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Department of Electrical and Computer Engineering
LAB INSTRUCTIONS
EES 612 – ELECTRICAL MACHINES AND ACTUATORS
EXPERIMENT # 2: SEPARATELY EXCITED DC MOTOR
Introduction
DC motors are widely employed in such devices as power shovels, printing presses,
traction equipment, golf carts, power wheelchairs, cooling fan drivers, car engine starters,
wind-shield wipers, and power mirrors, to name just a few. They are also used in
manufacturing and processing applications where easy speed (and/or torque) control is
needed. Small DC motors are also used as servomotors in position control applications.
This experiment investigates the characteristics of a separately excited DC motor.
In general, a DC motor is described by the two following fundamental equations:
𝑇 = 𝑘𝜙𝐼𝑎
𝐸 = 𝑘𝜙𝜔
(1)
(2)
where 𝑇 denotes the developed (internal) torque; 𝐸 denotes the counter electromotive
force (c-emf); 𝐼𝑎 denotes the armature current; 𝜔 is the shaft speed (in rad/s); and 𝑘𝜙 is
the so-called flux constant of the machine (in Nm/A or Volt-Second).
In a shunt or a separately excited motor, the armature terminal voltage 𝑉𝑎 is given by
𝑉𝑎 = 𝐸 + 𝑅𝑎 𝐼𝑎 (3)
where 𝑅𝑎 denotes the armature resistance. Thus, the so-called torque-speed
characteristic of a shunt or a separately excited DC motor can be found by combining
the Equations (1) through (3), as
𝑉
𝑅𝑎
𝜔 = 𝑎 − (𝑘𝜙)
2 𝑇 (4)
𝑘𝜙
The shaft speed expressed in rpm, 𝑛, is related to that expressed in rad/s, 𝜔, by
𝑛 = 9.55𝜔 or 𝜔 = 𝑛/9.55 (5)
In permanent-magnet machines the value of 𝑘𝜙 is fixed by the magnets establishing the
air-gap magnetic field, whereas in separately excited machines the value of 𝑘𝜙 is a
function of the field current 𝐼𝑓 and, therefore, can be varied by either the field voltage 𝑉𝑓 or
the field resistance 𝑅𝑓 , or by both.
Pre-Lab Assignment
The DC motor you will be using in the lab has been spun by another motor at a constant
speed of 1745 𝑟𝑝𝑚, and its open-circuit armature voltage (which is the same as 𝐸) has
been measured for different values of the field current 𝐼𝑓 . Table P1 shows the result.
Table P1: 𝑰𝒇 − 𝑬 characteristic at 𝒏 = 𝟏𝟕𝟒𝟓 𝒓𝒑𝒎.
𝑰𝒇 (𝒎𝑨)
𝑬 (𝑽)
0
10.9
50
27.8
100
50.4
150
76.5
160
88.4
175
94.3
200
94.6
225
111.8
250
112.1
275
125.0
300
127.0
325
136.1
350
137.8
370
143.6
375
144.1
400
146.8
410
148.6
𝒌𝝓 (𝑵𝒎/𝑨)
P1- Calculate 𝑘𝜙 corresponding to each value of 𝐼𝑓 , and complete Table P1. Then plot 𝑘𝜙
versus 𝐼𝑓 on Graph P1. Comment on the variation of 𝑘𝜙, as 𝐼𝑓 is increased.
𝒌𝝓 (𝑽𝒔)
Graph P1: Magnetizing curve of the DC motor.
𝑰𝒇 (𝒎𝑨)
P2- For the machine described above, calculate the no-load speed (in rpm) at an
armature voltage of 𝑉𝑎 = 120 𝑉 and a field current of 𝐼𝑓 = 250 𝑚𝐴. Ignore the
rotational losses.
P3- If the field current is reduced to 𝐼𝑓 = 175 𝑚𝐴, what should the armature voltage 𝑉𝑎 be
changed to, in order for the no-load speed to remain at the same value as that in P2?
Lab Work
1. General safety note
To prevent injuries or damage to equipment, the power source must be turned
OFF prior to wiring up the circuit. Ask your TA to check.
