Investigating a factor affecting the voltage output of a transformer.

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GCSE Physics Coursework

Plan

Introduction

I shall be investigating a factor affecting the voltage output of a transformer.

In order to do this I shall be measuring the range of voltages that are induced across the secondary coil of the transformer when one factor is varied. To do this as accurately as possible and to obtain a fair test I shall ensure that every other variable factor in the practical remains constant.

Background Theory

Transformers are used industrially to increase the low voltages produced in electricity generation (25 kV) to higher voltages to be transported in the grid electricity supply’s cables (250 kV), and then to decrease these voltages for use in domestic appliances (230 V).

A transformer is a device for changing the voltage of alternating current (a.c.) signals and power supplies. Two coils are wound around an iron core, which is preferably laminated so as to reduce energy loss via eddy currents. Iron is a magnetically soft metal, which thus allows it to easily be magnetised and demagnetised (i.e. it doesn’t retain a permanent magnetic field). Transformers utilise the effect of Electromagnetic Induction.  The alternating voltage in the primary coil creates an alternating current, leading to an alternating magnetic field in the primary coil. The magnetic field lines move back and forth and are cut by the secondary coils, inducing a voltage in them; thus a current flows in the secondary coil.

In a step-up transformer there are more turns on the secondary coil than on the primary; there is a larger voltage in the secondary coil than in the primary (i.e. larger secondary voltage).

             

In a step-down transformer the primary coil has more turns than the secondary, and so there is a smaller voltage in the secondary coil than in the primary (i.e. smaller secondary voltage).

Variables

In my experiment the secondary voltage is the dependent variable.

The independent variables are:

  • Number of turns on the primary coil

Increasing number of primary coil turns decreases secondary voltage

  • Number of turns on the secondary coil

Increasing number of secondary coil turns increases the number of times the magnetic field lines are cut, increasing the secondary voltage

  • Primary voltage

Increasing primary voltage increases the current, increasing the speed at which the magnetic field alternates, increasing the number of times the magnetic field lines are cut by the secondary coil in a given time, increasing the secondary voltage

  • Cross-sectional area of secondary coils

Increasing cross-sectional area of secondary coils increases the number of magnetic field lines cut in a given time, increasing secondary voltage

  • Distance of separation between primary and secondary coils

Increasing the distance leads to the attenuation of magnetic field lines, decreasing secondary voltage

  • Material of wire

Different metal wires have different resistances; increasing the resistance of the coil’s wire decreases the current, decreasing rate at which the magnetic field lines alternate, so less magnetic field lines are cut by secondary coil in given time, decreasing the secondary voltage

  • Softness of iron core

Core must be soft so as to be magnetised and demagnetised easily resulting in each reading of the secondary voltage being unaffected by a previous one via the prior build up of an electromagnetic field on the core (i.e. so each reading is reliable)

  • Spacing between coils

Increasing the space between coils increases the distance the magnetic field lines have to travel, decreasing their strength, decreasing the secondary voltage

The equation relating the number of coil turns and size of voltages is;

V1/V2=N1/N2

Where V=voltage, N=number of turns on coil

And 1 and 2 denote primary and secondary coils respectively.

This can be rearranged to give the secondary voltage;

V1/V2=N1/N2

  • V1=N1*V2 /N2
  • V1*N2=N1*V2
  • V1*N2 /N1=V2

V1*N2 /N1=V2

The variable I have decided to use is the primary voltage (V1). I intend to keep N1 and N2 constant so that V2 is only varied by V1. I shall also wind the same number of turns for the primary and secondary coils, with the result that whatever number of coils I decide upon for both N1 and N2 after the investigations of my preliminary experiments, N2 divided by N1 will be equal to one;

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e.g. if N1=50 and N2=50;

V1* 50/50=V2

 V1*1=V2

 V1=V2

Therefore when the primary voltage is equal to 20 V, the secondary voltage should also equal 20 V.

However, I believe that there will be some energy losses in the experiment I shall undertake. For this reason, I think that by ensuring N2/N1 is the constant ‘1’, effectively it follows that according to my transformer equation V1 should equal V2. In this way it will be easier to see how much energy is lost in the experiment by merely subtracting each secondary voltage from its relative primary ...

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