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LAB 12
OVERVIEW: WORK DONE BY A MOTOR
6/97
An electric motor is an electro-mechanical device. It converts electrical energy into mechanical
kinetic energy, or motion.
Assume you are using an electric motor to lift a load. The electrical energy used by the motor
is converted to mechanical energy, or work, when the load is lifted. The electrical energy input is
the product of the motor voltage, motor current, and the time required to lift the load. This
electrical energy is the electrical work input to the system. This can be expressed as follows:
𝑊𝑜𝑟𝑘𝑖𝑛 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝐸𝑛𝑒𝑟𝑔𝑦 = 𝑉 · 𝐼 · 𝑡
The mechanical work done by the motor is the product of the lifting force and the distance
the load was lifted. This is expressed in the equations below, where lifting force (output) equals
the weight of the load, and the output distance is the height through which the weight is lifted.
Mechanical Linear Work:
𝑊 = 𝐹•𝐷
𝑊𝑜𝑟𝑘𝑜𝑢𝑡 = 𝑊𝑒𝑖𝑔ℎ𝑡 · 𝐻𝑒𝑖𝑔ℎ𝑡
In this experiment you will use an electric motor to lift a load. You will measure the electrical
energy input to the motor. You will also measure the mechanical work output of the motor. Then
you will calculate the efficiency of the motor lifting system,
OBJECTIVES:
A) Measure the electrical energy input of a motor.
B) Measure the mechanical work output of a motor.
C) Find the efficiency of an electric motor lifting system.
EQUIPMENT:
Support Stand Set (following parts):
--Mechanical Breadboard
--Two 24" Support Rods with Base
--14 1/2" Crossbar
--Two 1-Screw Rod Connectors
Slotted Weight Set with Hanger
High Current AC/DC Power Supply
AC/DC Motor
Nylon Cord
Universal Lead Set
Stopwatch
Single pulley
Hook Sleeve
30-cm Ruler
PROCEDURE:
Figure 1 shows the lab setup you will use in this experiment. Refer to this and the detailed
figures that follow when assembling the equipment.
A) 1. Mount the two support rods to the breadboard, in the position shown in Figure 2.
2. Using the rod connectors, mount the crossbar to the support rods. Position the crossbar at the
top of the support rods. Have at least 311 of the bar extending over the edge of the breadboard.
(See Figure 3)
3. Attach the hook sleeve to the crossbar. Hang the single pulley from the hook sleeve.
4. Mount the electric motor to the breadboard, in the position shown in Figure 2. The feet of the
motor mounting board fit into the breadboard holes. The shaft of the motor should extend over
the edge of the breadboard.
5. Connect the 0-24 VDC jacks on the power supply to connections B1 and B2 on the motor. To
make this connection you will need the banana plug to spade leads from your Universal Lead
Set.
6. Attach jumper leads from B, and B. to the shunt field windings (marked C1 and C2,
respectively) on the motor, as shown in Figure 4.
7. Place the breadboard at the edge of the lab table. Tie the end of the string to the weight hanger.
Run the string over the single pulley, and then through the outside hole in the motor spool pulley.
8. Position the weight hanger on the floor, directly below the pulley. Take up the slack in the
string by pulling it through the motor spool pulley. Tie a knot in the. string next to the motor
spool pulley.
COLLECTING DATA:.
B) 1. Place a 100-gram mass on the weight hanger (total mass of 15.0 g). Turn on the power
supply. Slowly increase the voltage until the motor lifts the weight at a slow, constant rate. Turn
off the power supply, using the power switch. Return the weight to the floor. Turn the power
supply on again, and check to make sure the weight is lifted properly. If it is not lifting properly,
adjust the voltage again. if it is lifting properly, maintain the voltage setting and turn off the
power supply.
2. Measure the distance from the floor to the top of the mechanical breadboard, in centimeters.
This is the distance (h) that the weight will be lifted. Record in Data Table 1.
TRIAL 1
3.. Repeat the lifting process several times. Take voltage and current measurements from the
power supply during the lifting process. Record an average of these values in Data Table 1.
4. Using the stopwatch, time how long it takes the motor to raise the weight through the distance
(h). 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.
5. Turn off the power supply, and return the weight hanger to the floor.
TRIAL 2
6. Add 100 grams to the weight hanger, for a total mass of 250 grams. Turn on the power supply.
Adjust the voltage so that the weight is again being lifted at the same slow, constant rate. Turn
off the power supply, and return the weight to the floor.
7. Repeat steps 3,4, and 5.
TRIAL 3
8. Add 100 grams to the weight hanger for a total hanging mass of 350 grams. Turn on the power
supply and adjust the voltage again. Turn off the power supply, and return the weight to the
floor.
9. Repeat steps 3,4, and 5.
10. Take the lab setup apart.
LAB 12 WORK. DONE SY A MOTOR
6/97
p. 4 COMPIITATIONS:
C) 1. Convert the height measurement (h) from cm to meters.
2. Convert each mass (grams) to weight in newtons.
3. Find the mechanical output work done by the motor in each trial, using the equation:
𝑊𝑜𝑢𝑡 = 𝑤 · ℎ (Output Work = weight · height lifted)
4. Find the electrical energy input of the motor in each trial, using the equation:
𝑊𝑖𝑛 = 𝑉 · 𝐼 · 𝑡 (Input Work = Voltage x Current x time)
Note: The product of one volt (V) x one ampere (A) x one second (s) is equal to one joule (J).
5. Calculate the efficiency for each trial, using the equation:
n = (Wout ÷ Win ) · 100