2. Equipment
DC machine module EMS 8211
DC power supply module EMS 8821 (for applying armature and field voltages)
Dynamometer module EMS 8911 (for applying load torque)
Hand-held tachometer (for measuring shaft speed)
Bench-top digital multimeter (for measuring armature voltage)
Hand-held clamp-on ammeters (for measuring armature and field currents)
3. Circuit
Connect the circuit of Figure 1 which ensures that the DC machine is to be controlled
as a separately excited motor. In the circuit of Figure 1, the field voltage 𝑉𝑓 is constant
at about 120 𝑉, whereas the field resistance 𝑅𝑓 (and therefore the field current 𝐼𝑓 ) can
be varied by the “rheostat”. Clockwise rotation of the rheostat reduces the field
resistance and, thus, increases 𝑰𝒇 . The armature voltage, however, can be varied by
the “voltage knob”. The shaft torque applied by the dynamometer can be varied by
the “torque knob”. A clockwise rotation of each knob increases the
corresponding quantity that the knob controls. The circle labeled as 𝑉𝑎 represents
a voltmeter connection for armature voltage measurements, and the other two circles
represent armature and field current measurements by the clamp-on ammeters.
Important
 Make sure that, using the dedicated button of the clamp-on ammeter, you zero
the reading of an ammeter after you place its clamp around the wire, but
before you turn on the power supply.

If the ammeter goes to sleep (i.e., it turns off on its own) during the experiment,
do not turn it off and on. Rather, press the “Hold” button twice to wake it up.
Otherwise, you will have to zero its reading again while the circuit is deenergized.
Figure 1: DC machine configured as a separately excited motor.
4. Experiments
E1: Torque-Speed Characteristic at Full Field and Armature Voltage
E1.1 With the power supply module off, turn both knobs and the rheostat fully
counterclockwise (to ensure zero armature voltage, zero shaft torque, and minimum
field current). Then turn on the power supply and adjust the rheostat to bring the
field current up to 250 𝑚𝐴 (monitor the field current by the clamp ammeter). The
motor must not spin at this stage (since the armature voltage is zero); if it does,
something is terribly wrong!
E1.2 Gradually turn the voltage knob clockwise and raise the armature voltage to 120 𝑉.
This should result in clockwise rotation of the motor. The torque knob must still be
kept at its fully counterclockwise position, such that the dynamometer’s scale
displays zero. Thus, the motor experiences no shaft torque. However, it nonetheless
combats the rotational losses and, consequently, its armature current is not zero.
E1.3 Wait for a few minutes to allow the armature and field windings to warm up. This
mitigates the drift of the resistances. Thereafter, if needed, readjust the armature
voltage and the field current to, respectively, 120 𝑉 and 250 𝑚𝐴. Report the shaft
speed (measured by the tachometer) and armature current (measured by the
corresponding clamp ammeter) in Table E1.3.
Table E1.3: No-load shaft speed and armature current, for 𝑽𝒂 = 𝟏𝟐𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
𝒏 (𝒓𝒑𝒎)
𝑰𝒂 (𝑨)
E1.4 Gradually increase the shaft torque by turning the torque knob clockwise. Measure
the armature current and shaft speed for each of the dynamometer’s reading listed in
Table E1.4. If needed, readjust 𝑉𝑎 to 120 𝑉 and 𝐼𝑓 to 250 𝑚𝐴, before each
measurement.
Table E1.4: Different 𝑰𝒂 − 𝒏 pairs, for 𝑽𝒂 = 𝟏𝟐𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
Dynamometer’s
reading
𝑽𝒂 (𝑽)
𝑰𝒇 (𝒎𝑨)
0.1
120
250
0.2
120
250
0.3
120
250
0.4
120
250
0.5
120
250
0.6
120
250
0.7
120
250
0.8
120
250
0.9
120
250
1.0
120
250
1.1
120
250
1.2
120
250
1.3
120
250
1.4
120
250
𝑰𝒂 (𝑨)
𝒏 (𝒓𝒑𝒎)
E1.5 Turn the torque knob fully counterclockwise, but do not turn off the power supply.
