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Friday, March 28, 2014
Power is the rate of doing work or using energy. It is a measure of the amount of work done in a
certain unit of time. The defining equation is P W/t where W is the work done in Joules, and t is
the time required to do the work in seconds. The unit of Power will then be a Watt (W). A Watt
is one Joule per second.
Mechanical work is done when an object is moved. This motion can be caused by force or
torque. When a force is applied, linear motion is produced. When torque is applied, angular
motion is produced and the object rotates.
When a mechanical system does work or uses energy, some energy is lost to heat and friction. As
a result, the total output power that a system produces is actually less than the input power used.
The efficiency of the system can be calculated using the equation Efficiency Power Output
(mechanical)/Power Input (Electrical)
In this experiment, you will build a system that develops linear mechanical power. Then you will
put the system to work and find the power it produces. You will also measure the total power
input to the system, and find its efficiency.
Friday, March 28, 2014
OBJECTIVES: Upon successful completion of this experiment, you should be able to:
A) Find the electrical input power used by a motor as it performs work.
B) Figure the mechanical input power of a motor as it raises a weight at a constant speed.
C) Find the efficiency of a motor as it converts electrical power to mechanical power.
Mechanical Breadboard
14.5" Crossbar
Hook Sleeve
AC/DC Motor or DC Motor
Universal Lead Set
Weight Hanger
Slotted Weight Set
Meter Stick
Two 24" Support Rods with Bases
Two one-screw Rod Connectors
Single Pulley
Digital Stopwatch
Nylon Cord
AC/DC Power Supply
A) Lab setup The basic lab setup for this lab is shown in Figure 2 (for benches 1-6 or 7 and 8.
Mount the support rods at the left end of the breadboard. See Figure 2.
2. Using two 1-screw rod connectors, mount the crossbar near the top of the support rods. The
crossbar should extend over the edge of your lab table. See Figure 3 for proper bench.
3. Position the hook sleeve on the crossbar so it hangs over the edge of the table. Hang the
heavy-duty pulley on the hook sleeve. See Figure 3.
4. Mount the spool pulley to the motor shaft. Place the motor on the breadboard. Secure the
AC/DC motor to the breadboard with a C-clamp. Position the motor so the spool pulley is
aligned with the hook sleeve and heavy-duty pulley. See Figure 3.
5-. Connect the motor to the power supply. (For the AD/DC motor use the shunt configuration.
See Figure 4).
6. Tie a knot in one end of a meter-long string. Thread the other end of the string through the
spool pulley hole, and pull it tight. See Figure 5.
7. Tie the free end of the string to the weight hanger. Route the string over the pulley.' Place the
weight hanger on the floor, as shown in Figure 6.
8. Rotate the motor shaft by hand until the string is taut. The weight hanger should stay on the
9. Add the 500 gm weight to the weight hanger. Your total hanging mass is 550 gm, since the
hanger's mass is 50 gm. Record this as the value for Hanging Mass of Trial 1 in Data Table 1.
10. Turn on the power supply. Slowly increase the voltage until the motor lifts the weights at a
constant rate. Turn off the power supply using the power switch. Return the weight to the floor.
Turn the power supply on again. If the motor is not lifting properly, adjust the voltage again. If it
is lifting properly, maintain the voltage and turn off the power supply.
B) Collecting Data
1. Measure the distance between the floor and the top of the breadboard. Record this as the
Lifting Height (D) in Data Table 1. See Figure 7.
2. Turn on the power supply. Take voltage and current measurements from the power supply
during the lifting process. Record an average of these values in Data Table 1.
Friday, March 28, 2014
3. Using the stopwatch, time how long it takes the motor to raise the weight throUgh D. Start the
watch at the same time that you turn on the power supply. Stop the watch when the bottom of the
weight hanger reaches the top of the breadboard. Record the measured time, in seconds, in Data
Table 1.
4. Turn off the power supply, and return the weight hanger to the floor.
5. Add 200 gm to the weight hanger, for a total hanging mass of 750 gm. Turn on the power
supply. Adjust the voltage so the weight is again being lifted at the same constant rate. Turn off
the power supply, and return the weight to the floor.
6. Repeat steps B-2 through B-4.
7. Take apart your lab setup.
1. Convert the height measurement (D) from centimeters to meters, and record the value in the
Data Table.
2. Compute the weight of each hanging mass, in Newtona. Record these values as the Lifting
Force in Table 1.
3. Find the mechanical power used to lift the weight for both trials. This is the output power.
𝑃𝑜𝑢𝑡 = Force • Height - time
𝑃𝑜𝑢𝑡 = 𝐹 •
4. Find the electrical power used to lift the mass for both trials. This is the input power.
𝑃_𝑒𝑙𝑒𝑐 = 𝑉 • 𝐼
NOTE: one volt x one amp = one Watt
5. Find the efficiency of the motor-pulley system for both trials. Efficiency = output power =
input power
𝑛 = ( 𝑃𝑜𝑢𝑡) · 100
(n is the efficiency in %)
Record the results in Table 2.