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Efficiency of a Motor

Extracts from this document...

Introduction

Jessica Roper 11H

Efficiency of a Motor

Aim: To investigate the relationship between the efficiency of a motor and the load it is lifting.

Apparatus:

  • Motor
  • 50cm length of cotton
  • Blue tack
  • 0.01Kg plasticine
  • Clamp
  • Boss
  • Ammeter
  • Voltmeter
  • Low Voltage Unit
  • 0.01Kg masses
  • Sellotape
  • Wires
  • Mass hook
  • Stopwatch

Safety

You should not use an electrical source with wet hands. Sockets should have nothing inserted into them other than plugs. This is because the voltage of a mains supply is 240V and this is enough to kill a human being. Water should be kept away from sockets, as water is a good conductor of electricity and therefore could potentially be lethal. For the same reason all wires should be insulated. As a general safety rule there should be no running in the laboratory and all bags should be placed under desks to avoid tripping.

Diagrams image00.png

image01.png

Preliminary Results

From my preliminary experiments I discovered that the cotton would not fix itself to the spindle of the motor in such a way that it would wind up without the addition of blue tack to secure it. It is vital therefore that I use blue tack in this experiment or the efficiency of the motor cannot be calculated because the load would not move any distance whatsoever. I attempted to gain results for 0.

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Middle

The LVU should be switched on and at the same time the stop clock should be started.As the load is being lifted the amps recorded on the ammeter should be recorded.As soon as the mass has reached the motor the stopwatch should be stopped and the time recorded.Steps 4-7 should be repeated a further 2 times so we are sure that the results we have gained are accurate.Steps 4-8 should then be repeated again for the different masses.

Hypothesis

All energy must be transferred. Energy cannot be created or destroyed. Energy efficiency is how much of the energy that has been input is transferred into a useful energy output. In this case the motor is given electrical energy this is then transferred to rotational kinetic energy this rotational kinetic energy is then transferred into gravitational potential energy when the load is lifted.

To calculate energy efficiency we use the following formula:image04.png

The useful energy output is gravitational potential energy. The formula for this is:

Mass × g × change in height

G is gravity. We measure this as 10N. The total energy input is the power put into the motor. We calculate this using the formula:

Voltage × current × time

This means that to find the energy efficiency the formula I will need to use overall is:image02.png

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Conclusion

Also the cotton may have stretched as more mass was attached to it, this would have increased the time taken for the motor to lift the load as there was a further distance to lift and therefore decreased the efficiency. Nevertheless, I believe that if the cotton did stretch the difference would have been so little that it would have very little bearing on the results. To be convinced of this, the cotton should have been measured regularly to check this was not the case and if it was this should have been taken into account in the equation.

Another problem was that we could not control the ‘stickiness’ of the blue tack and this ‘stickiness’ decreased the more the experiment was repeated. Replacing the blue tack, however, would have been difficult and altered the set-up of the cotton the spindle which may have influenced the results more than the state of the blue tack would have done.

I feel that the results we used were well space, however, I think that it may have been a good idea to continue adding masses until the motor could no longer lift them. There would have been more to conclude from the graph, for example, to see if the efficiency continued to decrease at the same rate.

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