E2: Torque-Speed Characteristic at Full Field, but Reduced Armature Voltage
E2.1 Continuing from Step E1.5 above, reduce the armature voltage to 100 𝑉 by turning
the voltage knob counterclockwise, but maintain the field current at 250 𝑚𝐴
(readjust if necessary). Notice the shaft speed reduction. The dynamometer’s scale
should display a shaft torque of about zero. Thus, the motor operates with no shaft
load, at a reduced armature voltage.
E2.2 Note down the shaft speed and armature current in Table E2.2.
Table E2.2: No-load shaft speed and armature current, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
𝒏 (𝒓𝒑𝒎)
𝑰𝒂 (𝑨)
E2.3 Gradually increase the shaft torque by turning the torque knob clockwise. Measure
the armature current and shaft speed for each of the dynamometer’s readings listed
in Table E2.3. If needed, readjust 𝑉𝑎 to 100 𝑉 and 𝐼𝑓 to 250 𝑚𝐴, before each
measurement.
Table E2.3: Different 𝑰𝒂 − 𝒏 pairs, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
Dynamometer’s
reading
𝑽𝒂 (𝑽)
𝑰𝒇 (𝒎𝑨)
0.1
100
250
0.2
100
250
0.3
100
250
0.4
100
250
0.5
100
250
0.6
100
250
0.7
100
250
0.8
100
250
0.9
100
250
1.0
100
250
1.1
100
250
1.2
100
250
1.3
100
250
1.4
100
250
𝑰𝒂 (𝑨)
𝒏 (𝒓𝒑𝒎)
E2.4 Turn the torque knob fully counterclockwise, but do not turn off the power supply.
E3: Torque-Speed Characteristic at Reduced Field and Armature Voltage
E3.1 Continuing from Step E2.4 above, bring the field current down to 175 𝑚𝐴 by turning
the rheostat counterclockwise, but maintain the armature voltage at 100 𝑉 (readjust
if necessary). Notice that this increases the shaft speed. The dynamometer’s scale
should display a shaft torque of about zero. Therefore, the motor works with no shaft
load, at a reduced armature voltage and field current.
E3.2 Note down the shaft speed and armature current in Table E3.2.
Table E3.2: No-load shaft speed and armature current, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟏𝟕𝟓 𝒎𝑨.
𝒏 (𝒓𝒑𝒎)
𝑰𝒂 (𝑨)
E3.3 Gradually load the shaft by turning the torque knob clockwise. Measure the
armature current and shaft speed for each of the dynamometer’s readings listed in
Table E3.3. If needed, readjust 𝑉𝑎 to 100 𝑉, and 𝐼𝑓 to 175 𝑚𝐴.
Table E3.3: Different 𝑰𝒂 − 𝒏 pairs, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟏𝟕𝟓 𝒎𝑨.
Dynamometer’s
reading
𝑽𝒂 (𝑽)
𝑰𝒇 (𝒎𝑨)
0.1
100
175
0.2
100
175
0.3
100
175
0.4
100
175
0.5
100
175
0.6
100
175
0.7
100
175
0.8
100
175
0.9
100
175
1.0
100
175
𝑰𝒂 (𝑨)
𝒏 (𝒓𝒑𝒎)
E3.4 Turn the torque knob fully counterclockwise. Then, turn the voltage knob
counterclockwise, such that the motor comes to a standstill. Turn off the power
supply and all the meters.
Conclusions and Remarks
C1.1 Using equations (2), (3), and (5), and any two 𝐼𝑎 − 𝑛 points from Table E1.4,
calculate 𝑘𝜙 and 𝑅𝑎 of the machine, for 𝑉𝑎 = 120 𝑉 and 𝐼𝑓 = 250 𝑚𝐴; for better
accuracy, the two points should be the extremes, i.e., one from the top and the
other from the bottom of the table. Show all the work. Report the results in Table
C1.1, below. Then, compare this value of 𝑘𝜙 with the value of 𝑘𝜙 you found in P1.
Calculate their difference as a percent of the latter, i.e., as a percent of the value of
𝑘𝜙 you found in P1.
