Motors and Generators A topic (in two parts) about the interaction between magnetic fields.

Motors and Generators

A topic (in two parts) about the interaction between magnetic

fields

Assumed Knowledge from the Preliminary

• Demonstration 1: Electric currents produce magnetic fields

• The recognition that electric currents produce magnetic fields is essential.

• Use of hand rules should be well practised

Images from Andriessen et. al. Physics 2 Jacaranda p109, 110

The Motor Effect• A current carrying conductor in an external magnetic field

experiences a force.

• F=BIlsin calculates the magnitude of the force on a current carrying conductor in a magnetic field, whereF = the force on the conductor (N – newtons)B = the magnetic flux density of the external field (T – tesla)I = current in the conductor (A – amperes) l = the length of conductor in the field (m – metres) = angle between the conductor and the field (o)

• The right hand palm rule gives the direction of the force.

Image from Andriessen et. al. Physics 2 Jacaranda p113

Demonstration 2Modelling the Motor Effect

Focus area 1-Column 3 dot point 2-Column 2 dot point 1

Features of the DC electric motor

• Demonstration 3Aspects of the DC Motor

• Knowing the right equation


Electromagnetic induction

• Electromagnetic induction is producing an emf between the ends of a conductor when there is a magnetic flux change around it.[emf is electromotive force – a measure of the energy supplied to each coulomb of charge by the source (battery, generator etc)]

tn

Where = induced emf

n = no. of turns in the coil = change in magnetic flux

t = change in time (or time taken for the change in flux to occur)

Focus area 2- Column 3 dot points 1 and 2

Demonstration 4Modelling generation of electric current

Demonstration 5Lenz’s Law

Lenz’s Law

• Lenz’s Law: The direction of an induced emf is such that it opposes the change that causes it.


Demonstration 6Demonstrating production of AC current

Focus area 3

Column 3 dot point 1

Transformers• The purpose of a transformer is to increase or decrease AC

voltages.

• Transformers consist of two coils of insulated wire wound onto a soft iron core.


Demonstration 7Modelling structure of a transformer

Focus area 4

Column 3 dot point 1

AC Induction Motors

• An AC motor has - a stator, consisting of field coils (electromagnets) which provide the rotating external magnetic field- a rotor, consisting of conductors on the motor’s shaft.- a cooling fan

Demonstration 8 – Demonstrating AC induction motor principle

A bench pulley is placed between opposite poles of two strong magnets.

When a rotating magnetic field spins around a stationary disc,a. eddy currents are induced in the disc in the region of the fieldb. the induced magnetic field will start the disc moving.c. the change causing the induced current was the movement of the field relative to the disc, so the opposition to the change will be the moving of the disc to reduce the relative motion between the field and disc.

Disc on

shaftmagnets

Motors and Generators A topic (in two parts) about the interaction between magnetic fields.

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Transcript of Motors and Generators A topic (in two parts) about the interaction between magnetic fields.