SONNY P. DE LEONMT 1

Transformer

Electromagnets and the Electric Motor

Electromagnets are magnets that are created when electric current flows in a coil of wire.

A simple electromagnet is a coil of wire wrapped around a rod of iron or steel.

Because iron is magnetic, it concentrates and amplifies the magnetic field created by the current in the coil.

The Principle of the Electric Motor

An electric motor uses electromagnets to convert electrical energy into mechanical energy.

Electric Motors

Electric motors are very common.

All types of electric motors have three key components:

1. A rotating element (rotor) with magnets.2. A stationary magnet that surrounds the

rotor.3. A commutator that switches the

electromagnets from north to south at the right place to keep the rotor spinning.

Electric motors

The rotating part of the motor, including the electromagnets, is called the armature.

This diagram shows a small battery-powered electric motor and what it looks like inside with one end of the motor case removed.

Induction and the Electric Generator

Key Question:

How does a generator produce electricity?

Induction and the Electric Generator

If you move a magnet near a coil of wire, a current will be produced.

This process is called electromagnetic induction, because a moving magnet induces electric current to flow.

Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

Induction

Current is only produced if the magnet is moving because a changing magnetic field is what creates current.

If the magnetic field does not change, such as when the magnet is stationary, the current is zero.

Induction If the magnetic field is increasing, the

induced current is in one direction.

If the field is decreasing, the induced current is in the opposite direction.

Magnetic flux

A moving magnet induces current in a coil only if the magnetic field of the magnet passes through the coil.

Generators A generator is a device that uses induction to

convert mechanical energy into electrical energy.

Transformers

The transformer uses electromagnetic induction, similar to a generator.

Transformers

The induced emf in the primary and secondary coil is given by

EMF P = -Np Δ(BA) / Δt ( primary coil)

EMF S = -Ns Δ(BA) / Δt ( secondary coil)

Transformer efficiency is the ratio of the power output to the power input.

E = power output / power input

= Ve S Is / Ve P I p

Step-up transformer – transformer that produces a larger output voltage.

Step-down transformer - gives a lower output voltage.

Sample Problems

P1. The primary coil of a transformer has 150 turns. It is connected to a 120-V source. Calculate the number of turns on the secondary coil needed to supply these voltages; (a) 625 V, (b) 35 V (c) 6.0V

A step-up transformer has 80 turns on its primary coil. It has 1200 turns on its secondary coil. The primary circuit is supplied with an alternating current at 120 V.

a. What voltage is across the secondary circuit?

b. The current in the secondary circuit is 2.0 A. What current is in the primary circuit?

c. What are the power input and output of the transformer?

Solve:1. A friend being back from Europe a device that she

claims to be the world’s greatest coffee maker.Unfortunately, it was designed to to operate from 240 V line to obtain the 960 W of power that it needs.

a. What can she do to operate it at 120-V?

b. What current will the coffee maker draw from 120-V line?

2. An ac generator that delivers 20 A at 6000 V is connected to a step-up transformer . What is the output current at 120 000 V if the trnsformer efficiency is 100%?

3. A step-up transformer has 400 secondary turns and only 100 primary turns. A 120V alternating voltage is connected to primary coil.What is the output voltage?

Application: Trains that Float by Magnetic Levitation