Table C1.1: 𝑹𝒂 and 𝒌𝝓, for 𝑽𝒂 = 𝟏𝟐𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
𝑹𝒂 (𝛀)
𝒌𝝓 (𝑵𝒎/𝑨)
C1.2 Using the calculated value of 𝑘𝜙 from Table C1.1, and the measured armature
currents from Table E1.4, calculate the developed torque for each corresponding
shaft speed. Report the result in Table C1.2 below. Then, plot 𝑛 versus 𝑇 on Graph
C1 (show 𝑇 on the horizontal axis); label the curve as “experimental”. Use
appropriate data ranges and ticks for the axes, such that graphs’ space is efficiently
utilized (for example, 𝑻 should range from 0 to 2.5 𝑁𝑚, in steps of 0.1, etc.). Next, on
the same graph, plot the straight line that Equation (4) represents, and title it
“theoretical”. Again, assume the values of 𝑹𝒂 and 𝒌𝝓 from Table C1.1.
Comment on the torque-speed characteristic of the motor and the disagreements
between the “experimental” and “theoretical” curves. State your reasons for the
discrepancies.
Table C1.2: Calculated developed torque versus shaft speed, for 𝑽𝒂 = 𝟏𝟐𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
Dynamometer’s
reading from
Table E1.4
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
𝑰𝒂 (𝑨)
From Table E1.4
𝒏 (𝒓𝒑𝒎)
From Table E1.4
𝑻 = 𝒌𝝓𝑰𝒂 (𝑵𝒎)
Take 𝒌𝝓 from Table C1.1
n (rpm)
T (N.m)
Graph C1: Theoretical and experimental torque-speed curves
for 𝑉𝑎 = 120 𝑉 and 𝐼𝑓 = 250 𝑚𝐴.
C2.1 Using equations (2), (3), and (5), and any two 𝐼𝑎 − 𝑛 points from Table E2.3,
calculate 𝑘𝜙 and 𝑅𝑎 of the machine, for 𝑉𝑎 = 100 𝑉 and 𝐼𝑓 = 250 𝑚𝐴; show all the
work. Report the results in Table C2.1. Then, compare this value of 𝑘𝜙 with the
value of 𝑘𝜙 you found in P1; calculate their difference as a percent of the latter.
Table C2.1: 𝑹𝒂 and 𝒌𝝓, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
𝑹𝒂 (𝛀)
𝒌𝝓 (𝑵𝒎/𝑨)
C2.2 Using the calculated value of 𝑘𝜙 from Table C2.1, and the armature currents from
Table E2.3, calculate the developed torque for each corresponding shaft speed.
Report the result in Table C2.2 below. Then, plot 𝑛 versus 𝑇 on Graph C2 (show 𝑇
on the horizontal axis); label the curve as “experimental”. Use appropriate data
ranges and ticks for the axes, such that graphs’ space is efficiently utilized (for
example, 𝑻 should range from 0 to 2.5 𝑁𝑚, in steps of 0.1, etc.). Next, on the same
graph, plot the straight line that Equation (4) represents, and title it “theoretical”.
Again, assume the values of 𝑹𝒂 and 𝒌𝝓 from Table C2.1.
Comment on the torque-speed characteristic of the motor and the disagreements
between the “experimental” and “theoretical” curves. State your reasons for the
discrepancies.
Table C2.2: Calculated developed torque versus shaft speed, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
Dynamometer’s
reading from
Table E2.3
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
𝑰𝒂 (𝑨)
From Table E2.3
𝒏 (𝒓𝒑𝒎)
From Table E2.3
𝑻 = 𝒌𝝓𝑰𝒂 (𝑵𝒎)
Take 𝒌𝝓 from Table C2.1
n (rpm)
T (N.m)
Graph C2: Theoretical and experimental torque-speed curves
for 𝑉𝑎 = 100 𝑉 and 𝐼𝑓 = 250 𝑚𝐴.
C3.1 Using equations (2), (3), and (5), and any two 𝐼𝑎 − 𝑛 points from Table E3.3,
calculate 𝑘𝜙 and 𝑅𝑎 of the machine, for 𝑉𝑎 = 100 𝑉 and 𝐼𝑓 = 175 𝑚𝐴; show all the
work. Report the results in Table C3.1. Then, compare this value of 𝑘𝜙 with the
value of 𝑘𝜙 you found in P1; calculate their difference as a percent of the latter.
Table C3.1: 𝑹𝒂 and 𝒌𝝓, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟏𝟕𝟓 𝒎𝑨.
