Aim: To investigate how the efficiency of an electric motor varies when it is lifting different weights. Summary: I have investigated how the efficiency of an electric motor varies when it is lifting different weights. I found that the efficiency of my motor (using an input voltage of 4v) was maximum when it was lifting 500-800g weights. I also found that the efficiency of the motor depended on its internal resistance and friction. I also used my results to estimate the internal resistance of the motor and I found it was about 0. 5? In my extension experiment I used the electric motor as a generator.
I dropped a variety of weights from my pulley system and I measured the amount of energy produced using a joule meter. I found that the efficiency of the generator increased as the mass of the weight I dropped increased. The percentage efficiency of the generator also appeared to tend towards 14%. Finally I discussed whether my experiment was time reversible. I decided that it was to an extent but not completely. I decided this since I only retrieved about 1-2% of the energy I used to lift the weight when I dropped the weight again, using the motor as a generator.
Experiment 1: See Diagram1 for Apparatus and circuit diagram Experimental Method: I will use a 12v electric motor to lift a variety of weights a fixed height and I will measure the voltage and current in the motor coils. I will also measure the time taken to lift the masses. How will I make my experiment accurate? 1. In my experiments I will keep all variables constant except the one I am investigating. I will vary the mass of the load but I will keep the input voltage to the motor and field coils constant and I will lift the load to the same height each time. 2.
I will repeat each measurement in till I am satisfied that it is accurate and then I will use an average when I am analysing my results. I will also take a wide range of results to ensure I get a complete picture. 3. I will do a trial experiment to decide on appropriate values for the voltage in the motor and field coils. I will also need to decide on appropriate range of masses to test. I want the voltage in the motor coils to be large enough so that I can measure the current/voltage in the motor coils accurately with a 1-10 V/A meter however I want the motor to be able to lift a wide range of masses at this voltage.
Acknowledged Errors: 1. Friction in the pulley will reduce the efficiency of the motor . The pulleys are needed to reduce the torque the motor requires to lift the weights. By reducing the torque the motor can lift a wider range of weights. I have tried to reduce the friction in the pulleys by oiling them. 2. The radius of the cotton reel used to wind the string around the motor shaft increases as more string is wrapped round it. As a consequence the torque on the motor is increased. This error is unavoidable and I will attempt to correct for it by using an average value for the radius in my calculations.
3. The internal resistance of the power supply and the ammeter used in my experiment will affect the voltage across the motor. For this reason I will directly measure the voltage across the motor and I will not simply use the E. m. f of the power supply in my calculations. 4. The bottom pulley contributes to the weight of the load. It weighs 104. 8 grams (error +/-0. 1g) My results: Experiment1: Field coils voltage=9. 88V, Motor Coil input voltage (as show on power pack)=4V, the height the mass was lifted=1. 025 m (% error=0. 9), g=9. 8(Estimated error=1%)
I repeated each reading in till I got reliable results the value given in the table is the average value I recorded. Mass (including pulley) in grams Average Voltage (V) Average Current (A) Average time taken to lift mass I. 025m Estimated Error=+/- 1% Estimated Error==+/- 0. 05V Estimated Error=+/-0. 05A Estimated Error==/- Experiment 2:
Although the voltage and current remained roughly constant while I was lifting the weights at a steady speed, they varied by as much as . 3V/A. This variation was due factors such as the varying friction of the pulley system and it made taking accurate results difficult. In my second experiment I am going to use a joule meter to measure the amount of energy given to the motor. The joule meter will take in account the fluctuations in voltage/current that I observed and therefore it should give more accurate results.
This is assuming that the joule meter is accurate. See Diagram 2 for circuit diagram Experimental Method: I will attach different masses to the pulley and I will measure how high they are lifted if 20J of electrical energy is given to the motor-As soon as the joule meter reads 20J I will stop the weight and measure the distance it has moved. I have chosen this method since the joule meter I am using can only measure the number of joules received to the nearest 10J. In my experiments I was only using about 30-70 Joules and therefore the error in my results could be as high 33%.
However using the joule meter I can accurately tell when exactly 20J of energy has been used, and then I can then use the ruler to measure the height of the weight to the nearest millimetre. Using this method and assuming that the joule meter is accurate, I estimate the experimental error to be about 2-3%. My results:
Motor coil voltage (as shown)=4V, Field Voltage=9. 80 Mass (g) (inc pulley) Voltage (V) Height Lifted (Cm) Height lifted Exp2 (Cm) Average Height Lifted(Cm) Average Time taken to lift weight (s) Estimated Error=+/- 1% Estimated Error==+/- 0. 1V Estimated Error =+/-0. 5cm Estimated Error =+/-0. 5cm Estimated Error =+/-0.5cm Estimated Error=Testing the accuracy of the joule meter Before I analyse the results from my experiment I will test the accuracy of joule meter.
See Diagram 3: For Diagram of Apparatus used Experimental Method: I will supply the joule meter with a constant voltage and I will time how long it takes to record energy of 10J. From this information I will be able to compare the energy I put in to the joule meter and the energy it recorded and I will be able to draw a calibration curve. My Results: Load=10. 3? Voltage (V) I would have repeated my results for this experiment if I had more time Analysing the results of the calibration experiment: Voltage (V) Time for 10J (s) Power in (watts) Power recorded (W) Joules in Joules recorded % Error Voltage squared 1.
This is how I calculated the above values: Power in= V2/R R=10. 3? Power recorded=10J/Time taken Joules in=(Power in) (time taken for 10J) % Error=100(Joules recorded-Joules in)/Joules in Is the power recorded equal to? If the joule meter is accurate the power it records will be equal to . To test this I have plotted the power recorded against the voltage I recorded squared.