# Unit4 Application Notes EM

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Additional Notes (Electric and Magnetic Fields)

Electric motorsThe common electromagnetic machines are motors, generators and transformers. All three are describedin this episode but you will need to check your specification to find out which you need to cover and inwhat detail.

Motor torque

From this practical work (and previous knowledge), it should be clear to your students that a simple motoris a (rectangular) coil of wire that rotates in a magnetic field when a current is passed through the coil.

The diagram shows a section through a coil that is pivoted at so that it can turn about a horizontal axis.The coil has sides of length L and a width w, so that its area is A. There are N turns of wire in the coilcarrying a current I. B is the flux density between the magnetic poles.

The force F on each side of the coil is F = NBIL

(resourcefulphysics.org)

rubber bands

brushes

insulating tape

SN

1

2

SN

1

2

F

F

W/2

x

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The direction of the forces is found by Fleming's left hand rule and the two forces together produce acouple.

The torque produced = 2 (F w/2) = NBILw = NBIA

This picture is only valid if B is uniform and B and I are perpendicular. The design of commercial motorstries to make this true for a significant part of the rotation by including a lot of shaped soft iron, both in thearmature and in the pole pieces. At the same time this increases the value of B.

The current has to be reversed each time the coil is perpendicular to the field so that the forces reverseand the circular motion is maintained. A commutator and brushes are used for this.

Generators and transformers

In a generator, motion of a conductor in a magnetic field induces an emf. In a transformer, it is thechanging field that induces an emf in a fixed conductor.

Generators

The structure of a simple generator is essentially the same as a motor. The difference is that nowmechanical energy is converted into electrical energy. The electrical current to a load is via a commutatorfor an ac generator or slip rings if ac is required.

Basic ideas can be understood by thinking about a coil rotating in a uniform magnetic field.

Consider a coil of area A with N turns of wire rotating at a constant angular velocity in a uniformmagnetic flux density B. As the coil rotates, it cuts through the lines of flux. Another way to express this isto say that the flux linking the coil is changing.

At what point is the rate of flux-cutting greatest? (When it is horizontal in the diagram above; when it is

vertical, the rate of flux cutting is instantaneously zero.)

Rate of flux cutting = induced emf = BANcos t

with a maximum value, Eo = BANwhen the coil is parallel to the field.

coil

Eo

B

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Transformers

Experiments with transformers can be used as a way of investigating and confirming the laws ofelectromagnetic induction and could be done earlier. This work can also be a means of rounding off thewhole of this section of post-16 work.

The aim is to show that a transformer is an electrical machine that converts one ac voltage into anotherac voltage. Working through parts or all of the following presentation will illustrate both the structure and

the operation of a transformer.

Working PrincipleWhen an electric current passes through a long, hollow coil of wire there will be a strong magneticfield inside the coil and a weaker field outside it. The lines of the magnetic field pattern run throughthe coil, spread out from the end, and go round the outside and in at the other end.

Primary coilNP turns

Current, IP

ac Input, VP ac Output, VS

Secondary coilNS turns

Current, IS

soft iron core

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These are not real lines like the ones you draw with a pencil. They are lines that we imagine, as inthe sketch, to show the pattern of the magnetic field: the direction in which a sample of iron would bemagnetised by the field. Where the field is strongest, the lines are most closely crowded.With a hollow coil the lines form complete rings. If there is an iron core in the coil it becomesmagnetised, and seems to make the field become much stronger while the current is on.

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The iron core of a transformer is normally a complete ring with two coils wound on it. One isconnected to a source of electrical power and is called the 'primary coil'; the other supplies thepower to a load and is called the 'secondary coil'. The magnetisation due to the current in the

primary coil runs all the way round the ring. The primary and secondary coils can be woundanywhere on the ring, because the iron carries the changes in magnetisation from one coil to theother. There is no electrical connection between the two coils. However they are connected by themagnetic field in the iron core.

When there is a steady current in the primary there is no effect in the secondary, but there is aneffect in the secondary if the current in the primary is changing. A changing current in the primaryinduces an e.m.f. in the secondary. If the secondary is connected to a circuit then there is a currentflow.

A step-down transformer of 1,200 turns on the primary coil connected to 240 V a.c. will produce 2 Va.c. across a 10-turn secondary (provided the energy losses are minimal) and so light a 2 V lamp.

A step-up transformer with 1,000 turns on the primary fed by 200 V a.c. and a 10,000-turn secondarywill give a voltage of 2,000 V a.c.

The iron core is itself a crude secondary (like a coil of one turn) and changes of primary currentinduce little circular voltages in the core. Iron is a conductor and if the iron core were solid, theinduced voltages would drive wasteful secondary currents in it (called 'eddy currents'). So the core ismade of very thin sheets clamped together, with the face of each sheet coated to make it a poorconductor. The edges of the sheets can be seen by looking at the edges of a transformer core.

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