College of Engineering Technology

AEEP2211

Electrical Machines
Laboratory Manual
Rayyan Irshad 60302204
Mahmoud Firas 60099554
Haseeb Ahmad 60084195
Farzhan Kabeer 60103041

Laboratory Exercise #3
The Seperately-Excited DC Motor

1
 Table of Contents

SAFETY RULES AND OPERATING PROCEDURES ............................................................... 3

LABORATORY SPECIFIC SAFETY INFORMATION .............................................................. 4
LABORATORY EXERCISE OBJECTIVE ................................................................................... 5
DISCUSSION ................................................................................................................................. 5
PROCEDURE ................................................................................................................................ 10
EQUIPMENT REQUIRED ............................................................................................................. 10
CONCLUSION ............................................................................................................................... 20
REVIEW QUESTIONS ................................................................................................................... 20
FINAL COMMENTS ...................................................................................................................... 22
REFERENCES ................................................................................................................................ 24

2
SAFETY RULES AND OPERATING PROCEDURES
The safety of our employees and students is a core value of CNA – Qatar. No other business
objective has higher priority.

1. Note the location of the Emergency Disconnect (red button near the door) to shut off power
in an emergency. Note the location of the nearest emergency exit (map on bulletin board) and
safety equipment (fire extinguisher, safety shower etc.).

2. No shop tool, equipment or machine will be used unless the operator fully understands the
proper and safe operation. If the operator is not sure of correct procedures, help must be
requested from the instructor.

3. Report any broken equipment or defective parts to the lab instructor. Do not open, remove
the cover, or attempt to repair any equipment. No shop Tool, equipment or machine will be
used if safeguards are removed or the device is not operating properly.

4. Proper PPE must be worn at all times in the lab. In addition, appropriate attire must be worn
while operating or observing in the vicinity of operating shop and laboratory tools, machines,
equipment and processes. (No loose-fitting clothes, open shoes, long loose hair, excessive
jewelry or accessories.)

5. Students are allowed in the laboratory only when the instructor is present.

6. Open drinks and food are not allowed near the lab benches.

7. When the lab exercise is over, all instruments, except computers, must be turned off and the
laboratory station must be cleaned.

8. Do not move instruments from one lab station to another lab station or from the laboratory
without prior permission from instructor.

Hazards
Electrocution

3
LABORATORY SPECIFIC SAFETY INFORMATION

Introduction

The danger of injury or death from electrical shock, fire, or explosion is present while
conducting experiments in this laboratory. To work safely, it is important that you understand the
prudent practices necessary to minimize the risks and what to do if there is an accident.

Electrical Shock

Avoid contact with conductors in energized electrical circuits. The typical cannot let-go (the
current in which a person cannot let go) current is about 6-30 mA (OSHA). Muscle contractions
can prevent the person from moving away the energized circuit. Possible death can occur as low
50 mA. For a person that is wet the body resistance can be as low as 1000 ohms. A voltage of 50
volts can result in death.

Do not touch someone who is being shocked while still in contact with the electrical conductor
or you may also be electrocuted. Instead, press the Emergency Disconnect (red button located
near the door to the laboratory). This shuts off all power.

Make sure your hands are dry. The resistance of dry, unbroken skin is relatively high and thus
reduces the risk of shock. Skin that is broken, wet, or damp with sweat has a low resistance.
When working with an energized circuit, work with only your right hand, keeping your left hand
away from all conductive material. This reduces the likelihood of an accident that results in
current passing through your heart.

Be cautious of rings, watches, and necklaces. Skin beneath a ring or watch is damp, lowering the
skin resistance.

If the victim isn’t breathing, find someone certified in CPR. Be quick! Some of the staff in the
Department Office are certified in CPR. If the victim is unconscious or needs an ambulance,
contact the Security or call 2999. If able, the victim should go to the Student
Medical Services for examination and treatment.

Fire

Transistors and other components can become extremely hot and cause severe burns if touched.
If resistors or other components on your proto-board catch fire, turn off the power supply and
notify the instructor. If electronic instruments catch fire, press the Emergency Disconnect (red
button). These small electrical fires extinguish quickly after the power is shut off. Avoid using dry
powder fire extinguishers on electronic instruments. Try to use only CO2 extinguisher, if possible.