𝑹𝒂 (𝛀)
𝒌𝝓 (𝑵𝒎/𝑨)
C3.2 Using the calculated value of 𝑘𝜙 from Table C3.1, and the armature currents from
Table E3.3, calculate the developed torque for each corresponding shaft speed.
Report the result in Table C3.2 below. Then, plot 𝑛 versus 𝑇 on Graph C3 (show 𝑇
on the horizontal axis); label the curve as “experimental”. Use appropriate data
ranges and ticks for the axes, such that graphs’ space is efficiently utilized (for
example, 𝑻 should range from 0 to 2.5 𝑁𝑚, in steps of 0.1, etc.). Next, on the same
graph, plot the straight line that Equation (4) represents, and title it “theoretical”.
Again, assume the values of 𝑹𝒂 and 𝒌𝝓 from Table C3.1.
Comment on the torque-speed characteristic of the motor and the disagreements
between the “experimental” and “theoretical” curves. State your reasons for the
discrepancies.
Table C3.2: Calculated developed torque versus shaft speed, for 𝑽𝒂 = 𝟏𝟎𝟎 𝑽 and 𝑰𝒇 = 𝟏𝟕𝟓 𝒎𝑨.
Dynamometer’s
reading from
Table E3.3
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
𝑰𝒂 (𝑨)
From Table E3.3
𝒏 (𝒓𝒑𝒎)
From Table E3.3
𝑻 = 𝒌𝝓𝑰𝒂 (𝑵𝒎)
Take 𝒌𝝓 from Table C3.1
n (rpm)
Graph C3: Theoretical and experimental torque-speed
curves for 𝑉𝑎 = 100 𝑉 and 𝐼𝑓 = 175 𝑚𝐴.
T (N.m)
C4 Using the data of Tables E1.4 and C1.2, calculate the motor’s output power (shaft
power) and efficiency, for each value of the shaft torque. Complete Table C4, and
plot the efficiency versus the output power of the motor on Graph C4. Comment on
the variations of efficiency as a function of the output power.
Table C4: Input power, output power, and efficiency, for 𝑽𝒂 = 𝟏𝟐𝟎 𝑽 and 𝑰𝒇 = 𝟐𝟓𝟎 𝒎𝑨.
Dynamometer’s
reading
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
𝒏 (𝒓𝒑𝒎) 𝝎(𝒓𝒂𝒅/𝒔)
𝑷𝒊𝒏 = 𝑽𝒂 𝑰𝒂 (𝑾)
𝑷𝒐𝒖𝒕 = 𝑻𝝎 (𝑾)
𝜼 = 𝑷𝒐𝒖𝒕 /𝑷𝒊𝒏
%
Output Power
(W)
Graph C4: Efficiency versus shaft power, for 𝑉𝑎 = 120 𝑉 and 𝐼𝑓 = 250 𝑚𝐴.
C5 Plot all the three theoretical curves of C1.2, C2.2, and C3.2 on Graph C5, and
comment on the effect of the following practices on the torque-speed characteristic of
a separately excited DC motor: (1) only armature voltage reduction, and (2) both
armature voltage reduction and field weakening.
Also, explain why in this experiment we did not weaken the field alone, but also
reduced the armature voltage along with it.
n (rpm)
T (N.m)
Graph C5: Theoretical torque-speed curves resulted from the experiments
E1, E2, and E3.
Blank Page
C6 Using the measurements of E1.3, E2.2, and E3.2, and the results of C1.1, C2.1, and
C3.1, calculate the motor’s rotational power loss and its associated torque, for each of
the three test conditions. Show all the work. Report the results in Table C6.
Table C6: Rotational power loss and torque, for the three test conditions.
Dynamometer’s
reading
𝒏 (𝒓𝒑𝒎)
0
0
0
Last updated Jan. 28, 2014—AY
𝝎(𝒓𝒂𝒅/𝒔)
𝑰𝒂 (𝑨)
𝑷𝒓𝒐𝒕−𝒍𝒐𝒔𝒔 (𝑾)
𝑻𝒓𝒐𝒕−𝒍𝒐𝒔𝒔 (𝑵𝒎)