1. Gearmotor Efficiency W
table top
gearmotor
pulley
string

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3. Parts
gearmotor with male terminals; make sure positive and negative leads don’t touch!
Groups of students should check out a kit from their instructor.
Give your instructor your student ID when checking out the kit.
bag with jeans rivet
to attach string
extension wire with barrel jack on one end and female terminals on the other
power supply
fill bag with weight (keys, phones, …)

4. Parts Configuration
plug these terminals in lightly; unplug them by grasping the terminals themselves, not the wires (so that you don’t pull the wires out of the crimp connectors)
string loops around rivet

5. Inside the Gearboxes 38 RPM at 12V 5 RPM at 12V
Two different gearmotors are shown below, one that rotates at 5 revolutions per minute (RPM) and one that rotates at 38 RPM. The DC motors spin at the same rate. Notice that the 5 RPM motor has more gear reduction.
38 RPM at 12V
5 RPM at 12V

6. Variable Power Supply
This power supply can provide 3, 4.5, 5, 6, 7.5, 9 or 12 volts
Switch between voltages using the green knob
Different voltages will make the motor run at different speeds & torques 6

7. Calculating Gearmotor Efficiency
Energy to Lift Weight
table top
position after being lifted
We know that energy is lost due to friction in the gearbox and that “losses” occur in the electric motor. The overall efficiency of the system is . . .
𝜂= 𝑤𝑜𝑟𝑘 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑙𝑖𝑓𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑚𝑜𝑡𝑜𝑟 = 𝑊∙ℎ 𝑉∙𝐼∙𝑡 h
electrical energy is determined
the same way as for pump project W

8. Gearmotor Efficiency Assignment
Determine efficiency and speed as a function of four voltages
Measure height to be lifted using a ruler using same starting and ending point for all experiments
Choose a weight by adding items to the bag
Perform the experiments
Attach multimeter to measure voltage
Hold gearmotor on edge of table with hand, wrap string around pulley, and attach bag with weights to string
Plug in gearmotor to lift weight
Measure time required to cover the distance, and measure the voltage
Reconfigure the multimeter to measure current
Run the experiment again to measure the current
Compute the quantities listed in the table below

9. Return Kit to Instructor, and Collect your ID