First Aid
A first aid kit is located on the wall near the door. Proceed to Medical Services, if needed

4
Electrical Machines
Laboratory Exercise #3
The Separately-Excited DC Motor

EXERCISE OBJECTIVE When you have completed this exercise, you will be able to demonstrate the
main operating characteristics of a separately-excited dc motor using the
DC Motor/Generator.

DISCUSSION OUTLINE The Discussion of this exercise covers the following points:

▪ Simplified equivalent circuit of a dc motor

▪ Relationship between the motor rotation speed and the armature voltage
when the armature current is constant
▪ Relationship between the motor torque and the armature current
▪ Relationship between the motor rotation speed and the armature voltage
when the armature current varies

DISCUSSION Simplified equivalent circuit of a dc motor

Previously, you saw that a dc motor is made up of a fixed magnet (stator) and a
rotating magnet (rotor). Many dc motors use an electromagnet at the stator, as
Figure 2-8 shows.

Stator
(electromagnet)

IS

Rotor
(armature)

ES N S

Figure 2-8. Simplified dc motor using an electromagnet as stator.

When power for the stator electromagnet is supplied by a separate dc source, of

either fixed or variable voltage, the motor is known as a separately-excited
dc motor. Sometimes the term independent-field dc motor is also used.The

© Festo Didactic 88943-00

5
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Discussion

current flowing in the stator electromagnet is often called field current because it
is used to create a fixed magnetic field.

The electrical and mechanical behavior of the dc motor can be understood by

examining its simplified equivalent electric circuit shown in Figure 2-9.

IÆ RÆ

E
+ RÆ

+
EÆ ECEMF

Figure 2-9. Simplified equivalent circuit of a dc motor.

In the circuit, EÆ is the voltage applied to the motor brushes, IÆ is the current
flowing in the armature through the brushes, and RÆ is the resistance between
the brushes. Note that EÆ, IÆ, and RÆ are usually referred to as the armature
voltage, current, and resistance, respectively. ERÆ is the voltage drop across the
armature resistor. When the motor turns, an induced voltage ECEMF proportional
to the speed of the motor is produced. This induced voltage is represented by a
dc source in the simplified equivalent circuit of Figure 2-9. The motor also
develops a torque T proportional to the armature current IÆ flowing in the motor.
The motor behavior is based on the two equations given below. Equation (2-1)
relates motor speed n and the induced voltage ECEMF. Equation (2-2) relates the
motor torque T and the armature current IÆ.

n = K1 · ECEMF (2-1)

where n is the motor rotation speed, expressed in revolutions per

minute (r/min).
K1 is a constant expressed in r/min.
V
ECEMF is the voltage induced across the armature, expressed in
volts (V).

T = K 2 · IÆ (2-2)

where T is the motor torque, expressed in newton-meters (N·m) or in pound-

force inches (lbf·in).
K2 is a constant expressed in N·m or lbf·in.
A A
IÆ is the armature current, expressed in amperes (A).

© Festo Didactic 88943-00

6
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Discussion

Relationship between the motor rotation speed and the armature voltage
when the armature current is constant

When a voltage EÆ is applied to the armature of a dc motor with no mechanical

load, the armature current IÆ flowing in the equivalent circuit of Figure 2-9 is
constant and has a very low value. As a result, the voltage drop ERÆ across the
armature resistor is so low that it can be neglected, and ECEMF can be considered
to be equal to the armature voltage EÆ. Therefore, the relationship between the
motor rotation speed n and the armature voltage EÆ is a straight line
because ECEMF is proportional to the motor rotation speed n. This linear
relationship is shown in Figure 2-10. The slope of the straight line equals
constant K1.

Motor speed n
(r/min)

Slope = K1

Armature voltage EÆ
(V)

Figure 2-10. Linear relationship between the motor rotation speed and the armature voltage.

Since the relationship between voltage EÆ and the rotation speed n is linear, a
dc motor can be considered to be a linear voltage-to-speed converter, as shown
in Figure 2-11.

Input = armature voltage EÆ K1 Output = motor rotation speed n

Figure 2-11. DC motor as a voltage-to-speed converter.

© Festo Didactic 88943-00

7
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Discussion

Relationship between the motor torque and the armature current

The same type of relationship exists between the motor torque T and the
armature current IÆ, so that a dc motor can also be considered as a linear
current-to-torque converter. Figure 2-12 illustrates the linear relationship between
the motor torque T and the armature current IÆ. Constant K2 is the slope of the
line relating the two. The linear current-to-torque converter is shown in
Figure 2-13.

