Faculty of Engineering, Architecture and ScienceDepartment of Electrical and Computer Engineering

LAB INSTRUCTIONSEES 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=kϕ I a (1)E=kϕω (2)

where T denotes the developed (internal) torque; E denotes the counter electromotive force (c-emf); I a 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 V a is given by V a=E+Ra I a (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

ω=V akϕ

−Ra

(kϕ )2T (4)

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 I f and, therefore, can be varied by either the field voltage V f or the field resistance R f, or by both.

Pre-Lab AssignmentThe 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 I f . Table P1 shows the result.

Table P1: I f−E characteristic at n=1745 rpm.I f (mA) E (V ¿ kϕ (Nm /A)

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

P1- Calculate kϕ corresponding to each value of I f , and complete Table P1. Then plot kϕ versus I f on Graph P1. Comment on the variation of kϕ, as I f is increased.

Graph P1: Magnetizing curve of the DC motor.

kϕ ¿)

I f (mA)

P2- For the machine described above, calculate the no-load speed (in rpm) at an armature voltage of V a=120V and a field current of I f=250mA. Ignore the rotational losses.

P3- If the field current is reduced to I f=175mA, what should the armature voltage V a 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 8211DC 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 V f is constant at about 120V , whereas the field resistance R f (and therefore the field current I f ) can be varied by the “rheostat”. Clockwise rotation of the rheostat reduces the field resistance and, thus, increases I f . 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 V a 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 de-energized.

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 250mA (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 120V . 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, 120V and 250mA. 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 V a=120V and I f=250mA.n(rpm) I a(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 V a to 120V and I f to 250mA, before each measurement.

Table E1.4: Different I a−n pairs, for V a=120V and I f=250mA.Dynamometer’s

reading V a(V ) I f (mA) I a(A) n(rpm)

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

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 100V by turning the voltage knob counterclockwise, but maintain the field current at 250mA (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 V a=100V and I f=250mA.n(rpm) I a(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 V a to 100V and I f to 250mA, before each measurement.

Table E2.3: Different I a−n pairs, for V a=100V and I f=250mA.Dynamometer’s

reading V a(V ) I f (mA) I a(A) n(rpm)

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

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 175mA by turning the rheostat counterclockwise, but maintain the armature voltage at 100V (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 V a=100V and I f=175mA.n(rpm) I a(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 V a to 100V , and I f to 175mA.

Table E3.3: Different I a−n pairs, for V a=100V and I f=175mA.Dynamometer’s

reading V a(V ) I f (mA) I a(A) n(rpm)

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

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 I a−n points from Table E1.4, calculate kϕ and Ra of the machine, for V a=120V and I f=250mA; 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 V a=120V and I f=250mA.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 2.5Nm, 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 V a=120V and I f=250mA.Dynamometer’s reading from

Table E1.4

I a(A)From Table E1.4

n(rpm)From Table E1.4

T=kϕI a(Nm)Take kϕ from Table C1.1

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 )

T (N .m )

Graph C1: Theoretical and experimental torque-speed curvesfor V a=120V and I f=250mA.

C2.1Using equations (2), (3), and (5), and any two I a−n points from Table E2.3, calculate kϕ and Ra of the machine, for V a=100V and I f=250mA; 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 V a=100V and I f=250mA.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 2.5Nm, 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 V a=100V and I f=250mA.Dynamometer’s reading from

Table E2.3

I a(A)From Table E2.3

n(rpm)From Table E2.3

T=kϕI a(Nm)Take kϕ from Table C2.1

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

Graph C2: Theoretical and experimental torque-speed curvesfor V a=100V and I f=250mA.

n (rpm )

T (N .m )

C3.1 Using equations (2), (3), and (5), and any two I a−n points from Table E3.3, calculate kϕ and Ra of the machine, for V a=100V and I f=175mA; 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 V a=100V and I f=175mA.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 2.5Nm, 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 V a=100V and I f=175mA.Dynamometer’s reading from

Table E3.3

I a(A)From Table E3.3

n(rpm)From Table E3.3

T=kϕI a(Nm)Take kϕ from Table C3.1

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

Graph C3: Theoretical and experimental torque-speed curves for V a=100V and I f=175mA.

n (rpm )

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 V a=120V and I f=250mA.Dynamometer’s

reading n(rpm) ω (rad /s ) P¿=V a I a(W ) Pout=Tω(W ) η=Pout /P¿

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

%

O utput P o w e r (W )

Graph C4: Efficiency versus shaft power, for V a=120V and I f=250mA.

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.

Graph C5: Theoretical torque-speed curves resulted from the experiments E1, E2, and E3.

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n (rpm )

T (N .m )

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) ω (rad /s ) I a(A) Prot−loss(W ) T rot−loss(Nm)

0

0

0

Last updated Jan. 28, 2014—AY