous motors, solenoids, and most other electrical machines.

Michael Faraday is generally credited with the discovery of the induction phenomenon in 1831 though it may have been anticipated by the work of Francesco Zantedeschi in 1829.[citation needed] Around 1830[1] to 1832[2] Joseph Henry made a similar discovery, but did not publish his findings until later.

Laws of electromagnetism

English scientist Michael Faraday proposed three laws of electromagnetic induction:

(1) a changing magnetic field induces an electromagnetic force in a conductor;

(2) the electromagnetic force is proportional to the rate of change of the field;

(3) the direction of the induced electromagnetic force depends on the orientation of the field.

Lenz's law

The direction of an electromagnetically-induced current (generated by moving a magnet near a wire or by moving a wire in a magnetic field) will be such as to oppose the motion producing it.

This law is named after the German physicist Heinrich Friedrich Lenz (1804–1865), who announced it in 1833.

Alternate Current

Alternating Current is the movement of electrons in a wire backwards then forwards repeatedly and thus whose current constantly changes direction.In Europe this change repeats 50 times per second (or 50 Hz). In the USA, the frequency is 60 Hz.

AC is remarkably useful because it allows us to change electricity very easily using transformers which cannot work with DC.

The UK mains supply is about 230V. It has a frequency of 50Hz (50 hertz), which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an AC supply.

Alternate Current Generators

Regardless of size, all electrical generators, whether dc or ac, depend upon the principle of magnetic induction. An emf is induced in a coil as a result of

(1) a coil cutting through a magnetic field, or
(2) a magnetic field cutting through a coil.

As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor.

That part of a generator that produces the magnetic field is called the field.
That part in which the voltage is induced is called the armature.

For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts - a rotor and a stator. The ROTor is the part that ROTates; the STATor is the part that remains STATionary. In a dc generator, the armature is always the rotor. In alternators, the armature may be either the rotor or stator.

Making AC electricity

When a wire is moved in the magnetic field of a generator, the movement, magnetic field and current are all at right angles to each other. If the wire is moved in the opposite direction, the induced current also moves in the opposite direction.

Remember that one side of a coil in a generator moves up during one half turn, and then down during the next half turn.

This means that as a coil is rotated in a magnetic field, the induced current reverses direction every half turn. This is called alternating current (AC).

It is different from the direct current (DC) produced by a battery, which is always in the same direction.

In practical generators, the coil is fixed, and mounted outside the magnet, and it is the magnet which moves.

The size of the induced voltage can be increased by:

* rotating the coil or magnet faster
* using a magnet with a stronger magnetic field
* having more turns of wire in the coil
* having an iron core inside the coil

The mains electricity is an AC supply. The voltage it supplies to our homes is 230V.

Transformers

A transformer is an electrical device that changes the voltage of an ac supply. A transformer changes a high-voltage supply into a low-voltage one, or vice versa.

* A transformer that increases the voltage is called a step-up transformer.
* A transformer that decreases the voltage is called a step-down transformer.
* Step-down transformers are used in mains adapters and rechargers for mobile phones and CD players.
* Transformers do not work with dc supplies.

A transformer consists of a pair of coils wound on an iron core. The AC in one coil produces a changing magnetic field. This changing magnetic field induces a voltage in the other coil of the transformer.

## How transformers work

A transformer needs an alternating current that will create a changing magnetic field. A changing magnetic field also induces a changing voltage in a coil. This is the basis of how a transformer works:

• The primary coil is connected to an AC supply.
• An alternating current passes through a primary coil wrapped around a soft iron core.
• The changing current produces a changing magnetic field.
• This induces an alternating voltage in the secondary coil.
• This induces an alternating current (AC) in the circuit connected to the secondary coil.

It's important to know that:

• There is no electrical connection between the primary and the secondary coils.
• Transformers only work if AC is supplied to the primary coil. If DC was supplied, there would be no current in the secondary coil.
• As the current in the primary coil increases steadily or decreases steadily, there is a constant voltage induced in the secondary coil.
• As the voltage in the primary coil reaches maximum strength the voltage induced in the secondary coil is at its weakest (zero volts).

## Calculating voltages

The ratio between the voltages in the coils is the same as the ratio of the number of turns in the coils.

primary voltage / secondary voltage = turns on primary / turns on secondary

This can also be written as:

Vp/Vs = Np/Ns

Step-up transformers have more turns on the secondary coil than they do on the primary coil.

Step-down transformers have fewer turns on the secondary coil than they do on the primary coil.

Question

A transformer has 20 turns on the primary and 400 on the secondary. What is the output voltage if the input voltage is 500V?

Vp/Vs = Np/Ns Therefore Vs/Vp = Ns/Np

Vs/500 = 400/20

Vs = 500 x (400/20)

Vs= 10,000 Volts

Ideal power equation

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit.

If this condition is met, the incoming electric power must equal the outgoing power:

giving the ideal transformer equation

Transformers normally have high efficiency, so this formula is a reasonable approximation.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio.[29]

For example, if an impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.

Transmission

Electricity is generated on a large scale at power stations and then transmitted through cables (called the National Grid) to factories and homes.

Copper cables carrying the electricity are buried in the ground or aluminium cables are suspended from pylons. Aluminium is used because it has a low density
and can safely be suspended from inexpensive thin pylons. Pylons have the disadvantage that they look ugly on the landscape but have the advantage of easy access to the cables for maintenance and repair. Transmission using pylons is cheaper than burying cables underground.

Transformers are used to produce a very high voltage for the transmission of electricity, to minimize energy loss.

A generator at a power station might produce electricity with a voltage of 25,000V and a current of 8,000A.

Such a large current would cause the cables of the National Grid to get hot because of the heating effect of current.

Energy could be lost due to :

1. heat loss due to resistance in coils
2. leakage of magnetic field lines between primary and secondary coils
3. heat loss due to eddy currents induced in iron core
4. hysteresis loss caused by the flipping of magnetic dipoles in the iron core due to the a.c.

To reduce the energy loss, a step up transformer at the power station is used to raise the voltage to 400,000V.This is 16 times the input voltage of 25,000V.