Motor torque T
(N·m or lbf·in)

Slope = K2

Armature current IÆ
(A)

Figure 2-12. Linear relationship between the motor torque and the armature current.

Input = armature current IÆ K2 Output = motor torque T

Figure 2-13. DC motor as a current-to-torque converter.

Relationship between the motor rotation speed and the armature voltage
when the armature current varies

When the armature current IÆ increases, the voltage drop ERÆ (RÆ · IÆ) across the
armature resistor also increases and can no longer be neglected. As a result, the
armature voltage EÆ can no longer be considered equal to ECEMF, but rather the
sum of ECEMF and ERÆ, as Equation (2-3) shows:

EÆ = ECEMF + ERÆ (2-3)

© Festo Didactic 88943-00

8
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Discussion

Therefore, when a fixed armature voltage EÆ is applied to a dc motor, the voltage

drop ERÆ across the armature resistor increases as the armature current IÆ
increases, and thereby, causes ECEMF to decrease. This also causes the motor
rotation speed n to decrease because it is proportional to ECEMF. This is shown in
Figure 2-14, which is a graph of the motor rotation speed n versus the armature
current IÆ for a fixed armature voltage EÆ.

Motor speed n (r/min)

Fixed armature voltage = EÆ

Armature current IÆ (A)

Figure 2-14. The motor rotation speed drops as the armature current increases (fixed armature
voltage EA).

Figure 2-15. Example of a separately-excited dc motor used in a racing kart.

© Festo Didactic 88943-00

9
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure Outline

PROCEDURE OUTLINE The Procedure is divided into the following sections:

▪ Set up and connections

▪ Determining the armature resistance
▪ Motor speed versus armature voltage
▪ Motor torque versus armature current
▪ Speed decrease versus armature current
▪ Additional experiments (optional)
Motor speed-versus-armature voltage and motor torque-versus-armature
current characteristics for reversed armature connections.

PROCEDURE

High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.

Set up and connections

In this section, you will mechanically couple the DC Motor/Generator to theFour-

Quadrant Dynamometer/Power Supply and set up the equipment.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of

equipment required to perform the exercise. Install the equipment in the
Workstation.

a Before performing the exercise, ensure that the brushes of the

DC Motor/Generator are adjusted to the neutral point. To do so, connect a
variable-voltage ac power source (terminals 4 and N of the Power Supply) to
the armature of the DC Motor/Generator (terminals 1 and 2) through current
input I1 of the Data Acquisition and Control Interface (DACI). Connect the
shunt winding of the DC Motor/Generator (terminals 5 and 6) to voltage
input E1 of the DACI. In LVDAC-EMS, open the Metering window. Set two
meters to measure the rms values (ac) of the armature voltage EÆ and
armature current IÆ at inputs E1 and I1 of the DACI, respectively. Turn the
Power Supply on and adjust its voltage control knob so that an ac current
(indicated by meter I1 in the Metering window) equal to half the nominal
armature current flows in the armature of the DC Motor/Generator. Adjust the
brush adjustment lever on the DC Motor/Generator so that the voltage across
the shunt winding (indicated by meter E1 in the Metering window) is minimal.
Turn the Power Supply off, close LVDAC-EMS, and disconnect all leads and
cable.

Mechanically couple the DC Motor/Generator to the Four-Quadrant

Dynamometer/Power Supply using a timing belt.

Before coupling rotating machines, make absolutely sure that power is turned off
to prevent any machine from starting inadvertently.

© Festo Didactic 88943-00

10
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

2. Make sure that the main power switch of the Four-Quadrant

Dynamometer/Power Supply is set to the O (off) position, then connect its
Power Input to an ac power wall outlet.

3. On the Power Supply, make sure that the main power switch and the 24 V
ac power switch are set to the O (off) position, and that the voltage control
knob is set to 0% (turned fully counterclockwise). Connect the Power Supply
to a three-phase ac power outlet.

4. Connect the Power Input of the Data Acquisition and Control

Interface (DACI) to the 24 V ac power source of the Power Supply.

Turn the 24 V ac power source of the Power Supply on.

5. Connect the USB port of the Data Acquisition and Control Interface to a
USB port of the host computer.

Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to

a USB port of the host computer.

