Activity 4.4b - Motor-Generator

Purpose

In thermodynamics we learned no conversion of energy is perfect. In this activity we will explore what makes a DC motor function, how to create a generator from it, measure the power, and calculate the efficiency.

Motors and generators are electromagnetic mechanical devices.Electricity flowing through a conductor creates a magnetic field. If the conductor is made into a coil it will concentrate a magnetic field that has a north and south pole. The laws of magnets states that like poles repel and unlike poles attract. These forces can be used to make an electric motor that converts electrical energy into rotational mechanical energy. If a coil of wire is rotated through a magnetic field it will generate an electric current as the coil cuts through the magnetic lines of force. A generator uses this principle to convert rotational mechanical energy to electrical energy.

Equipment

1- Small base plate,

2 – motors,

2 – building blocks,

8 – banna plugs mounted on 4 inch wires,

1 – light bulb and socket

1 – digital volt and amp meter

Power supply

2 inches of 3/16 diameter shrink plastic tubing

small soldering iron

Procedure

  1. Below is the sketch of the setup we will use.

  1. Secure all parts and equipment and assemble the system on a Fischertechnik base plate as shown by the diagram.
  1. Mount two building blocks on the base plate about 4 inches apart.
  1. Slide one motor on the top of each block. (one of the motors will be used as a the generator)
  1. Cut a 2 inch section of 3/16 inch diameter plastic heat shrink tubing.
  1. Slide tubing over and between motor and generator shafts.
  1. Adjust shafts so tubing covers the threaded parts of both shafts.
  2. Using a heat source (soldering iron) slowly heat the shrink tubing on to the two shafts. The tubing should shrink down tightly and grip both shafts.
  1. The motor shaft should now be able to turn the generator shaft without slipping. Test this by turning the motor shaft with your finger.
  2. Mount the light bulb and socket to the base plate.
  1. Wire the input circuit using a Fischertechnik power supply; check the mechanical connection between the motor and generator. Disconnect the power.
  2. Wire output from the generator to the light bulb.
  3. Test the system by turning on the motor to see if the generator lights the light bulb. If the system fails: check the shrink tubing for slipping, all wiring, light bulb filament, and input motor voltage.
  4. Set up voltmeter to measure 20 volts DC. Ask for help if needed.
  5. Power up the system and connect the voltmeter in parallel across the motor circuit as shown by the diagram. Record input voltage below.
  6. Connect the voltmeter in parallel to the output of the generator as shown by the diagram. Be sure to observe + and – polarity. Measure the output voltage across the glowing light bulb. Record the output voltage below.
  1. Now set up the meter to measure direct current in amps. Ask for help if needed.
  1. Connect the amp meter in series with the motor as shown by the diagram. You must disconnect the plug to the motor and place the amp meter into the circuit with a series connection. Be sure to observe + and – polarity.
  1. Turn on the system and measure input current. Record current below.
  1. Connect the amp meter in series with the light bulb as shown by the diagram. Disconnect the wire from the lamp and place the amp meter in series with the bulb.
  1. Turn on the system and measure the output current in amps. Record the measurement below.

Disassembly:

  1. Remove the shrink tubing from the motor and generator shafts by carefully cutting the tubing with a knife.
  2. Disconnect meter and turn it to the off position. Disconnect all wires.
  3. Disassemble all fisher tech parts and return them to storage boxes.

Conclusion

  1. Using the information you gathered in the activity, perform the following calculations:

Calculations: Power = Voltage x Current or P = V x I

Power input of system P = ______volts x ______amps P=______watts

Power output of system P = ______volts x ______amps P =______watts

Efficiency = power output divided by power input times 100%

Eff. = PO / PI x 100

Eff. = ______%

  1. Relate your experience in this activity in a paragraph using the following terms:

Voltage – Resistance – Current – Power – Polarity – DC – AC

  1. Create a sketch of a Generator in the space below. Explain what a DC generator does and how it works?
  1. What was the percent efficiency of your motor generator system? Why do you think it was so low?Where was the rest of the energy lost to?
  1. What impact do generating plants have on our environment?

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Copyright 2007

POE – Unit 4 – Activity 4.4b – Motor Generator– Page 1