6. Connect the equipment as shown in Figure 2-16. Use the variable dc voltage
output of the Power Supply to implement the variable-voltage dc power
source ES. Use the fixed dc voltage output of the Power Supply to implement
the fixed-voltage dc power source. E1, I1 and I2 are voltage and current
inputs of the Data Acquisition and Control Interface (DACI). Leave the circuit
open at points A and B shown in the figure.

7. On the Four-Quadrant Dynamometer/Power Supply, set the Operating Mode

switch to Dynamometer. This setting allows the Four-Quadrant
Dynamometer/Power Supply to operate as a prime mover, a brake, or both,
depending on the selected function.

Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main

power switch to the I (on) position.

© Festo Didactic 88943-00

11
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

IÆ

+ DC Motor/ Two-quadrant,
ES EÆ Generator constant-torque
armature brake

Iƒ
A B

DC Motor/
Generator
shunt winding

DC Motor/
Generator
rheostat

Figure 2-16. Separately-excited dc motor coupled to a brake.

8. Turn the host computer on, then start the LVDAC-EMS software.

In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation function is
available for the Data Acquisition and Control Interface module. Select the
network voltage and frequency that correspond to the voltage and frequency
of the local ac power network, then click the OK button to close the LVDAC-
EMS Start-Up window.

© Festo Didactic 88943-00

12
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply

window, then make the following settings:

− Set the Function parameter to Two-Quadrant, Constant-Torque Brake.

This setting makes the Four-Quadrant Dynamometer/Power Supply
operate as a two-quadrant brake with a torque setting corresponding to
the Torque parameter.

− Set the Pulley Ratio parameter to 24:24. The first and second numbers in
this parameter specify the number of teeth on the pulley of the Four-
Quadrant Dynamometer/Power Supply and the number of teeth on the
pulley of the machine under test (i.e., the DC Motor/Generator),
respectively.

− Make sure that the Torque Control parameter is set to Knob. This allows
the torque of the two-quadrant brake to be controlled manually.

− Set the Torque parameter to the maximum value (3.0 N·m or 26.5 lbf·in).
This sets the torque command of the Two-Quadrant, Constant-Torque
Brake to 3.0 N·m (26.5 lbf·in).

a The torque command can also be set by using the Torque control knob in the
Four-Quadrant Dynamometer/Power Supply window.

− Start the Two-Quadrant, Constant-Torque Brake by setting the Status

parameter to Started or by clicking the Start/Stop button.

10. In LVDAC-EMS, open the Metering window. Set two meters to measure the
dc motor armature voltage EÆ (E1) and armature current IÆ (I1). Set a meter
to measure the dc motor armature resistance RÆ [RDC (E1, I1)]. Finally, set a
meter to measure the dc motor field current Iƒ (I2).

Click the Continuous Refresh button to enable continuous refresh of the

values indicated by the various meters in the Metering application.

Determining the armature resistance

In this section, you will measure the armature resistance RA of the

DC Motor/Generator. It is not possible to measure the armature resistance RÆ of
the DC Motor/Generator with a conventional ohmmeter because the non-linear
characteristic of the motor brushes causes incorrect results when the armature
current IÆ is too low. The general method used to measure RÆ consists in
connecting a dc power source to the motor armature and measuring the voltage
required to make nominal current flow in the armature windings. No power
source is connected to the motor stator to ensure that the motor does not rotate,
and that ECEMF equals zero. The ratio of the armature voltage EÆ to the armature
current IÆ yields the armature resistance RÆ directly.

a The motor will not start rotating because it is mechanically loaded.

© Festo Didactic 88943-00

13
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

11. Turn the Power Supply on by setting the main power switch to the I (on)
position. Set the voltage control knob of the Power Supply so that the
armature current IÆ (indicated by meter I1 in the Metering window) flowing in
the DC Motor/Generator is equal to the rated armature current.

a The rating of any of the supplied machines is indicated in the lower section of
the module front panel.

Record the value of the armature resistance RÆ [indicated by

meter RDC (E1, I1) in the Metering window].

Armature resistance RÆ =30.40 Ω

12. On the Power Supply, set the voltage control knob to 0%, then set the main
power switch to the O (off) position. (Leave the 24 V ac power source of the
Power Supply turned on.)

Interconnect points A and B in the circuit of Figure 2-16.

Motor speed versus armature voltage

In this section, you will measure data and plot a graph of the separately-excited
dc motor speed n as a function of the armature voltage EÆ to demonstrate that
the motor speed is proportional to the armature voltage under no-load conditions.

13. In LVDAC-EMS, open the Data Table window. Set the Data Table to record
the dc motor rotation speed n and torque T (indicated by the Speed and
Torque meters in the Four-Quadrant Dynamometer/Power Supply window),
as well as the dc motor armature voltage EÆ, armature current IÆ, and field
current Iƒ (indicated by meters E1, I1, and I2 in the Metering window).

14. In the Four-Quadrant Dynamometer/Power Supply window, set the

Torque parameter to 0.0 N·m (or 0.0 lbf·in).

15. Turn the Power Supply on by setting the main power switch to the I (on)
position.

On the DC Motor/Generator, set the Field Rheostat knob so that the field
current Iƒ (indicated by meter I2 in the Metering window) is equal to the value
indicated in Table 2-1 for your local ac power network.

© Festo Didactic 88943-00

14
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

Table 2-1. Field current If .

Local ac power network

Field current If
Frequency (mA)
Voltage (V)
(Hz)

120 60 300

220 50 190

240 50 210

220 60 190

16. On the Power Supply, vary the voltage control knob setting from 0% to 100%
in 10% steps in order to increase the armature voltage EÆ by steps. For each
setting, wait until the motor speed stabilizes, then record the motor armature
voltage EÆ , armature current IÆ , and field current Iƒ , as well as the motor
rotation speed n and torque T in the Data Table.

17. When all data has been recorded, stop the DC Motor/Generator by setting
the voltage control knob to 0% and the main power switch of the Power
Supply to the O (off) position. (Leave the 24 V ac power source of the Power
Supply turned on.)

In the Data Table window, confirm that the data has been stored, save the
data table under filename DT211, and print the data table if desired.

18. In the Graph window, make the appropriate settings to obtain a graph of the
dc motor speed n as a function of the armature voltage EÆ. Name the
graph “G211”, name the x-axis “Armature voltage”, name the y-axis “Motor
speed”, and print the graph if desired.

What kind of relationship exists between the armature voltage EÆ and

dc motor speed n?
Linear

Does this graph confirm that the separately-excited dc motor is equivalent to

a linear voltage-to-speed converter, with higher voltage producing greater
speed?

❑ Yes ❑ No

19. Use the two end points to calculate the slope K1 of the relationship obtained
in graph G211. The values of these points are indicated in data table DT211.

n2 — n1 — r/min
K1 = = =
E2 — E1 — V

© Festo Didactic 88943-00

15
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

20. In the Data Table window, clear the recorded data.

Motor torque versus armature current

In this section, you will measure data and plot a graph of the separately-excited
dc motor torque T as a function of the armature current IÆ to demonstrate that the
motor torque is proportional to the armature current.

21. In the Four-Quadrant Dynamometer/Power Supply window, make sure that

16
 the Torque parameter is set to 0.0 N·m (0.0 lbf·in).

22. Turn the Power Supply on by setting the main power switch to the I (on)
position.

On the DC Motor/Generator, slightly readjust the Field Rheostat knob, if

necessary, so that the field current Iƒ (indicated by meter I2 in the Metering
window) is equal to the value indicated in Table 2-1 for your local ac power
network.

On the Power Supply, set the voltage control knob so that the motor rotation
speed n is 1500 r/min. Note and record the value of the motor armature
voltage EÆ (E1).

Armature voltage EÆ (n = 1500 r/min) =262 V

Note and record the value of the motor torque T indicated by the Torque
meter in the Four-Quadrant Dynamometer/Power Supply.

Motor torque T (minimum) =0.202 N·m (lbf·in)

23. In the Four-Quadrant Dynamometer/Power Supply window, set the Torque

parameter to the minimum value measured in step 22. Record the motor
rotation speed n and torque T, as well as the motor armature voltage EÆ,
armature current IÆ, and field current Iƒ in the Data Table.

Increase the Torque parameter from the minimum value to about 1.9 N·m
(about 16.8 lbf·in) if your local ac power network voltage is 120 V, or from the
minimum value to about 2.3 N·m (about 20.4 lbf·in) if your local ac power
network voltage is 220 V or 240 V, in steps of 0.2 N·m (or 2.0 lbf·in). For
each torque setting, readjust the voltage control knob of the Power Supply so
that the armature voltage EÆ remains equal to the value recorded in step 22,
readjust the field current Iƒ to the value given in Table 2-1, then record the
motor rotation speed n and torque T, as well as the motor armature
voltage EÆ, armature current IÆ, and field current Iƒ in the Data Table.

The armature current IÆ will exceed the rated value while performing this manipulation.
Therefore, perform this manipulation in less than 5 minutes.

© Festo Didactic 88943-00

17
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

24. When all data has been recorded, stop the DC Motor/Generator by setting
the voltage control knob to 0% and the main power switch of the Power
Supply to the O (off) position. (Leave the 24 V ac power source of the Power
Supply turned on).

In the Four-Quadrant Dynamometer/Power Supply window, set the Torque

parameter to 0.0 N·m (0.0 lbf·in).

In the Data Table window, confirm that the data has been stored, save the
data table under filename DT212, and print the data table if desired.

25. In the Graph window, make the appropriate settings to obtain a graph of the
dc motor torque T as a function of the armature current IÆ. Name the
graph “G212”, name the x-axis “Armature current”, name the y-axis “Motor
torque”, and print the graph if desired.

What kind of relationship exists between the armature current IÆ and the
dc motor torque T as long as the armature current does not exceed the
nominal value?
Linear

Does this graph confirm that the separately-excited dc motor is equivalent to

a linear current-to-torque converter (when the armature current does not
exceed the nominal value), with higher current producing greater torque?

❑ Yes ❑ No

a The torque-versus-current relationship is no longer linear when the armature

current IÆ exceeds the nominal value because of a phenomenon called
armature reaction. This phenomenon is described in the next unit of this
manual.

26. Use the two end points of the linear portion of the relationship obtained in
graph G212 to calculate the slope K2. The values of these points are
indicated in data table DT212.

T2 — T 1 — N · m (lbf · in)
K2 = = =
I2 — I1 — A

2.3-0.2 = 1.2 N.m/A

1.8-0.17

18
19
20
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

Speed decrease versus armature current

In this section, you will demonstrate that when the armature voltage EÆ is set to a
fixed value, the speed of the separately-excited dc motor decreases with
increasing armature current or torque because of the increasing voltage drop
across the armature resistor.

27. Using the values determined previously for the armature resistance RÆ
(step 11), constant K1 (step 19), and armature voltage EÆ (step 22), calculate
the motor rotation speed n for each of the three armature currents IÆ given
in Table 2-2 for your local ac power network.

ERÆ = IÆ × RÆ

ECEMF = EÆ — ERÆ

n = ECEMF × K1

Table 2-2. DC motor armature currents IA.

Local ac power network

Armature current IA
Frequency (A)
Voltage (V)
(Hz)

120 60 1.0 2.0 3.0

220 50 0.5 1.0 1.5

240 50 0.5 1.0 1.5

220 60 0.5 1.0 1.5

When IÆ = A:

ERÆ = V

ECEMF = V

n= r/min

When IÆ = A:

ERÆ = V

ECEMF = V

n= r/min

© Festo Didactic 88943-00

21
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Procedure

When IÆ = A:

ERÆ = V

ECEMF = V

n= r/min

Based on your results, how should voltage ECEMF and the dc motor speed n
vary as the armature current IÆ is increased?

28. In the Graph window, make the appropriate settings to obtain a graph of the
dc motor speed n as a function of the armature current IÆ, using the data
recorded previously in data table DT212. Name the graph “G212-1”, name
the x-axis “Armature current”, name the y-axis “Motor speed”, and print the
graph if desired.

Does graph G212-1 confirm the prediction you made in the previous step
about the variation of the dc motor speed n as a function of the armature
current IÆ?

❑ Yes ❑ No

Briefly explain what causes the dc motor speed n to decrease when the
armature voltage EÆ is fixed and the armature current IÆ increases.

22
29. In the Graph window, make the appropriate settings to obtain a graph of the
dc motor speed n as a function of the dc motor torque T using the data
recorded previously in data table DT212. Name the graph “G212-2”, name
the x-axis “Motor torque”, name the y-axis “Motor speed”, and print the
graph. This graph will be used in the next exercise of this unit.

a If you want to perform the additional experiments, skip the next step, then
return to it when all additional manipulations are finished.

30. On the Power Supply, make sure that the main power switch is set to
the O (off) position, then turn the 24 V ac power source off. Close the
LVDAC-EMS software. Turn the Four-Quadrant Dynamometer/Power Supply
off. Disconnect all leads and return them to their storage location.

23
© Festo Didactic 88943-00

24
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Conclusion

Additional experiments (optional)

Motor speed-versus-armature voltage and motor torque-versus-armature current

characteristics for reversed armature connections

You can obtain graphs of the dc motor speed n as a function of the armature
voltage EÆ, and dc motor torque T as a function of the armature current IÆ, with
reversed armature connections. To do so, make sure that the Power Supply is
turned off [main power switch set to the O (off) position] and reverse the
connections at the variable dc voltage output (voltage source ES) in Figure 2-16.
Make sure that the voltage control knob of the Power Supply is set to 0%. Refer
to steps 13 to 25 of this exercise to record the necessary data and obtain the
graphs. This will allow you to verify that the linear relationships between the
motor speed n and armature voltage EÆ, and between the motor torque T and
armature current IÆ, are valid regardless of the polarity of the armature
voltage EÆ. Recalculating constants K1 and K2 will show you that their values are
independent of the armature voltage polarity.

CONCLUSION In this exercise, you learned how to measure the armature resistance of a
dc motor. You saw that the rotation speed of a separately-excited dc motor is
proportional to the armature voltage applied to the motor. You saw that the
torque produced by a dc motor is proportional to the armature current. You
observed that the dc motor speed decreases with increasing armature current
when the armature voltage is fixed. You demonstrated that this speed decrease
is caused by the increasing voltage drop across the armature resistor as the
armature current increases.

If you performed the additional experiments, you observed that the speed-
versus-armature voltage and torque-versus-armature current relationships are
not affected by the polarity of the armature voltage. You also observed that the
direction of rotation is reversed when the polarity of the armature voltage is
reversed.

REVIEW QUESTIONS 1. What kind of relationship exists between the rotation speed and armature
voltage of a separately-excited dc motor?

2. What kind of relationship exists between the torque and armature current of a
separately-excited dc motor as long as the armature current does not exceed
the nominal value?

3. Connecting a dc power source to the armature of a dc motor that operates

without field current and measuring the voltage that produces nominal
current flow in the armature allows which parameter of the dc motor to be
determined?

© Festo Didactic 88943-00

25
Ex. 2-1 – The Separately-Excited DC Motor ⬥ Review Questions

4. Does the rotation speed of a separately-excited dc motor increase or

decrease when the armature current increases?

5. The armature resistance RÆ and constant K1 of a dc motor are 0.5 Ω and

5 r/min/V, respectively. A voltage of 200 V is applied to this motor. The no-
load armature current is 2 A. At full load, the armature current increases to
50 A. What are the no-load and full-load speeds of the motor?

© Festo Didactic 88943-00 61

26
Final Comments:

..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................

..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................
..................................................................................................................................

..................................................................................................................................

27
 Appendix A

Equipment Utilization Chart

The following equipment is required to perform the exercises in this manual.

Equipment Exercise
Model Description 1-1 2-1 2-2 2-3 3-1 3-2
8134(1) Workstation 1 1 1 1 1 1
8211 DC Motor/Generator 1 1 1 1 1 1
8254 Universal Motor 1
8311(2) Resistive Load 1 1 1 1
8821 Power Supply 1 1 1 1 1 1
8942 Timing Belt 1 1 1 1 1 1
8951-L Connection Leads 1 1 1 1 1 1
8960-C(3) Four-Quadrant Dynamometer/Power Supply 1 1 1 1 1 1
8990 Host Computer 1 1 1 1 1 1
9063-B(4) Data Acquisition and Control Interface 1 1 1 1 1 1
(1) The Mobile Workstation, Model 8110-2, can also be used.
(2) Resistive Load unit with voltage rating corresponding to your local ac power network voltage.
Use model variant -00, -01, -02, -05, -06, -07, or -0A.
(3) Model 8960-C consists of the Four-Quadrant Dynamometer/Power Supply, Model 8960-2, with

control functions 8968-1 and 8968-2.

(4) Model 9063-B consists of the Data Acquisition and Control Interface, model 9063, with

control function set 9069-1.

© Festo Didactic 88943-00

28
REFERENCES

1. LabVolt/Festo Student Manual 88943-00: Exercise 2-1

29